Secondary batteries have become important for smart grid and electric vehicle applications, and massive effort has been dedicated to optimizing the current generation and improving their energy density. Multi‐electron chemistry has paved a new path for the breaking of the barriers that exist in traditional battery research and applications, and provided new ideas for developing new battery systems that meet energy density requirements. An in‐depth understanding of multi‐electron chemistries in terms of the charge transfer mechanisms occuring during their electrochemical processes is necessary and urgent for the modification of secondary battery materials and development of secondary battery systems. In this Review, multi‐electron chemistry for high energy density electrode materials and the corresponding secondary battery systems are discussed. Specifically, four battery systems based on multi‐electron reactions are classified in this review: lithium‐ and sodium‐ion batteries based on monovalent cations; rechargeable batteries based on the insertion of polyvalent cations beyond those of alkali metals; metal–air batteries, and Li–S batteries. It is noted that challenges still exist in the development of multi‐electron chemistries that must be overcome to meet the energy density requirements of different battery systems, and much effort has more effort to be devoted to this. Understanding of the multi‐electron mechanisms of electrochemical processes is crucial to guide the design of advanced electrode materials and their corresponding application in secondary batteries. In this review, we discuss the multi‐electron redox reactions that occur during the electrochemical processes of different battery technologies. Although there are many excellent reviews regarding the electrochemistry of different battery chemistries, our aim is to explore new battery systems and novel electrode materials from the perspective of multi‐electron reactions. We discuss the theoretical foundation of multi‐electron reactions in Section 2For a given chemical reaction (1), electrochemical energy storage occurs when charge transfers.ΔrGΘ can be calculated according to the Nernst equation:The Gibbs free energy of the reaction under standard conditions n refers to the number of the charge transferred during the reaction per mole of reactant, E is the thermodynamic equilibrium voltage, and F is the Faraday constant. Energy density can always be expressed by the gravimetric energy density Wh kg−1 or the volumetric energy density Wh L−1. Thus, the gravimetric energy density εM and volumetric energy density εV of a battery can be calculated using Equation Here, ∑M represents the sum of the formula mole weights of the two reactants and ∑VM is the sum of the formula mole volume of the two reactants.Where M refers to the mole weight of the electrode material):The specific capacity of a given electrode material can be calculated from Equation Using Equations According to Equations FigureFigure+2/Ti+4, V+2/V+5, Cr+2/Cr+6, Mn+2/Mn+4, Fe+2/Fe+4, Co+2/Co+4, Ni+2/Ni+4, Cu+1/Cu+3, Nb+3/Nb+5, and Mo+3/Mo+6.4)n− have been extensively studied and are known to exhibit the characters required for an multi‐electron reaction mechanism. Transition metal oxide‐based materials can be classified as conversion‐type active materials and have high theoretical capacity as anodes for LIBs and NIBs (sodium‐ion batteries). Active elements  capable of electrochemically alloying with Li and Na have high charge capacity owing to the relatively high stoichiometric ratio of Li that electrode can accommodate. A comparison of these capacities is shown in Figure Careful analyses of the periodic table of elements have been performed to explore active elements suitable for multi‐electron electrochemical reactions. In these systems, Li and Na are charge carriers, and charge transfers from the variably valent transition metal cations upon Li and Na intercalation into the hosts. Thus, intercalation more than one Li or Na ion is required for multi‐electron transfer during the electrochemical process. Multi‐electron reactions can be achieved by insertion of a polyvalent cation. Comparisons of the physical properties of alkali metals such as Li, Na, and K, and multivalent cations such as Mg, Ca, Zn, and Al as charge carriers for rechargeable batteries are shown in Figure Multi‐electron reaction materials are of obvious interest for high capacity battery materials. During the past few years, in‐depth investigation of multi‐electron mechanisms has promoted the further exploration of novel electroactive materials. Moreover, it need to meet new challenges of innovation of the design and optimization of new materials. It has been said that “technology is always limited by the materials available”, and this is reasonably applicable to battery technology and science at present.3+ or Na+ into the host structure. This corresponds to a reduction in the valence of the metal redox center to maintain electroneutrality.FigureMultivalent chemistries have been intensively researched since multi‐electron reactions were first proposed to offer the possibility of improving battery energy density.3.1+ or Na+ into a crystalline host lattice. Intercalation reactions involve layered host lattices owing to their structural flexibility and free adjustment of their interlayer spacing according to the geometry of the intercalated guest species.Intercalation refers to the reversible intercalation of mobile guest ions such as Li2 and LiNi1/3Co1/3Mn1/3O2. LiCoO2 is the most commonly used positive material, and has a high theoretical capacity of 274 mAh g−1. However, LiCoO2 yields a specific capacity of no more than 150 mAh g−1 because its structure tends to collapse when more than 50% of the Li‐ions have de‐intercalated from the host lattice.xCoO2, Co3+ is oxidized to unstable Co4+, and a high concentration of Co4+ damages the cathode crystallinity and leads to a phase transformation.1/3Co1/3Mn1/3O2, a more stable structure against collapse at higher voltage with a sustainably reversible capacity of 190 mAh g−1. However, during lithium insertion these 3d metal oxides still cannot reversibly change their oxidation state by accommodating more than one lithium ion per metal without the destruction of their crystalline structure. Intercalation in this review refers to the reversible intercalation of mobile guest ions such as Li+ and Na+ into the crystalline host lattice.Research on cathode materials for lithium‐ion batteries has been almost completely focused on layered LiCoO3.2.12O5) belongs to the transition metal oxide family and is often employed as a secondary battery cathode. V2O5 has a layered structure held together by weak van der Waals interactions, described by a square‐pyramidal coordination of V5+ with five oxygen atoms  at 0.35 < x < 0.7, δ‐ phase (3.15 V) at 0.7 < x < 1, and γ‐ phase (2.21 V) at 1 < x < 2.2O5 leads to the irreversible formation of ω‐phase (2 < x < 3) LixV2O5, which has a rock salt type structure.] Reports have demonstrated that all these phases can be reversibly cycled in the stoichiometric range of 0 ≤ x ≤ 3, even though the structure undergoes irreversible transformation.Li–V2O5 combined with structural change during the deep charge–discharge process hinders its development as cathode for secondary batteries.] Electrochemical performance is influenced by a number of physicochemical properties such as structure and morphology. The fabrication of electrode materials in nanoscale and architectured forms is a common approach used to improve electrochemical performance. Nanostructured V2O5 modified towards a more open structure  interlayer spacing of 11.53 Å  can accommodate Na insertion and extraction. The crystal structure is described in Figure 2O5 nanobelts have exhibited a high capacity of 231.4 mAh g−1 at 80 mA g−1 .V+ or 2 Na+ can be inserted into crystalline V2O5. It is worth noting that V2O5 aerogel stores charge through a special mechanism which enables it to reversibly uptake more than 2 Li+. Chemical lithiation studies have shown that up to 5.8 equivalents of Li per mole of V2O5 aerogel can be inserted, corresponding to a specific capacity of 650 mAh g−1.2O5 aerogel delivered 345.5 , 324, 291, and 240 mAh g−1 at C/20, C/10, C/4 and 1 C rate, respectively. When used as a sodium‐ion battery cathode, the electrode delivered about 150 mAhg−1, corresponding to the insertion of 1 Na+ equiv. per mole of V2O5, indicating a more pronounced spread of sodium‐site energies.Under the classical charge‐storage mechanism discussed above, only 2 Li2O5 aerogels provide enhanced capacity for both lithium and polyvalent cation n−, in which tetrahedral (XO4)n− and MO6 octahedra combine via strong covalent bonding, have become a very extensive area of cathode research besides oxides.During charge and discharge, the reactants transforms from the initial phase to another, such as LiMPO4, M = Fe, Mn, Co, Ni, or V), which contain phosphorus atoms tetrahedrally coordinated by oxygen, have been the most intensively studied. LiFePO4 was first reported by J. Goodenough+2/Fe+4 two‐electron redox couple, only the Fe3+/Fe2+ redox energy lies close to the Fermi energy; the +3/+4 voltage is too high for current electrolytes, thus restricting it to a suitable level of 3.4 V that allows one Li+ reversible extraction/insertion per formula unit. This voltage issue excludes the practical use of LiFePO4 based on the +2/+4 couple.Phosphates (LiMPO3V2(PO4)3 (LVP), which has two polymorphs, i.e., rhombohedral and monoclinic phases. Rhombohedral Li3V2(PO4)3 was first prepared by ion exchange from Na3V2(PO4)3 and first investigated for its electrochemical behavior upon Li extraction.[[qv: 50b]] The charge–discharge profile of Li3V2(PO4)3 confirms that two electrons can be extracted during the first charge process, the potential of which is up to 4.1 V. However, a polarization is observed in the discharge curve and only 1.3 Li are reinserted. These results suggest a kinetic limitation to the reinsertion of Li, which is caused by structural change upon the extraction of two Li+. The monoclinic phase  is thermodynamically stable and preferable for use as a cathode owing to its 3D open framework for fast lithium ion diffusion and accommodation of up to 2 Li+ in the 3.0–4.3 V vs Li/Li+ window.Another extensively studied polyanion compound is Li3V2(PO4)3 → Li2.5V2(PO4)3 (≈3.6 V vs Li/Li+), Li2.5V2(PO4)3 → Li2V2(PO4)3 (≈3.7 V vs Li/Li+), and Li2V2(PO4)3 → LiV2(PO4)3 (≈4.1 V vs Li/Li+) are exhibited in Figure+.] The third lithium, extracted at voltage of 4.6 V vs Li/Li+, induces a two‐phase reaction between LiV2(PO4)3 and V2(PO4)3 associated with partial oxidation of V4+ to V5+.2(PO4)3 display fairly close average bond distances and average vanadium valencies of +4.5, resulting in a disordered lithium reinsertion 3, further leading to rapid specific capacity fading.In this window, no solid solution region is observed. The corresponding phase transition processes of Li2MSiO4, M = Fe, Mn, Co, Ni) are another type of polyanion cathode material. The 3d metal elements usually have multiple oxidation states, which allow for multi‐electron reactions during the charge–discharge process. Orthosilicates are able to allow the transfer of more than one electron per 3d metal owing to their special framework.4)4− group can theoretically allow the 3d metal to exchange its valence state between +2 to +4, thus enabling two lithium to be reversibly de‐/inserted during the electrochemical process.[[qv: 50a]] Li2FeSiO4 provides the ability to exchange two‐electron reaction for Fe (II) to Fe (IV) with a theoretical specific capacity of 331 mAh g−1. During Li+ extraction/insertion between Li2FeSiO4 and LiFeSiO4 , the volume change of the crystal structure is reported to be only 2.8%.3+, the resulting significant structural collapse of the FeSiO4 leads to drastic capacity fading. Moreover, the calculated de‐intercalation potential of the second Li ion in Li2FeSiO4 is 4.79 V, which is beyond the limit of most electrolytes. Hence, this two‐electron reaction is hard to access at room temperature. Intergrown nanocomposite Li2FeSiO4•LiFePO4–C has been successfully synthesized using the strategy of introducing an electrochemically active component, LiFePO4, to Li2FeSiO4. This composite has been reported to reversibly react with 1.26 Li at 15°C and 1.72 Li at 45 °C.Orthosilicates  with two‐dimensional pathways for Li‐ion diffusion and monoclinic forms (P21/n and Pn) with Li‐ion pathways that are interconnected in three dimensions.+ intercalation potentials for Pmnb, Pmn21, and P21/n are are 4.18, 4.19, and 4.08 V, respectively.−1, was achieved during the first cycle, and the discharge capacity suffered drastic fading on subsequent cycles.[[qv: 59a]] When more than 1.66 Li+ was delithiated from the host, the crystalline structure could not be recovered after the subsequent discharge. The main reason for this large irreversible capacity and drastic capacity fading of Li2MnSiO4 is the formation of amorphous‐like regions and continuous changing of the lattice parameters during the extraction and insertion of Li ions. Another factor leading to the destruction of the crystalline structure is the Jahn–Teller effect associated with Mn3+ of Li2MnSiO4. Thus, limited progress has been attained for Li2MnSiO4 so far.Another key member of the silicate family is Li3)3− is the lightest polyanion group, borate compounds would be very attractive cathode materials. Although LiMBO3  cathode materials have been easily synthesized by solid state reaction,Because (BOThe key limiations of polyanion compounds have been their intrinsic low electronic conductivity and slow lithium‐ion diffusion rate. Various methods have been adopted in attempts to improve their conductivity: 1) introducing guest atoms into the crystal structure;2+/Fe3+, Mn2+/Mn3+, and Cu1+/Cu2+ exhibit only one‐electron reactions combined with the safety requirements for phosphate. Only vanadium (V3+ to V5+) and molybdenum (Mo3+ to Mo6+) have redox couples that could provide two‐electron activity in phosphates. Hence, much improvement could be expected for multi‐electron phosphates if the voltage window could be increased to a larger region. A voltage design strategy, computed by state‐of‐the‐art ab initio calculations, involving mixing a metal active on the +2/+3 couple  with an element which can be active at up to +5 or +6, such as V and Mo, has been proposed.Meanwhile, it is known that the choice of synthetic method can greatly affect the structure and morphology, and subsequently the electrochemical performance of cathode materials. Thus, the synthesis–structure–performance relationship of novel electrode materials should be further investigated. Investigations in the development of polyanion materials into matured electrode materials that to meet the requirements of high energy density are still in their early stages. Although polyanion compounds provide the possibility of multi‐electron reaction with specific characteristics, the development of stable polyanion frameworks for hosting multi‐electron reactions is still a challenge. As discussed above, in the 3.0–4.5 V voltage window, active redox couples Fe3.2.14Ti5O12 (LTO) is considered to be a promising anode material for Li‐ion batteries because of its excellent characteristics such as low cost, safety, negligible volume change during the charge–discharge process, and long life cycling.d3–m space group, which forms a 3D tunnels with octahedral sites sharing edges.+ ions, while the octahedral 16d sites are occupied by both Li+ ions and Ti4+ ions randomly, with a Li+:Ti4+ ratio of 1:5.] Spinel‐LTO can be described as Li3(8a)[LiTi5]c(16d)O12(32e) , leading to a theoretical capacity of 175 mAh g−1. The corresponding electrochemical reaction can be expressed as shown in Equation:Spinel Li4+ to Ti3+ occurs during insertion, causing a phase transition from spinel LTO to rock‐salt‐Li4Ti5O12.+ to transfer from 8a to 16c sites, resulting in an end composition of Li6(16c)[LiTi5](16d)O12(32e) , and the additional Li+ inserts at octahedral (16c) and tetrahedral (8a) sites.7Ti5O12 at 1.0–0.01 V vs Li/Li+ as can be presented as:As stated previously, spinel‐LTO can accommodate up to three lithium ions, giving a theoretical capacity of 175 mAh g−1 within 2.5–0.01 V vs Li/Li+ at 0.2 C. The whole insertion process is shown in Figure 9T5O12 still has tetrahedral (8a) sites.The additional two Li‐ions inserted at vacant tetrahedral sites at low potential enhance the reversible capacity to 215.1 mAh g4Ti5O12 has also been reported as an anode material for Na‐ion batteries.+ insertion/extraction potential is ≈0.91 V. The reaction can be represented as:Very recently, spinel Li+ ions occupy 16c sites exclusively to form a Na6Li phase, while at the same time, Li8a ions are pushed to the nearest neighbor Li4 phase to form a Li7 phase, thus two new phases are created [[qv: 77a]] compared with that of Li+ (10−13–10−9 cm2 s−1).4Ti5O12 can be resolved by forming the Li4+δTi5O12 phase, which has a higher conductivity during Li+ and Na+ intercalation. Li4Ti5O12 has a relatively low energy density when compared with conventional anodes such as hard carbon, but its high operating voltage dispels the concern of metal dendrite formation. As an anode, the practical specific capacity of LTO is about 150–160 mAh g−1, which is lower than that of other anode materials such as graphite. Strategies that have been reported to improve the anode performance of LTO include nanostructuring,] lattice ion‐doping with metal cations] such as Co3+, V5+, Cu2+, and Mn4+, non‐metallic ion doping−, Cl−, and F−, or surface coating.The diffusion of Na3.3Organic electrode materials usually provide a high transfer electron number, low weight, and flexible structure resulting in a great potential for high energy density and rapid charge and discharge.Although the mechanisms of sodium ion storage in the following two kinds of organic materials are different, previous studies have found that both can be used as electrode materials with two‐ or multi‐electron transfer during charge and discharge processes. Especially for Na‐ion batteries, these flexible host materials can provide a retractable space for storage of the large ionic radius Na ions.3.3.1FigureFigureOrganic carbonyl compounds represent the earliest discovered organic electrode materials. They deliver a good multi‐electron transfer process and high theoretical specific capacity. The sodium storage mechanism of carbonyl compounds is regarded to be a reversible chemical bonding reaction that involves the breakage of the C=O bond and connection between Na and O  (PAQS)–graphene composite electrode synthetized by Wang et al. was found to discharge about 50% of its theoretical capacity within a very short period of 16 s.Carbon materials have high electronic conductivity, which can effectively promote the alkali metal ion diffusion kinetics of organic materials. Chen and co‐workers reported that a AQ/CMK‐3 cathode for sodium ion batteries had an optimized mass ratio of 1:1.[[qv: 84b]] The composite showed a discharge capacity of 214 mAh g−1 after 100 cycles. In contrast, latest research results have shown that there is a nanoeffect for Na2C8H4O4 anodes that causes the insertion/extraction process of Na+ to involve a one‐step electron transfer mechanism.−1. Its unprecedented cycling stability meant that 94.7% of its initial capacity was retained after 50 cycles at 0.2C.Using polymerization to increase the molecular weight of the material can restrict the dissolution of small molecules. Poly (PDBM), reported by Owen et al.,3.3.2In recent years, organic radical polymers—which mainly refers to aliphatic or non‐conjugated polymers—have attracted wide attention owing to their high charge and discharge capacity, excellent rate performance, and long cycle‐life. These advantages are attributed to the electrochemically active “nitroxide radical” group, which has rapid electron‐transfer ability and high chemical stability.The first reported organic radical cathode was poly (PTMA).[[qv: 91c]] It was reported to exhibit a high operating voltage of 3.6 V, high capacity  and long cycle life . In addition to being used as a cathode material, some radical polymers have also been proved to function as anode materials. For example, the poly(nitroxylstyrene)s produced by Nishide et al. could be used as both positive and negative electrodes.[[qv: 91b]]−1 after 5 cycles), the composite delivered an increased initial capacity of about 300 mAh g−1 and about 250 mAh g−1 after 100 cycles. The multi‐electron transfer process of PTMA can be illustrated as the transformation of an n‐type aminoxy anion into a p‐type oxoammonium cation through an intermediate with one free radical. Overall, the capacities of organic radical materials still need to be further improved.Although there are a lot of highlights to organic radical materials, the development of these batteries is still subject to some problems. The most important factor is their poor conductivity. In an attempt to overcome this problem, poly (PTMA)–Ketjenblack developed by Lemmon et al. achieved a good conductivity determined by electrochemical impedance spectroscopy (EIS).[[qv: 91a]] Compared with pure PTMA , metalloids , and polyatomic nonmetal P can alloy with lithium and sodium or form A‐Me  binary compounds.[[qv: 9b]] These electrode materials have the ability to interact with more than one lithium and sodium per metal atom, leading to a much higher specific capacity than that of common carbon anodes. Thus, these materials seem to be ideal candidates for the next generation of high energy density anode materials.4.4Si:4200 mAh g−1, Li4.4Ge: 1600 mAh g−1, Li4.4Sn:990 mAh g−1, Li3Sb:665 mAh g−1, Li3P: 2596 mAh g−1). As in the case of Li‐ion batteries, alloy based electrodes have been extensively studied in past decades because of their much higher reversible capacity than that of Li‐graphite systems .+/Na, are detrimental to Na‐ion batteries because the resulting large polarization imposes kinetic limitations on the electrochemical insertion of Na ions, which causes rapid capacity fading. Additionally, no experimental report has yet been published on the electrochemical sodiation of Si‐based materials, although it is known that Si can theoretically alloy with Na based on the formation of NaSi phase.] Note that Na alloying compounds possess about two‐fold lower capacity density than Li alloying compounds at the same volume expansion rate. This is because the ionic radius of Na is considerably larger than that of Li. The above issues indicate the significant challenges in employing alloying compounds as negative electrodes.As in the case for Li‐ion batteries, alloy materials as electrodes have been extensively studied in the past decades because of the much higher reversible capacity than that of the Li‐graphite systems  at full lithiation could have a significant negative impact on stress generation and fracture, leading to capacity fading.] Si anodes have been extensively studied, including both their electrochemical performance advantages and operational limitations, and the knowledge gained could serve as a foundation to further understand other types of anode electrodes undergo similar alloying process.This leads to an extremely high theoretical lithium storage capacity of 3579 mAh gStudies have revealed that during the initial charge, amorphous phase growth in the crystalline Si leads to the formation of sharp interfaces (≈1 nm thick).[[qv: 94a]] Investigation into the atomistic mechanism of dynamic lithiation using in situ transmission electron microscopy (TEM) has revealed that the lithiation kinetics are anisotropic and controlled by the migration of the interface.Several strategies have been experimentally evidenced to solve these problems:] 1) nanostructured particles such as nanowires,] thin films,] and mesoporous structuresThese results indicate that the combination of composite structure and geometry to overcome the challenges confronting silicon anode electrodes still needs further investigation to meet high energy density requirements.3.4.222Sn5 in the highest lithiation, leading to a theoretical capacity of 990 mAh g−1, which was later clearly identified by Mössbauer spectroscopy.Research on Sn has been focused on tin‐based oxides, which has shown significantly improved cyclic performance compared with that of stand‐alone Sn electrodes.15Sn4, delivering a theoretical capacity of 847 mAh g−1.FigurexSn phase was close to that of the NaSn2 phase. The third α‐NaxSn phase was identified a‐Na3Sn based on volumetric expansion measurements.As an anode electrode, Sn is expected to react with sodium to form fully sodiated crystalline Na3.4.34 times higher than that of silicon) and fast lithium diffusion (400 times faster than in Si at room temperature).] These advantages ensure a high rate capability and efficient charge transport, which offsets the effect of the relatively large atomic size of Ge. The Ge lithiation process ultimately forms Li15Ge4,−1, and the delithiation process results in the formation of a porous amorphous phase.+/Na with a theoretical capacity of 369 mAh g−1,Germanium is an extensively studied anode material owing to its superior intrinsic electrical conductivity  does not form during delithiation owing to its high melting point of 825 °C. Recently, many research groups have demonstrated that Sb based Na‐ion batteries show very stable capacity retention over 100 cycles and a high reversible capacity of about 500–600 mAh g−1, with current peaks observed at 0.77 and 0.90 V vs Na+/Na.3Sb despite the bulk Sb particles used, and showed better cycle performance in Na‐ion batteries than in Li‐ion batteries.[[qv: 113c]]3Sb and Na3Sb are about 135% and 293%, respectively.3Sb . In Sb based Li‐ion batteries, the Sb electrodes suffer from detrimental volume changes during cycling, which degrades the electrochemical performance.As for the study of Sb, a large amount of research has been published despite its high voltage plateau at over 0.8 V vs lithium. The lithiation processAlloy binary compounds have been explored to improve electrochemical performance. Active‐inactive and active‐active binary alloys are two types of binary compounds widely investigated. Active elements can be alloyed with inactive elements to alter electrochemical performance, grain structure, and reduce alloy volume change during electrochemical processes.[[qv: 8b]] Addition of an active phase can lead to higher energy density, and even take part in lithiation reactions in some examples. Alloys of two active elements possess characteristics different from those of single‐active materials. Here, we only address Sn‐based alloy examples, because different alloy combinations are complex. Sn‐Sb alloys have attracted great attention owing to their superior electrochemical performance as an active/active binary compound. SnSb alloys undergo a two‐step reaction as shown in Equations −1. In the first step, the conversion reaction occurs first and forms Li3Sb rearranged in Sn domains, which buffer the volume expansion of the Sn phase. Li–Sn alloy forms in the subsequent step , which are beneficial for realizing both high power and energy density.] Black P can be synthesized from commercially available amorphous red P through a simple high energy mechanical milling technique at high temperature and pressure. This simple transformation method is expected to be widely applicable in the electronics industry, owing to the semiconducting nature of black P. It has a similar structure to graphite, with a layered structure of puckered sheets. Graphite is comprised of stacks of six membered carbon rings located in the same plane and has an interlayer distance of 3.35 Å. In the case of black P, each phosphorus atoms is bonded with two other atoms lying on the same plane and with one phosphorus atom in a different plane. This creates a network of six‐membered P6 rings that further forms a flaky layered material in which the interplanar distance is 3.09 Å.] However, black P does not undergo the intercalation reaction characteristic to graphite. The uptake of lithium ions or sodium ions by black P to the final compositions of Li3P and Na3P, respectively, occurs with the breaking of P–P bonds, which is different to the intercalation reaction in graphite, where the lithium ions are stored between the graphene layers without inducing any C–C bond breakage.Black phosphorus is the most thermodynamically stable allotrope of P, and has a high bulk electrical conductivity  pattern shown in Figure −1 over 100 cycles. The reactions involved in this voltage range followed the order Black P → LixP → LiP, corresponding to one electron transfer. Cui's group also successfully demonstrated high‐performance black phosphorus–graphite composite anodes for lithium‐ion batteries prepared by mechanochemical reaction through high energy mechanical milling.[[qv: 118a]] Robust P–C bonds have been generated between black P and a variety of carbon materials. The choice of carbon structure was found to play an essential role in the formation of stable P–C bonds, with graphite the best, retaining a reversible capacity of 1840 mAh g−1 after 100 cycles. Very recently, the discovery of a new 2D compound, monolayer black P, has attracted a great deal of attention.Black P was first used as an anode material for lithium‐ion batteries in 2007, prepared by the high‐energy mechanical milling method.[qv: 116b]] The obtained black P–carbon composites , which causes poor electrical contact between phosphorus particles and the conducting matrix, pulverization of the phosphorus particles, and continuous growth of the solid electrolyte interphase. A novel phosphorus/graphene nanosheet (P/G) hybrid nanostructures anode was developed through the ball‐milling of graphene stacks and red phosphorus.[[qv: 117a]] The graphene attached to the P particles via chemical bonding behaved as a conductive matrix that maintained electrical contact with the phosphorus during the volume change. This strategy dramatically improved the electrochemical performance of the P, with a reported specific capacity of 2077 mAh g−1 and enhanced retention of 95% after 60 cycles.However, the battery performance of these materials is still unsatisfactory, particularly in regards to fast capacity fading and low Coulombic efficiency. The main reason for this is the large volume change  and the level of lithiation and sodiation.Alloying anodes are promising electrode materials exhibiting high energy density because of their ability to uptake more than one Li atom. Si has especially been studied owing to its favorable properties. P and Ge also show high specific capacity and good rate capability. Other metals such as Sn possess some of the desired characteristics but also face challenges such large polarization and pulverization. However, these alloying anodes always exhibit rapidly decaying specific capacity during continuous cycling owing to their dramatic volume expansion and contraction during lithiation and sodiation. As for Li‐based alloys, Li atoms occupy a volume of 14.8 Å Li3.53.5.1The growing demand for electric vehicles and electronic devices has heightened research enthusiasm for high performance positive materials. If we want to obtain a breakthrough in cathode materials, complete utilization of high oxidation state transition metals should be key. The reversible electrochemical conversion reaction of high valence metal compounds can be described as follows:+, Na+, K+, etc.), M represents the transition metal ions , and X denotes certain anions . In particular, owing to their transfer of three electrons during the reaction process, metal fluoride‐based cathodes meet the target of light weight and multi electron transferring electrodes.Table− ions relative to the covalent characteristics of the Me–O and Me–S bonds.In which A represents the alkali metal ions  can be used in secondary cells.−1, far below their theoretical capacity.In fact, metal fluoride‐based cathodes were originally used for the primary batteries reported by Amatucci and co‐workers.Figure3 through one electron transfer, which is an intercalation mechanism like that of conventional transition metal oxide positive materials; secondly, the LiFeF3 product of the first step and two further lithium ions are transformed into LiF and Fe, which is a conversion mechanism. Researchers have designed batteries with high capacity and cycling stability based on this series of reactions. Fichtner and co‐workers−1 that remained at 270 mAh g−1 after 200 cycles. Their experiment proved that mixing FeF3 with carbon material can effectively improve the performance of FeF3. Subsequently, FeF3 nanoflowers on CNT branches obtained by a self‐assembly process were reported to display a high specific capacity of 210 mAh g−1 based on the conversion reaction process and excellent rate performance of about 150 mAh g−1 even at 500 mA g−1 current density  as an example, full reduction of the Co2+/Co3+ to form Co and Li2O leads to a capacity of 890.4 mAh g−1. In comparison, reducing Co4+ to Co3+ in the classical LiCoO2 intercalation compound leads to a capacity of 273.8 mAh g−1 .Transition metal oxides, including binary and mixed oxides, have been the most widely investigated high‐performance anode materials for both LIBs and NIBs in recent years because of their good safety, environmental friendliness, abundance, and notably high specific capacity achieved by conversion reactions.] Taking Co+ and Na+ via a redox conversion reaction, which results in the reversible formation of metallic nanoparticles dispersed intimately within the Li2O matrix that maintain electronic conductivity.] However, it is well known that metal oxides go through an electrochemically induced volume expansion upon the formation of Li2O during initial charging.With the rapid development of materials design strategies and synthesis techniques, great progress has been achieved in the fabrication of transition metal oxides with various nanostructures and enhanced electrochemical performance.] As anode materials for LIBs and NIBs, metal oxides typically store Lix, M = Ni, Fe, Mn, Co, etc.), in which M may have a variety of chemical valences, react with alkali ions (Li+ and Na+) via a multi‐electron conversion process, as given in Equation .Binary transition metal oxides , low cost, low toxicity, and natural abundance.SnO2 nanowires during electrochemical charging have suggested that the following electrochemical reactions occur:The results of in situ observation, using a nanoscale electrochemical device inside a transmission electron microscope, of the lithiation of single SnO+/Li, the SnO2 is transformed to Li2O and Sn.2O causes instant volume expansion. The subsequent charging and discharging are related to the second reversible alloying reaction between LixSn and Sn (24); amorphous Li2O does not participate in the electrochemical reaction. The in situ experiments described above confirmed that the nanowire consisted of nanocrystalline LixSn and Sn dispersed in an amorphous Li2O matrix. The nanowire underwent dynamic structural evolution after charging, as can be seen in Figure2O and LixSn.2 upon initial charging is summarized by the schematic drawing in Figure 2O.2O behaves as buffering domain during Li–Sn alloy formation and decomposition, thus the dispersion of nano‐Sn in amorphous Li2O can maintain the structural integrity of the anode. It also provides ionically conducting medium for the Li ion migration and helps to keep the electrochemically formed nano‐Sn metal particles apart and prevents their agglomeration.2 based electrodes. Sn precipitation causes loss of contact of the Sn particles, possible short‐circuit hazard, and Sn‐catalyzed electrolyte decomposition.2 electrodes is an effective strategy to prevent Sn further growth that may short the battery, by changing the kinetics of Sn precipitation. A number of composite electrodes have been developed and extensively studied in an attempt to obtain a more homogenous current density for better electrochemical performance.Upon initial charging to ca. 0.9 V vs Li−1. Further, Wang and co‐workers showed that octahedral SnO2 nanoparticles (≈60 nm) obtained using a hydrothermal method delivered a reversible capacity of about 500 mAh g−1 with stable cycle performance as an anode for sodium‐ion batteries. The improved cyclability was attributed to the retardation of Sn aggregation during cycling by the Na2O matrix. However, the actual deployment of its bulk counterpart is restricted by some intrinsic drawbacks: (1) The huge volume change upon alloying and de‐alloying (≈300 %) leads to pulverization of the electrode materials and loss of electrical contact with binders and current collectors. (2) A poor electrical conductivity at room temperature further restricts the electrode kinetics at high rates and comprises the cyclability of the electrode.]The theoretical reversible lithium storage capacity for the second reaction is readily calculated to be 790 mAh g2/carbon composites have been extensively studied. These composites not only accommodate the volume change and protect the SnO2 from aggregation and pulverization, but also enhance the electronic conductivity of overall electrode.2‐electrodeposited porous carbon nanofiber (PCNF@SnO2@C) composite was introduced as a sodium‐ion battery anode material.[[qv: 146d]] Electrochemical performance studies revealed that the as‐prepared composite anode possessed high‐capacity (374 mA g−1), good capacity retention (82.7%), and large Coulombic efficiency (98.9% after the 100th cycle). A free‐standing composite of carbon foam encapsulated SnO2 nanocrystallites with tunable size range was developed as an anode for LIBs, in which the carbon foam served as a build‐in hierarchical porous current collector to enhance the electrical conductivity of the anode.−1), impressive rate capability of up to 8 A g−1, and long cycle life of 250 cycles. This good cycling performance was attributed to the synergistic effect of the electrically conductive network, percolating macro‐pores, large surface area, and SnO2 nanocrystallites with tunable size range. These encouraging results emphasize that the synergistic effects produced by multiple modification strategies such as nanoscale and morphology engineering, integrated electrode design, and hierarchical porosity control can be comprehensively employed in the right balance to obtain facile kinetics and excellent structural robustness.[[qv: 150b]]Nanostructuring is therefore necessary to relieve the stress at the electrode surface and provide necessary void space for expansion.2SnO4, M = Co, Mn, Mg, Zn, have been studied. In this structure, tetravalent Sn ions occupy tetrahedral sites and bivalent M ions occupy octahedral sites. Doping of electrochemically inactive element prevents structural changes and enhances internal conductivity of Sn to involve all redox sites in conversion reaction.] In some cases, the inactive element can also take part in lithiation reactions, which can significantly affect the voltage curve, cycling performance, and lithiation kinetics.A good number of tin oxides with the inverse spinel structure, M2SnO4 prepared by solid state reaction or by hydrothermal synthesis, as well as by co‐precipitation, have been reported, either in pure form or as composites with carbon.] It has a large theoretical reversible capacity of 1231 mAh g−1 as a result of contributions from the alloying and de‐alloying of Sn and Zn and the conversion reactions involving Sn, Zn and Li2O. Spinel Zn2SnO4 particles prepared via a hydrothermal reaction with 0.4 m NaOH solution at 200 °C for 24 h are composed of uniform cube‐shaped crystallines.−1 in the first lithium insertion process, and 580 mAh g−1 was retained after 50 cycles. Moreover, HRTEM (high resolution transmission electron microscopy) and XRD data further confirmed the co‐existence of metallic Sn and Zn, and alloy precipitates of about 20 nm in size around the amorphous Li2O matrix.In recent years, studies on the Li cycling of Zn2SnO4 have confirmed that both alloying and de‐alloying processes and conversion reactions of Sn and Co with Li2O are involved in the Li cycling. However, pure cubic spinel Co2SnO4 (1105 mAh g−1) suffers from particle agglomeration and severe volumetric changes that result in poor cyclability.2SnO4/C composites with a mixed amorphous and crystalline structure were prepared for the first time, using a stepwise synthesis technique based on high‐energy ball milling, and showed better capacity retention and rate performance.3@C nanoboxes synthesized by thermal annealing have exhibited exceptional long‐term cycling stability over 400 cycles for highly reversible lithium storage.Also, EXAFS (extended X‐ray absorption fine structure) and Sn Mossbauer studies conducted on Co2O4  and CdFe2O4 are another kind of oxide that has been exploited for both alloying and de‐alloying and conversion reactions.Of particular note, mixed transition metal oxides with a spinel structure like ZnM2O4 is a cubic spinel structure oxide with bivalent Zn ions occupying the tetrahedral sites and Co ions occupying the octahedral sites. ZnCo2O4 has been considered an attractive candidate for the substitution of conventional graphite anodes in lithium ion batteries owing to its superiorities such as better reversible capacity, enhanced cycling stability, and good environmental benignity. So far, nanoparticles and porous nanoflakes of ZnCo2O4 have been synthesized and shown good first‐cycle reversible capacity as Li‐ion battery anode materials.For example, ZnCoThe metallic Zn can form an alloy with Li, and with cobalt can participate in the conversion as follows below:]2O4 are electrochemically discharged with Li‐metal, destruction of their crystal structure occurs followed by the formation of metallic Zn and Co nanoparticles and Li2O. Upon deep discharge, Zn can reversibly react with Li to form Li–Zn alloy at potentials below 1.0 V, and contribute to the anodic capacity, as shown in Equation 2O via the displacement reaction to form nanoparticles of the respective metal oxides. In the most favorable case, CoO can further react reversibly with Li2O to form Co3O4, as shown in Equation When nanosized particles of ZnCo2O4 prepared by a simple low‐temperature urea based combustion method was reported to yield a reversible capacity of ≈900 and ≈960 mAh g−1 when cycled at 25 °C and 55 °C, respectively, corresponding to 8.35 ± 0.05 mol of recyclable Li per mol of ZnCo2O4.[[qv: 163b]] This is the first time that Zn has been made to contribute to reversible capacity through both alloy formation and displacement reaction in a mixed oxide, and both metallic Zn and Co appear to act as matrix ions. However, the simple structures, poor electric conductivity, and large volume change of the ZnCo2O4 nanostructures during the electrochemical reaction led to fast capacity decrease.Nano‐phase ZnCo2O4, cadmium can also form an alloy with Li in a similar manner to Zn, and under optimal conditions Li3Cd is formed during Li cycling.[[qv: 163a]] 8.7 mol of Li can be reversibly cycled (810 mAh g−1) at 0.07 C in the range 0.005–3.0 V at ambient temperature. The underlying reaction mechanism is combination of alloying, de‐alloying and conversion reactions of the “Li–Cd–Fe–Li2O” composite. However, cadmium is toxic and not suitable for wide application.As for CdFeIn conclusion, transition metal oxides tend to exhibit large polarization and suffer from low electrical conductivity. Additionally, poor capacity retention and unstable solid electrolyte interphase formation at high current densities are common for these electrode materials because of their large volume changes during cycling processes. Effective strategies that have been employed to tackle these problems include fabricating carbon‐based composites to enhance electrical conductivity and designing unique nanostructures that can withstand volume changes.4Research into lithium‐ion batteries has striven for multi‐electron reactions, which offer the promise of improved energy density.−1, which is lower than that of lithium (3861 mAh g−1). However, this inferiority is compensated for by its superior volumetric capacity of 3833 mAh mL−1 , enhanced safety, low cost, and easy handling of magnesium metal.] Magnesium secondary batteries (MIB) based on the construction Mg|organic electrolyte|intercalation cathode are analogous to the above‐described alkali metal battery systems. Interestingly, a magnesium rechargeable battery offers the possibility of two‐electron reaction through the intercalation of one Mg2+ accompanied by a two‐electron reaction per transition metal center.Metallic magnesium has been considered to be an attractive anode material owing to advantageous characteristics such as natural abundance, low cost, high melting point (649°C), and very negative potential of ≈2.4 V vs SHE.] The ionic radius of the divalent magnesium cation is 0.86 Å, similar to the 0.9 Å of monovalent lithium. Magnesium electrochemistry is better understood than that of lithium electrodes in non‐aqueous electrolyte systems. In battery applications, the charge capacity of Mg is 2205 mAh g2+ ion transport.3SO2)2 as a salt with high stability and safety were successfully used in a magnesium ion battery, which showed an ambient temperature conductivity of 4.42 × 10−4 S cm−1, as good as that of an liquid electrolyte.2) with glyme–diglyme has been reported.2+/Mg anodic stability of this electrolyte enables it to be used with high‐voltage cathode materials.However, the construction of a MIB faces many challenges, despite the number of similarities in the nonaqueous electrochemistries of Mg and alkali metals. Moreover, magnesium electrochemistry is more complicated. First, in the case of Li electrodes, the passivating surface film that forms on the Li metal may be composed of Li‐ion conductors, products of reaction between polar aprotic solvents and commonly used salt anions, and metallic Mg which blocks Mg+2 to a large extent and thus allows for a high theoretical energy density. A number of excellent reviews on rechargeable magnesium batteries have been conducted, covering either individual cathodes or whole systems.] Results obtained in previous studied were tested under different experimental conditions. Nevertheless, in light of very recent research attention, an up‐to‐date review of the breakthroughs in MIBs has become very necessary. Herein, we briefly review the cathode materials which allow reversible electrochemical intercalation of Mg2+ and further discuss the problems that hamper the commercialization of MIBs.Moreover, selecting suitable cathode materials for Mg ion batteries with high reversible capacity and operating voltage is a thorny issue, even though extensive work has been carried out.+2 intercalation hosts , are a unique class of host materials that allow fast and reversible insertion of a variety of cations, such as Li+, Na+, Cu+, Fe2+, Zn2+, Ni2+, Co2+ and Mg2+, at ambient temperatures.6S8 has been proposed as the model cathode material for rechargeable Mg batteries owing to its unusual structure capable of reversibly electrochemically intercalating Mg ions with relatively fast kinetics. Chevrel phases have rhombohedral hexagonal crystal symmetry, space group R3¯, which consists of octahedral MO6 clusters : 6 tightly packed Mo atoms situated inside an almost regular cubic cage of 8 S atoms.6S8 is intrinsically deficient by 4 electrons. Thus, 4 neutralizing electrons are adopted upon 2 Mg intercalation to stabilize the electron deficient structure and the MO6 clusters do not act as classical redox centers like those of other transition metal compounds such as Co3O4 and MnO2.6S8 host can therefore provide fast redistribution of electronic charge and easily compensate for the charge imbalance during insertion, ensuring a high mobility and reversible reaction of Mg2+ in the host material.Various types of materials have been proposed as MgxMo3S4 cathodes and an electrolyte based on Mg organohaloaluminate salts.2+ insertion  and outer ring (site 2) owing to geometrical and electrostatic limitations  and practical capacity of less than 100 mAh gMg insertion is much more complicated and various phase compositions are formed during the whole intercalation process 6]3). Redox‐active organic materials have been investigated because their weaker intermolecular forces than those of inorganic materials may cause them to interact less strongly with Mg2+. 2,5‐dimethoxy‐1,4‐benzoquinone (DMBQ) was evaluated for Mg2+ storage in a cell with the structure Mg|0.5 m Mg(ClO4)2‐γ‐butyrolactone|DMBQ‐acetylene black‐PTFE.2+ per mole. The two voltage plateaus of the discharge curve correspond to successive one‐electron reduction steps.3D structures are promising cathode materials for rechargeable magnesium batteries. Prussian blue analogs (PBAs) with an open 3D frame structure are able to support the passage of a large number of MgxMSiO4  with olivine structure 3 is possible, at up to one Mg atom per host molecule with negligible lattice expansion.A variety of olivine‐type polyanion compounds have been evaluated as cathodes for MIBs owing to their good energy density and production from relatively “green” source materials and synthesis.as Figure16b all+2 intercalation ability of a number of transition‐metal oxides, sulfides, and borides by reduction with dibutylmagnesium.2 cathode  and HR‐TEM studies, it has been concluded that the magnesium insertion process is a conversion reaction on the surface of MnO2 rather than an intercalation reaction.2 with BET surface area above 70 m2 g–1 delivered a discharge capacity of 250 mAh g−1, suggesting that a higher surface area makes it easier to conduct the conversion reaction and enables a greater capacity, although the discharge capacity faded quickly to less than 50 mAh g−1 after 10 cycles. Another two dimensionally structured cathode material, molybdenum disulfide, has also attracted wide attention owing to its layered structure  has attracted great attention for many decades owing to its multiple possible valence states (V5+ → V3+), associated with multiple electron transfer, and its excellent lithium and sodium insertion properties as mentioned above. Research on V2O5 has shown that it also has potential as a host for polyvalent cationic guests such as Mg2+, Al3+, and Zn2+.2O5 .2+ intercalation, the c lattice parameter of MgxV2O5 shows a steady expansion, and high Mg concentration intercalation leads to phase transition of α‐V2O5 to δ‐V2O5, which involves V2O5 layers sliding along the a‐direction.2O5]n layers in the a‐direction, as shown in Figure x = 0 (α‐V2O5) to x = 1 (δ‐V2O5). Still, α‐V2O5 is a promising cathode material that shows reversible cycling of Mg, although the computed α phase migration barrier indicates poor Mg mobility. So far, all reported experimental efforts have succeeded in reversibly inserting about 0.5 Mg per V2O5 mole. Reducing the grain size of V2O5 is an effective method of improving the insertion stoichiometry to x ≈ 0.6.Because Mg intercalation is accompanied by twice the number of electrons of Li intercalation, the Mg–V5 Figure d. The in2O5 is calculated to be 1.26 eV using the nudged elastic band method , much higher than the 0.35 eV for Li ions, which explains the slow diffusion observed in experiments.2+ depends on the amount of H2O in the electrolyte.2O in the structure of the cathode material was illustrated by the use of vanadium bronzes; variations in the water content of identical bronzes dried at different temperature were found to be responsible for the difference in their electrochemical properties.[[qv: 198a]] The positive effect of incorporating water into the host crystal structure is believed to originate from a shielding mechanism of Mg2+ from direct intercalation with lattice oxygen atoms that further improves the Mg‐ion kinetics.Moreover, divalent Mg ion insertion into the host structure is difficult owing to its slow solid‐state diffusion, caused by the higher charge density arising from its divalent nature and variations in experimental conditions.[[qv: 15c]] Furthermore, the hopping barrier of Mg ions in α‐V2O5, promising polyvalent cation insertion results have been reported for V2O5 aerogels. Aerogels are attractive hosts because of their high surface area and small diffusion distances for a solid‐state material.2O5 aerogels are able to reversibly insert ions of high charge and large ionic radius beyond lithium, such as Na+, K+, Mg2+, Ba2+, and Al3+.2O5 aerogels have been found to reversibly insert only 0.6 mol of Mg2+, which is in agreement with the values reported for nanocrystalline V2O5. The reason for this may be chemically bound water, which can react with the inserted metal in the aerogels and create hydrate compositions of up to V2O5•5H2O. The major difference between aerogel and bulk material is that the diffusion of Mg2+ ions into bulk V2O5 is slow and incomplete, and the majority of the Mg becomes located on the surface of the V2O5 crystals, whereas the high surface area and short diffusion distance of aerogels yields improved Mg2+ diffusion. The measured capacity of 180 mAh g−1 corresponds to a reduction of V from +5 to +3 oxidation state. Additionally, aerogels are excellent and versatile hosts for multi‐valent cations such as Mg2+, Ba2+, and Al3+.2+ per mole, as shown in Figure 2+, which can be reversibly inserted into the host structure, depends on temperature, morphology, particle size, synthesis method, and electrolyte system.Besides crystalline V2+ ions.2+ ions into the traditional cathodes. Much effort has been concentrated on the modification of ionic transport channels to improve the mobility of Mg2+ within intercalation cathodes.[[qv: 15a]] Three main strategies can be concluded to improve the kinetics of Mg transport: 1) Downsizing the particle size of the materials; 2) shielding of the Mg2+ ion charge with additional anion groups such as the oxygen of H2O molecules; 3) using cluster compounds as unique hosts, such as the Chevrel phases discussed above, that easily attain local electroneutrality.[[qv: 175d]]Research on rechargeable Mg batteries remains a challenge, even though Mg ion batteries are expected to become a new generation of batteries for large‐scale energy storage. The primary reason for the poor electrochemical performance of these batteries is the sluggish diffusion of Mg2+ cations and Xn− anions during electrochemical cycling has been proposed.2+ cations, and therefore ultra‐high rate capability is expected for this battery. This concept has provided a new viewpoint from which to develop Mg and other multivalent  batteries.A conceptual rechargeable Mg battery in which charge transfer is achieved via simultaneous transport of Mg−1.Calcium batteries are multivalent systems which have a higher cell voltage than magnesium systems and possess a high volumetric capacity of 2073 mAh mL−1, making aluminum a strong candidate for energy storage.[[qv: 15b]] The concept of a nonaqueous rechargeable Al‐ion battery in which Al cations can be reversibly intercalated has been proposed.Aluminum is the most abundant metallic element in the earth's crust, and its cost is significantly lower than that of most other metals.2O5 nanowires, anatase TiO2 nanotube arrays, copper hexacyanoferrate, and Chevrel phase M6S8 as mentioned above.3/1‐ethyl‐3‐methylimidazolium chloride ([EMICl]) electrolyte has realized a new revolution in battery technology.[[qv: 15b]] The respective reactions that occurred on the positive and negative materials can be expressed as follows:During the few past years, many kinds of cathode materials have been explored for Al‐ion batteries, such as VFigure−1 and amazing cycle life of more than 7500 cycles at a current density of 4000 mA g−1. The cell also exhibited well‐defined discharge voltage plateaus in the ranges of 2.25–2.0 V and 1.9–1.5 V, which were unprecedented when compared with past Al‐ion batteries. Moreover, the Al‐ion batteries had an ultrafast Al3+ ions insertion/extraction ability that exhibited stable cycling performance both at low and high current densities. Based on the “multi‐coordination ion/single ion intercalation and deintercalation” theory described above, a similar battery system was designed using Al foil as the anode and carbon paper as the cathode  than that that of the Li+ cation (7.6 Å) and Mg2+ cation (7.2 Å), which indicates a possibility of using Al3+ cation as a guest in intercalation chemistry and with lower volume expansion of the host compounds upon tri‐valent aluminum ion insertion.The aluminum electrode and pyrolytic graphite electrode, which had a foam structure  electrode and an oxygen evolution reaction (OER) porous electrode, and operates using the continuous and almost inexhaustible oxygen supply from the surrounding air.] The overall chemical reaction occurring inside a metal–air battery can be described as follows Equation .211 MultThe reason why metal–air batteries can be regarded as multi‐electron transport systems is related to the series of complex electrochemical reactions that occur on the air cathode. These reactions involve the participation of multiple electrons and generate several oxygen‐containing compounds.FigureTableThere are several kinds of metal–air batteries based on different reaction mechanisms with different metal species.−1 and a high theoretical voltage of 3.09 V. However, practically attainable values are much lower owing to the parasitic corrosion reaction that evolves hydrogen gas at the metallic negative electrode.[[qv: 16c]] Considerable efforts have been directed towards the development and commercialization of primary and mechanically rechargeable batteries. Mg–air batteries are not electrically rechargeable with the reversible oxygen reduction and evolution reactions because the electrodeposition of magnesium is not thermodynamically feasible in aqueous electrolytes.[[qv: 16c]] These versions are also applicable to Al–air batteries.−1.−1 is relatively low.Mg–air batteries have a high theoretical energy density of 2840 Wh kg2, Na–O2, and K–O2 are gaining rapidly increasing attention. In fact, most non‐aqueous metal–air systems based on alkali metals can be referred to as A–O2 batteries because most laboratory research has involved the use of an O2 tank, although many papers refer to the systems as metal–air. Non‐aqueous A–O2 batteries have a short history and research is still in progress. The general operating principles of an A–O2 battery are shown in Figure + cation at the anode/electrolyte interface, and the electron is transferred to the outer circuit. At the cathode side, oxygen is reduced to an O2− species  that may form an alkali metal superoxide (AO2) in the presence of A+ cations. In fact, the reactions of lithium, sodium, and potassium with oxygen are quite different, despite their close chemical relationship.Non‐aqueous metal–air batteries such as Li–O2 is highly unstable, it is only found as an intermediate species in Li–O2 cells. The Li–O2 system has a theoretical specific capacity of 3458 Wh g−1, which will fully meet the future battery needs. The possible reaction mechanism in Li–O2 batteries based on non‐aqueous electrolyte systems is envisioned as follows:Because LiO2O2 or Li2O. These reactions are usually called “ORRs” (oxygen reduction reactions). The generated lithium oxides are electrochemically decomposed to Li and O2 on charging. Generally, the formation of Li2O2 is an accepted mechanism which may involve two intermediate processes:The two respective electron transfer processes involve four‐electron and two‐electron pathways, which produce different reduction products. Oxygen is reduced on discharging to form Li2 intermediate is an important factor in the pathway of O2 reduction.It cannot be ignored that the solubility of the LiO2O2 to Li and O2 on charging reported by Ogasawara and co‐workers,Meanwhile, the electrochemical decomposition of Li2.2O2 can be divided into three stages.2‐like species on the surface  and multi‐walled carbon nanotube (CNT) composites synthetized by simple hydrothermal method have exhibited excellent electrochemical performance.2O4 exhibited high catalytic activity while the CNTs offered facile electron transport pathways. Lu and co‐workers designed sandwich structured Pd/MnOx/Pd nanomembranes synthetized by a roll‐up technique.x layer by the two Pd outer layers improved the power efficiency of the battery to 86%, and lowered the overpotential to 0.2 V. This work can be regarded as the first attempt to create a composite of two different types of catalyst.The current status of catalytic materials and structures for air electrodes has been reported in many important journals. Oxygen catalyst materials still face great challenges. Electrocatalysts for the ORR and OER play key catalytic roles and determine the power, energy density, and energy efficiency of metal–air systems based on both aqueous and nonaqueous electrolytes. Several groups of oxygen catalysts, including transition metal oxides, functional carbon materials, metal oxide–nanocarbon hybrid materials, metal–nitrogen complexes, transition metal nitrides, conductive polymers, and precious metal alloys have been discussed.2 g−1) and ORR activity has exhibited excellent practicability in metal–air batteries.A novel nitrogen‐doped exfoliated graphene air electrode with high surface area (2980 m1+xAlyGe2−y(PO4)3 (LAGP) as the electrolyte and a LAGP/single‐walled carbon nanotube (SWCNT) composite as the air cathode.Besides catalysts, non‐aqueous solvents for metal–air batteries have also been studied in detail. Many kinds of organic solvents such as dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dimethoxyethane (DME), and tetraethylene glycol dimethyl ether (TEGDME) have been tested in Li–air batteries.] Zhou and co‐workers developed an all‐solid‐state Li–air battery using Li2 still exist, and there is a big gap between the actual and theoretical capacity. Rational design of the electrode structure is important because the reactions involve the diffusion of O2 and precipitation of the discharged products. Pure oxygen is too difficult to handle in practical application, and substituting pure oxygen with air  may lead to irreversible reactions. For the sake of improving the actual capacity of air batteries, further research aiming at overcoming these technical issues is urgently needed, and the details of the operating mechanism should be deeply investigated.In summary, metal–air batteries are a promising energy storage technology owing to their high potential energy density. However, technical issues relating to the catalysts and the use of O6−1 within the next few years.−1 with a two‐electron redox reaction per atom.[[qv: 234b]] Additionally, the abundant resources and environmental friendliness of these systems are beneficial in terms of sustainable development.Lithium sulfur batteries have been extensively studied because they have the potential to reach practical energy densities of 500–600 Wh kg2S in an ambient environment, the total reaction can be inferred as follows:The first room temperature Li–S systems can be traced back to 2009, which successfully enhanced the capacity and cycling stability capable for batteries using ordered mesoporous CMK‐3 composited with sulfur.8 molecules and S42− ions with 4e− transfer, which delivers a capacity of 419 mAh g−1. The low plateau provides a high capacity of 1256 mAh g−1 and corresponds to the reduction from S42− to S2− with 12e− transfer. Therefore, Li–S batteries exhibit a high theoretical capacity of 1675 m Ah g−1 corresponding to the transfer of 16 mol of electrons. According to the further reported results of density functional theory (DFT),8 → S42− → S22− → S2−, as shown in FigureThrough theoretical calculation using the Nernst equation, the reaction process can be divided into a two‐stage scheme.8 gains two electrons to form S82− ions at 2.4 V. However, these S82− ions may undergo a disproportionation reaction to form S52− and S3•−. In the second step, sulfur free radicals S3•− and S62− species are reduced to S42− and S32− ions at 2.1 V. The disproportionation reactions occurring during the first two steps can be summarized as follows:The complex reaction process at the sulfur electrode during the lithiation can be distinguished from other multi‐electronic materials, which is a key factor for classifying it as a separate category. Actually, the various forms of sulfur will be obtained in different reaction environments .] Many studies have identified the presence of abundant polysulfide species in solution arising from disproportionation reactions. For instance, Barchasz et al.−1. In the third step, the S42− ions obtain six electrons to generate S2− ions at 2 V, producing 1256 mAh g−1. This process involves three separate transformation steps:Together, these reactions provide a capacity of 419 mAh g32−, S22−, and S2−, can be detected in the final products. Additionally, advanced in situ X‐ray absorption near edge structure (XANES) spectroscopy was applied to analyze the mechanisms of redox behavior at sulfur electrodes.42− and S62− despite their very short period. Subsequently, Lowe and co‐workers3•− and S4•−, which appeared at nearly 2.1–1.6 V. They also proposed that elemental sulfur is initially reduced to S8− rather than S82−. Therefore, there is no definite conclusion on the types of polysulfide involved. However, we have determined that the chemical equilibria of these polysulfides , graphene series materials, alloy materials  etc. can be used as superior electrode for LIBs with high energy density since they contain the most of multi‐electron elements in our summary , chalcogenides (MoSy Figure . These e


NFE2L2) is essential for preventing type 2 diabetes mellitus (T2DM)‐induced complications in animal models. This case and control study assessed genetic variants of NFE2L2 for associations with T2DM and its complications in Han Chinese volunteers. T2DM patients with (n = 214) or without (n = 236) complications, or healthy controls (n = 359), were genotyped for six NFE2L2 single nucleotide polymorphisms  with TaqMan Pre‐Designed SNP Genotyping and Sequence System. Serum levels of heme oxygenase‐1 (HMOX1) were determined through enzyme‐linked immunosorbent assay. Informative data were obtained for 341 cases and 266 controls. Between T2DM patients and controls, the genotypic and allelic frequencies and haplotypes of the SNPs were similar. However, there was a significant difference in genotypic and allelic frequencies of rs2364723, rs10497511, rs1962142 and rs6726395 between T2DM patients with and without complications, including peripheral neuropathy, nephropathy, retinopathy, foot ulcers and microangiopathy. Furthermore, HMOX1 levels were significantly higher in T2DM patients with complications than in controls. Multiple logistic regression analysis, however, showed that only rs2364723 significantly reduced levels of serum HMOX1 in T2DM patients for the GG genotype carriers compared with participants with CG+CC genotype. The data suggest that although NFE2L2 rs2364723, rs10497511, rs1962142 and rs6726395 were not associated with T2DM risk, they were significantly associated with complications of T2DM. In addition, only for rs2364723 higher serum HMOX1 levels were found in the T2DM patients with CG+CC than those with GG genotype.The transcription factor nuclear factor erythroid 2‐like 2 ( Type 2 diabetes mellitus (T2DM) is characterized by insulin resistance, with or without defects in insulin production and secretion. Globally, up to 90% of all diabetes cases are T2DM, and the disease is a leading world heath challenge To date, there is no effective treatment for T2DM. Clinically, T2DM management consists of changes in lifestyle, lowering other cardiovascular risk factors and maintaining blood glucose levels in the normal range The development of T2DM and its complications are likely because of multiple factors, including the lifestyle and genetic changes To date, there have been numerous studies regarding links between SNPs and the risk of T2DM. Through genetic association research, some SNPs of different genes have been found to increase the risk of developing T2DM in certain populations The protein nuclear factor erythroid 2‐like 2  is a basic leucine zipper During non‐stressed conditions, NFE2L2 is sequestered in the cytoplasm by the repressor protein KEAP1 (kelch‐like ECH [erythroid cell‐derived protein with CNC homology]‐associated protein 1) and targeted for proteasomal degradation In this study, therefore, we investigated NFE2L2 polymorphisms for associations with risk of T2DM and its complications in a cohort of Chinese volunteers of Han descent. We also investigated whether NFE2L2 SNPs are related to expression of its downstream gene, HMOX1.For this study we recruited volunteers of Han Chinese descent: 450 patients (214 cases with complications and 236 cases without complications) and 359 healthy controls with the same age range (Table 2).The T2DM patients were originally from Northeast China, and the age‐matched healthy control participants (with neither T2DM nor family history of diabetes) were randomly selected from the health examination clinics of our hospital. During enrolment, each participant filled out a questionnaire regarding health, diagnosis of type of diabetes and T2DM duration, family history of diabetes and ethnic background Table . Clinicahttp://www.hapmap.org) to download NFE2L2 SNP data from the Han Chinese in Beijing database. Data were processed with Haploview software 4.2.0.0 . Linkage disequilibrium blocks were constructed in accordance with a previous study r2 of 0.80 were applied to capture the common variations within the blocks covering NFE2L2. Most of these sites were also checked in PubMed for its consistency with published studies We used the website of the International HapMap (Haplotype Mapping) Project  in accordance with the manufacturer's protocol, and quantified by Nanodrop .The Tag SNPs were genotyped with TaqMan Pre‐Designed SNP genotyping kits  and the ABI PRISM 7300 Sequence Detection System (Applied Biosystems). The polymerase chain reaction (PCR) amplification was performed in a 25 μl reaction mixture under the following conditions: 40 cycles of 95°C for 10 min., 92°C for 15 sec. and 60°C for 1 min. The primers to detect NFE2L2 SNPs were synthesized by Applied Biosystems. The ABI probe codes were C_351878_10for rs2364723; C_31613510_10 for rs13001694; C_30175556_20 for rs10497511; C_11634983_10 for rs1806649; C_11634984_10 for rs1962142 and C_155538_10 for rs6726395.The PCR products were randomly selected for DNA sequencing analysis by Shanghai Sangon Biological Engineering Technology & Services. For each SNP we choose three cases and the results were compared with the results of Taqman genotyping.Serum HMOX1 levels were measured in 111 randomly selected T2DM patients (27 with complications and 84 without complications) and 53 controls with commercial ELISA kits  in accordance with the manufacturer's protocol.P‐value >0.05 indicated Hardy–Weinberg equilibrium. Genotype frequency and distribution in patients and controls were analysed by the chi‐squared test with statistical software Statistical Packages for Social Sciences . The association between the combined effects of NFE2L2 SNPs and T2DM (with or without diabetic complications) was analysed by SHEsis software as in a previous study The genotype frequency of NFE2L2 was assessed for Hardy–Weinberg equilibrium by the chi‐squared goodness‐of‐fit test. A t‐test and analysis of variance, respectively, and the data presented as mean ± S.D. Multivariate logistic regression analysis for the influence of clinical and genetic factors for serum HMOX1 levels was also performed. A P‐value ≤0.05 was considered statistically significant.Differences in serum HMOX1 levels or SNPs between patients and controls were analysed by Student's 2, respectively). The total cholesterol and total triglyceride in T2DM  were higher than control participants and control participants (55.7 ± 10.3 years) were similar, as were the gender distributions and BMI (25.7 ± 3.2 and 25.6 ± 2.7 kg/m3.2 yearsn = 115), nephropathy (n = 85), retinopathy (n = 33), foot ulcers (n = 8), microangiopathy (n = 9) and neurogenic bladder (n = 1). There was no difference for age, gender, BMI, total cholesterol and high‐density lipoprotein , suggesting the suitability of this sample pool for genetic analysis.The six NFE2L2 SNPs genotyped in this study were highly polymorphic and their genotypic distributions were in Hardy–Weinberg equilibrium (P > 0.05). However, there were significant differences both in genotypic frequencies and in allelic frequency of rs2364723, rs10497511, rs1962142 and rs6726395 between the T2DM patients without and with complications .Haplotypes of NFE2L2 were constructed for the patients and controls, and six major haplotypes with frequencies >3% were identified Table . The datWe randomly selected a subset of T2DM patients and control participants to evaluate serum HMOX1 levels with regard to its association with these NFE2L2 SNPs Tables  and 7. TP = 0.038], suggesting that NFE2L2 polymorphism of rs2364723 may cause an increase in serum HMOX1 level in T2DM patients.Multiple logistic regression analysis was performed to examine the correlations of HMOX1 level with NFE2L2 SNPs or other variables including gender, diabetic complication and age Table . We founNuclear factor erythroid 2‐like 2 is a transcription factor and functions to up‐regulate expression of antioxidant genes in response to oxidative stress The first issue is that we did not find the significant difference between NFE2L2 SNPs and T2DM in this study. This may be related to the diversity of risks responsible for the development of T2DM, including diet, age, environmental alterations and genetic factors. Although these risks may exist with the presence of oxidative stress, the latter is not the pivotal step for the development of T2DM. In a published study, the association between different NFE2L2 SNPs and human cancers was established Although diabetic complications were developed based on diabetes, the susceptibilities of diabetic individuals to diabetes‐caused secondary complications are different. For instance, generally speaking the risk for heart disease is six times higher for women with diabetes, but only two‐ to threefold in men with diabetes, compared with those without diabetes In addition, to our knowledge, T2DM can increase oxidative stress that is a major reason to induce T2DM complications It has been well‐reported that the oxidative stress was increased in the patients with diabetes and diabetic animals, reflected by enhanced oxidative stress biomarkers in the bloods and tissues, such as malondialdehyde, 3‐nitrotyrosine, 4‐hydroxynonenal and reactive oxygen species via up‐regulation of IL‐10 and IL‐1R antagonist expression et al. At the cellular level, activation of NFE2L2 results in the up‐regulated expression of many cytoprotective proteins, including HMOX1. The latter is an essential enzyme in heme catabolism and has anti‐inflammatory properties Limitations of this study include the high number of non‐informative samples, which was because of inadequate samples or PCR readings. Moreover, our study population was limited to the geographic area of Northeast China, which contains certain specific factors such as temperature (cold at winter and cool at summer compared with Southern populations of China), dietary and environmental differences, and even genetic background. All these may cause certain impacts on the serum antioxidant or oxidative stress marker patterns. In addition, we also found a significantly higher plasma level of HMOX1 in those carrying with the rs2364723 CG or CC allele than in those carrying the rs2364723 GG allele. As a result of relatively small sample size specifically for this observation in this study, we could not have exact explanation now. Thus, further studies with a larger sample size including diverse populations from different geographic areas are needed to verify differences relative to controls.In summary, this study investigated the association of NFE2L2 polymorphisms with T2DM and its complications. We found that multiple mutations of NFE2L2 rs2364723, though not associated with T2DM, were significantly associated with the prevalence of complications in T2DM, indicating that this gene locus may predispose towards diabetic complications. The mutation of NFE2L2 rs2364723 G allele was significantly associated with increased serum HMOX1 levels in T2DM patients.The authors have no conflicts of interest to declare for this work.Lu Cai and Lining Miao initiated and designed the study. Xiaohong Xu, Jing Sun, Xiaomin Chang, Ji Wang and Manyu Luo participated in recruiting samples and/or performed laboratory studies. Lu Cai, Lining Miao and Kupper A. Wintergerst periodically discussed the progression of project, data interpretation and wrote the draft of manuscript. Lu Cai and Kupper A. Wintergst revised and formed the final manuscript. All authors contributed to review of the draft and revised manuscript with certain suggestions.


Our computational efforts rely on the well-tested unbiased CALYPSO  method combined with density functional theory calculations. We find that all ground-state Re2X82− clusters have cube-like structures of Dh4 symmetry with two Re atoms encapsulated in halogen framework. The reasonable agreement between the simulated and experimental photoelectron spectrum of the Re2Cl82− cluster supports strongly the reliability of our computational strategy. The chemical bonding analysis reveals that the δ bond is the pivotal factor for the ground-state Re2X82−  clusters to maintain Dh4 symmetric cube-like structures, and the enhanced stability of Re2Cl82− is mainly attributed to the chemical bonding of 5d orbital of Re atoms and 3p orbital of Cl atoms.Dirhenium halide dianions received considerable attention in past decades due to the unusual metal–metal quadruple bond. The systematic structural evolution of dirhenium halide clusters has not been sufficiently studied and hence is not well-understood. In this work, we report an in-depth investigation on the structures and electronic properties of doubly charged dirhenium halide clusters Re Understanding the properties of the metal–metal multiple bond is significant to further elucidate the chemistry of transition metal complexes. As a prototypical metal–metal multiple bonded ion, Re2Cl82− has been studied extensively in the past decades22. The experimental and theoretical investigations mainly focused on the preparation6, the molecular structure8, the metal–metal multiple bond14 and the electronic properties22 of Re2Cl82− in the octachlorodirhenate salts. Previous studies13 have determined the geometry structure of Re2Cl82− to be of Dh4 symmetry, and confirmed the formation of Re–Re quadruple bond is due to the σ2π4δ2 subset of the bonding orbitals. Due to the difficulty of preparing the Re2Cl82− dianion in the gas phase, Wang et al.21 first measured the photoelectron spectrum (PES) of gaseous Re2Cl82− several decades after the discovery of the metal−metal multiple bond, leading to a thorough understanding of the Re–Re quadruple bond and its electronic structure.The discovery2Cl82−, similar treatments of the heavier halogen derivative Re2Br82−, which contains the Re–Re quadruple bond as well, are limited24. To our knowledge, sparse studies have been reported on the lighter halogen derivative Re2F82−. Peters et al.25 initially synthesized (n-Bu4N)2[Re2F8]·4H2O, which contains Re2F82−, via Re2Cl82− and (n-Bu4N)F·3H2O reacting in CH2Cl2 solution. Conradson et al.26 later confirmed the presence of the Re–Re quadruple bond in Re2F82− using X-ray absorption fine structure and resonance Raman spectroscopy. The structure of Re2F82− was determined to be eclipsed Dh4 geometry by Henkel et al.27. Recently, Balasekaran et al.28 prepared the (NH4)2[Re2F8]·2H2O salt in nearly 90% yield by the reaction of (n-Bu4N)2[Re2Cl8] with molten NH4HF2. They also investigated the molecular and electronic structure along with the electronic absorption spectrum of Re2F82− relying on multiconfigurational quantum chemical calculations. Compared to Re2F82−, studies on Re2I82− are sparse as well. Glicksman et al.29 firstly reported a synthesis route of Re2I82− via the reaction of the dirhenium complex Re2(O2CC6H5)4Cl2 and HI with Bu4N+ in ethanol or methanol to produce the (Bu4N)2Re2I8 salt. Preetz et al.30 prepared (Bu4N)2Re2I8 successfully for the first time in a simple procedure by the reaction of (Bu4N)2Re2X8 with HI in dichloromethane. Preetz et al.31 also reported measurements of the resonance Raman spectrum of (Bu4N)2Re2I8, while Cotton et al.32 studied its crystal structure until a few years later. Extensive studies on Re2X82− have been reported, but almost all previous studies focused on Re2X82− dianions in crystals. There has been no systematic work on Re2X82−  clusters until now, so the following questions attract our interests: (i) It is not clear whether Re2X82− clusters are characterized by the same molecular geometry as Re2X82− dianions in crystals. (ii) Does the chemical bonding in Re2X82− clusters differ from that of dianions in crystals? (iii) What is the relative stability of Re2X82− clusters? Consequently, we turn our attention to the systematic study of lowest-energy geometries and electronic structures of Re2X82−  clusters.Compared to the extent of the investigations on Re2X82− clusters, we perform a search for the the low-energy structures of Re2X82−  by means of the CALYPSO  code combined with DFT  calculations. We get the ground-state structures of Re2X82− clusters from the above calculations and subsequently investigate their relative stabilities and chemical bonding. This paper is organized as follows. The obtained results and a pertinent discussion are in the following. Then we summarize our conclusions. Last, a detailed presentation of the computational method is described.As a first step of our study on the structural evolution of Re2X82−  clusters, together with other three typical low-lying isomers are shown in Fig. 2X82−  clusters are all collected in Table 2F82−, Re2Cl82−, Re2Br82− and Re2I82−, respectively. The electronic states, point group symmetries, average binding energies, HOMO−LUMO  energy gaps, together with charge on Re atoms and halogen atoms of the ground-state Re2X82− clusters are summarized in Table The lowest-energy structures of the Re2X82−  all possess cube-like geometric structures with Dh4 symmetry. The geometric structures of ground-state Re2X82−  clusters favor low-spin 1A1g state and have the same symmetry with those in crystals32. With the increase of the halogen atom size, the Re–Re bond length in Re2X82− varies from Re2F82− to Re2I82− as 2.20 Å, 2.21 Å, 2.22 Å, 2.24 Å, showing a little difference to those of Re2X82− dianions in crystal2. The 2a isomer has a distorted cube-like structure with C2 symmetry and the energy is 1.14 eV higher than its ground-state structure 1a. The metastable isomer 3a of Re2F82− has Cs symmetry, at 1.71 eV higher than its global minimum 1a. Interestingly enough, the structure of 3a is a small square-bottom boat, and the two rhenium atoms are located in its interior. The point group symmetry of the 4a isomer is Cv2, and its energy is 3.36 eV higher than its lowest-energy structure 1a. It is obvious that the two rhenium atoms are exposed outside in the geometry structure of 4a. It can be clearly seen from Fig. C1 symmetry with its energy 1.16 eV higher than the ground-state structure 1b. The isomers of 3b and 4b display cage-like structure with the two Re atoms encapsulated in a chlorine framework. They possess Cs and Cv2 symmetry with the energy 1.22 eV and 1.47 eV, respectively, higher than their lowest-energy structures 1b. The isomer 2c has Cv2 symmetry, with total energy 0.28 eV higher than the ground-state 1c, whose structure is similar to that of 2b. The 3c structure of Cv2 symmetry looks similar to the structure of 4b, with total energy higher than 1c by 1.31 eV. The Cv2 symmetry 4c isomer is 3.07 eV less stable than the ground-state 1c. The 2d isomer of Cv2 symmetry has similar structure to the isomers 2b and 2c with the total energy 0.43 eV higher than 1d structure. The structure of 3d, which possesses Dd2 symmetry with total energy higher than the 1d one by 1.50 eV, is similar to that of 4b and 3c isomers. The Cv2 symmetry 4d isomer, at 2.43 eV higher in energy than the ground-state 1d, has similar structure to 4c. It is worth noting that, except for the 4a structure of Re2F82−, the two rhenium atoms are invariably inclined to stay inside for all ground and excited state structures of Re2X82− . In order to confirm the validity of our calculations, we have also optimized the Re2X82−  clusters at the BP86 level of theory. The corresponding lowest-energy geometries of Re2X82− clusters are presented in Figure Dh4 symmetry and the same electronic state of 1A1g. The respective ground-state structures are in agreement with those obtained by our B3LYP optimization, further supporting our claim for the reliability of our theoretical approach.It can be seen in Fig. 2X82−  and compared to the available experimental PES. Moreover, the values of adiabatic detachment energy (ADE), extracted from the threshold of the first peak, and the values of vertical detachment energy (VDE), obtained from the first peak maximum of the spectra, are summarized in Table 21.Photoelectron spectrum (PES) is a powerful tool, which can provide important information about the electronic configuration of ground-state structures. In order to verify the validity of our obtained structures in this work, as shown in Fig. 2X82− to the ground and excited states of singly charged Re2X8−. Overall, the binding energies, spacings and intensities are different in each spectrum, but the spacings and intensities of spectra features for Re2Br82− and Re2I82− are nearly similar to each other. The most prominent spectral feature of Re2F82− is the observation of a lower adiabatic detachment energy measured as 0.05 eV. This phenomenon is normal for multiply charged anions, largely due to the presence of intramolecular Coulomb repulsion34. It can be clearly seen that the first two peaks of the Re2F82− spectrum are very weak and appear in very lower binding energies. The following relatively intense peaks locate at 1.25 and 1.70 eV, respectively, and the most intense peak locates at 3.80 eV. For the simulated PES of Re2Cl82−, the ADE, namely the second electron binding energy, is 0.98 eV, which is consistent with the experimental datum of 1.00 eV. The first peak is located at 1.19 eV, in reasonable agreement with the first peak location of the experimental spectrum of Re2Cl82− centered at 1.16 eV. The following three peaks are relatively intense, located at 2.21, 2.80, and 3.24 eV, respectively. In the simulated PES of Re2Br82−, the first peak is quite intense, and emerges at 2.09 eV. In addition, there is a relatively sharp peak, which is the most intense one, located at 3.95 eV. The simulated spectrum of Re2I82− exhibits three obviously intense peaks and the first relatively weak peak emerges at 1.21 eV. The agreement of the ADE and VDE values of the simulated and experimental PES for Re2Cl82− supports the validity of our computational approach. We hope that the simulated PES of dirhenium halides can provide a helpful reference for the spectroscopic studies of Re2X82−  dianions.As shown in Fig. Eb). For Re2X82−  clusters, the Eb can be defined as follow:E(Re2X82−), E(Re), E(X), and E(X−) represent the energy of Re2X82− clusters, neutral Re atom, neutral X atom, and anionic X−, respectively. The average binding energies of Re2X82−  clusters are displayed in Fig. 2F82− is relatively stable due to its large average binding energy value. Besides, the Eb values of Re2X82− decrease monotonically from Re2F82− to Re2I82−, indicating that the relative stabilities of Re2X82−  decrease with increasing size of the halogen atom.To explore the relative stabilities of the ground state structures of the dirhenium halide system, we have calculated the average binding energies (2X82− clusters is the HOMO−LUMO energy gap (Egap), which represents the ability of an electron to jump from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). A large value of Egap implies the corresponding molecular structure is chemically inert. The evolution of the HOMO−LUMO energy gap for the ground-state Re2X82− clusters is shown in Fig. Egap decrease monotonically, that is to say, the clusters are relatively less reactive as the halogen atom size increases, in agreement with the obtained results for the average binding energies. However, in contrast to the average binding energy rapid decrease from Re2F82− to Re2I82−, the HOMO−LUMO energy gap decreases less steeply.Another important parameter related to the relative stabilities of the Re39 is an important chemical property to describe the ability of an atom to attract electrons. A large electronegativity value is interpreted as a strong ability to attract electrons. In order to investigate the charge transfer between Re atoms and halogen atoms for ground-state Re2X82− clusters and compare to the electronegativity, we utilize natural population analysis (NPA) to obtain the charge on each atom. The results are summarized in Table 2F82− to Re2I82−. This phenomenon agrees with the decreasing values of Pauling electronegativity, which are 3.98, 3.16, 2.96 and 2.6640 for F, Cl, Br and I, respectively. Meanwhile, the charge on Re changes from positive to negative and the negative charge on Re is increasing with the halogen atom size increasing. All this indicates that it becomes more competitive for Re to attract electrons as the electronegativity values of halogens decrease. Although the electronegativity value of Re 36 is lower than that of all halogen atoms, the negative charge on Re atoms is more than that of Br and I for Re2Br82− and Re2I82−, clearly not in accord with the electronegativity. This is perhaps related to the weak ability of Br and I halogen atoms to attract electrons caused by their large atomic radii and the strong intramolecular Coulomb repulsion observed in multiply charged anions34. In summary, the two excess negative charges shift from peripheral halogens to Re atoms in the case of heavier halogen atoms.Electronegativity2X82−, we have carried out a detailed analysis on the HOMO−LUMO molecular orbitals (MOs). Figure 2X82− clusters together with their orbital energy levels, which feature their HOMO−LUMO energy gaps. Herein we take the Re–Re bond as z axis and the Re–Cl bond as ± x and ± y axes to analyze the chemical bonding. As shown in Fig. 2F82− displays a large overlap between the 5dxy orbitals of the two Re atoms, and the overlap forms a δ bond. The HOMO–1 and HOMO−2 are doubly degenerate, which feature π bonds formed by the overlap of the 5dyz orbitals. From the HOMO of Re2Cl82−, it can be seen that the 5dxy orbitals of the Re atoms overlap and this leads to the formation of the δ bond. In Re2Cl82−, the HOMO-2 and HOMO–3 are doubly degenerate, leading to π bonds formation by the overlap of the 5dyz orbitals. In the HOMO of Re2Br82−, the 5dxy orbitals of Re atoms have an overlap, leading to the formation of the δ bond. As for Re2I82−, there is no bonding between Re and Re in HOMO, while the HOMO-2 reveals an overlap between the 5dxy orbitals of the Re atoms, forming a δ bond. Both the HOMO of Re2X82−  and the HOMO−2 of Re2I82− are δ bonds. Compared to σ and π bonds21, the δ bond is weak and contributes relatively a little to the total strength of the Re–Re bond. Nevertheless, it plays a vital role in geometric structure. The δ bond strength achieves its maximum only when the conformation is eclipsed. Its strength reduces as the conformation becomes staggered19. Thus, we can deduce that the δ bond is the pivotal factor for the ground-state Re2X82−  clusters to maintain Dh4 symmetric cube-like structures.In order to explore the nature of the chemical bonding in Re2X82−, we select Re2Cl82− as a representative example in order to perform a chemical bonding analysis using the adaptive natural density partitioning (AdNDP) method, which is particularly suitable to decipher the nature of the chemical bond. The AdNDP results displayed in Fig. 8 framework, which strengthen the interactions between the inside Re atoms and the peripheral Cl atoms. In addition, the high occupation number among the localized and delocalized bonds may point to its stability. Overall, the enhanced stability of Re2Cl82− is mainly caused by the chemical bonding of 5d orbital of Re atoms and 3p orbital of Cl atoms.To further understand the bonding mechanism in Re2X82−  all possess cube-like geometry with Dh4 symmetry. This finding has been further tested by performing calculations at the BP86 level. The simulated PES of Re2Cl82− shows reasonable agreement with the experimental spectrum, which further confirms the validity of our computational approach. According to the average binding energy and HOMO−LUMO energy gap, Re2X82− clusters are progressively less stable as halogen atoms become heavier. The chemical bonding analysis indicates that the δ bond is the pivotal factor for the ground-state Re2X82−  clusters to retain Dh4 symmetric cube-like structures, and the enhanced stability of Re2Cl82− is mainly due to the chemical bonding of 5d orbital of Re atoms and 3p orbital of Cl atoms. We expect our findings provide stimulations and guidance to researchers for further experimental or theoretical investigations on binuclear transition metal halide clusters.We have performed a comprehensive research on the structural and electronic properties of dirhenium halide clusters via unbiased CALYPSO structure searches combined with DFT optimizations. The results clearly reveal that the ground states of Re2X82−  clusters are determined via the CALYPSO method which is based on the particle swarm optimization (PSO) algorithm. This theoretical method has been described in sufficient detail previously43. In brief, it can explore the global minima of the potential energy surfaces of predicted system only based on the chemical compositions at given external conditions. It has achieved great success in predicting the ground-state structures of various systems49. Herein, initial structures for Re2X82− clusters are achieved by the CALYPSO method. Each generation contains 50 structures and 60% of those are generated by PSO, while the other structures are generated randomly. We have followed 30 generations for each cluster to achieve convergence to the global minima on the potential energy surfaces. The candidates within 4 eV of the lowest-energy structure are further optimized using the B3LYP51 exchange-correlation functional. The 6-311 + G(d) basis set for F and the LANL2DZ53 basis with effective core potentials for Cl, Br, I and transition-metal Re are employed throughout all calculations. All these calculations are carried out using the Gaussian 09 software package54. The choice of the B3LYP functional and The LANL2DZ basis leans heavily on experience acquired from the previous investigations55. Different spin multiplicities are taken into consideration in the geometric optimization process . The harmonic vibrational frequencies are calculated in order to ensure that the optimized structures are true minima without any imaginary frequencies. We also perform the natural bond orbital (NBO)56 and adaptive natural density partitioning (AdNDP)57 to get further insight into the chemical bonding mechanism.The low-lying isomers of Re2X82− clusters obtained by BP86 method are in accordance with our B3LYP results. In addition, compared with the Re–Re bond length of Re2X82− dianions in previous study2  method58. The reasonable agreement between the simulated and the experimental PES of Re2Cl82− strongly supports our choice of B3LYP functional.In order to identify the reliability of our calculations, we have optimized the structures of dirhenium halide clusters with the non-hybrid BP86 functional and the same basis set as above. It is readily seen that the optimized results of Rey2 Table , we can Supplementary information