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[Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404)

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[New Insights into Fundamental Processes and Physical Degradation Mechanisms in Rechargeable Lithium‐Oxygen Batteries Providing Suitably High Energy Densities](https://mdr.nims.go.jp/datasets/8c6e3301-ff05-470d-bd19-219f2f702175)

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New Insights into Fundamental Processes and Physical Degradation Mechanisms in Rechargeable Lithium‐Oxygen Batteries Providing Suitably High Energy DensitiesNew Insights into Fundamental Processes and PhysicalDegradation Mechanisms in Rechargeable Lithium-OxygenBatteries Providing Suitably High Energy DensitiesShoichi Matsuda*[a]Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 126/133] 1ChemElectroChem 2024, 11, e202300605 (1 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemwww.chemelectrochem.orgConceptdoi.org/10.1002/celc.202300605http://orcid.org/0000-0002-0640-3404http://crossmark.crossref.org/dialog/?doi=10.1002%2Fcelc.202300605&domain=pdf&date_stamp=2024-01-22Lithium-oxygen batteries (LOBs) can exhibit high energydensities and so have attracted much attention as next-generation energy storage devices. Although 500 Wh/kg LOBshave recently been demonstrated, the performance of thesebatteries could still be greatly improved. This Concept articlehighlights the importance of exploring new technologies, suchas gas-diffusion layers and lightweight protective membranesfor lithium metal electrodes, as an approach to realizing LOBshaving energy densities sufficient for practical applications. Inaddition, the physical degradation phenomenon specific toLOBs designed with high energy density is also introduced,such as the volume change of the positive electrode and themovement of the electrolyte associated with the progress ofrepeated discharge/charge cycles. We believe that the topicspresented in this article will promote new research directions inthe future development of high energy density and long cyclelife LOB.1. IntroductionLithium-oxygen rechargeable batteries (LOBs) are promisingcandidates for next-generation energy storage devices due totheir potential to provide superior energy density values.[1,2] Infact, recent studies have demonstrated the stable operation of500 Wh/kg class LOBs that could exceed the energy densities ofconventional lithium-ion batteries (LiBs).[3] Even so, varioustechnical issues still limit the practical commercialization of thistechnology, such as poor cycle lifespans (<50 cycles), lowpower densities (<50 Wh/kg) and slow charging rates (<0.5 C).In addition, much higher energy densities above 700 Wh/kg atthe cell level are highly demanded for various societalapplications. To realize the improvement of these batteryperformances, the deep understanding of the reaction mecha-nism and the development of novel electrode/electrolytematerials are crucial. Recent advances in the main componentsof LOBs, such as electrolyte, carbon electrode and lithium metalelectrode, are well summarized in recent reviews.[4,5] At thesame time, it is also necessary to develop the materials otherthan the above main materials, including gas diffusion layer(GDL), current collector and separator. Although these sidecomponents also have a large impact on the performance ofLOBs, especially in practical high energy density conditions, thelimited studies have been done for the development of thesematerials. In addition, with such material development, thedeep understanding of the complicated reaction in LOBs is alsocrucial. Although previous studies mainly focused on thechemical/electrochemical reaction in oxygen positive electrodeand lithium negative electrode, not only such chemical reactionbut also physical degradation occurs in LOBs, in which thereaction proceeds by formation and decomposition of solid-state products (Li2O2 and Li metal). Thus, there is concern aboutthe large volume change of the electrode during repeateddischarge/charge cycles. However, due to the difficulty ofanalyzing such a macroscopically complicated reaction in LOBs,the details remain unclear. Based on these considerations, inthe present concept article, the present article summarizes therecent research progress on the unexplored fundamentaltechnologies and physical degradation mechanism in LOBs withpractically high energy density.2. Design strategy for fabrication of LOBspossessing high cell-level energy density2.1. Energy Density SimulationsTypical LOBs contain metallic lithium, which has a low redoxpotential of � 3.04 V (vs. standard hydrogen electrode (SHE))and a high theoretical capacity of 3860 mAh/g, and atmospher-ic O2 as active materials. For example, assuming an operatingvoltage of 2.7 V, an energy density of 10422 Wh/kg can beobtained based on the mass of lithium metal (3860 mAh/g).This value corresponds to the energy density based on theactive material (excluding the mass of oxygen in the atmos-phere) before the discharge state. However, in practicalconditions, the cell components such as electrolyte in theseparator and porous carbon electrode are also taken intoaccount to estimate the energy density based on the mass ofthe whole LOB cell.To quantitatively understand the importance of the sidecomponent on the cell-level energy density of the LOBs, weestimate the energy density of the LOBs. The parameters usedfor this simulation are summarized in Table 1. The eachcomponents in LOBs were schematically illustrated in Figure 2a.For this energy density calculation, the capacity and massloading of the carbon electrode are set to be 3000 mAh/g and2 mg/cm2, which are typical value as carbon electrode for highenergy density LOBs.[3] As a result, the areal capacity of theoxygen positive electrode is 6 mAh/cm2. Polyolefin membranewith 20 μm thickness and lithium metal foil with 10 μm thick-ness were used as separator and negative electrode. Theamount of electrolyte was estimated to completely fill the porespace in the carbon electrode and separator. In the following,we calculate the cell-level energy density of LOB in the fully[a] S. MatsudaCenter for Green Research on Energy and Environmental Materials, NationalInstitute for Material Science, 1-1 Namiki, 305-0044 Tsukuba, Ibaraki, JapanandCenter for Advanced Battery Collaboration, Center for Green Research onEnergy and Environmental Materials, National Institute for MaterialsScience, 1-1 Namiki, 305-0044 Tsukuba, Ibaraki, JapanandNIMS-SoftBank Advanced Technologies Development Center, NationalInstitute for Materials Science, 1-1 Namiki, 305-0044 Tsukuba, Ibaraki,JapanE-mail: matsuda.shoichi@nims.go.jp© 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.Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 127/133] 1ChemElectroChem 2024, 11, e202300605 (2 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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 Licensedischarged state (excluding the mass of oxygen) and averagevoltage of 2.7 V. Figure 1a exhibited the mass-based proportionof each component in standard LOB setup (Cell A). There can beseen that 10-μm-thick Cu foil with density of 8.8 mg/cm2 and200 μm-thick carbon fiber based GDL with density of 8.4 mg/cm2 accounted for the greatest relative mass contribution. Here,the areal electric energy (Wh/cm2) can be obtained by multi-plying the average discharge voltage (V) and the areal capacity(mAh/cm2). By dividing the areal electric energy by the totalweight of the cell components, the cell level energy density ofLOB can be obtained. In this energy density calculation, theweight of the sealing film, tab and lead are not contained. Incase these weights are also included, the estimated energydensity becomes approximately 10% lower value.[6]Next, let us consider the situation, in which the two heaviestcomponents are replaced to lightweight materials (Cell B). Thisheavy current collector can be replaced with a Cu foil with athickness of 2 μm. In addition, the GDL and current collectorcan be replaced with an integrated gas-diffusible currentcollector with mass loading of 1.3 mg/cm2. As results, theenergy density of cell B reached to 563 Wh/kg (Figure 1b).Furthermore, replacing the lithium metal protective membraneof 10 mg/cm2 with a membrane of 2 mg/cm2 provided a valueof 671 Wh/kg (Cell C). These results clearly demonstrate thatdecreasing the masses of various cell components, other thanthe carbon electrode and electrolyte, can greatly affect the cell-level energy density of an LOB. The following section providesan overview of recent research concerning the development oflightweight GDLs and protective membranes for lithium metalelectrodes. Current collectors for negative electrodes havingreduced masses have already been widely investigated basedon research regarding LiBs, and the details of such work can befound in recent publications. Thus, a discussion of technologicalissues related to current collectors is outside the scope of thisarticle.2.2. Gas Diffusion Layer and Current CollectorThe GDL is a specific material for LOBs, which function tofacilitate efficient oxygen transport from outside the cell intothe porous carbon electrode. Despite the importance ofdeveloping a lightweight and highly oxygen diffusible GDLmaterial, the evaluation protocol for the GDL is not wellestablished. From a practical point of view, multiple LOB cellsneed to be densely stacked, similar to conventional LiBs. Insuch stacked situations, proper cell configuration is required toensure that oxygen can effectively pass through the entirepositive electrode via the GDL.[2] In particular, oxygen must betransported horizontally in the GDL. Then, oxygen must befurther transported in the vertical direction to pass through thewhole part of the positive electrode (Figure 2a). Therefore, theperformance of the GDL should be evaluated in a suitable cellconfiguration in which the oxygen supply is allowed onlythrough the horizontal direction of the GDL. However, in a coin-type cell or a swagelok-type cell, which are widely used for LOBstudy in the laboratory, the oxygen is supplied from the upperpart of the positive electrode (Figure 2b). Therefore, theperformance of the GDL cannot be properly evaluated in such acell configuration.We have recently proposed a single-layered cell with a2 cm×2 cm sized electrode as a possible candidate for standardLOB cells, in which oxygen gas first needs to transport throughthe GDL in the vertical direction (Figure 1c). And then, oxygengas further transports to inside of cell in the horizontaldirection. Using such a setup, the concept of a gas-diffusiblecurrent collector was recently demonstrated, which combinesthe functions of oxygen mass transport and electron transferwith minimal mass loading.[7] For the gas-diffusible currentcollector for high energy density LOB, the following threefactors are crucial: (i) light weight, (ii) high electrical conductiv-ity to function as an electron transport channel, and (iii) suitablestructure for an oxygen transport channel. To verify thisconcept, nickel-coated polymer fiber meshes were fabricated, inwhich the PET fiber with diameter of 27 μm is fully coveredwith nickel with 0.5 μm thickness (Figure 2d–2g). The batteryperformance evaluation test revealed that LOB cells equippedwith an ultralight gas-diffusible current collector exhibit equiv-alent discharge capacity with that of cells equipped withconventional heavy components at 0.4 mA/cm2,although, itsweight is much smaller than that of conventional one (Fig-ure 2h). These results suggesting that the use of nickel-coatedpolymer fiber meshes improve the energy density of LOBwithout diminishing their battery performance. As described inSection 2.1, the development of a lightweight GDL is critical tothe realization of high energy density LOBs at the cell level.However, at present, the research attention for the develop-ment of GDL is quite limited. Intensive research attention isrequired for the development of advanced GDL materials toimprove the performance of LOBs.2.3. Separators Supressing Chemical Cross-OverFor realizing long cycle life LOB, the protection of lithium metalelectrode against atmospheric components and chemical crossovered compounds from positive electrode is crucial. Inaddition, many studies to date have utilized redox mediators todecrease the over-potential associated with charging. In suchcases, it is helpful to apply a protective membrane to thelithium metal electrode to limit self-discharge due to the cross-Shoichi Matsuda is a Team Leader at theNational Institute for Materials Science (NIMS),Japan. He studied chemistry in The Universityof Tokyo and received his PhD in 2015. Afterthat, he joined NIMS as an ICYS (InternationalCenter for Young Scientists) researcher andthen got a tenured position in 2017. He wasawarded the Young Researcher Award of TheElectrochemical Society of Japan (2023). Hiscurrent research interests are lithium-oxygenrechargeable batteries and data-driven auto-mated experiments.Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 128/133] 1ChemElectroChem 2024, 11, e202300605 (3 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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 Licenseover of redox mediators. The impact of chemical cross-overbecomes much more prominent in the case of an LOB celldesigned to provide a high energy density, in which thedistance between electrodes is reduced and the areal capacityis high. These features lead to the formation of greater amountsof side products on the oxygen electrode side that subse-quently pass through the negative electrode. In the majority ofprior LOB studies, the distance between the negative andpositive electrodes exceeded 100 μm because of the use ofseparators comprising thick glass fibers. In contrast, an LOBTable 1. Parameters used for energy density simulation of LOB.Cell A B CCarbon membrane Weight 2 2 2 mg/cm2Thickness 100 100 100 μmPorosity 95 95 95 %Separator Weight 1.1 1.1 1.1 mg/cm2Thickness 20 20 20 μmPorosity 45 45 45 %Electrolyte Weight 14 14 14 mg/cm2Gas diffusion layer Weight 8.4 8.4 8.4 mg/cm2Thickness 220 220 220 μmPorosity 89 89 89 %Current collector Weight 3.5 3.5 3.5 mg/cm2Gas diffusible current collector Weight 1.3 1.3 mg/cm2Cu foil (10 μm) Weight 8.8 mg/cm2Thickness 10 μmCu foil (2 μm) Weight 1.8 1.8 mg/cm2Thickness 2 2 μmStandard protective membrane Weight 6.5 6.5 mg/cm2Thickness 20 20 μmLightweight protective membrane Weight 1.9 mg/cm2Thickness 6 μmLi foil (50 μm) Weight 2.67 2.67 2.67 mg/cm2Thickness 50 50 50 μmTotal weight 46.3 28.7 24.1 mg/cm2Average voltage 2.7 2.7 2.7 VAreal capacity 6 6 6 mAh/cm2Areal electric energy Wh/cm2Energy density 349 563 671 Wh/kgFigure 1. Weight fractions of LOB components calculated using the parameters listed in Table 1.Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 129/133] 1ChemElectroChem 2024, 11, e202300605 (4 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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 Licensedesigned to produce a high energy density may include a thinseparator and have an electrode distance of less than 20 μm.Recent work using a three electrode setup was able todeconvolute the over-potentials associated with the positiveoxygen electrode and negative lithium metal electrode anddemonstrated that deterioration of the latter was the mainreason for the limited cycle lifespan of an LOB having a highenergy density.[7] In addition, assessments using in situ massspectrometry have established that the reaction efficiency ofthe lithium electrode decreases over time primarily because ofthe cross-over of compounds such as H2O and CO2 from thepositive oxygen electrode side (Figure 3a). These compoundsare considered to be generated as side products of solventdecomposition during positive electrode reaction.[8–10]To protect the lithium metal electrode from such undesir-able products from the positive electrode side, the use oflithium-ion selective membrane as a separator is an effectiveapproach. As an example, the use of a solid-state electrolytebased on LATP can provide superior Li ion conductivity (>10� 4 mS/cm2) and resistance to attack by chemical cross-overcompounds. The concept of protecting a lithium metalelectrode using an LATP pellet was demonstrated by Viscoet al.[12] As described in Section 2.1, reducing the mass of theprotective membrane applied to the lithium metal electrodewithout diminishing its protective ability is an essential aspectof producing LOBs with high cell-level energy densities.Although a 20-μm-thick LATP pellet with a mass loading of6.5 mg/cm2 is commercially available from the Ohara Company,further reduction of weight and thickness of LATP pellet isrequired. Recent studies have demonstrated the application ofa 6-μm-thick LATP pellet for LOBs (Figure 3b). Notably, Li ionconductivity of this material is equivalent to that of standardthick LATP pellet. The use of this 6-μm-thick LATP pellet asseparator in 400 Wh/kg class LOB, the cycle life of LOB is largelyimproved over 20 cycle (Figure 3c), which is the best perform-ances among the LOB reported so far, especially in the regionof energy density higher than 200 Wh/kg.[11]As the lightweight flexible protective separator, the hybridmembrane made of micron-sized LAPT particles embedded in apolymeric matrix is also alternative candidate.[13] The uniquestructure of the membrane enables both high Li-ion conductionand gas-impermeability (Figure 3d and 3e). Notably, by use ofthis membrane as separator, the operation of 500 Wh/kg classLOB was demonstrated, although its cycle life is not reported.The development of lightweight, flexible and scalable protec-tive membranes for lithium metal electrodes is important to thefuture development of high-performance LOBs and furtherintensive studies are highly demanded.Figure 2. (a) Schematic illustration of LOB wtih stacked cell configuration. (b) Schematic illustration of LOB wtih coin-type cell configuration. (c) Photographicimage of a single-layered LOB cell with a 2 cm×2 cm sized electrode. (d) Illustration of a metal-coated polymer fiber. (e) Photographic and (f, g) SEM images ofNi-coated PET fiber mesh. Scale bars are (c) 1 cm, (e) 2 cm (f) 100 μm and (g) 0.5 μm.Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 130/133] 1ChemElectroChem 2024, 11, e202300605 (5 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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 License3. Physical degradation mechanisms3.1. Electrode Volume ChangesIn LOBs, both chemical and physical degradation processes canoccur. Importantly, the reactions in LOBs proceed on the basisof the formation or decomposition of active solid-statecompounds (specifically, Li2O2 in the positive electrode and Limetal in the negative electrode). As such, the electrode volumechange in an LOB will be much larger than that in a standardLiB, which operates via an intercalation mechanism. Consideringthe fact that the volume change of the active material in LiBhas a large impact on the cycle life, the correct understandingof how the volume change of the electrode occurs in LOB iscrucial for the design of materials that realize the LOB with longcycle life. Typically, the performance of LOBs is evaluated underlow areal capacity conditions (<1 mAh/cm2). In such cases, theamount of Li2O2 that is formed and decomposed duringdischarging and charging, respectively, is relatively small(<0.5 mm3/cm2). Thus, the impact of electrode volume changesis minimal. Conversely, LOBs exhibiting high cell-level energydensities also demonstrate high areal capacity values (>4 mAh/cm2). Consequently, the reversible formation and decomposi-tion of large amounts of Li2O2 (>1 mm3/cm2) can have asignificant effect on the porous carbon electrode. In such high-energy-density LOB cell, the electrode materials (porous carbonpositive electrode and lithium metal negative electrode) afterlong cycle test can be easily damaged during cell disassemblingand sample washing process. Therefore, non-destructive techni-ques play a crucial role in acquiring quantitative informationabout the volume change of electrodes.Recently, X-ray computed tomography (X-ray CT) has beenapplied as a very promising nondestructive observation methodthat can provide three-dimensional imaging of samples byirradiation with an X-ray beam from different directions, andthe use of XCT analysis of the LOBs were reported.[14–16]Figure 4a and 4b shows the cross-sectional XCT images of theLOB cell before and after 10th cycles. Associated with theprogress of 10th cycles, there can be seen the largely volumechange of electrodes. For negative electrode, thickness in-creased from 50 to 180 μm, a greater than 3-fold difference. Incontrast, the positive electrode thickness decreases from 160 to75 μm. As a constant pressure was maintained in the cell, thisdecrease can be attributed to the compression of the porouscarbon membrane owing to the corresponding increase in thenegative electrode thickness. Overall, the XCT analysis imagesprovide crucial information on the evolution of depositformation and the corresponding electrode volume changesover multiple discharge/charge cycles, thereby demonstratingFigure 3. (a) schematic illustiration of chemical cross-over phenoemenon in LOBs. (b) Photographic image of 6-μm-thick Li conductive solid-state ceramic-based separator (2 cm in square). (c) Discharge/charge profiles of the LOB with (blue) and without (back) the lightweight solid-state ceramic-based separator.(d) Schematic illustration of the membrane with aligned lithium-ion conducting channel. (e) Photographic image of ion-channel aligned membrane (8 cm indiameter).Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 131/133] 1ChemElectroChem 2024, 11, e202300605 (6 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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 utility of this technique for studying physical changes in theLOB components.Confocal microscopy is also effective technique for analyz-ing the volume change of electrodes in LOBs in non-destructivemanner. A prior study by the present authors using operandoconfocal microscopy confirmed a very large change in thevolume of the porous carbon electrode during discharge underhigh current density conditions (>1 mAh/cm2).[14] In theseexperiments, the cell voltage abruptly dropped to 2.4 V andthen gradually increased to 2.55 V (Figure 4c), which is consid-ered to be originated in the increase of effective electro-chemical surface area by formation of Li2O2 on the electrode.Following this, the discharge voltage gradually decreased to theareal capacity limit. Notably, a considerable volume change wasobserved in the positive electrode over the duration of thedischarge, with an increase in thickness from 110 to 250 μm bythe halfway point of the process (Figures 4d and 4e). In thiswork, a self-standing, binder-free, single-walled carbon nano-tube (CNT) membrane was utilized as the carbon electrode.Because this membrane was prepared via a simple vacuumfiltration method, the CNT fibers were not strongly connectedto one another. During discharge under high current densityconditions, the concentrations of oxygen and lithium ionsrapidly decreased as the oxygen reduction reaction proceededat the electrode. In addition, Li2O2 was formed at localized siteson the electrode, inducing macroscopic expansion of thecarbon electrode. This inhomogeneous process could explainthe sudden change in the electrode volume. For this reason,the porous carbon must possess sufficient mechanical proper-ties to resist variations in volume during repeated discharge/charge cycles. The complex reactions occurring in LOBs havenot yet been fully explored, especially in the case of cellsdesigned to provide high energy densities. An improvedunderstanding of the complicated phenomena associated withthese devices is therefore crucial to establishing principles forthe design of electrode materials.3.2. Electrolyte Movement PhenomenonElectrolyte movement phenomenon is another physical degra-dation mechanism unique to LOBs having high cell-level energydensity designs. If the available pore volume in the carbonelectrode is completely filled with electrolyte, a portion of theelectrolyte will be forced out as Li2O2 is formed duringdischarge, assuming that the electrode volume does notchange. This phenomenon has been demonstrated experimen-tally using cross-sectional confocal microscopy.[14] Research hasconfirmed the formation of electrolyte droplets on the carbonelectrode during discharge. Following this, as charging occurs,these droplets are absorbed back into the electrode. In additionto this electrolyte movement in the horizontal direction, asimilar process is believed to occur in the vertical direction. Insuch cases, the electrolyte can migrate to the negative lithiumelectrode and/or GDL. The latter has a large volume that canaccommodate a significant amount of electrolyte, and so someof the electrolyte may not return to the carbon electrode duringcharging. Although study for experimental confirmation ofelectrolyte depletion is limited, several studies have suggestedthat this is possible.[17,18] As an example, a sudden increase involtage during the initial part of the charging process has beenobserved in trials with an LOB incorporating a lean electrolyteand with a high areal capacity.[19,20] Notably, replacing theconventional carbon fiber GDL with a hydrophobicpoly(tetrafluoroethylene) layer suppresses increases in thecharging voltage. These results suggest that interactionsbetween the GDL and the electrolyte can modify the chargingvoltage profile. Ideal GDL materials should therefore be hydro-phobic and lightweight.Figure 4. XCT images of the LOB cell (a) before cycling and (b) after the 10th discharge/charge cycle. (c) Voltage profie of LOB cell for confocal microscopicanalylsis. (d, e) Cross-sectional confocal microscopy images of a fabricated LOB cell taken at various time intervals during the discharge process.Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 132/133] 1ChemElectroChem 2024, 11, e202300605 (7 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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 License4. Summary and OutlookThis article provided an overview of future research trendsrelated to the development of GDLs for efficient oxygentransport and protective membranes for lithium metal electro-des. Although there has been limited research to dateconcerning these materials compared with work related toporous carbon electrodes and electrolytes, the development ofthese components is crucial to the realization of LOBs exhibitingenergy densities suitable for practical applications. Althoughthe details are not mentioned in this article, the developmentof oxygen permeable membrane is also crucial for realizingpractical LOBs.[21] This article also highlighted the importance ofphysical degradation phenomena in LOBs, which should also betaken into account when attempting to prolong the cycle life.As an example, a case study examining volume changes of thepositive electrode and movement of the electrolyte waspresented. Both degradation mechanisms are unique to LOBs.Despite work to date, additional knowledge concerning thecomplicated degradation mechanisms occurring in LOBs isneeded, based on the use of non-destructive analyticaltechniques. Such information is thought to be vital to thedevelopment of LOBs with long cycle life.Conflict of InterestsThe authors declare no conflict of interest.Data Availability StatementData sharing is not applicable to this article as no new datawere created or analyzed in this study.Keywords: Metal-air batteries · Volume change · Energyconversion · Electrochemistry[1] K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1–5.[2] S. Matsuda, M. Ono, S. Yamaguchi, K. Uosaki, Mater. Horiz. 2022, 9, 856.[3] S. Matsuda, E. Yasukawa, T. Kameda, S. Kimura, S. Yamaguchi, Y. Kubo,K. Uosaki, Cell Reports Phys. Sci. 2021, 2, 100506.[4] W.-J. Kwak, Rosy, D. Sharon, C. Xia, H. Kim, L. R. Johnson, P. G. Bruce,L. F. Nazar, Y.-K. Sun, A. A. Frimer, M. Noked, S. A. 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Qiao, S.Sun, Energy Storage Mater. 2023, 58, 94–100.Manuscript received: October 29, 2023Revised manuscript received: December 26, 2023Version of record online: January 22, 2024Wiley VCH Donnerstag, 07.03.20242406 / 335394 [S. 133/133] 1ChemElectroChem 2024, 11, e202300605 (8 of 8) © 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbHChemElectroChemConceptdoi.org/10.1002/celc.202300605 21960216, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202300605 by National Institute For, Wiley Online Library on [18/06/2024]. 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/10.1149/1.1836378https://doi.org/10.1039/D1MH01546Jhttps://doi.org/10.1016/j.xcrp.2021.100506https://doi.org/10.1021/acs.chemrev.9b00609https://doi.org/10.1021/acs.chemrev.9b00545https://doi.org/10.1021/acs.chemrev.9b00545https://doi.org/10.1039/D0MH00067Ahttps://doi.org/10.1021/acsaem.2c03841https://doi.org/10.1021/acsaem.2c03841https://doi.org/10.1021/acs.jpcc.2c07847https://doi.org/10.1021/acs.jpcc.2c07847https://doi.org/10.1021/acsenergylett.3c00132https://doi.org/10.1039/D2RA07670Ehttps://doi.org/10.1021/acs.jpcc.3c01094https://doi.org/10.1021/acs.jpcc.3c01094https://doi.org/10.1021/acsenergylett.8b02242https://doi.org/10.1039/C7SC02519Jhttps://doi.org/10.1039/D3FD00082Fhttps://doi.org/10.1016/j.ensm.2023.03.018 New Insights into Fundamental Processes and Physical Degradation Mechanisms in Rechargeable Lithium-Oxygen Batteries Providing Suitably High Energy Densities 1. Introduction 2. Design strategy for fabrication of LOBs possessing high cell-level energy density 2.1. Energy Density Simulations 2.2. Gas Diffusion Layer and Current Collector 2.3. Separators Supressing Chemical Cross-Over 3. Physical degradation mechanisms 3.1. Electrode Volume Changes 3.2. Electrolyte Movement Phenomenon 4. Summary and Outlook Conflict of Interests Data Availability Statement