Materials exhibitinghigh energy/power density are currently neededto meet the growing demand of portable electronics, electric vehiclesand large-scale energy storage devices. The highest energy densitiesare achieved for fuel cells, batteries, and supercapacitors, but conventionaldielectric capacitors are receiving increased attention for pulsedpower applications due to their high power density and their fastcharge–discharge speed. The key to high energy density in dielectriccapacitors is a large maximum but small remanent (zero in the caseof linear dielectrics) polarization and a high electric breakdownstrength. Polymer dielectric capacitors offer high power/energy densityfor applications at room temperature, but above 100 °C they areunreliable and suffer from dielectric breakdown. For high-temperatureapplications, therefore, dielectric ceramics are the only feasiblealternative. Lead-based ceramics such as La-doped lead zirconate titanateexhibit good energy storage properties, but their toxicity raisesconcern over their use in consumer applications, where capacitorsare exclusively lead free. Lead-free compositions with superior powerdensity are thus required. In this paper, we introduce the fundamentalprinciples of energy storage in dielectrics. We discuss key factorsto improve energy storage properties such as the control of localstructure, phase assemblage, dielectric layer thickness, microstructure,conductivity, and electrical homogeneity through the choice of basesystems, dopants, and alloying additions, followed by a comprehensivereview of the state-of-the-art. Finally, we comment on the futurerequirements for new materials in high power/energy density capacitorapplications. Although the increasing deploymentof renewable energies is encouraging, there still are many barriersto the replacement of power generation from traditionally high CO2-emitting sectors based on coal and gas, which is still acritical and large portion of the energy generation, due to the intermittentnature of renewables. Hence, to simultaneously move away from fossilfuels and to circumvent the unpredictability inherent in clean energyresources, it is necessary to integrate energy-harvesting technologieswith energy storage devices.To limit global warming to <1.50 °C, as set out in theParis agreement, carbon dioxide emissions need to decrease ∼45%by 2030 and reach net-zero by 2050.3 Technologies alreadyexist to store energy, such as batteries, electrochemical supercapacitors,and electrostatic capacitors.16 The latter are electrical energy-storage devices belonging to thecategory of passive components, which are ubiquitous in electronics.Indeed, every year more than 3 trillion multilayer ceramic capacitors(MLCCs) are manufactured from BaTiO3 (BT), the prototypicalferroelectric (FE) ceramic.22Energy storage, therefore, is emergingas a key enabler for sustainablerenewable technologies, particularly for the electrification of transportationbut also in more specialized applications such as heart defibrillatorsand active armor.4–105 W/kg) resulting fromtheir faster charging/discharging characteristics (∼μs),which are advantageous for power electronics in electrical vehicles(EVs) and pulse power applications  and conversionefficiencies (η > 90%); (ii) endure wider temperature ranges(−50–250 °C) and frequency ranges (1–1000Hz); (iii) exhibit greater reliability (>105 cycles)andfatigue resistance (<5% change over capacitor lifetime); and (iv)be compatible with cost-effective internal electrodes and be easilyintegrated with other components.In comparison with Li-ion batteries or fuel cells, the nonpolarizedelectrostatic or dielectric capacitors possess high power density, and for the latter, thermal stability.28 from the Clevite Corp., USA. One decade later, Burn and Smyth29 from Sprague Electric Company evaluated theenergy storage performance in SrTiO3 (ST) and BT with appliedelectric fields up to 400 kV cm–1. Until that point,quantitative data of energy storage on these materials were limitedto fields generally smaller than 150 kV cm–1 dueto the relatively low dielectric BDS of the fabricated ceramics. Theyemphasized that the maximum energy density for a ceramic should beobtained for thinner dielectric layers due to the lower probabilityfor the occurrence of defects ,which are well-known sources of dielectric breakdown. Later in 1990,Love,30 also from Sprague Electric Company,revisited energy storage in ceramic capacitors and highlighted empiricaldesign principles to achieve enhanced energy storage in capacitors,as shown in C0G-type capacitors are manufacturedfrom low relative permittivity (εr) linear dielectrics but may achieve an energy storage of 1 J cm–3, by virtue of their intrinsically high BDS. The significanceof the BDS, to achieve high energy storage becomes apparent in thecase of X7R-type capacitors, fabricated from high εr BT. An important correlation betweendielectric BDS and the thickness (t) can be extractedfrom t of the dielectric layers, the energystorage appears to increase >3 fold. This effect has been recentlycaptured by Yang and co-workers,31 whocompiled BDS data from literature for several dielectric materialsof different t and observed decay inversely proportionalto (t)a, where a was determined as 0.5. Finally, when comparing the energystorage of Z5U and X7R, it becomesapparent that high εr alone is nota sufficient parameter to achieve high energy storage. Interestingly,Love30 stressed that the capacitor industrywas rather conservative in terms of perfecting the BDS of ceramicsto reach values near those of single-crystals, which would significantlyenhance the energy storage in ceramic capacitors.The prospectsof employing ceramic capacitors for energy storagecan be traced back to the 1960s work by Jaffe30 proposed that maximumenergystorage density can be achieved in intermediate rather than high εr materials since they exhibit larger BDS. Fletcher and co-workers32 convincingly postulated that greater energystorage densities can indeed be achieved in FE materials, whose Curietemperature (Tc) is adjusted to ensurethat the material is operated in the paraelectric regime, where itshows a relatively small zero-field εr, an approachalready mentioned by Jaffe in 1961.28Love33 proposedthe use of so-called weakly coupled relaxors, such as 0.7BaTiO3–0.3BiScO3 (0.7BT–0.3BS), to fabricateenergy storage devices. This new conceptual approach aimed at exploitingthe extraordinary temperature stability of εr exhibit by this family of materials. When compared withcommercial X7R capacitors, 0.7BT–0.3BS capacitorsdisplayed superior performance, reaching a recoverable energy density(Wrec) of 6.1 J cm–3 at 730 kV cm–1. Again, the large dielectric BDSplayed a decisive role in this performance. More recently, in 2019Wang, Reaney and co-workers34 unveileda novel approach to enhance energy storage characteristics via thefabrication of chemically heterogeneous but electrically homogeneousceramics, with Wrec reaching 10.5 J cm–3, as detailed later in this review.In 2009, Ogihara and co-workersHere, wepresent the principles of energy storage performance inceramic capacitors, including an introduction to electrostatic capacitors,key parameters for evaluating energy storage properties, microstructuralconsiderations, and critical electrical factors. Second, we will reviewthe current state-of-the-art for lead and lead-free electroceramicsfor energy storage capacitors with bulk ceramics, ceramic multilayers(MLs), ceramic films and glass ceramics evaluated separately. Third,we will describe strategies for optimizing energy storage in electroceramics.Finally, we will demonstrate, with appropriate examples, a guide tothe future development for electroceramics in energy storage capacitors.22.1C , of an ideal capacitor is given by the ratio of the charge, Q, stored on each metallic plate and the applied voltage, V, as shown by C of a realdevice, as illustrated in A, separated by a distance, d, subject to a V, can be obtained throughthe application of Gauss’s lawThe simplestdielectric capacitor consists of two parallel metallic plates separatedby an insulator, which becomes polarized under the application ofan electric field. This is the defining behavior of a dielectric material.The actual capacitance, 2.2V, the electrostatic energy stored, W, in the dielectriccan be estimated byQmax isthe maximum charge achieved at the end of the charging cycle and dq is the incremental charge increase during the chargingcycle. The volumetric energy density, Wst ,is a common key performance indicator, expressed byE is the electric fieldand Dmax is the electric displacementin the material under the maximum applied field, Emax. The electrical displacement (D)corresponds to the charge density (Q/A) on the metallic plates and is expressed by D =ε0E + P (P is the polarization(surface charge density).During the application of a ε0E + P 2, where D is approximately equal to P, and it followsthat D = εE = ε0εrE, where ε0 is the permittivity of free space (=8.854 × 10–12F m–1) and εr is the relative permittivity, which is the ε/ε0 ratio. This approximation allows storedenergy density (Wst) to be defined interms of P, as followsPmax isthe maximum polarization reached at the Emax. From a practical viewpoint, Wst because several experimental methods exist to determine P under an applied E. In 1961, Jaffe28 pointed out that the recoverable energy (Wrec) corresponds to the area above the dischargingcurve, whose upper limit is given by the Pmax. Essentially, the mathematical integration of the area above a polarization-electric(P–E) loop provides an estimate of Wrec, as schematically illustrated in For high ε materials, 2O3, where εr is independentof the applied E. The calculation of Wrec fromthe P–E response illustrated in Wrec is dependent on εr and E. Parts b–d of Wrec needs to be carried out using 35 Already in 1961, Jaffe28 stressed thatin FEs, charging energy is mainly absorbed by domain switching andis retained as remanent polarization (Pr). The typically high remanence of classical FEs can be effectivelyminimized via chemical doping, giving rise to the response shown in 1/3Nb2/3)O3.36For linear dielectrics such as AlE. RFEs remain unsaturated at high applied E, and therefore, any increment of the E will havea contribution to energy storage. Remanence-free materials are therefore,preferable for achieving high Wrec. Lineardielectric materials meet this requirement but due to their low εr, energy storage is limited. Antiferroelectrics(AFEs) display low-remanence under low E but at large E the P–E loop opens due to thestabilization of an FE with respect to AFE phase and they displaya saturated polarization, as illustrated in 28 AFEs should afford advantagesfor high energy storage, providing that dielectric breakdown issuesare eliminated .It is now generally accepted that relaxor behavior originates fromthe response of polar nanoregions (PNRs) to an alternating Wloss); therefore, the Wrec is actually the most important parameter, as schematically illustratedin Wrec becomesWloss isthe energy loss during discharging, which correlates to the area enclosedby the P–E loop  is the forbidden energy between thetop of the valence band and bottom of the conduction band. Eg is commonly used to define insulator (Eg > 4.0 eV), semiconductor (0.0 eV < Eg < 4.0 eV), and metal (Eg = 0.0 eV). For semiconductor, the intrinsic BDS canbe defined asEg.38 Thus, semiconductorswith wider Eg have higher intrinsic BDS.The electronic structure and band gaps of semiconductor can be studiedtheoretically using, e.g., linear discriminant analysis, or experimentally,e.g., absorbance spectroscopy and diffuse reflectance spectroscopy.39 A general rule of thumb is that the activationenergy (Ea) for conduction is approximatelyhalf Eg. Both may beincreased by doping or through the formation of solid solutions, oftendelivering higher BDS and Wrec.41The band gap, as shown in 42 Thus, NNwas introduced into Na0.5Bi0.5TiO3 (NBT) and BiFeO3–BaTiO3 (BF–BT)to enhance Eg. The Eg for BF–BT–xNN ceramics increasedfrom 2.5 eV up to 2.95 eV for x ≤ 0.15, asshown in Wrec ∼ 8.12 J cm–3 under electric field ∼400kV cm–1, along with greater thermal stability  and ultrafast discharge rate (t0.9 < 100 ns), 43For example, the highest 2.3.2εr are important aspects of the so-called “electricalmicrostructure” of electroceramics.44 In many instances, such as the core–shell microstructureor grain boundary response of BT based ceramics, the distributionsare markedly heterogeneous and lead to localization of the electricalfield strength in lower εr regionsor pathways for breakdown through interconnected conducting regions.In 2019, electrical homogeneity was for the first-time proposed byWang, Reaney and co-workers in the BF–BT system as a key factorto optimize BDS and as a consequence Wrec. Electrical heterogeneity was effectively eliminated by alloyingwith a third end-member so that it became more difficult to form aconductive pathway at high field, resulting in higher BDS and Wrec.34The distributionof regions with different conductivity and 2, Air,O2) provided the type and magnitude of electrical conductivityis affected by oxygen stoichiometry. Practically, however, in production,a suitable dopant strategy is utilized once the conduction type isknown (p vs n type). For example,the conductivity of BF-ST-based compositions is suppressed by dopingwith 3 mol % Nb on the B-site to compensate for Bi volatilizationand the formation of oxygen vacancies (VO..), through variationof the Fe valence (Fe3+ to Fe4+).45A homogeneouselectrical microstructure may be obtained in many different ways suchas heat-treatment in the appropriate atmosphere (NR) and C can be extracted.53 Both the volume fraction and difference in magnitude of R and C for multiple electrical responsesare equally important in influencing energy storage performance. Giventhe importance of the electrical microstructure, a brief outline ofthe role of IS is described and its advantages with respect to directcurrent methods are emphasized.For materials with more than one bulk-likeregion, e.g., phasemixtures, core–shell microstructures, or surface layers, alternatingcurrent (AC) impedance spectroscopy (IS) is able to show multipleresponses and the resistance  electricalmeasurements are the most commonlyemployed technique to evaluate the electrical characteristics of materials.However, they merely give the overall response instead of the propertiesof specific regions  unless microprobetechniques are employed.–2 to 107 Hz,is applied on the sample.56 The small voltage preventsany permanent change to the sample as well as yielding a (near) linearrelationship between input and output. The wide range of frequenciesallows separation of the response of different electro-active regionsaccording to their relaxation times. For energy storage capacitors,impedance is capable of: (i) establishing the contributions to theelectrical microstructure  and determine theirindividual conductivity and εr which give an insightinto the distribution of electrical components within the sample;(ii) verifying the origin of the dominant electrical behavior ;58 and (iii) determining the conduction mechanism and charge carriertype which helps further interpret the electrical response of thematerial.47An alternative and much more convenient techniqueis IS. In ISmeasurements, an AC signal with small voltage over a wide range offrequency, typically 10R and a C, usually in parallel. Then the electric relaxation time or timeconstant, τ, of each region can be expressed as its R and Cωmax, it holds the relation:R and C values, electro-active regions can be separated in thefrequency domain. Once the value of R and C are extracted, they can then be assigned to appropriateregions of the sample.Impedance can be definedas a complex number which usually containsboth resistive and reactive (capacitive and/or inductive) components:pO2) rangeto gain a better understanding of the conduction mechanism and thecharge carrier. The associated activation energy, Ea, can be estimated using Arrhenius equationσ0 is pre-exponential factor, k is the Boltzmann constant, and T is temperature. Ea may be related to predominant charge carrierand conduction mechanism. The type of charge carrier may also be determinedto some extent by the pO2 dependence ofconductivity, i.e., p-type: conductivity increaseswith increasing pO2; n-type: conductivity decreases with increasing pO2; ionic charge carrier: conductivity is independent with pO2.Normally the impedance measurement needsto be taken over a temperatureand/or oxygen partial pressure O3 (KNN–BMN),small amounts of CuO help densify ceramics through the formation ofa transient liquid phase, as reported by Qu and co-workers , two-step sintering,2.3.4G) on energy storage properties has been discussedfor several electroceramics because of the relationship between BDSand G, expressed in eq a is the exponent valuesbeing in the range of 0.2–0.4.84 Waser explained that leakage current in fine-G ceramicsis lower than coarse-G ceramics due to the high grainboundary density which act as barriers for charge carriers.85 Thus, dielectric materials with high densityand fine-G are required to optimize energy storage. G may be tailored by chemical doping and the formation ofsolid solution. It may also be modified by the application of an ultrathincoating on the primary particles prior to sintering via chemical coatingmethods, e.g., SiO2 on BT ceramics.89 The optimization on Emax and Wrec via grain size-engineeringfor several materials is illustrated in The effect of grainsize  or immiscibility on cooling from high temperaturefor perovskite end members with dissimilar ion size and bonding (BFbased ceramics).33.13.1.1Wrec and η.101 The energy storage properties of RFE and AFE lead-based ceramicsare summarized in Lead-basedceramics are used commercially as energy storage materials for high-powerpulsed capacitors due to their excellent 3.1.1.11/3Nb2/3)O3–PbTiO3 (PMN–PT), Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT),and TiO3 (SPBT)-based materials, have been reportedas potential candidates for energy storage capacitors.110 Zhang and co-workers investigated the relaxation behavior and energystorage properties of (1–x)PMN–xPT ceramic, obtaining Wrec ∼0.47 J cm–3 at room temperature.102 Li and co-worker probed the effect of domain structureon Wrec and thermal stability of 0.2PMN–0.8Pb(SnxTix1–)O3 (PMN–PSxTx1–) ceramics, as illustrated in Wrec ∼ 0.85 J cm–3 withexcellent thermal stability which was attributed to the coexistenceof ferroelectric domains and PNRs.111Many lead-based RFEs,including Pb(Mg3.1.1.23 (PZ) is the first known AFE and exhibits a double P–E hysteresis loop below TC. However, the high critical switching field required foran AFE–FE phase transition at room temperature limits applicationsfor energy storage. Chemical substitution to reduce switching fieldis an effective strategy to overcome the problem and three well-knownPbZrO3 based compositions are reviewed: (i) O3 (PLZT);115 (ii) O3 (PLZST);129 and (iii) O3 (PLZS).131 Additionally, some new AFEs have also been identified based on PbHfO3, Pb(Lu0.5Nb0.5)O3, Pb(Yb0.5Nb0.5)O3, and Pb(Tm0.5Nb0.5)O3.138PbZrO3.1.1.2.13–PbTiO3 solid solution.140According to the phase diagram 9, PLZT eVpb..) ionic compensation model, an orthorhombic AFE phase similar toPbZrO3 occurs for Zr rich compositions. However, only PLZTwith <10 mol % La is typically utilized for energy storage applications147 since higher concentrations have lower polarization and thereforelower Wrec. Li and co-workers prepared(Pb0.97La0.02)(Zr0.95Ti0.05)O3 ceramics via a solid-state reaction route, yielding Wrec ∼ 0.83 J cm–3 andη ∼ 70% under an electric field of 55 kV cm–1.114 Jo and co-workers found that AFEand RFE behavior can both be obtained by substitution of La and excessPbO in PLZT, resulting in the enhancement of Wrec by promoting a slim and slanted hysteresis loop. Both high Wrec ∼ 3.04 J cm–3 andη ∼ 92% were obtained along with no performance degradationafter 105 cycles.112 Tuningthe Zr/Ti ratio has also shown to be an effective way to improve Wrec of PLZT ceramics. Qiao and co-workers reportedslimmer P–E loops giving a Wrec ∼ 3.38 J cm–3 in (Pb0.895La0.07)(ZrxTix1–)O3 ceramics by changingthe Zr/Ti ratio O3 with 2 mol %La has been most commonly studied in which the Zr/Ti/Sn ratio is variedto give a complex phase diagram that contains FE tetragonal (FT), high-temperature nontilted FE rhombohedral (FR(HT)), low temperature FE rhombohedral (FR(LT)) AFE tetragonal(AT), and AFE orthorhombic (AO) phases.To further optimize the energy storage propertiesof PLZT ceramics,Sn may be substituted on the B-site of PLZT, which broadens the AFEcompositional range,rom 1989 11.167  and weakening ferroelectricity.171Wrec ∼ 5.6J cm–3 and high thermal stability have been obtainedin PLZST ceramics with a Zr/Sn/Ti ratio of 0.5:0.45:0.05116 while Wang and co-workers reported, in theirstudy of the (Pb0.97La0.02)(Zr0.5Snx0.5-Tix)O3 solid solution, superior Wrec of 4.2 J cm–3 with η of 82% for(Pb0.97La0.02)(Zr0.5Sn0.44Ti0.06)O3 ceramics with good temperature stability.118 Recently, a ferrielectric (FIE) configurationwas reported in PLZST which consists of ferroelectric ordering segmentswith either magnitude or angular modulation of dipoles.172 The net polarization of field-induced FE ordercan be tailored by adjusting the Sn/Ti ratio.We note that the Antration 12.118 Ad126 The AFE phasebecomes more stable with a commensurate increase in the AFE–FEswitching field as La concentration increases. The energy storageproperties of (Pbx1–1.5Lax)(Zr0.5Sn0.43Ti0.07)O3 ceramics were optimized (Wrec of 4.2 J cm–1) by Dan and co-workers for compositionswith x = 0.03 due to a large switching electric fieldand high BDS.126 Furthermore, doping with Baand Sr (A-site) improves fatigue behavior and temperature stability,suppresses the stress sensitivity, and enhances energy storage.173The performance of PLZST ceramics is also influenced by the Pb/Laratio.3.1.1.2.3E-inducedmultiphase transition in PLZS for which a conventional AFE–FEphase transition at low E, followed by a second FE-FEphase transition at a higher E, leads to an increasein polarization.131Wrec of 10.4 J cm–3 and η of 87% wereachieved at 400 kV cm–1 for (Pb0.98La0.02)(Zr0.55Sn0.45)0.995O3 ceramics, along with superior discharge current density of1640 A cm–2 and ultrafast discharge speed (75 nsdischarge period) (Zr0.9Sn0.1)0.995O3 ceramics,as illustrated in (130Wang and co-workers reporteda unique  period) 13a,b.131rated in 13c,d. Th3.1.2176 As a result, there has been a steady but continuous improvementin their energy storage performance,180 with a view to replacing existing lead-basedmaterials. Several lead-free ceramic systems are considered as potentialcandidates to replace PLZT for energy storage applications, includingthose based on BT, ST, KNN, BF, NBT, AgNbO3 (AN), and NN.In the lastdecades, extensive research has focused on lead-free electroceramicsdue to concerns over the toxicity of lead/lead oxide-based compounds.3.1.2.1181 Several studies have reported improvements in energy storage performanceof BT-based ceramics through (i) substituting oxides to improve BDS,such as Al2O3, La2O3,MgO, SiO2;184 (ii) employing different sintering techniques,such as SPS, citrate precursor, and cold sintering (CS) to increaseceramic density or control grain growth;185 (iii) adding sintering aids such as ZnNb2O6 and NiNb2O6 to increasedensity;187 (iv) introducing further end-membersin the solid solution, Bi(Mg1/2Ti1/2)O3,189 BiYbO3,190 BMN,193 NBT–Na0.73Bi0.09NbO3,194 Nd(Zn1/2Ti1/2)O3,195 Bi(Zn2/3Nb1/3)O3,196 Bi(Li1/2Nb1/2)O3,198 Bi(Zn1/2Zr1/2)O3,199 Bi(Zn1/2Ti1/2)O3,200 YNbO4,201 Bi0.9Na0.1In0.8Zr0.2O3,202 Bi(Li1/2Ta1/2)O3,203 Bi(Mg1/2Zr1/2)O3,204 Bi(Zn1/2Sn1/2)O3,205 K0.5Bi0.5TiO3–KNbO3,206 andK0.5Bi0.5TiO3–NN207 to promote RFE behavior. The energy storageproperties of BT-based materials are summarized in BT-based dielectricceramics have been studied for decades and dominatethe commercial market of ceramic capacitors.2 configuration into the Ba-site increases Pmax. Pr is minimized by forminga so-called “weakly coupled relaxor” state in whichlong-range polar coupling is disrupted by the multiple B-site ionswhich have different valence and ionic radius to Ti. Using this principle,Hu and co-workers208 reported high Wrec of 4.49 J cm–3 with aη of 93% for 0.6BT–0.4 Bi(Mg0.5Ti0.5)O3 (BT–BMT) ceramics that were temperature stableto 170 °C (variation Wrec < 5%and η < 4%).The most effective approach, however, is the introduction of aBi-based perovskite end member in which the B-site contains multiplecations. Doping with a Bi ion that has a lone pair electronic 6s2/3(Mg1/3Nb2/3)O3 (BT–0.06B2/3MN) but incompositions compatible with Ag/Pd electrodes due to the presenceof only 4 mol % Bi on the A-site.209 Similarenergy storage properties, Wrec ∼4.6 J cm–3 and η ∼ 92%  andlow dielectric loss (<1%) at room temperature. Extensive effortshave been made to improve the energy storage performance of ST-basedceramics, including (i) doping with Ba,216 Dy,217 Mg,218 Ce,64 and Bi222 on the A-site or Mn223 and Sn225 on the B-site; (ii) using sintering aids, such as ZnO,65 MgO,228 SiO2,229 SrO–B2O3–SiO2,230 ZnO–Li2O,231 Bi2O3–B2O3–SiO2,232 B2O3–SiO2–Bi2O3–CaO–BaO,233 Al2O3,234 BaO–TiO2–SiO2,75 BaCu(B2O5),235 and NiNb2O6;236 (iii) employingdifferent sintering techniques such as microwave sintering75 and SPS;70 and (iv) introducing complexend-members, such as NBT,238 NBT–Ba(Al0.5Nb0.5)O3,239 NBT–BT,241 and Bi(Mg0.5Hf0.5)O3.242 The energy storage properties of ST-based materialsare summarized in ST, which is an incipient ferroelectric, is consideredas a promisingcandidate for energy storage applications due to its relatively highpermittivity TiO3–0.1 Bi(Mg0.5Hf0.5)O3 ceramic at 360 kV cm–1, 242 which was also fatigue-resistant up to 105 cycles and temperature stable from 25 to 200 °C, 242From a review of the literature, doping commonly increasesboth 3.1.2.33 solid solutions were first investigated,and Wrec ∼ 0.54 J cm–3 was achieved at 129 kV cm–1.249 ZnO and CuO were introduced as sintering aids improved Wrec in KNN-based ceramics250 by decreasing porosity and restricting grain growth. BDS of 400kV cm–1 was obtained in 0.8KNN-0.2Sr(Sc0.5Nb0.5)O3 ceramic with 0.5 mol % ZnO250 and CuO reduced the sintering temperature andenhanced the density of 0.9KNN–0.1BMN ceramics.66 A third perovskite end-member, e.g., ST, BF,and BMN, has also been shown to optimize energy storage properties.251Wrec of 4.03 J cm–3 at 400 kV cm–1 was obtained in 0.85KNN–0.15ST bulk ceramics92 with similar energy storage performance realized in 0.90KNN–0.10BMNceramic with a large Pmax (41 μCcm–2) obtained at 300 kV cm–1, 91 The energy storage properties of KNN-based materials aresummarized in In 2016, the energy storage propertiesof KNN–HfO3.1.2.4TC and large spontaneouspolarization.259 Compared with other lead-free ceramics, BF-based were not initiallyconsidered as good candidates for energy storage applications dueto their high leakage current.260p-type electrical conductivity due to the volatilizationof Bi and the variation of Fe valence states has been reported frequentlyin BF system, which limits the BDS and restricts energy density.263 However, the introduction of dopants and alloying with end-memberssuppresses the leakage current, reduces the electrical conductivity,increases intrinsic Eg and induces transitionsfrom a FE to RFE state. The energy storage properties of BF-basedmaterials are summarized in BF-basedceramics are best known as multiferroics but have also beenexplored for high-temperature ferroelectric and piezoelectric applicationsdue to their high Pmax.90 Undoped BF–xBT ceramics exhibit optimizedferroelectric and piezoelectric properties in a mixed-phase regionof 0.25≤ x ≤ 0.35.264 The majority of studies have focused on this region andmodified compositions either by (i) A and/or B-site chemical doping,including Nd, Nb, Zn0.5Zr0.5, Zn2/3Nb1/3, and Li0.5Nb0.5 or (ii) alloyingwith a third end-member, such as Nd(Zn0.5Zr0.5)O3, Nd(Mg2/3Nb1/3)O3, La(Mg0.5Ti0.5)O3, Ba(Mg1/3Nb2/3)O3, AN, and NN.269BF–BT-based materials are purported as potentialenergystorage compositions due to their excellent BDS and high Wrec.34 Two electrical components with similar capacitance value, C∼ 8 × 10–10 F cm–1 and 1.4 × 10–9 F cm–1,were found in 0.7BF–0.3BT ceramics, as illustrated R1 ∼ 3.8kΩ cm and R2 ∼ 1.3 MΩcm, were vastly different. The presence of a large volume fractionof conductive cores (R1) easily led toelectrical breakdown at lower electrical field. In contrast, onlyone single electrical component with C ∼ 1.87× 10–9 F cm–1 and resistivityof ∼2.3 MΩ cm was observed for 8% Nd(Zr0.5Zn0.5)O3 (NZZ)-doped BF–BT ceramics(–1).As a result, optimized Wrec ∼ 2.45and 10.5 J cm–3 were realized at Emax ∼ 240 and 700 kV cm–1 for0.08NZZ-BF–BT bulk ceramics and MLs, respectively.The importance of electrical homogeneity was first postulatedin2019 by Wang, Reaney, and co-workers as a key factor to optimize BDSas well as ceramics17c,d. ThxNb–yBMN ceramics, by promoting electrical homogeneity, enhancing Ea and suppressing the p-typeconduction, all of which resulted in significantly enhanced BDS. Theceramic without Nb  exhibits a broadened arc in Z* at room temperature concentrationto reduce polar coupling, results in Wrec ∼ 8.2 J cm–3 at Emax ∼ 460 kV cm–1 for BF–ST–0.03Nb–0.1BMNceramics, 45Recently, superior energydensity through tailored dopant strategieswas achieved in BF–ST–perature18a withicplots 18b.By d samples18c,d, suanced Ea 18e. A re kV cm–1 18g which3.1.2.5Pmax and Tc. However, theirlarge hysteresis and low BDS are not ideal for high energy densitycapacitor applications.284 Attempts to improve their properties generally fall into the followingapproaches: (i) doping on the A-site 296 and B-site (commonly Nb)297 and codoping;321 (ii) forming solid solution with other end-members, such as AN,NN, and SBT;325 (iii) using additives such as MnO, Fe2O3,MgO, SnO2, ZnO, CaO, and ZrO2334 and (iv) employing different processing methods such as hot-pressing59 and synthesis using sol–gel derived powders.336 The energy storage propertiesof NBT-based materials are summarized in NBT-basedceramics are promisingcandidates of lead-free dielectrics due to their high 0.7Bi0.2)TiO3(SBT) achieved Wrec of 2.5 and 9.5 J cm–3 with η> 90% for bulk ceramic and MLs at 200 and 720 kV cm–1  utilization of a highly polar base system ; (ii) disruption of long-range polar coupling through formingsolid solutions with, e.g., SBT and BMN without sacrificing averageionic polarizability, Emax ∼ 470 kV cm–1, Wrec ∼ 7.5 J cm–3, and η∼ 92% for 0.62NBT–0.30SBT–0.08BMN, Recently, Ji and co-workers3.1.2.6Pmax and small Pr. Thereare only a handful of lead-free AFE systems, with AN showing particularpromise because it possesses a large saturation polarization of 52μC cm–2 under an Emax ∼ 220 kV cm–1.343 Recent research on AN ceramics has focused on stabilizing the AFEphase so that switching field is moved to higher fields while simultaneouslyoptimizing Pmax.345AFEs have long been considered as the prime candidatefor energystorage capacitors due to their large 2 and WO3;347 (ii) doping Ba, Sr, Ca, Bi,La, Sm, and Gd on the A-site356 often with isovalent Ta doping on the B-site;359 and (iii) forming solid solutions with end-members, such as BiMnO3 and Bi(Zn2/3Nb1/3)O3.361 Most dopants reduce the G of AN which maximizesBDS but delay the onset of the AFE-FE transition to higher field whilesimultaneously narrowing hysteresis in the induced FE phase. Wrec of 4.4, 4.5, and 5.2 J cm–3 with η of 70, 63, and 69.2% has been obtained for La, Gd,and Sm A-site doped AN ceramics,362 respectively, and B-site Ta-doped AN was reported to exhibit Wrec of 4.2 J cm–3 with ηof 69%  substitution of aliovalent B-site oxides such as MnOηof 69% 21a,b.3573.1.2.7E-induced FE phase is metastable. Thus, AFE behavior inNN based materials is commonly stabilized by chemical substitutionwith end members such as BS and CaHfO3.370Recently, AFE NN has received attention as a potentialcandidatefor energy storage applications. An AFE double hysteresis loop isdifficult to observe in NN because (i) the energy difference betweenthe AFE phase and field-induced FE phase is very small and (ii) Emax >250 kV cm–1 and Wrec > 2.5 J cm–3 have been reported for NN in solidsolution with BMN, ST, Bi(Mg2/3Ta1/3)O3, and Bi(Mg0.5Ti0.5)O3 by stabilizingthe AFE phase or inducing relaxor behavior,374 as summarized in 42 have alsoproposed the concept of an “AFE relaxor” to explainthe energy storage properties of 0.78NN–0.22NBT ceramics. Theyargue that the local AFE phase transforms reversibly into an FE monoclinicphase at∼ 400 kV cm–1, giving a large ΔP (Pmax > 50 μCcm–2 and Prem < 5μC cm–2). Wrec of ∼12.2 J cm–3 was reported with η∼ 69%, at 680 kV cm–1, 42 However, the term “AFE relaxor” has little physicalsignificance since an antipolar phase cannot form short-range polarfeatures characteristic of a relaxor. 0.78NN–0.22NBT may, therefore,be better described as either a relaxor or a short-range AFE phasethat undergoes a field induced transition. This intriguing behavioris interesting, but it is the large Emax (680 kVcm–1) that is most likely responsible forthe exceptional Wrec rather than the intrinsiccrystal chemistry. The underpinning reasons for the large Emax most likely relate to the defect chemistry,band gap and electrical homogeneity, consistent with the key factorsproposed by Ji and co-workers.3373.1.3Tg) and growth > Tg382 The microstructure of a glass ceramic is typically dominated bya largely 2D and 3D defect-free  glassphase and a uniformly distributed (provided the system undergoes homogeneousrather than heterogeneous nucleation) ceramic phase. Wrec and η are both large due to the high BDS associatedwith the absence of 2D and 3D defects accompanied by a near zero valueof Pr. The energy storageproperties of glass-based glass ceramics are summarized in Glass-ceramics arecomposed of one or more crystallized phases (ceramics) dispersed uniformlyin amorphous phase (glass). They often exhibit the combined propertiesof ceramics and glass depending on the induced crystalline phasesand their microstructures. Glass-ceramics are prepared by meltingthe requisite raw materials, cooling to room temperature to form aglass, followed by two step annealing to induce crystal nucleation glass ceramics at the optimized crystallizationtemperature of 900 °C, as shown in 384 With increasingcrystallization temperature from 850 to 1000 °C, the BDS decreasedfrom 1890 to 1440 kV cm–1 and the crystallinityincreased from 64.5 to 97.3%  glass ceramicsfirst increased and then decreased as shown in 6Nb10O30 y on the phase structure, dielectric properties andBDS in BaO–R2O–Nb2O5–SiO2 glass ceramics, where the highest Wrec ∼ 19 J cm–3 wasachieved with εr ∼ 22 andsuperior BDS of ∼4400 kV cm–1 in compositionof BaO–Na2O–K2O–Nb2O5–SiO2.396Each constituent oxidein the glass matrix has an important effecton the crystal phase, microstructure, BDS, and energy storage properties.For example, SiO6Nb10O30 25b. The l % SiO2 25a,c, reEmax ∼ 12000 kV cm–1 and Wrec ∼ 35 J cm–3 in BaO–B2O3–Al2O3–SiO2 glass.397 The effect of Al/Si ratio on the modification of the microstructureand properties of SrO–BaO–Nb2O5–SiO2–Al2O3 glassceramics was also studied by Xiu and co-workers.398Compared with alkali based compositions, alkali-free glass compositionswere found to deliver lower dielectric loss and fewer defect microstructure.For example, Smith and co-workers reported both ultrahigh 2O3,401 Sm2O3,402 Yb2O3,404 and Gd2O3,405 arealso commonly substituted into glass formulations for energy storageapplications. Rare-earth oxides are mainly reported to act as nucleatingagents402 or crystal growth inhibitors.406 Zhang and co-workers revealed that La2O3 leadsto a homogeneous microstructure in the BaO–SrO–TiO2–Al2O3–SiO2 glass-ceramics which improved BDS ∼ 1600 kV cm–1 and Wrec ∼ 3.2 J cm–3 (2.5 timesof the glass-ceramics without La2O3).399 Zheng and co-workers reported that 0.5 mol% La2O3 in SrO–BaO–Nb2O5–B2O3–ZnO glass-ceramicsalso optimized εr (∼130)and Wrec ∼ 7.1 J cm–3 through achieving a BDS ∼ 1100 kV cm–1 dueto a reduction in crystallite size and precipitation of high εr phase, Sr0.5Ba0.5Nb2O6.400 A similareffect was reported for Sm2O3 by Chen and co-workersin the SrO–BaO–Nb2O5–B2O3–ZnO glass ceramics with Wrec of 8.2 J cm–3 at 1100 kV cm–1.402 Moreover, Yb2O3 is reported to eliminate the impurity phases and forma uniform microstructure in BaO–SrO–TiO2–Al2O3–B2O3–SiO2 glass-ceramics, leading to Wrec of 3.5 J cm–3, ∼ 1.8 times higher thanundoped compositions.404Additionally, rare-earth oxides, such as LaxSrx1-TiO3−(Ba−B−Al−Si−O) (BST−BBAS)glass-ceramics .408 Xiao and co-workers further reported that theprecipitation of impurity phases in the K2O–SrO–Nb2O5–SiO2–Al2O3–B2O3 glass-ceramics waslimited by controlling the crystallization time using microwave sintering,with optimum εr, BDS of 1400 kVcm–1 and maximum theoretical Wrec (∼9 J cm–3) obtained after 10min.407Apart fromthe conventional annealing, novel methods such as microwavetreatment have been reported to improve the energy storage propertiesof glass ceramics. Zhang and co-workers found that microwave treatmentrestrained the formation of the dendritic microstructure in Baceramics 26, leadi3.1.4415The debate over whether lead-free electroceramicscan replace theirlead-based counterparts has been ongoing for over two decades. Leadbased compositions generally outperform their lead-free counterpartson most metrics. Moreover, lead-free compositions are disparate witha large number of different formulations potentially required to coverthe properties achieved with essentially doped PZT. Provided, however,that the performance, reliability and cost of lead-free are competitivewith PZT, it is highly likely that lead-free electroceramics willbegin to replace their lead-based equivalents and attain large scaleproduction in the coming years as a consequence of environmental legislation.416 This latter statementdoes not hold for piezoelectric ceramics market which is dominatedby PZT and its derivatives.419Of all the applications,lead-free high energy density capacitors are the most likely to seelarge-scale production since (i) the performance of lead-free compositionsis approaching that of lead-based; (ii) reduction in intrinsic electricalproperties may be compensated by increasing the BDS often throughdecreasing layer thickness see 3.1.2; anEmax, ΔP, Wrec, andη, for lead-based and lead-free ceramics are summarized andplotted in Wrec vs Emax fordifferent lead-based/lead-free bulk ceramics is displayed in Emax (up to ∼400 kV cm–1) and Wrec (up to ∼12 J cm–3) with respectto lead-free candidates. NN-based ceramics currently offer the highest Wrec under high E (>350 kVcm–1) for lead-free compositions, followed by AN-,NBT-,BF-, and KNN-based materials. BT and ST-based ceramics display thelowest Wrec in spite of their high Emax of ∼450 kV cm–1, but they are perhaps the most appealing dielectrics commerciallysince they are the current basis of MLCC production.The energy storage performances, ΔP vs Wrec iscompared in ΔP (up to ∼60 μC cm–2), followed by AFE NN- and AN-based (up to ∼40 μC cm–2) and KNN-based materials (up to ∼35 μCcm–2) with the lowest (up to ∼25 μCcm–2) for BT- and ST-based materials. Bicontainingelectroceramics such as NBT and BF, have been heavily studied recentlyas potential lead-free electroceramic materials due to their largepolarization.423 A high ΔP (60 μC cm–1) is obtained in NBT and BF based by reducing the Pr through chemical substitution withother perovskite end members to form a relaxor with an ultraslim P–E loop. To some extent therefore, the advantageof a high intrinsic polarization end member such as BF is weakened.Intermediate ΔP ∼ 40 μC cm–2 values are observed for AFEs such as AN- and NN-basedmaterials but Pmax is often limited ascompositions exhibit polarization saturation as a function of appliedfield.Wrec vs η for a widerange of compositions.ST-based materials display the best η (∼90%) due to theirlinear-like dielectric behavior. For BT-, NBT-, and lead-based materials,η varies with the material composition since it is a functionof many factors. Dielectric loss associated with defects such as VO.. play a role but primarily at high field andhigh frequency, energy is dissipated during the transition to a fieldinduced long-range ordered state which is manifested by the openingof the P–E loop. If the transition is smearedover for the operational range through alloying or doping to createa so-called “weakly coupled relaxor state”, η> 90% can be achieved.337p-type conductivity in BF-based ceramics.45 In addition, AFE based materials generally suffer from opening ofthe polarization loop above switching field to form a field inducedFE phase which is detrimental to η. Compositional modificationsto AN and NN ceramics aim not only to push the AFE-FE transition tohigher field and stabilize the AFE phase but also to disrupt the long-rangeordering in the field induced FE phase, thereby creating a slimmerportion of the P–E loop (higher η) thanthat being observed in unmodified materials.366High leakage current and electrical conductivity are consideredas major challenges in BF and KNN-based materials but are addressedby appropriate doping, e.g. donor doping to mitigate Emax, ΔP, Wrec, and η), temperature and frequency stability arealso importantfor practical applications. In the future, high energy density ceramiccapacitors will be placed closer to the core engine electronics tooptimize the equivalent circuit resistance. Therefore, the temperaturerequirement for energy storage ceramics is anticipated to increase.According to the white paper “Multilayer Ceramic Capacitorsfor Electric Vehicles” published by Knowles capacitors in 2017,424 the explosive development of EVs has promptedthe appearance of new 200 °C-stable C0G type I dielectric ceramiccapacitor on the market. However, these materials still do not fulfillthe required high power/voltage, energy density, and temperature requirements(∼250 °C) to facilitate use near-engine. Better frequencystability from 100 Hz to 100 kHz is required to reduce power fluctuationswhen capacitors are used for DC/DC conversion for battery chargingand DC/AC conversion for propulsion.429 Enhanced frequency stability also enablesthe capacitor to be compatible with diodes and thyristors for powerswitching and control.430Apart from the parameters discussed above are rarely reported. Wang and co-workers, for example, discussed frequencystability from 10–2 to 102 Hz for 0.57BF-0.3BT–0.13BLNwith Wrec ∼ 8 J cm–3 and η ∼ 81% at 400 kV cm–1. All electroceramicsfor capacitors appear to deliver a charging–discharging speedat or faster than 1 μs. Short times of τ0.9 ∼ 0.15 μs (90% of energy discharge in 0.15 μs)were reported by Li and co-workers in BT based ceramics at 150 kVcm–1197 while Qi et al.described even faster charging–discharging speeds (τ0.9 ∼ 97 ns) in BF-based ceramics at 200 kV cm–1.42 Increasing attention has also beenfocused on fatigue-resistant behavior as a performance metric in practicalapplications with 104 cycles (Wrec variation <10%) reported in BF-based ceramic multilayers from20 to 100 °C, coupled with a low value of electrostrain (<0.03%).276Most compositions have been shownto deliver 3.23.2.1432 This fabrication technology is a powder-based approach that accommodatesscale-up from laboratory research to commercial manufacturing. Themarket of ceramic MLs ∼ $5.3 billion in 2017 but will reach∼ $7.8 billion by 2024 for electronic applications, includingbut not limited to mobile phones, laptops and motor vehicles.433 Advanced high energy density ceramic MLs, basedon AFEs and RFEs materials, are being developed to facilitate powerelectronics within hybrid electric vehicles which require higher Wrec and operating temperature. Simultaneously,research into low cost internal electrodes is required so that thehighest performant ceramics can be developed. The energy storage propertiesfor different ceramic MLs are summarized in Ceramic MLsare fabricated by a series of processing steps which include slurrypreparation, tape-casting, screen printing, lamination, cosintering,and termination, as shown in –3 and Wrec∼ 3 Jcm–3 was obtained in Pb(Zr0.95Ti0.05)0.98Nb0.02O3 MLs usingPt as internal conductive electrode.434 Optimized performance of Wrec ∼3.8 J cm–3 was also reported by Hao and co-workersfor (Pb0.88Ba0.05La0.02Dy0.04)(Zr0.68Sn0.27Ti0.05)O3 MLs using 5%Ag/95%Pd internal electrode. However, this was accompaniedby a large electric field-induced strain ∼0.71% at 300 kV cm–1,435 a major drawback forpractical applications due to inferior mechanical stability in operation.To date, Hao and co-workers have reported the record-high Wrec ∼ 12.6 J cm–3 andη ∼ 80% under Emax ∼560 kV cm–1 for Pb0.98La0.02(Zr0.7Sn0.3)0.995O3 ceramicML, as shown in 436Several lead-basedAFE ceramic MLs have been reported using differentinternal electrodes. A giant power density ∼2000 kW cmWrec and fatigue-resistant behavior accompaniedby negligible electric field induced strain. 0.87BT–0.13 Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 ceramic MLs have been fabricated with a dielectric layer t of 11 μm using 60Ag/40Pd internal electrodes thatexhibit excellent Wrec ∼ 8.13 Jcm–3 and η ∼ 95% at 750 kV cm–1.437 The energy storage properties of0.87BT–0.13 Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 were further enhanced by Zhao and co-authorsby decreasing the dielectric layer to ∼5 μm, achieving Wrec ∼ 10.12 J cm–3 at1012 kV cm–1, 438 NBT–0.45SBTceramic MLs have also been reported to exhibit Wrec ∼ 9.5 J cm–3 with η ∼95% at 720 kV cm–1,222 which were further improved by forming a solid solution with a thirdperovskite end-member, 10%BMN to give Wrec ∼ 18 J cm–3 with η > 90% at 1000kV cm–1, by Ji and co-workers, 337 The highest among all lead/lead-free ceramic MLs, Wrec ∼ 21.5 J cm–3 at∼1030 kV cm–1, however, was achieved fortextured NBT–0.3SBT ceramic MLs.439 Texturing was achieved through the use of <111>-oriented STplateletswhich reduced field-induced strain at high field thus enhancing theBDS greatly. Combination of texturing with alloying with a third endmember (BMN) in NBT–SBT may well represent an exciting pathto achieve yet higher energy densities.Although lead-free AFE ceramics show great promise,there are noreports of ceramic MLs in the literature. Instead, lead-free RFEshave dominated recent ceramic ML research due to their excellent 416 In 2018,Nd-doped BF-0.3BT was reported to exhibit Wrec ∼ 6.74 J cm–3  and η ∼ 77% at 540 kV cm–1 with a layer t of 33 μm.90 On the other hand, alloying BF–0.3BT with 8 mol% Nd(Zr0.5Zn0.5)O3 resulted in Wrec ∼ 10.5 J cm–3 withη of 87% at 700 kV cm–1 with a dielectriclayer t of ∼17 μm, 34 Further studies focused on promoting electrical homogeneity, whichwas considered to prevent conductive pathways developing in thesecomposition, thereby avoiding the breakdown at high field and facilitatingthe improvement of Wrec by reducing thedielectric layer thickness. Finally, Wrec ∼ 13.8 J cm–1 at 953 kV cm–1 was achieved for BF–0.3BT–0.13 Bi(Li0.5Nb0.5)O3 (BF–BT–0.13BLN) ceramicMLs at a dielectric layer t of 8 μm, as shownin 276Wang, Reaney, and co-workers have employed a rangeof differentchemical dopants and alloying additions to investigate BF-T-basedceramic multilayers.3.2.2Wrec compared to MLs fabricated through powder-basedtechnology have been reported for ceramic films deposited on LaNiO3/Si(100) or Pt/Ti/SiO2/Si substratesby physical vapor or chemical deposition techniques, such as radiofrequency magnetron sputtering,449 spincoating,451 pulsed laser deposition,452 and chemicalsolution deposition.453 The BDS of ceramicfilms is significantly improved due to the reduction of t (<1 μm) often attributed to fewer defects (grain boundaries)and/or pore/void concentration. Not only have higher figures of meritbeen reported for ceramic films, but several researchers have proposednovel underlying mechanisms (beyond reduction in defect density) behindthe enhancement, such as the formation of polymorphic nanodomains.452 However, nonpowder based techniques are difficultto scale up into MLCCs, which may constrain these extraordinary resultsto lab-based fundamental research rather than promising practicaloutput for commercial exploitation. The energy storage propertiesfor ceramic films are summarized in Higher BDS and 0.94La0.04)(Zr0.98Ti0.02)O3 ceramic film with t of 2 μm was reportedto exhibit 47.4 J cm–3 under electric field of 3700kV cm–1 on a Pt(111)/TiO2/SiO3/Si(100) substrate.454 Further increasing La concentration resulted in superior Wrec ∼ 40.2 J cm–3 andη ∼ 62%, achieved at Emax∼ 1998 kV cm–1 for Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 on a LaNiO3/F-Mica substrates.449 In addition, for Pb0.97La0.02Zr0.66Sn0.23Ti0.11O3, an improved Wrec ∼ 46.3 J cm–3 wasachieved at ∼4000 kV cm–1, accompanied byexcellent temperature stability (up to 380 K) and cyclic reliability(up to 105), 453Lead-basedceramic films have been studied heavily in the pastdecade using different preparation methods, particular for PLZT. (Pb3/Si(100) substrateswith t = 1.2 μm were reported to exhibit excellent Wrec ∼ 30.2 J cm–3 under Emax ∼ 2310 kV cm–1.455 Similar properties, Wrec ∼ 33.3 J cm–3 under Emax ∼ 2300 kV cm–1, were alsoobtained for Fe doped NBT–K0.5Bi0.5TiO3 ceramic film.456 Recently, evenhigher Wrec ∼ 50.1 J cm–3, η ∼ 63.9% accompanied by fast charge–dischargespeed (∼210 ns) were achieved simultaneously at ∼2200kV cm–1 in relaxor 0.6NBT–0.4 Bi(Ni0.5Zr0.5)O3 films.457 In addition, an ultrahigh Wrec ∼112 J cm–3 with η ∼ 80% was reportedby Pan and co-workers in BF–BT–ST ceramic films recentlyand cyclic fatigue resistant (up to 109 time) was reportedby Park in a 9.2 nm thick Hf0.3Zr0.7O2 film458 and Wrec ∼ 63 J cm–3 with η ∼ 85% wererealized in 50 nm thick Al doped HfO2 ceramic films withexcellent temperature and frequency stability.459In the past few years, there has been increased focus on lead-freeceramic films due to concerns over toxicity of PbO. NBT and BF basedrelaxor compositions have dominated research and have competitiveor even superior energy storage performance to lead-based films. Mndoped NBT ceramic films on a LaNiOrecently34. Polym3.2.3Wrec, η, and ΔP, between bulkceramics, ceramic MLs, and ceramic films are shown in Wrec (∼130 J cm–3) and Emax (∼5800 kV cm–1)are obtained in ceramic films, followed by ceramic MLs (∼21J cm–3 and Emax ∼1000 kV cm–1) and bulk ceramics (Wrec (∼12 J cm–3 and Emax ∼ 650 kV cm–1) and scaleprimarily with t of the dielectric layer  with respect to ceramic MLs (up to ∼70 μC cm–2) and bulk ceramics (up to ∼60 μC cm–2), as shown in VO.. and associated defect dipoles.The energy storage performances, ic layer 35d. WrecWrec > 15 J cm–3 at ∼1000 kV cm–1. However, the selectionof inner electrodes for these materials is limited to Pt, currentlytoo costly for mass production. Commercial focus therefore, currentlyremains mainly on modified BT compositions which are compatible withNi, Ag and Ag/Pd electrodes depending on composition and pO2 during fabrication. A huge stride forward in the industrywould be the development of a low cost equivalent to Ag/Pd electrodeswhich could permit the fabrication of a wider range of MLs at higher pO2 which would inhibit the formation of VO.. and maintain high BDS.From a commercial perspective,energy storage performance of lead-freeceramic MLs has improved significantly in the past few years withBF and NBT based ceramic MLs now rivalling lead-based ceramic MLs,delivering 4The review of the state-of-the-art of ceramics, MLs, and filmspresented in 4.1498 If this concept is married to dopant strategies to induce or maintaina homogeneous electrical microstructure, ΔP and BDS can be optimized leading to a large Wrec.The most commonly utilized strategyto optimize energy storageproperties is through inducing a relaxor state within a system thatcontains highly polarizable ionic species. It is typically carriedout through strategic doping or alloying to form a pseudoternary solidsolution. Levin and co-workers proposed that long-range ferroelectriccorrelations can be effectively “blocked” by using designeddopants with enhanced local polarizability.εr was observed after doping xBi2/3(Mg1/3Nb2/3)O3 (B2/3MN)into BT ceramics, along with reduction on maximum dielectric constant(εm) and associated temperature(Tm).209 Afterfitting the εr and Tm using modified Curie–Weiss law (as follows)2/3MN ceramics with a Burns temperature∼154 °C. In addition, XRD revealed a transformation fromtetragonal to an average pseudocubic structure as increasing x concentration, coupled with a reduction in Pr, confirming a relaxor state at roomtemperature, These two simple precepts can be applied to mostsystems; e.g., a frequency dispersion of Wrec ∼ 4.55 J cm–3 at 520 kV cm–1 was recorded forBT–0.06B2/3MN (209 with similarproperties reported for Bi(Mg0.5Ti0.5)O3,208 Bi(Li0.5Ta0.5)O3,203 Bi(Zn0.5Zr0.5)O3,199 and K0.73Bi0.09NbO3499 doped BT ceramics. However, the advantage ofBT–B2/3MN is in the comparatively low concentrationof dopant required to induce a relaxor state. The polar coupling isdisrupted through a combination of A-site (Bi and VO..) and B-site (Nband Mg) dopants which provide a range ofdifference in ionic size, charge and electronegativity. However, only4 mol % Bi is present on the A-site which minimizes reaction withAg/Pd  to ∼2 μm (Nd doped BF–BT).As a result, enhanced Wrec ∼ 1.8J cm–3 was obtained for 15 mol % Nd doped BF–BT,which was further improved to 6.74 J cm–3 by multilayering(90 with similar optimization reported in BF dopedwith Bi(Zn2/3Nb1/3)O3 and Bi(Zr0.5Zn0.5)O3.266 The same design strategy of inducing an RFE state is also used inthin film BF–BT–ST compositions.452Similar compositionalmodifications to induce a relaxor state havebeen adopted in BF-based solid solution. For example, in FE BF–BTcompositions, macroscopic herringbone-type domains are observed in37a but dlayering37b,c90 w272Pmax is often unsaturated in RFE and increases with E which means ΔP is not only a function ofthe slimness of the P–E loop but also of theapplied E. RFEs are therefore, among the most promisingcandidates for capacitors in power electronics (Emax > 300 kV cm–1).In summary, the inferior energy storage performance of all FEscan be improved by forcing a RFE state through strategic doping oralloying, 4.2353 althoughsimilar strategies date back to early studies of lead-based PLZT andPLZST AFE ceramics.145 The most comprehensive study of AN AFE ceramicswas performed by Lu and co-workers in which they used A and B-sitesubstitutions to develop, Ag0.97Nd0.01Nb0.80Ta0.20O3 which yielded Wrec ∼ 6.5 J cm–3 at 370 kV cm–1 with η∼ 71%, 366 In theirstudy, Lu and co-workers defined several key points required to optimizeAN-based ceramics:i)Pmax through localstrain/field coupling around the smaller(with respect to Ag) Nd ion the A-site and its compensating VA, Optimization of ii)Stabilization ofthe AFE structurethrough a combination of Nd and Ta doping which leads the inducedAFE/FE transition to higher fields. iii)P–E loop.This was also achieved, through Nd doping that disrupted polar andantipolar coupling which manifested itself as a decrease in domainwidth from ∼1 to 0.5 μm and streaking of ±1/4(001)c superstructure reflections in electron diffraction patternsfor x = 0.03, Inducing a slim hysteresis curvein the field induced region of the Many recent publications have focused on optimization of energystoragein lead-free AN-based materials,P–E loop to the left of the curve in the positive quadrant and thusoptimize Wrec, All the above maximize the area of the Stabilization of the AFE structurewas also confirmed by First-principlescalculation and Ginzburg–Landau–Devonshire (GLD) phenomenology,as illustrated in Wrec (∼1.5–2 J cm–3) and η (∼40%), which can be optimizedto Wrec > 3 J cm–3 andη > 50% (enhanced-AFEs) by strategies described in the workof Lu and others.366 Similar values of ΔP (30–40 μCcm–2) are found for AFEs and enhanced-AFEs whichreflects an intrinsic limitation of AFE materials, i.e. when antiparallelpolar coupling is fully switched to polar under electric field, Pmax reaches saturation and is difficult to enhanceunlike for RFEs (–1), much higher ΔP and Wrec are obtainedfor AFEs compared with RFEs, indicating that AFEs are more suitablefor low/intermediate-voltage energy storage applications.for RFEs 4.1. η is4.34.3.181 Smaller grainsizes are typically obtained (on the order of a few nm), leading tothe enhanced density and BDS. BT ceramics, for example, with a 4 nmthick SiO2 coating, have a good value of Wrec (∼1.43 J cm–3),233 in which SiO2 coating inhibits graingrowth, thereby modifying the microstructure and reducing DC currentleakage. The effect of SiO2 layer thickness on BT particleshas been systematically investigated (Wrec(∼4.8 J cm–3) and η (∼99.1%)were obtained for 20 wt % SiO2 coated BT, as shown in 88 Other coating materials such as Al2O3 and La2O3,184 are also reported to optimize energy storage properties, as listedin Chemical coatingis commonly reported as a strategy to optimize energy storage propertiesin BT ceramics. In general, chemical coatings are synthesized andapplied using wet-chemical and sol–gel methods.stigated 43a-c. Th4.3.2r are optimized with the final propertiesrelated to the type of electroceramic material and the thickness ofeach layer. The BDS of BT-based ceramics was enhanced to >300 kVcm–1 by laminating layers between BT–x wt % SiO2 layers (t ∼20 μm)and BT layer (t ∼ 25 μm).441 εr decreased but this wascompensated by an increased BDS with increasing SiO2 concentrationin the BT–x wt % SiO2 layers. ST+ Li2CO3 (t ∼ 50 μm)and 0.93NBT–0.07Ba0.94La0.04Zr0.02Ti0.98O3 (t ∼ 33 μm)layered structure were also fabricated via tape-casting with improved Wrec ∼ 2.72 J cm–3 at294 kV cm–1.502 Enhanced Wrec ∼ 2.41 J cm–3 at237 kV cm–3 was also obtained for layer-structureceramics with ST + Li2O3 (t ∼ 50 μm) and NBT–0.06BT (t ∼50 μm), 442 The interface between the ST +Li2O3 and the NBT–0.06BT layer was furtherinvestigated using the finite element analysis. BDS was improved byreducing the breakdown paths between the ST + Li2O3 and the NBT–0.06BT layer, 442 with electrical field redistribution and interface blocking playingessential roles.503 BDS was also influencedby the difference in εr and thickness ratio betweenthe adjacent layers.Layer-structurescomposed of multiple materials have been reported to optimize energystorage properties and are typically tape cast, followed by lamination.Both BDS and ε55.1–1) is often attributed tothe volatilisation of lead/lead oxide which leads the formation oflead vacancy (Vpb..) and VO.. that results in current leakage. Such issues may be partially solvedby a combination of improved processing and dopants but achievingthe values of BDS observed in lead-free materials has proved elusive.The low η in lead-based AFE-type ceramics (<80%) is mainlya result of opening of the hysteresis loop at high field due to thestabilization of a field induced FE phase. This results in a changein crystal class from tetragonal (AFET) to rhombohedral(FER) giving large strains (>0.3%) which may preventlong-termcycling through mechanical fatigue.Lead-based ceramicshave great potential as energy storage materialsin modern microelectronics where high voltage and temperature arerequired, such as in pulsed power and power electronic applications.Lead-based AFE-type ceramics exhibit extremely high energy densitybut optimizing BDS, η and minimizing electrostrain is problematic.Low BDS , as well as utilizing advanced aberration corrected TEM tostudy the local structure.The lack of popularity inresearching lead-based compared with lead-free materials in the academiccommunity has meant that exploration of novel systems is rather limited,but there are, for example, interesting mixed Pb- and Bi-based systemswith high ε5.2Eg and as a consequence Ea, reducing the total electrical conductivity and promotingelectrical homogeneity through the use of strategic dopants to modifydefect chemistry. If these strategies are married with a reductionin the dielectric layer thickness, high values of Wrec ∼ 20 J cm–3 and η ∼90% can be achieved. Recent work on texturing of ceramic MLs has alsoproved successful in enhancing Wrec butthe complexity of this approach may inhibit commercial uptake. However,the two overriding issues with the majority of lead-free compositions,particularly those whose Wrec are >10J cm–3 are (i) the need to find an effective lowcost internal electrode system that permits their commercial exploitationand (ii) pushing their operating window to >200 °C and >100Hz.Interestingly, electrostrain, a major drawback in lead-based materials,is broadly speaking not an issue in most of the lead-free RFEs andAFEs since the measured values of strain are often significantly lower(<0.2%) even at high fields. As with lead containing ceramics,there are only a few comprehensive investigations of the energy storagemechanisms which require high field in situ studiesto be performed. A greater understanding of the role of defect chemistry,doping and alloying is also required, particular on how this influences, Eg, resistivity and electrical homogeneity, andthus the Wrec and η, thermal stabilityand cyclic reliability. In addition, in commercial MLCCs, the ripplecurrent, equivalent series R, failure mode, voltagerating, the reliability in high humidity need to be evaluated andexplored.Lead-free candidates, including BT, ST, BF, KNN,NBT, AN and NN-basedsystems, are extensively studied and summarized in this review. Researchinto lead-free materials far outweighs that in lead-based, due tohow the potential environmental legislation surrounding manufacturingand the end use of lead-based products has influenced funding bodiesand awards. As a result, the optimization of energy storage propertieshas progressed rapidly in the last 5 years. Successful strategiesto improve properties include, disrupting long-range polar couplingparticularly if the average ionic polarizability is increased or unaffected,construction relaxor feature (PNRs) in FEs and AFEs, enhancing 5.3Wrec, ultrahigh η (low energy dissipation), ultrafastcharge–discharge speed, excellent temperature/frequency stability.However, there are still challenges/problems. Generally, increasingthe volume fraction of the crystal phase will increase the ε/P but decrease BDS. It is critical to balance the εr and BDS to obtain the highest Wrec. The mechanism of crystallization and controlof crystal phase/microstructure is still ambiguous, which should befurther investigated using, advanced TEM and in situ XRD/TEM as a function of applied field and temperature. Furthermore,although the theoretical Wrec  of glass-ceramics are much higher than other bulkceramics andeven MLs (15–20 J cm–3), the measured/calculated Wrec by P–E loops anddischarging processes is low (<2 J cm–3) dueto the lower applied electric fields. As a result, we recommend usingthe same test method (P–E loops and dischargingprocess) to evaluate the practical energy storage performance forglass-ceramics, consistent with other dielectrics.Glass-ceramics havethe advantages of facile manufacture, high


Oreochromis niloticus as the experimental animals, we found that HPD and Pi incubation reduced triglyceride (TG) content (P ≤ 0.05), suppressed lipogenesis, activated AMP-activated protein kinase (AMPK) pathway and autophagy (P ≤ 0.05), and increased fatty acid β-oxidation and lipolysis in tilapia liver and hepatocytes (P ≤ 0.05). Our further investigation indicated that Pi treatments activated the lipophagy and facilitated mitochondrial fatty acid β-oxidation, and according reduced TG deposition (P ≤ 0.05). Mechanistically, phosphorus increased the AMPKα1 phosphorylation level at S496 and Beclin1 phosphorylation at S90, and Beclin1 phosphorylation by AMPKα1 was required for phosphorus-induced lipophagy and lipolysis. Our study revealed a mechanism for Beclin1 regulation and autophagy induction in response to high-phosphorus diet, and provided novel evidences for the link between dietary phosphorus addition and lipolytic metabolism via the AMPK/Beclin1 pathway. Our results also suggested that AMPK should be the potential target for the prevention and control of lipid metabolic disorders. Overall, these results suggested that HPD reduced hepatic lipid deposition by activating AMPK pathway and Beclin1 phosphorylation levels to activate lipophagy, which provided potential targets for the prevention and control of fatty liver in fish.High-phosphorus diet (HPD) reduces lipid deposition and significantly influences lipid metabolism. However, the relevant mechanism is unknown. Herein, using widely-cultured teleost tilapia  Phosphate is a necessary microelement and plays many functions in important biological processes, such as bone formation, the biosynthesis of phospholipids and nucleic acids, intracellular signaling and energy metabolism –3. Studimetabolic disease worldwide, and hepatic TG accumulation makes the liver susceptible to the mediators of inflammatory cytokines, potentially leading to hepatitis, fibrosis, cirrhosis and finally liver failure , 7. At pAutophagy is an evolutionarily conservative and degradation process essential for the maintenance of the energy levels within the cells , 9. MoreThe deposition and utilization of lipids are critical for the maintenance of energy homeostasis within the cells. AMP-activated protein kinase (AMPK), a serine/threonine protein kinase, is an evolutionarily conserved cellular energy sensor that maintains cellular energy homeostasis . AMPK isOreochromis niloticus is a critical economic fish all over the world, whose annual yields amount to 4.5 million tons in 2018 . They were kept in a recyclable system for 2 weeks to acclimate the experimental conditions.2PO4·2H2O at the inclusion levels of 0 , 0.53 g/100g  and 1.05 g/100g  , 1.75 g/100g  and 2.66 g/100g .Three diets were produced with dietary phosphorus addition in the form of NaHet, HPD) . The Pi et, HPD) , 30. Whe4-N 0.05–0.08 mg/L. The water temperature was measured with thermometer. Dissolved oxygen was measured with portable DO instrument. pH was measured with portable pH meter, and NH4-N with Nessler reagent spectrophotometry (GB/T 7479-1987). After the 10-week experiment, before collecting the samples, tilapia was fasted for 24 h. All sampled tilapia were anesthetized using MS-222 (100 mg/L water). Then, the liver tissues from ten fish were obtained from each tank, immediately frozen in liquid nitrogen and stored at −80°C for the RNA and protein isolation. The liver of another 3 fish was collected in each tank. They were fixed in 2.5% paraformaldehyde and 10% neutral buffered formalin for the histological, histochemical and ultrastructural analysis, respectively. Another 3 fish were collected and their livers were obtained for analysis of TG content and enzyme activities. The livers of another 6 fish were frozen in the liquid nitrogen and stored at −80°C for subsequent analysis.After acclimating the experimental conditions, 225 tilapia  were randomly divided into nine circular fiberglass tanks . Each experimental diet was assigned into three tanks randomly. The fish were fed to satiation twice a day  for 10 weeks. The experiment was conducted at natural photoperiod (approximately 12 h of light and 12 h of darkness). During the experiment, the parameters of the water quality in the tanks were followed below: water temperature 28.0–30.5°C, dissolved oxygen 6.10–6.52 mg/L, pH 7.05–7.59 and NHCommercial kits were used to determine TG  and NEFA contents  according to the manufacturer's instructions, respectively. Phosphorus content of diets and the liver was determined using the molybdovanadate method (spectrophotometry) as described previously . Fatty aORO staining was conducted according to Spisni et al. . H&E staTM reagent , and transcribed into the cDNA with reverse transcription kit . qPCR assays were carried out in a 10 μL reaction system, 2 × SYBR®Premix Ex TaqTM (TaKaRa) 5 μL, 10 mM each of forward and reverse primers 0.2 μL, 0.6 μL diluted cDNA template and 4 μL double distilled H2O. The specific primer sequences for the qPCR analysis are listed in b2m, rpl7, hprt, β-actin, ubce, 18s rRNA and tuba) were selected to screen out two stable genes as the endogenous controls, based on the analysis of the geNorm. The methods 2ΔCt−Δ was used to calculate the relative mRNA expression of each gene.The qPCR assays were performed to quantify the mRNA expression of genes, based on these in our previous research . In brieWestern blotting analysis was conducted to analyze the protein expression according to our recent publications . In brie2HPO4·12H2O/NaH2PO4·2H2O. The Pi concentrations were used based on previous studies (ampkα1 and beclin1 genes to determine the effect of the AMPK/Beclin1 pathway on lipid metabolism in tilapia. Tilapia hepatocytes were incubated with the inhibitor chloroquine (CQ) of the autophagy-lysosomal pathway  to investigate whether and how autophagy mediated dietary phosphorus-induced lipolysis. The primary hepatocytes were incubated in the control or Pi group in the L-15 medium for 48 h with or without 2-h pre-incubation with 5μM CQ.To explore the mechanism of dietary phosphorus on lipid metabolism, we isolated the primary hepatocytes from the tilapia and cultured them as described previously . Approxi studies  and our We used the 3--2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay to test the cell viability. The protocols were based on the publication .TM transfection reagent . For Beclin1 and AMPKα1-knockdown experiment, the qPCR analysis was used to determine the knockdown efficiencies of these siRNA sequences. The siRNA sequences were given in To identify the phosphorylation sites of Beclin1 and AMPKα1, we constructed the vectors of Beclin1 and AMPKα1 expression according to our publication . The opeThe Bodipy 493/503 staining was performed by our published methods . At firsThe immunoprecipitation analysis was performed based on these protocols . The hept tests  were performed. The data among the three groups were evaluated by one-way analysis of variance and then by the post-hoc Duncan's multiple range test to determine the statistical significance. For four groups, the homogeneity of the variances was analyzed by the Levene's test, and one-way ANOVA determined statistical significance with the Bonferroni post-hoc test. P ≤ 0.05 was considered statistically significant.The data were presented as mean ± S.E.M. The Prism 8 software  was used to analyze the data. For two groups, Student's P ≤ 0.05). Hepatosomatic index (HSI) showed no significant differences among the three treatments.The survival was not influenced significantly by dietary phosphorus . Weight P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05), but increased the activity of lipolytic enzyme CPT1 (P ≤ 0.05); the HPD group also reduced the mRNA expression of lipogenic genes  (P ≤ 0.05), but increased the mRNA expression of the lipolytic gene pparα (peroxisome proliferator-activated receptor α) (P ≤ 0.05)  . Compare ≤ 0.05) . The HPD ≤ 0.05) . These rbeclin1, atg1a, atg1b, atg101, atg13, atg3, atg4b, atg4d, atg5, atg7, atg8a and atg8b (P ≤ 0.05) (P ≤ 0.05)  (P ≤ 0.05) (P ≤ 0.05)  , increas ≤ 0.05) , and up- ≤ 0.05) , and inc ≤ 0.05) . Thus, tampkα1, ampkα2, ampkβ1, ampkβ2, ampkγ1 and ampkγ2 (P ≤ 0.05) (P ≤ 0.05)  , and inc ≤ 0.05) . These iP ≤ 0.05) (P ≤ 0.05)  . Thus, w ≤ 0.05) . These dampkα1, ampkα2, ampkβ1, ampkβ2, ampkγ1 and ampkγ2 (P ≤ 0.05) (P ≤ 0.05)  , and inc ≤ 0.05) . These iP ≤ 0.05) (P ≤ 0.05) . Compared to the control, Pi incubation increased the protein level of the p-Beclin1 and LC3B-II, and reduced the p62 expression ( ≤ 0.05) . Lyso-Tr ≤ 0.05) .acadm, acadvl, acadl, acox3, hadh and hadhb) (P ≤ 0.05)  . Thus, oP ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05)  . Meanwhi ≤ 0.05) , and att ≤ 0.05) . Taken tbeclin1 expression (P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05) (We investigated the possibility of whether Beclin1 was required for Pi-induced lipophagy by knockdown experiment. The siRNA-774 was chosen from three sequences because of its capacity for inhibiting  ≤ 0.05) . Compare ≤ 0.05) . The con ≤ 0.05) . Moreove ≤ 0.05) . Meantim ≤ 0.05) , which w ≤ 0.05) . These fampkα1 by siRNA was used. The siRNA-1374 was chosen from three sequences because of its capability for inhibiting the ampkα1 mRNA expression (P ≤ 0.05) (ampkα1 knockdown inhibited AMPKα1 protein expression (P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05) (P ≤ 0.05)  . Compare ≤ 0.05) . Moreove ≤ 0.05) . AMPKα1  ≤ 0.05) . AMPKα1  ≤ 0.05) . The res ≤ 0.05) . These ibeclin1 and ampkα1 genes were constructed and transfected into HEK 293T cells to investigate their interaction. We found that AMPKα1 coprecipitated with Beclin1 , and increased the protein expression of autophagy markers (LC3B-II and Beclin1). During the autophagosome formation, the cytosolic LC3-I is conjugated to the phosphatidylethanolamine (PE) to form the LC3-II, which is then bound to the autophagosome , and increased NEFA content. Our further investigation suggested that Pi treatments activated the lipophagy and facilitated mitochondrial fatty acid β-oxidation, and according reduced TG deposition. In our in vitro study, compared to the control, Pi up-regulated the p-Beclin1 and LC3B-II protein levels, down-regulated p62 expression. Beclin1 phosphorylation is required for the autophagy induction and LC3 accumulation  can negatively regulate lipid synthesis in the liver, thereby preventing the occurrence of NAFLD, which could provide the potential target for the treatment of NAFLD in practical.In conclusion, HPD-induced autophagy and lipid turnover involved the activation of AMPK pathway via the AMPKα1 phosphorylation at S496. AMPK promotes phagophore nucleation by phosphorylation Beclin1 at S90. Our data demonstrated that dietary phosphorus supplementation .The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.


Penicillium digitatum is one of the most devastating diseases during post-harvest stages of citrus fruit. In this study, a potential endophyte Bacillus subtilis L1-21, isolated from healthy citrus plants, was assessed for its biocontrol activity against the pathogen P. digitatum. Based on an in vitro crosstalk assay, we suggested that B. subtilis L1-21 inhibits the pathogen with an inhibition zone of 3.51 ± 0.08 cm. Biocontrol efficacy was highest for the fermented culture filtrate of B. subtilis L1-21. Additionally, using GC-MS analysis, 13 compounds were detected in the extract of this endophyte. The culture filtrate in Landy medium could enlarge and deform pathogen spores and prevent them from developing into normal mycelium. Accordingly, the Landy culture filtrate of B. subtilis L1-21 was stable in the temperature range of 4–90 °C and pH of 3–11. Further, MALDI-TOF-MS for B. subtilis L1-21 detected surfactin, fengycin, bacillaene and bacilysin as potential antifungal compounds. GFP-tagged B. subtilis L1-21 easily colonized in citrus fruit peel and pulp, suggesting its role in eliminating the fungal pathogen. Altogether, it is highly expected that the production of antifungal compounds, and the colonization potential of B. subtilis L1-21 are required against the post-harvest P. digitatum pathogen on citrus fruit.Citrus is among the most important plants in the fruit industry severely infected with pathogens. Citrus green mold caused by  Penicillium is one of the most severe diseases with losses of at least 10% and, in severe cases, up to 90% [P. digitatum is the most devastating. The necrotic fungus gains entry in the fruit tissues through wounds and has a short life cycle of 3 to 5 days but produces a profound number of conidial spores that could initiate a new life cycle [Citrus is a general term for citrus fruit trees, mainly including pomelo, mandarin, tangerine, lemon, sweet orange, grapefruit and lime . The plap to 90% ,6. Amongfe cycle , spreadiPenicillium largely depend upon chemical fungicides such as pyrimethanil, imazalil, thiaben-dazole, prochloraz or different mixed formulations of these compounds [Penicillium [Bacillus spp. have been widely studied and reported for many years to manage plant diseases and improve plant survival [B. amyloliquefaciens, B. subtilis, B. velezensis, B. megaterium, B. pumilus and B. cereus have effective antagonistic activity against a variety of plant pathogens, including fungi, bacteria and oomycetes [Bacillus spp. could control green mold caused by P. digitatum [Pseudomonas [Paenibacillus [Aureobasidiuma [Traditional practices widely used for the management of ompounds . Howeverompounds . Thus, aicillium . Additioicillium ,12. It aicillium . As it isurvival ,14,15. Iomycetes ,17. Poteigitatum ,19. Otheudomonas , Paenibabacillus  and Aureasidiuma .Bacillus could control green mold through different interesting mechanisms and strategies. These mechanisms include induced resistance, wound colonization, competition with a pathogen for sites and nutrients, antifungal compounds, inhibition of spore germination, deformation of mycelia and disruption of the cell wall [P. digitatum [ell wall ,23,24. Nell wall ,26, macrB. subtilis L1-21 regulates important metabolites in huanglongbing (HLB)-affected citrus plants [B. subtilis L1-21. The colonization potential of this endophyte was revealed in detail during the pathogen interaction inside the citrus fruit. Furthermore, we elucidated the mode of action in conjunction with metabolic compounds produced by B. subtilis L1-21 to develop an efficient strategy for protection against citrus fruit green mold diseases.In our previous study, we reported that endophyte s plants . HoweverBacillus subtilis L1-21 was previously isolated from healthy citrus plant [B. subtilis L1-21 was prepared in Luria Bertani (LB) medium at 30 °C at 160 rpm for 16 h, and 1% (v:v) was inoculated in the above medium at 160 rpm at 30 °C. Samples were taken every day until the 4th day. After fermentation, the medium was centrifuged at 10,000 rpm for 15 min and then the supernatant was filtrated using 0.22 μm filter paper. The green fluorescence protein (GFP) labelled B. subtilis L1-21 was obtained through transforming the vector pYC127 [B. subtilis L1-21-GFP.us plant . The endCitrus reticulata cv. Shatangju) fruits were harvested from an orchard located in Guiling Guanxi Province, China. The citrus plants present in the orchard were not given any chemical treatment. The fruit brought to the laboratory were rinsed for 3 min with flowing water followed by surface sterilization with 2.5% of sodium hypochlorite for 2 min, washed with distilled water, 75% of ethanol (v/v) for 1 min, washed three times with sterile distilled water, and air-dried at room temperature in laminar flow hood for further use. Mandarin  method was used to extract genomic DNA [The conidia from the diseased citrus fruit were picked with a sterilized wire loop on an ultra-clean workbench and cultured on potato dextrose agar (PDA) medium at 25 ± 1 °C for 3 d. After purification, the conidia were stored at 4 °C for a short period. In order to check the pathogenicity of isolate, spores were inoculated at the equator of citrus fruit with concentrations of 10omic DNA . ITS1 (TB. subtilis L1-21 was obtained on NB agar after 24 h. Petri dishes (9 cm) containing 15 mL of PDA were prepared. In total, 100 μL of pathogen suspension (1.0 × 105 spores mL−1) was applied to the PDA medium and kept for few minutes for drying [B. subtilis L1-21 was inoculated at four symmetrical points 2.5 cm from the center for 2 d at 25 ± 1 °C. After 2 d, the diameter of the inhibition zone was measured accordingly.Culture of r drying . B. subtB. subtilis L1-21 cells and P. digitatum spores were suspended in a 0.85% sodium chloride solution. The punctures were inoculated with the following treatments: (i) control 1, 10 μL of P. digitatum spore suspension (1 × 104 spores mL−1) and 10 μL of 0.85% sodium chloride solution; control 2, 10 μL of P. digitatum spore suspension (1 × 104 spores mL−1) and 10 μL of NB culture; (ii) 10 μL of P. digitatum spore suspension 1 × 104 spores mL−1) and 10 μL of B. subtilis L1-21 cell suspension ; (iii) 10 μL of P. digitatum spore suspension and 10 μL of B. subtilis L1-21 culture filtrate; and (iv) 10 μL of P. digitatum spore suspension and 10 μL of B. subtilis L1-21 fermentation (cell and culture filtrate). Control 1 was set for treatment (ii) and control 2 was set for treatment (iii) and (iv). Each replication consisted of 30 citrus fruits and each treatment consisted of three replications. The modified disease fruit grading standard was as following [One puncture was made with a sterile tip at a 1-cm depth at the equator of citrus fruit. ollowing : Grade 0B. subtilis L1-21 displayed a marked inhibitory effect against the pathogen, B. subtilis L1-21 was cultured in NB medium and incubated at 30 °C and 160 rpm for 4 d. The cells were removed after centrifugation, 200 mL of supernatant was extracted with 200 mL of ethyl acetate 3 times, 600 mL of ethyl acetate was evaporated by rotation, and cells were finally dissolved in 4 mL of ethyl acetate. GC–MS analysis was operated using a gas chromatography  equipped with DB-5MS (30 m × 0.25 mm × 0.25 μm) and a mass spectrometer (EI with replaceable horn). The temperature program was set at an initial temperature of 40 °C for 1 min, and then increased to 130 °C at 10 °C min− 1, maintained for 5 min and then increased to 230 °C at 8 °C min−1, and maintained for 5 min; in total, 30.5 min. High-purity helium was used at a constant flow rate of 1 mL min−1. The mass spectrometer was equipped with a 70 eV electron impact ionization  and a quadrupole analyzer. The ions were detected in the range of 50–600 m/z. Finally, results of mass spectra were analyzed using Chemistation software .In order to prove how this −1; glucose, 1 g L−1; CaCl2, 1 g L−1; K2HPO4, 1 g L−1; MgSO4, 0.2 g L−1; FeCl3, 0.01 g L−1; pH = 7.0), R2A medium, tryptic soya gar (TSA) medium, 10% TSA medium and Landy medium were selected for screening. The culture filtrate of B. subtilis L1-21 was prepared in LB medium at 30 °C and 160 g for 16 h, and 1% (v:v) was inoculated in the above medium at 160 rpm and 30 °C. Samples were taken every day until the 4th d. After fermentation, the medium was centrifuged at 10,000 rpm for 15 min and then the supernatant was filtrated through 0.22 μm filter paper. After mixing  culture filtrate with PDA (filtration) containing 0.8% (w:v) agar, 50 μL of the mixture was put onto the sterilized glass slide; 40 μL of spore suspension (106 spores mL−1) was added to the solidification drop, and was incubated at 25 ± 1 °C; the spore germination was observed and counted after 32 h. The medium without inoculating B. subtilis L1-21 was used as the control. Prochloraz was used as positive control with a concentration of 450 mg/L . The spore germination was observed under microscope (ZEISS) and the spore germination rate was calculated. More than 200 spores were counted for each treatment and three replications were used for each. The morphology of spores was observed using scanning electron microscope .NB medium, MT medium  to 365-nanometer UV light for 15, 30, 45, 60 and 121 min at a distance of 30 cm, and the experiment was repeated three times. In addition, L1-21 culture filtrate was adjusted to pH 1–12 using 0.5 M HCl and NaOH and placed at room temperature for 24 h, followed by an adjustment of pH 7 for bioassay, and the pH 6 served as the control group. Each treatment consisted of three replicates. After all the above treatments, effects of culture filtrate on spore germination rate of P. digitatum were determined to evaluate the stability of culture filtrate.Temperature tolerance of B. subtilis L1-21 grown in 200 mL of Landy was dried at 50 °C, followed by an addition of 4 mL of methanol to dissolve the compounds. Chromatographic separation was carried out using a column  with a flow rate of 1 mL min−1. A mobile phase consisting of 0.5% (v/v) trifluoroacetic acid (TFA) (A) and methanol (B) was used. The following linear gradient elution was used: 5% methanol at 0 min, increased to 100%, kept at 100% methanol from 60 to 70 min. The UFLC-MS-IT-TOF apparatus and method were used as performed previously by Li et al. [m/z and 800–2000 m/z, respectively.The antifungal compounds were detected using LC-MS. Culture filtrate of i et al. . The ionB. subtilis L1-21 was put on shaking in NB medium (with chloramphenicol 10 μg mL−1) at 160 rpm with 37 °C for 2 d. The fruit were soaked in GFP-L1-21 (1 × 108 CFU mL−1) for 30 min and kept at 25 ± 1 °C. Samples were taken at 1, 3, 5 and 7 d with three replicates in each treatment. After surface sterilization, 1 g of the fruit peel and pulp was ground and 9 mL of sterilized 0.85% (w/v) sodium hypochlorite was added. The 100-μL tissue solution was inoculated on a chloramphenicol (10 μg mL−1) NB plate and incubated at 37 °C for 24 h; the number of GFP colonies was counted. The colonization ability was further validated using a scanning electron microscope. In this experiment, one puncture was made with a sterile tip at a 1 cm depth at the equator of citrus fruit. The experiment was carried out in the following order: Control (CK) for 12 and 24 h; 10 μL of B. subtilis L1-21 cell suspension  for 12 and 24 h; 10 μL of P. digitatum spore suspension for 24 h; 10 μL of P. digitatum spore suspension (1 × 104 spores mL−1) and 10 μL of B. subtilis L1-21 cell suspension for 12 and 24 h. The fruits were kept at 25 ± 1 °C. The wound of citrus fruit was cut and then observed using a scanning electron microscope at 12 and 24 h.GFP-tagged p < 0.05. All data acquired from the repeated experiments were expressed using the mean ± standard deviation. All data were statistically analyzed using SPSS version 22  using a one-way analysis of variance (ANOVA). Duncan’s multiple range test was applied to determine the significant difference at Penicillium with depictions of conidiophores and spores (18S rRNA gene sequence was deposited to NCBI and a GenBank accession number was assigned (MZ881937). The results of the identity analysis of the sequences confirmed that they belonged to Penicillium digitatum  treatment, cell suspension and culture filtrate treatment were 98.15 ± 3.21%, 54.47 ± 12.33% and 85.10 ± 13.45%, respectively  is a dipeptide antibiotic that shows antagonistic activity against a wide range of fungi and bacteria. Bacilysin not only inhibits fungi and bacteria, but also antagonizes cyanobacteria (Microcystis aeruginosa) [P. digitatum has not been reported before.To gain further insights into the identification of antifungal compounds, we analyzed membrane . Li et amembrane  reportedi et al.  also higeen mold . In thisuginosa) . HoweverB. subtilis L1-21 using GC-MS. Among them, phenol attained significant antifungal activity, also reported against Fusarium oxysporum f. sp. cubense, produced by B. amyloliquefaciens NJN-6 [B. amyloliquefaciens NJN-6 exhibited antifungal activity against F. oxysporum f. sp. cubense. Phenylethyl alcohol showed antifungal activity against Fragaria ananassa cv. Maehyang and prolonged the postharvest life of strawberries [Salmonella Typhi pathogen was successfully inhibited with hexadecanoic acid [B. subtilis L1-21 can be cultured in Landy medium to control citrus green mold. Future work will focus on optimizing a cheaper medium that can promote a higher production of antagonistic substances and a mode of action of these components against pathogens.In this work, we identified 13 compounds produced by ns NJN-6 . Howeverwberries . Salmoneoic acid ,42 whereoic acid ,44. HoweB. subtilis L1-21, previously used against the pathogen causing citrus huanglongbing, can play a prominent role to reduce the pathogen P. digitatum during fruit storage. Bacillus subtilis L1-21 produces important antifungal compounds in the form of surfactins, fengycins, bacillaene and bacilysin to mitigate the pathogen. We also highlighted that a low concentration of the B. subtilis L1-21 Landy culture filtrate was able to deform and expand the pathogen spores, which resulted in a loss of vitality. The endophyte can easily colonize in peel and pulp, providing no room for the pathogen to invade easily. The potential endophyte can serve as an important bioagent to reduce the incidence of pathogens through citrus–endophyte pathogen interaction on other kinds of citrus fruits present in the food industry. Further characterization of the molecular mechanism of B. subtilis L1-21 to control this pathogen will provide an exciting venue for future research.In conclusion, we suggest that