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Kaiming Xue, Huimin Wang, [Denis Y. W. Yu](https://orcid.org/0000-0002-5883-7087)

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[Emerging Battery Systems with Metal as Active Cathode Material](https://mdr.nims.go.jp/datasets/8489aa45-65c5-4adb-a441-5f8f88891729)

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Emerging Battery Systems with Metal as Active Cathode MaterialEmerging Battery Systems with Metal as Active CathodeMaterialKaiming Xue,[a] Huimin Wang,[a] and Denis Y. W. Yu*[a, b]Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 1/16] 1ChemElectroChem 2024, e202300661 (1 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemwww.chemelectrochem.orgReviewdoi.org/10.1002/celc.202300661http://orcid.org/0000-0002-5883-7087http://crossmark.crossref.org/dialog/?doi=10.1002%2Fcelc.202300661&domain=pdf&date_stamp=2024-06-21The high-cost and limited availability of raw materials forlithium-ion batteries hinder their future development and urgeresearchers to explore alternative battery systems. Amongthem, batteries utilizing the electrochemical redox reaction ofmetals such as Cu, Fe, Sn, etc. as the cathode to reversibly storeand release energy are attractive because their raw materialsare common and abundant. This review examines this type ofnovel battery system, introduces its basic mechanism andproblems, analyses the strategies that are used to improve itsreversibility and cycling stability and also proposes somepossible future directions of investigations.1. IntroductionAs an important device to reversibly store and release electricalenergy, battery has become an indispensable part of our dailylife to power consumer electronics such as cell phones, laptops,cameras and supplement the electricity grid.[1,2] Especially, thefast advancement of electrical vehicles in this decade furtherfosters the growth of the battery industry.[3] However, lithium-ion battery (LIB), one of the most common battery systems,faces an obstacle for large-scale applications because of its highcost caused by limited reserves of its raw materials.[4] Due to theutilization of lithium, whose price has witnessed a constantincrease in recent years, and other expensive transitional metalelements such as Ni and Co, the active cathode materialsaccount for the largest portion (about 30%) of the raw materialscost for the battery.[5]In this respect, other novel battery systems using alternativecathodes are being intensively investigated by the researchcommunity.[6–11] For example, lithium-O2 (Li� O2)[6] and lithium-sulfur (Li� S)[7] batteries that use inexpensive carbon electrode ashost for O2 and sulfur electrode, respectively, as the cathodesare potential low-cost systems. Recently, the use of commonmetals as electrodes is also attracting more and more interestfrom researchers. Metals such as Li, Al, and Sn are frequentlyinvestigated as the battery anode in organic electrolytes,whereas Zn and Fe are studied as anode with aqueouselectrolytes.[12–14] In comparison, the use of metal as activecathode materials is still in its infancy. Therefore, a review ofnovel battery systems using metal cathodes would be timelyand helpful for the advancement of these unconventionalbattery systems. In this review, we systematically summarizedvarious kinds of metal cathodes from copper, which is the mostcommonly used metal cathode because of its low price andhigh redox potential, to other metals ranging from silver andstainless steel to tin and other Group IV, V, and VI metals. Theperformances of these batteries are demonstrated in differenttypes of electrolytes, such as aqueous-based electrolyte,organic-based electrolyte, aqueous/organic mixed electrolyte,and molten salt electrolyte. Such variability allows the potentialutilization of metal cathodes in a wide spectrum of electro-chemical energy storage applications.In general, metals can undergo redox reactions at a certainpotential and reversibly convert between metal atoms andmetal cations by giving out electrons during charging andaccepting electrons during discharging to store and releaseelectrical energy, as demonstrated in the following reactions:charge : M � ne� ! Mnþdischarge : Mnþ þ ne� ! MSince different metals have different electrode potentials, asshown in Table 1, the output voltage of the resulting batterysystems using metal as active cathode materials is determinedby the electrode potential difference between the cathode andthe anode. For example, since the electrode potential of Sn/Sn2+ and Li/Li+ is � 0.13 and � 3.04 V (relative to a standardhydrogen electrode), respectively, the working voltage of thecorresponding Sn� Li battery with Sn cathode and Li anode isabout 2.8 V (the actual value may vary slightly from thetheoretical value due to different electrolytes).The advantages of metals are that they have excellentelectrical conductivity and often have high specific capacity asmultiple electrons can be extracted.[15] They also have goodprocessibility so that the direct utilization of metal foils aselectrodes is possible. So, when preparing the electrode,polymer binders, conductive additives, and toxic organicsolvents are not necessary, which simplifies the manufacturingprocess and increases the loading amount of active cathodematerials. If metal powder is used, they can still be slurry-coated[a] Dr. K. Xue, Dr. H. Wang, Prof. D. Y. W. YuSchool of Energy and Environment, City University of Hong Kong Kowloon,Hong Kong, ChinaE-mail: yu.denis@nims.go.jpHomepage: https://samurai.nims.go.jp/profiles/yu_denis?locale=en[b] Prof. D. Y. W. YuResearch Center for Energy and Environmental Materials (GREEN), NationalInstitute for Materials Science, 305-0044 Tsukuba, Ibaraki, Japan© 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbH. Thisis an open access article under the terms of the Creative Commons Attri-bution License, which permits use, distribution and reproduction in anymedium, provided the original work is properly cited.Table 1. Standard potentials of different metals.Half reaction Standard Potential (V)Ag+ +e� , Ag +0.80Cu2+ +2e� , Cu +0.342H+ +2e� , H2 0Sn2+ +2e� , Sn � 0.13Ni2+ +2e� ,vNi � 0.25Fe2+ +2e� , Fe � 0.44Zn2+ +2e� , Zn � 0.76Ti3+ +3e� , Ti � 1.37Na+ +e� , Na � 2.71Li+ +e� , Li � 3.04Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 2/16] 1ChemElectroChem 2024, e202300661 (2 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://samurai.nims.go.jp/profiles/yu_denis?locale=enin the same way as traditional LIB electrodes. In this review, thespecific capacity and current density are expressed in mAhcm� 2and mA cm� 2 for metal foil electrodes or mAhg� 1 and mA g� 1for metal powder electrodes.The main issue of using metal as the cathode material in abattery is that of the self-discharge caused by the shuttling ofcathode metal cations that are generated in the charge processfrom the catholyte to the anolyte, which are spontaneouslyreduced at the anode. This will lead to low Coulombic efficiency(CE), large energy loss, and poor cycling stability. In addition,the cathode materials will be continuously consumed, causingthe gradual disintegration of the electrode. Moreover, theanode will be poisoned by the “alien” cathode material,affecting the deposition process of the anode material.Strategies to overcome the shuttling problem mainly target atregulating and modifying the separator and electrolyte, whichwill be illustrated in detail in this review.2. Copper CathodeCopper has both advantages of low cost and high redoxreaction potential over common metals. In addition, it is stablein both aqueous and organic electrolytes. Therefore, mostinvestigations to date utilize copper as the active cathodematerials.2.1. Cu-BasedBattery Coupled with low Electrode PotentialAnodes such as Li and Al� Li Slloy2.1.1. Charge-Discharge mechanismWhen a Cu-based cathode is coupled with a low electrodepotential anode such as Li and Al� Li alloy, the correspondingbattery will have an output voltage of about 3 V. Though,organic electrolytes with larger electrochemical window willhave to be used in these batteries to keep the anode stable.Huang et al. first demonstrated the feasibility of a coin cell withCu and Li foils as the cathode and anode, respectively, with acarbon-coated polypropylene (PP) separator in an electrolyte of1.0 M LiClO4 in ethylene carbonate (EC) : propylene carbonate(PC)=1 :1 (volume ratio).[16] The voltage profiles of the batteryare shown in Figure 1a. They attributed the oxidation processto a one-step reaction from Cu to Cu2+ whereas the reductionprocess is a two-step reaction first from Cu2+ to Cu+ and thento Cu from the incremental charge versus potential (dQ/dV)curve (Figure 1b). Though, their obtained areal capacity is onlyabout 0.014 mAhcm� 2.In 2019, Wang and Yu proposed that Cu undergoes a one-electron transfer to Cu+ in a lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI)/dimethyl carbonate(DMC)-based organic electrolyte during charging if the chargecapacity is limited.[17] They verified it by quantifying the amountof Cu cations in the electrolyte solution at different states ofcharge with inductively coupled plasma mass spectroscopy(ICP-MS) and comparing the experimental value with thetheoretical amount of Cu+, as shown in Figure 1c.It is worthy to note that the mechanism of Cu cathode isstill controversial and perplexing as the stability of differentcations (Cu+ or Cu2+) may depends on the choice of electrolyte,i. e. lithium salts (e.g. LiClO4 vs. LiTFSI) and solvents (e.g. EC/PCvs. DMC). A systematic investigation in the future is necessaryto obtain a thorough understanding of the reaction mechanismof Cu cathode in organic electrolytes.2.1.2. Suppression of the Cross-Over of Cu CationsThough, if the Cu ions from the catholyte cross-over to theanolyte, the CE of the battery will be decreased because the Cuions can be readily reduced at the anode, which poses a largeDr. Kaiming Xue received his bachelor degreefrom Zhejiang University in 2019. In 2023, heobtained his Ph.D. degree from City Universityof Hong Kong under the supervision of Dr.Denis Yu. His research interest focuses on theseparator modification and electrolyte designfor different metal-cathode batteries.Dr. Huimin Wang is a researcher specializingin the field of energy storage, specifically inthe area of metal cathodes, electrolyte designand novel battery systems. She obtained herbachelor‘s degree from Anhui University andsuccessfully completed her Ph.D. studies in2020 at City University of Hong Kong, underthe guidance of Dr. Denis Y.W. Yu.Prof. Denis Yu is the group leader of theRechargeable Battery Materials Group at theNational Institute for Materials Science inJapan. He received his bachelor‘s degree fromPrinceton University and his Ph.D. degreefrom Harvard University. His research interestincludes understanding fundamental reactionmechanisms in battery materials and electro-des, centering on examining the effect ofsurface chemistry and structure on electro-chemical performances, as well as on thelong-term stability and safety of batteries.Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 3/16] 1ChemElectroChem 2024, e202300661 (3 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseproblem to the reversibility of the battery. Conventionalseparators commonly used in LIBs such as polypropylene (PP)membranes have high porosity, but they are not ion-selective,which means any ions (cations and anions) can freely passthrough them.[18] Figure 1d shows the voltage profiles of aCu� Li with PP membrane in 3 M LiTFSI in DMC electrolyte at acurrent rate of 0.01 mAcm� 2.[19] The CE of the battery isextremely low because of the cross-over of Cu ions. Techniquesto suppress the cross-over of Cu ions are necessary to improvethe reversibility of the battery with the Cu cathode. So far, thestrategies that are demonstrated are shown in Figure 2, by(a) tuning the interaction between the electrolyte and separa-tor, (b) addition of the Cu trapping layer, (c) use of anionexchange membrane, and (d) use of a ceramic separator.2.1.2.1. Interaction Between Electrolyte and SeparatorWang et al. have found that the migration of Cu ions throughPP (Celgard 2400) membrane in fact depends on the type ofelectrolyte and its interaction with the membrane.[20] As shownin Figure 3a, by screening different kinds of solvents commonlyused for battery electrolytes, they found that the Cu� Al batterywith 3 M LiTFSI in fluoroethylene carbonate (FEC) electrolytedisplays the highest CE. They also noted that there is acorrelation between the CE of the battery and the contact angleof the electrolyte on the PP membrane, with higher contactangle giving higher CE. Since the contact angle reflects thesurface energy between the electrolyte and the separator, theseresults suggest that the high surface energy between the PPmembrane and the electrolyte suppresses the disadvantageousshuttling of Cu ions across the PP separator. However, the FECelectrolyte shows poorer wetting compatibility with the PPmembrane, leading to a larger cell resistance and limiting therate performance of the battery. Nonetheless, the workdemonstrated that the interaction between the electrolyte andmembrane can significantly influence the ion transport throughthe membrane, which is an area that needs further investigationfor batteries with metal cathode.2.1.2.2. Trapping of CuTo stop the Cu cations from moving from the catholyte to theanolyte, another method is to add a coating layer that can trapthe Cu cations on the electrode or the separator to preventthem from moving across to the anode.Wang et al. first coated the Cu cathode with a layer ofpolyacrylonitrile (PAN), which improves the CE of the corre-sponding Cu� Al battery. The PAN polymer has a large amountof electron-rich nitrile (� C� N) groups in its molecular structure,which is a Lewis base and is able to interact with Cu+ (LewisFigure 1. (a) Charge-discharge curves of Cu/Li coin cell with a C-coated Celgard separator; (b) the incremental charge versus potential (dQ/dV) curve ofCu@CNFs electrode during the charge/discharge process (re-print with permission from [16] Copyright 2018, Elsevier); (c) variation of Cu contents inelectrolyte (re-print with permission from [17] Copyright 2019, American Chemical Society); (d) fifth cycle charge–discharge curves at a current density of0.01 mAcm� 2 for Cu� Li batteries with different separators (re-print with permission from [19] Copyright 2021, American Chemical Society).Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 4/16] 1ChemElectroChem 2024, e202300661 (4 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseacid) in the electrolyte solution to anchor the Cu+ ions andprevent them from passing through the electrolyte andparticipating in possible side reactions with the anode.[22] Asshown in Figure 3b and 3c, the CE of the Cu� Al battery with thePP membrane tested in the 3 M LiTFSI FEC electrolyte at acurrent rate of 0.014 mAcm� 2 can be increased from about 92%to 98% after the introduction of the PAN layer. Although thePAN polymer coated on the cathode surface may helpimmobilize Cu+, the intactness of the PAN layer in the cyclingprocess is questionable since it may be detached from thecathode surface along with the dissolution of Cu during thecharging process.Besides coating on the electrode, in another work, Xue et al.coated the commercial PP separator with polyacrylic acid (PAA),a chelating polymer, to fabricate a composite membrane ofPAA/PP to confine the shuttling of Cu+ through the modifiedmembranes (Figure 2b).[19] By utilizing the chelating interactionbetween the carboxyl groups in the polymer and copper ions,the chelating polymer can prevent the movement of Cu+ fromthe catholyte to the anolyte in the cell and suppress the self-discharge of the corresponding battery. The Cu� Li battery withthe PAA/PP membrane (86%) shows much higher CE than theone with the pristine PP separator (20%) and commercial anionexchange membrane (AEM) (70%) at a rate of 0.01 mAcm� 2, asshown in Figure 1d. By contrast, other polymer coatings such aspolyethylene oxide (PEO) show low CE as they do not containthe same carboxyl groups as the PAA polymer that can trap theCu cations.Though, the physical coating of a PP separator with func-tional polymers can produce other problems, such as theincrease in the resistance of the battery. In Xue et al.’s work,although the PAA/PP membrane improves the reversibility, thebattery exhibits unsatisfactory rate performance whose averagedischarge voltage drops significantly from 3.08 V at 0.1 mAcm� 2to 2.07 V at 1 mAcm� 2 as the PAA layer limits the transport ofother ions besides Cu+ through the membrane as well (Fig-ure 3d), especially at increased current rates.[19] To reduce thevoltage polarization, the authors introduced barium titanate(BTO) nanoparticles into the PAA/PP membrane, which is shownto enhance the ionic transfer through the membrane andincrease the average discharge voltage to 2.85 V at 1 mAcm� 2.Since Cu cations are being trapped into the polymer layer,one of the other concerns is whether the effect of thesepolymer coating can be long-lasting as the Cu cations willaccumulate inside the polymer with cycling, and eventuallysaturating the layer. Further work is necessary to study how theion blocking capability of the polymers is influenced by theaccumulation of Cu ions. In addition, efficient ion transportchannels through the trapping layer need to be constructed tokeep the resistance of the battery low.2.1.2.3. Use of Anion Exchange Membrane (AEM)With the new developments in ions-selective membranes, it isnow possible to tune the type of ions that are transportedthrough them. So, in principle, the cross-over of Cu cations canbe stopped with the use of an AEM, in which the chargebalance within the battery will be maintained by the transportof anions.Wang et al. demonstrated that the use of a commercially-available anion exchange membrane (AEM – FAPQ-310-PP(F310) ) as the separator can improve the CE of a Cu� Albattery.[17] Interestingly, the effectiveness of the F310 mem-Figure 2. Strategies to solve the cross-over problem: (a) tuning the interaction between the electrolyte and separator (re-print with permission from [20]Copyright 2020, Wiley-VCH); (b) addition of the Cu trapping layer (re-print with permission from [19] Copyright 2021, American Chemical Society); (c) use ofanion exchange membrane (re-print with permission from [17] Copyright 2019, American Chemical Society) and (d) use of a ceramic separator (re-print withpermission from [21] Copyright 2014, Springer Nature).Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 5/16] 1ChemElectroChem 2024, e202300661 (5 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensebrane depends on the concentration of the electrolyte.Specifically, CE is 19%, 80%, and 88% at a rate of0.0225 mAcm� 2 with an areal capacity limitation of0.225 mAhcm� 2 for cells tested in 1 M, 3 M, and 6 M electrolytewith LiTFSI and DMC, respectively and the cycle performance isbetter with higher salt concentrations (Figure 3e and 3 f). Theimprovement is ascribed to the different solvation structureswithin the electrolyte. With the increase in the salt concen-tration, the Cu ions are most likely incorporated into largesolvation structures such as contact ion pairs (CIP) andaggregate (AGG), which are harder to cross through the AEM.However, the increased interaction between the salt and thesolvent molecules also limits the transfer of anions through theAEM, resulting in higher overpotential (Figure 3e). This workdemonstrates that the synergistic effect of a AEM membrane,which only conducts anion but blocks cations, with a highlyconcentrated electrolyte can reduce the shuttling of Cu+ andincrease the CE of the battery.Though, commercial AEMs are typically developed for theuse in the aqueous medium with counter anions such as SO42�or Cl� , which may not be directly suitable for Cu-based batteryin the organic-based aprotic electrolyte.[23] In view of this, Xueet al. designed a membrane with a poly(ionic liquid) (PIL) ofpoly(diallyldimethylammoniumbis(trifluoromethylsulfonyl)imide) (PDADMA+TFSI� ) coated on acommercial PP separator (PIL/PP) that can be used in organicelectrolytes.[24] The PIL contains a positively-charged polymerbackbone with TFSI� anions, with a molecular structure asshown in Figure 4a, which can block the shuttling of copperions via an electrostatic repulsion effect. It is also compatiblewith the LiTFSI salt typically used in electrolytes for Cu-basedbatteries.Figure 3. (a) Voltage profiles of Cu� Al batteries with a PP membrane at a constant rate of 0.014 mAcm� 2 and a capacity limit of 0.14 mAhcm� 2 in 3 M LiTFSI inDMC, EMC, DEC, PC, and FEC; photographs showing the contact angles (CA) of 3 M LiTFSI DMC, EMC, DEC, PC, and FEC electrolytes on a PP membrane atroom temperature (re-print with permission from [20] Copyright 2020, Wiley-VCH); (b) charge/discharge curves of a Cu� Al cell and a Cu/PAN� Al cell in 3 MLiTFSI FEC at a current density of 0.014 mAcm� 2 and a capacity limit of 0.14 mAhcm� 2; (c) cycle performances of the Cu� Al and Cu/PAN� Al cells; andelectrochemical performance of Cu� Al full cell (re-print with permission from [22] Copyright 2021, Elsevier); (d) average discharge voltage of Cu� Li batteries atdifferent current rates with different separators (re-print with permission from [19] Copyright 2021, American Chemical Society); (e) voltage profiles and(f) cycle performance of Cu� Al full cells at a rate of 0.0225 mAcm� 2 and a capacity limitation of 0.225mAhcm� 2 in 1, 3, and 6 M LiTFSI DMC electrolytes,respectively, (re-print with permission from [17] Copyright 2019, American Chemical Society).Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 6/16] 1ChemElectroChem 2024, e202300661 (6 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseSpecifically, the PIL is first dissolved in N-methyl-2-pyrroli-done (NMP) solvent and coated on a commercial PP separatorto make a PIL/PP membrane for easy handling. Xue et al. thenconstructed a reversible 3.3 V Cu� Li battery in 3 M LiTFSI DMCelectrolyte with the membrane.[24] The battery demonstratedsuperior reversibility and cycling stability with CE of 99% forover 1000 cycles (Figure 4b and 4c) at the current rate of0.2 mAcm� 2 and capacity limit of 0.1 mAhcm� 2. In addition,since the positively charged polymer backbone can provideadditional channels to transport anions (Figure 4d), the batteryshows fast kinetics and reduced voltage polarization, with onlya 0.15 V drop in discharge voltage when the current rate isincremented from 0.1 mAcm� 2 to 1 mAcm� 2 (Figure 4e).2.1.2.4. Use of a Ceramic SeparatorApart from ion-selective membranes, there are also solid-stateconductors that can allow selective transport of a certain typeof ions. For example, Li1+xAlxTi2� x(PO4)3 (LATP) ceramic is a typeof sodium super ionic conductor (NASICON) solid electrolytethat allows the transfer of Li+ but not other cations. So, it ispossible to use a ceramic separator to prevent the cross-over ofCu ions.For example, Wang et al. tested a Cu� Li battery with asandwiched solid-state electrolyte of DES gel/LATP/DES-FEC gel.The deep-eutectic-solvent-based gel (DES gel) was prepared bymixing a DES solution composed of LiTFSI and succinonitrile(SCN) with ethoxylated trimethylolpropane triacrylate (ETPTA)monomer and 1 wt% azobisisobutyronitrile (AIBN) initiator andpolymerizing it at 70 °C (Figure 4f). The Li1.4Al0.4Ti1.6(PO4)3 (LATP)interlayer can help inhibit the passing through of coppercations. The Cu powder cathode of the Cu� Li battery canexhibit a specific capacity as high as 500 mAhg� 1 (Figure 4g).This work inspires future application of solid-state electrolytesin batteries with metal cathodes.Because a ceramic is non-porous, it also allows the use oftwo different electrolytes, an aqueous electrolyte on the Cucathode side with an organic electrolyte on the Li anode side.Wang et al. has demonstrated such a battery with 2 M LiNO3(aq) electrolyte for the Cu cathode and 1 M LiClO4 in EC/DMCelectrolyte for the lithium anode using a LISICON film (Fig-ure 5a).[25] As shown in Figure 5b, the Cu� Li battery with suchconfiguration can achieve a discharge voltage about 3 V.Besides Cu, Ag and Ni were also studied in such an organic/aqueous hybrid electrolyte and exhibit specific capacity ofabout 250 mAhg� 1 and 640 mAhg� 1, respectively.[26] Though,one of the drawbacks of this configuration is the potential riskFigure 4. (a) Molecular structure of PDADMA-TFSI; (b) The 20th cycle charge–discharge curves and (c) cycle performances of Cu� Li batteries at a rate of0.2 mAcm� 2 with different separators; (d) schematic diagram of the anion transport mechanism through the PIL polymer; (e) average discharge voltage ofCu� Li batteries with the PIL/PP membrane at different current rates (re-print with permission from [24] Copyright 2023, Wiley-VCH); (f) schematic diagram of asolid-state Cu� Li battery with a Cu cathode, a Li anode, and a multilayer electrolyte consisting of DES gel, LATP pellet, and DES-FEC gel; (g) charge-dischargecurves of the solid-state Cu� Li battery at a rate of 200 mAg� 1 between 2–4.1 V. (re-print with permission from [15] Copyright 2022, Wiley-VCH)Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 7/16] 1ChemElectroChem 2024, e202300661 (7 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseof electrolyte leakage. In the event that the aqueous electrolyteseeps into the anode side, the Li anode will react violently withwater, potentially causing a fire.2.1.3. Other Factors Affecting Cycle StabilityCross-over of Cu cations through the membrane is one of thefactors affecting the cycle stability of the batteries with the Cucathode. In addition, the uniformity of the stripping anddeposition of Cu on the cathode and the change in thesolvation structure of the electrolyte with cycling will also affectthe cycling performance of the cathode.Huang et al. made a composite cathode with a network ofCu nanoparticles and carbon nanofibers (Cu@CNFs) thatshowed a specific discharge capacity of 635 mAhg� 1 in the firstcycle but with fast capacity fading with cycling.[16] Theyattributed the capacity lost to the aggregation of re-depositedCu. By coating the composite cathode with Au nanoparticles,which can provide nucleation sites for Cu deposition, theydemonstrated improved discharge capacity and cycle stability,as shown in Figure 5c.In general, uneven stripping and deposition of Cu duringcharge and discharge will cause the formation and growth ofpits, islands, and dendrites, which will eventually lead to thedisintegration of the electrode. Therefore, to improve thestability of Cu cathodes, methods such as electrode coating orelectrode additives will have to be explored in the future.In addition to the electrode, the status of the electrolyteand its interaction with the membrane while cycling can alsoaffect the reversibility of the battery with Cu cathode. Forexample, even though the use of a PIL/PP membrane canenable a high CE, Xue et al. showed that the Cu� Li battery withDMC-based electrolyte gives inferior and gradually deterioratingdynamical performance at a higher current rate of 1 mAcm� 2(Figure 5d). They attributed it to the buildup of a concentrationgradient upon charge-discharge at the high current rate in thepresence of the PIL/PP membrane, which only conducts anionsFigure 5. (a) Schematic diagram and operating principles of a Cu� Li battery with solid-state separator; (b) cycle performance of the Cu� Li battery: continuouscharge–discharge curve (re-print with permission from [25] Copyright 2009, Elsevier); (c) cycling performance of Cu@CNFs and Au� Cu@CNFs electrodes at200 mAg� 1 (re-print with permission from [16] Copyright 2018, Elsevier); (d) selected voltage profiles of a Cu� Li battery tested in 3 M LiTFSI DMC with PIL/PPmembrane; (e) proposed concentration gradient in the battery with the 3 M LiTFSI DMC electrolyte; Raman spectra of (f) 3 M LiTFSI DMC and (g) 3 M LiTFSIDME/PC=3 :7 electrolytes extracted from cycled Cu� Li batteries; (h) cycle performance of the Cu� Li battery with 3 M LiTFSI DME/PC electrolyte at 2 mAcm� 2(re-print with permission from [27] Copyright 2023, Elsevier).Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 8/16] 1ChemElectroChem 2024, e202300661 (8 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License(Figure 5e).[27] Besides, the accumulation of Cu+ in the electro-lyte changes its solvation structure, causing the formation ofmore CIP in the electrolyte and reducing the amount of freeanions in the electrolyte (Figure 5f), resulting in higher resist-ance for TFSI� transport through the PIL/PP membrane andincreasing the voltage polarization of the Cu� Li battery withcycling. The authors found that the solvation structure of theelectrolyte and the interaction of it with the membrane differwith different electrolyte. They then designed a novel mixedsolvent with dimethoxyethane (DME) and propylene carbonate(PC) to replace the original DMC solvent, which shows higherionic conductivity with more free TFSI� anions that reduces theresistance of ion transport both in the electrolyte and throughthe membrane. In addition, the electrolyte can help keep astable solvation structure upon cycling (Figure 5g). Theseadvantages of the new DME/PC based-electrolyte help theCu� Li battery to cycle stably for more than 1500 cycles at a rateof 2 mAcm� 2 (Figure 5h).Unlike LIBs, where the overall composition of the electrolyteremains unchanged during charge and discharge due to arocking chair mechanism, that of the electrolyte in batterieswith metal cathodes involving the dissolution and deposition ofmetal cations will change within a cycle, and possibly uponcycling as well due to side reactions. Thus, further character-izations and analyses are needed to understand how theinteraction between the electrolyte and membrane and thetransport mechanism of ions inside the cell change with cyclingand affect the performances of the battery.2.2. Cu-Based Battery Coupled with High Electrode PotentialAnode such as ZnThe use of copper as the cathode and Zn as the anode in anaqueous battery dated back to 1836, when the Daniell cell wasinvented by John Frederic Daniell, a UK chemist. The Daniell cellhas two separate electrolytes of CuSO4 solution and ZnSO4solution for the Cu cathode and Zn anode, respectively. Theyare connected by a salt bridge, which serves to maintain thecharge balance and complete the internal circuit duringoperation. The battery functions on the basis of the electro-chemical reduction reaction on the Cu cathode ofCu2þ þ 2e� ! Cu and electrochemical oxidation reaction on theanode of Zn � 2e� ! Zn2þ with an overall voltage of about1.1 V. The Daniell cell is a primary battery, which cannot berecharged due to the severe cross-over of the Cu cations in thecatholyte to the anode, a similar problem as described in theCu� Li battery with organic electrolyte.There are some recent works on improving the reversibilityof Cu� Zn battery to make it rechargeable. For example, Donget al. utilize a thin ceramic film made of lithium super-ionicconductor (LATSP, Li1+x+yAlxTi2� xSiyP3� yO12) as the separator ofthe Cu� Zn battery and two distinct electrolytes of 2 M LiNO3and 1 M Zn(NO3)2 solutions as the catholyte and anolyte,respectively.[21] The LATSP allows the transfer of Li and blocksthe cross-over of Cu cations. Figure 2d displays a schematicdiagram of the working mechanism and assembly, while aphoto of the battery is shown in Figure 6a. The reported batterycan be normally charged and discharged for about 150 cycles(Figure 6b). Even though the CE is improved, the Cu� Zn batteryexhibits high polarization voltage (voltage difference betweencharge/discharge voltages) of about 0.5 V at a current rate of0.25 mAcm� 2 probably due to high resistance of the LATSPceramic. The LATSP ceramic film is also brittle, which isvulnerable to breaking during the battery assembly andoperation.Similarly, Zhang et al. used a composite separator of PVDF/PMMA� LiClO4/PVDF and two electrolytes of 0.1 M CuSO4/1 MLi2SO4 and 0.1 M ZnSO4/1 M Li2SO4 for the cathode and anodecells, respectively.[28] The schematic diagram of the workingmechanism and photo of the corresponding H-cell are exhibitedin Figure 6c and 6d, respectively. However, the cell needs acomplex composite separator with two different electrolytes,which restricts the form of the battery to an H-cell. New celldesign is needed to make it more suitable for practical use.Jameson et al. further adopted a CIMS membrane (ASTOMcompany) as the separator and used 1 M ZnSO4 solution asanolyte and 0.1 M CuSO4/1 M Na2SO4 mixed solution ascatholyte.[29] CIMS is a specially designed monovalent cationexchange membrane which allows the conduction of mono-valent cations and block divalent cations. The membranetherefore permits the shuttling of Na+ between the cathodeside and anode side to maintain the charge balance (Figure 6e).They used 3D printing to fabricate two identical parts toassemble the Cu� Zn battery, which can be cycled for about100 cycles (one component is shown in Figure 6f). Though, theyonly demonstrated a cell with cycling stability of up to100 cycles with an average CE of about 70% and polarizationvoltage of about 0.35 V at 0.5 mAcm� 2 (Figure 6g), so morework is needed to study its long-term cycle stability.Similarly, the use of an AEM is another possible method tosuppress the Cu ion cross-over in the aqueous electrolyte. Forexample, He et al. demonstrated an ampere-hour level Cu� Znpouch cell with the use of an AEM to enhance the CE of thebattery (Figure 6h).[30] For the catholyte side, they used anelectrolyte with 1 M CuSO4 (pH=1) to improve the reversibilityof the Cu stripping/plating processes. On the other side, theystabilized the dissolution/deposition of Zn in a ZnSO4 electro-lyte by coating it with a 1 μm thick Sn layer.Alternatively, instead of utilizing a membrane or ceramic toseparate the catholyte and anolyte, Zhu et al. demonstrated aconcept to use an alkaline-based electrolyte of 1 M KOHsolution for both cathode and anode to limit the solubility ofCu2+ (Figure 6i).[31] Specifically, Cu will become insoluble Cu-(OH)2 and CuO in the alkaline medium during the chargeprocess and the Cu metal is recovered during the dischargeprocess. The reactions are shown below:Cathode : Cu OHð Þ2=CuOþ 2e� $ CuAnode : Zn � 2e� $ Zn OHð Þ2=ZnOOverall : Cu OHð Þ2=CuOþ Zn$ Cuþ Zn OHð Þ2=ZnO:Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 9/16] 1ChemElectroChem 2024, e202300661 (9 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseThe resulting alkaline Cu� Zn battery shows a high specificcapacity of 718 mAhg� 1 at 0.1 Ag� 1 with a discharge voltage of0.76 V (Figure 6j). However, the battery experiences a fastcapacity decay from 538 mAhg� 1 in the first cycle to about300 mAhg� 1 after 20 cycles while cycling at 1 Ag� 1 (Figure 6k).Even though the reversibility of aqueous Cu� Zn batterieshas been significantly improved in recent publications, so farthey all require the use of either two different electrolytes forthe cathode and anode cells, which makes the battery difficultto be put into practical usage, or an alkaline solution that isharmful to both the environment and human health. Ideally, asingle and benign electrolyte for both catholyte and anolytewould be desirable to simplify the structure of the Cu� Znbattery and make it more environmentally friendly. In addition,more investigations are needed to understand the interactionsamong the electrodes, electrolytes, and membranes to increasethe long-terms stability of the battery.3. Other Metal CathodesApart from Cu, there are other metals with high electrodepotential that can be used as the cathode in a battery, and theyare summarized here.3.1. SilverOne of the problems of a metal cathode is the migration ofdissolved metal cations from the cathode to anode. Thisproblem can be suppressed if the product is insoluble in theelectrolyte. Amongst different metal compounds, AgCl is a well-known material with low solubility. As a demonstration, Wanget al. investigated the use of a Ag/AgCl redox reaction as thecathode of a battery. Lithium is used as the anode with 1 MLiPF6 and 0.05 M LiCl in EC:DEC (diethyl carbonate)=1 :1Figure 6. (a) Photographs of the LATSP-based Cu� Zn battery and (b) cyclic profile of the rechargeable Cu� Zn battery: cell voltage vs. time (re-print withpermission from [21] Copyright 2014, Springer Nature); (c) schematic diagram of an aqueous rechargeable Cu� Zn Daniell type battery using Li+ for chargetransport; (d) the photograph of the Zn jZnSO4+Li2SO4 j ion-block membrane jCuSO4+Li2SO4 jCarbon system (re-print with permission from [28] Copyright2015, Royal Society of Chemistry); (e) schematic diagram of the rechargeable Cu� Zn battery using the CIMS membrane, (f) design of the battery testing deviceconsisting of two identical components and (g) the voltage profile of the 80th to 100th cycle at 0.5 mAcm� 2 with 0.1 M CuSO4/1 M Na2SO4 on the cathode sideand 1 M ZnSO4 on the anode side (re-print with permission from [29] Copyright 2020, Elsevier); (h) schematic diagram of the Cu� Zn battery using AEM (re-print with permission from 30 Copyright 2023, Royal Society of Chemistry); (i) the rechargeable Cu� Zn battery with the alkaline electrolyte and (j) dischargecurves of the Cu� Zn battery at 0.1, 0.5, 1, and 5 Ag� 1; (k) the cycle performance of Cu� Zn battery in 1 M ZnSO4 or 1 M KOH solution at the current rate of1 Ag� 1 (re-print with permission from [31] Copyright 2019, Wiley-VCH)Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 10/16] 1ChemElectroChem 2024, e202300661 (10 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensesolution as the electrolyte.[32] As illustrated in Figure 7a, duringthe charge process, Ag is electrochemically oxidized, loseselectrons, and becomes insoluble AgCl in the presence of Cl� inthe electrolyte solution. During the discharge process, areversed reaction occurs, with AgCl electrochemically reducedto Ag. (Figure 7b) The charge/discharge reactions of the Agcathode are shown below:Ag � e� þ Clþ $ AgClThough, because the reaction involves the incorporation ofCl� into Ag, a solid-state reaction, the available capacitydepends strongly on the particle size and surface area of Ag, asshown in Figure 7c. Cathode with nano-particles Ag exhibits ahigher specific capacity (about 70 mAhg� 1) than that of micron-sized Ag (about 10 mAhg� 1). Utilization of Ag can be furtherincreased by growing nano-sized Ag particles on carbon nano-tubes (n-Ag/CNT) (Figure 7d). The higher electrical conductivityof the electrode and smaller particle size can help facilitate theredox reaction of Ag/AgCl.Figure 7. Schematic diagram of the Ag� Li system in the (a) charge process and (b) discharge process; (c) voltage profiles of micro- and nano-Ag powders;(d) charge-discharge profiles of Ag� CNT with different m(Ag)/m(CNT) ratios (re-print with permission from [32] Copyright 2019, Wiley-VCH); (e) schematic ofthe stainless-steel/lithium system; (f) voltage profiles of SS410 L half cells with F310 AEM membrane with a charge capacity limitation of 100 mAhg� 1 at10 mAg� 1 (re-print with permission from [33] Copyright 2019, Elsevier); (g) schematic diagram of a Sn� Li battery; (h) selected charge/discharge curves of Sn� Libatteries at 0.2 mAcm� 2 with an areal capacity limitation of 0.1 mAhcm� 2; (i) difference in polarization voltage of Sn� Li, Cu� Li, and Ni� Li batteries at differentcurrent densities; (j) Arrhenius plot of the average charge voltage (relative to values at 303 K) with respect to the battery temperature and (k) illustrationsexplaining the origin of the difference in the performance of different metal–lithium batteries (re-print with permission from [34] Copyright 2023, RoyalSociety of Chemistry).Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 11/16] 1ChemElectroChem 2024, e202300661 (11 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseThe cycling stability of the n-Ag/CNT� Li battery in the 1 MLiPF6 EC/DEC+LiCl electrolyte, however, is poor. This isattributed to the dissolution of AgCl into the electrolyte.Changing the electrolyte can enhance cycle stability, whichsuggests that the interaction between the electrolyte and theelectrode material is critical for these types of batteries. Eventhough the utilization of an expensive noble metal, in this caseAg, is not practical, the study is an interesting demonstration ofa metal conversion reaction for cathode applications and therecould potentially be other similar reactions with more sustain-able materials.3.2. Iron/Stainless SteelFe is an inexpensive material that has a high electrodepotential. Though, metallic Fe tends to be reactive to theenvironment, forming iron oxides spontaneously. To demon-strate the use of Fe as the cathode of a battery, Wang and Yuinstead used stainless-steel powder with a large content of Feas the starting material and developed a stainless-steel-Libattery, as shown in Figure 7e.[33] The solution of 1 M LiPF6+0.05 M LiCl in EC :DEC=1 :1 was used as the electrolyte.Because stainless steel is rather inactive, LiCl is needed tofacilitate the dissolution of Fe from it. During the chargeprocess, the Fe atom in the stainless-steel cathode will lose twoelectrons, become Fe2+, and dissolve into the electrolytesolution. Li+ in the electrolyte solution will transform into Liand deposit on the anode surface. Reverse reactions will occurduring the discharge process with Fe2+ deposited on thecathode surface and Li+ released from the anode, respectively.To suppress the transfer of Fe2+ from the catholyte to theanolyte, the authors utilized a AEM (F310) in the cell, whicheffectively improved the CE to over 80% at a current rate of10 mAg� 1 (Figure 7f). Further work is needed to increase the CEby other strategies mentioned earlier in this review.3.3. TinXue et al. recently reported Sn foil as the cathode of a Sn� Libattery in a carbonate–ether based electrolyte with PC/DME(Figure 7g).[34] Using the PIL/PP AEM as the separator, theresulting Sn� Li battery gives a discharge voltage of about 2.8 Vand can be cycled for more than 1500 times with an average CEof about 99.5% at a current density of 0.2 mAcm� 2 with anareal capacity limitation of 0.1 mAhcm� 2, as shown in Figure 7h.The Sn� Li battery shows fast kinetics, with only a plateauvoltage drop of 0.05 V even when the current rate is increasedfrom 0.2 mAcm� 2 to 1 mAcm� 2 (corresponding to a current of10 C).Interestingly, the electrochemical performances of metal-Libatteries depend on the type of metals. Xue et al. comparedthe voltage polarization, activation energy, and CE of Sn, Cu,and Ni cathodes coupled with the Li anode under the same cellconfiguration, separator, and electrolyte and found that theredox reaction of Sn and Cu has lower activation energy(Figure 7i) and the voltage polarization is smaller than that of Ni((Figure 7j). On the other hand, average CE of Sn and Ni ishigher than that of Cu, suggesting that it is less likely for Sn2+and Ni2+ to move through the PIL/PP membrane than Cu+(Figure 7k). Overall, Sn performs better than Cu and Ni as thecathode material using the PIL/PP membrane and the DME/PC-based electrolyte. Even though Sn has lower voltage than Cuand Ni, it could be a good cathode candidate consideringkinetics and stability.4. Liquid Metal batteries with Group IV, V, andVI MetalsLiquid metal battery (LMB) is another type of novel batterysystem using metals or alloys as the cathode, but requires ahigh working temperature (larger than 200 °C) to keep thecathode, anode and electrolyte in a liquid state.[35] As shown inFigure 8a, the liquid cathode, liquid anode, and liquid electro-lyte are immiscible with each other, forming three distinctivelayers due to the difference in densities. Lithium with lowerdensity normally serves as the anode (top layer), while for thecathode side, metals from group IV, V, and VI with higherdensity (bottom layer) are typically adopted to form Li� Te,Li� Bi, Li� Sb, Li� Sn, and Li� Pb liquid metal batteries. Theelectrolyte is commonly a mixture of multiple molten halidesalts such as LiCl� KCl, LiI� KI, LiCl� KCl� CsCl, etc. with a lowermelting point than the single constitutes, which can act as boththe electrolyte and separator. During the discharge process, theanode metal (Li) will be oxidized and release Li+ into themolten salt electrolyte, which will then alloy with the cathodemetal. A reversed process will take place during the chargeprocess with the de-alloying of Li from the cathode, where theLi+ will be transported through the electrolyte and reduced atthe anode. Though, because the electrode potential for cathodemetal-Li alloying is low, the nominal voltage of the battery istypically less than 1 V, as shown in Figure 8b.[36] Currently, thebattery still experiences large voltage polarization with increas-ing current rates. In addition, the high-temperature workingenvironment leads to higher energy consumption to maintainthe metal electrodes and molten salt electrolytes in a liquidstate and also limits the type of applications of such batterysystem.5. SummaryThe detailed information of all reviewed metal-cathode bat-teries are summarized in Table 2. In general, the charge transferoccurring in the batteries is originated from the redox reactionsbetween metal and metal cations. Though, its performances arehighly dependent on the type of the electrolyte, which willrequire more systematic investigations in the future. Cationcrossover from the cathode side to the anode side isdetrimental to the reversibility and long-term cycling stability ofmetal cathode batteries. Up to now, four strategies have beenWiley VCH Freitag, 21.06.20242499 / 337050 [S. 12/16] 1ChemElectroChem 2024, e202300661 (12 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseproposed to solve the problem. The cycling stability of thebattery is also affected by other factors such as the stripping/plating of the metal cathode.The advantages of metal cathode batteries (Figure 8c)include (1) the low-cost and abundant raw materials; (2) theeasy processibility of metal into foils which allows high massloading and reduces production cost; (3) high specific capacitiesof the active materials as many metal cathodes can undergo amulti-electron transfer reactions and (4) customizable operatingvoltages by choosing metal cathodes and anodes with desirableelectrode potentials. The disadvantages are (1) the dissolvedcations are at a higher electrode potential than the anode, thusthe cross-over of cations to the anode need to be prevented toreduce the self-discharge and electrode degradation, and(2) the large dependency of cell performance on electrolyte. Inparticular, a larger amount of electrolyte is needed to accom-modate the cathode metal cations, which can limit the energydensity of the battery. Regarding the disadvantages, futureadvances in the membrane or solid-state separator and thedevelopments of the electrolyte with higher solubility arepossible to overcome them.6. Perspectives on Future Developments ofMetal CathodesThere are several crucial topics that need to be investigated topush the novel battery into practicability.(a) Although the utilization of conductive metal foils ascathode increases the mass loading of active cathode materialsand does away with the inactive current collector, right now,the areal capacity of the metal cathodes is still limited, typicallylower than 0.1 mAhcm� 2, which means that the utilization rateof the metal cathode is rather low. There are currently someissues with the larger areal capacity. First, increasing arealcapacity will typically lead to quicker failure of the battery dueto the degradation/disintegration of the metal electrode. Morestudies to clarify the dissolution and deposition processes andalso investigations on electrolyte additives and solvationstructure of the electrolytes are needed to enable uniformstripping and plating of the metal to prolong cycle life. Second,the metal cations that are stripped from the cathode need tobe stored in the catholyte, so larger areal capacity means largeramount of electrolyte is needed. In particular, organic electro-lytes tend to have lower metal ions solubility than aqueouselectrolytes, so the higher cell voltage when coupled with ametal anode with low electrode potential such as Li is negatedFigure 8. (a) Schematic diagram of the discharge and charge processes of a liquid metal battery (re-print with permission from [35] Copyright 2016, Wiley-VCH); (b) discharge curves of Li j jLiCl� LiF j jBi liquid metal battery at T=550 °C with different current densities(re-print with permission from [36] Copyright2015, Elsevier); (c) advantages and disadvantages of the metal-cathode battery system.Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 13/16] 1ChemElectroChem 2024, e202300661 (13 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseby the lower solubility of the electrolyte. Thus, the practicalenergy density of the battery with metal cathode is expected tobe lower than that of LIBs, but can be comparable or evenexceed that of redox flow batteries with further optimizations.Further work on exploring electrolytes with higher solvatingability towards metal cations are essential for the futuredevelopment for metal cathode battery systems.(b) Further works are needed to improve the CE of thebatteries with metal cathode to stop the cross-over of metalcations from the catholyte to the anolyte. Currently, solidelectrolyte separators tend to have poor kinetics, while polymermembranes cannot achieve CE of 100%. More effectiveseparators and electrolytes are desired to reduce the self-discharge of the batteries.(c) Aside from problems related to the metal cathodes, thereare also significant issues such as dendrite formation and gasgeneration if it is coupled with another metal anode such as Lior Zn. At the end, strategies that can be compatible with boththe metal cathode and anode are required for full celloperations in the future.AcknowledgementsThis study was supported by a Research Matching GrantScheme (PJ9229008) by the government of Hong Kong SpecialAdministrative Region.Conflict of InterestsThe authors declare no conflict of interest.Table 2. A summary of different metal cathode batteries.Battery Cathode Anode Separator Electrolyte Capacity Voltage(V)Current(mA)CoulombicefficiencyRef.Cu� Li Cu powder Li disc DES gel/LATP/DES-FECgelDES gel/LATP/DES-FECgel500 mAhg� 1 2.75and 4200 mAg� 1 99.6% [15]Cu� Li Au� Cu@CNFs Li foil C-coatedseparator1.0 M LiClO4 in EC/PC 755 mAhg� 1 3.5 200 mAg� 1 99% [16]Cu� Al Cu foil Al foil AEM F310 6 M LiTFSI DMC 0.225 mAhcm� 2 3 0.0225 mAcm� 2 88% [17]Cu� Li Cu foil Li foil PAA(BTO)/PP3 M LiTFSI DMC 0.1 mAhcm� 2 3.3 0.01 mAcm� 2 86% [19]Cu� Al Cu foil Al foil PP 3 M LiTFSI FEC 0.14 mAhcm� 2 3 0.014 mAcm� 2 97% [20]Cu� Zn Cu plate ZnplateLATSPfilm2 M LiNO3 water and1 M Zn(NO3)2 water1.5 mAhcm� 2 0.8 0.25 mAcm� 2 98% [21]Cu� Al Cu/PAN Al foil PP 3 M LiTFSI FEC 0.14vmAhcm� 2 3 0.014 mAcm� 2 98% [22]Cu� Li Cu foil Li foil PIL/PP 3M LiTFSI DMC 0.1vmAhcm� 2 3.3 0.2 mAcm� 2 99% [24]Cu� Li Cu foil Li foil LISICONfilm1 M LiClO4 EC/DMCand 2 M LiNO3 water16 mAhcm� 2 3.1 1 mAcm� 2 99% [25]Cu� Li Cu foil Li foil PIL/PP 3 M LiTFSI DME/PC 0.1 mAhcm� 2 3.3 2 mAcm� 2 99% [27]Cu� Zn Cu plate ZnplatePVDF/PMMA-Li-ClO4/PVDF0.1 M CuSO4/1 M Li2SO4water and 0.1 MZnSO4/1 M Li2SO4water330 mAhg� 1 0.96 1 mAcm� 2 100% [28]Cu� Zn Cu plate ZnplateCIMS 0.1 M CuSO4/1 MNa2SO4 water and 1 MZnSO4 water763 mAhg� 1 0.8 0.5 mAcm� 2 100% [29]Cu� Zn Cu foil Zn foil AEM FAA-3- PK-1301.0 M CuSO4/H2SO4water and 2 M ZnSO4water10 mAhcm� 2 0.86 10 mAcm� 2 100% [30]Cu� Zn Cu clusters Zn foil NKK 1 M KOH water 718 mAhg� 1 0.76 0.1 Ag� 1 100% [31]Ag� Li nano-Ag/CNT Li foil glass fiber 1 M LiPF6/0.05 M LiClEC/DEC180 mAhg� 1 2.8 10 mAg� 1 95% [32]Fe� Li Stainless-steel powderLi foil AEM F310 1 M LiPF6/0.05 M LiClEC/DEC100 mAhg� 1 2.5 10 mAg� 1 90% [33]Sn� Li Sn foil Li foil PIL/PP 3 M LiTFSI DME/PC 0.1 mAhcm� 2 2.8 0.2 mAcm� 2 99.5% [34]Li� Bi Liquid Bi LiquidLiLiCl-LiFmoltedsaltLiCl-LiF molted salt 3.33 mAhcm� 2 0.68 200 mAcm� 2 100% [36]Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 14/16] 1ChemElectroChem 2024, e202300661 (14 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseData Availability StatementThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.Keywords: Cation crossover · dissolution/deposition ·electrolyte · membrane · metal cathode[1] Y. Liang, C.-Z. Zhao, H. Yuan, Y. Chen, W. Zhang, J.-Q. Huang, D. Yu, Y.Liu, M.-M. Titirici, Y.-L. Chueh, H. Yu, Q. Zhang, InfoMat. 2019, 1, 6–32.[2] B. Dunn, H. Kamath, J.-M. Tarascon, Science 2011, 334, 928–935.[3] J. Deng, C. Bae, A. Denlinger, T. Miller, Joule 2020, 4, 511–515.[4] M. Li, J. Lu, Z. Chen, K. 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PowerSources 2015, 275, 370–376.Manuscript received: November 11, 2023Revised manuscript received: December 27, 2023Version of record online: ■■■, ■■■■Wiley VCH Freitag, 21.06.20242499 / 337050 [S. 15/16] 1ChemElectroChem 2024, e202300661 (15 of 15) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemReviewdoi.org/10.1002/celc.202300661 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://doi.org/10.1002/inf2.12000https://doi.org/10.1126/science.1212741https://doi.org/10.1016/j.joule.2020.01.013https://doi.org/10.1002/ese3.47https://doi.org/10.1002/ese3.47https://doi.org/10.1021/acs.chemrev.9b00609https://doi.org/10.1038/s41560-022-01001-0https://doi.org/10.1002/anie.201905875https://doi.org/10.1039/D0SC00022Ahttps://doi.org/10.1016/j.nanoen.2018.09.064https://doi.org/10.1021/acsaem.9b00627https://doi.org/10.1021/acsami.1c11181https://doi.org/10.1021/acsami.1c11181https://doi.org/10.1016/j.electacta.2021.138595https://doi.org/10.1002/polb.23395https://doi.org/10.1002/polb.23395https://doi.org/10.1016/j.elecom.2009.07.031https://doi.org/10.1002/cssc.201000123https://doi.org/10.1016/j.jechem.2023.07.024https://doi.org/10.1039/C5CC00575Bhttps://doi.org/10.1039/C5CC00575Bhttps://doi.org/10.1016/j.jpowsour.2020.227873https://doi.org/10.1039/D3EE02786Dhttps://doi.org/10.1002/celc.201900969https://doi.org/10.1016/j.electacta.2018.12.064https://doi.org/10.1039/D2TA05305Ehttps://doi.org/10.1039/D2TA05305Ehttps://doi.org/10.1016/j.jpowsour.2014.10.173https://doi.org/10.1016/j.jpowsour.2014.10.173REVIEWMetal-cathode battery is a novelbattery system where low-cost,abundant metals with high electrodepotential can be used as the positiveelectrode material. Recent progresseswith emphases on the cathode,anode, electrolyte, and separator ofthe batteries are summarized andfuture research directions areproposed in this review paper.Dr. K. Xue, Dr. H. Wang, Prof. D. Y. W.Yu*1 – 16Emerging Battery Systems withMetal as Active Cathode MaterialWiley VCH Freitag, 21.06.20242499 / 337050 [S. 16/16] 1 21960216, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300661 by Cochrane Japan, Wiley Online Library on [22/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Emerging Battery Systems with Metal as Active Cathode Material 1. Introduction 2. Copper Cathode 2.1. Cu-BasedBattery Coupled with low Electrode Potential Anodes such as Li and Al−Li Slloy 2.1.1. Charge-Discharge mechanism 2.1.2. Suppression of the Cross-Over of Cu Cations 2.1.2.1. Interaction Between Electrolyte and Separator 2.1.2.2. Trapping of Cu 2.1.2.3. Use of Anion Exchange Membrane (AEM) 2.1.2.4. Use of a Ceramic Separator 2.1.3. Other Factors Affecting Cycle Stability 2.2. Cu-Based Battery Coupled with High Electrode Potential Anode such as Zn 3. Other Metal Cathodes 3.1. Silver 3.2. Iron/Stainless Steel 3.3. Tin 4. Liquid Metal batteries with Group IV, V, and VI Metals 5. Summary 6. Perspectives on Future Developments of Metal Cathodes Acknowledgements Conflict of Interests Data Availability Statement