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Xiaozhou Huang, Matthew Li, [Yanan Gao](https://orcid.org/0000-0003-0217-6512), Moon Gyu Park, [Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404), [Khalil Amine](https://orcid.org/0000-0001-9206-3719)
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[Discharge Rate‐Driven Li2O2 Growth Exhibits Unconventional Morphology Trends in Solid‐State Li‐O2 Batteries](https://mdr.nims.go.jp/datasets/3133c87d-56e9-488b-bfbf-a47124bab47f)
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Discharge Rate‐Driven Li2O2 Growth Exhibits Unconventional Morphology Trends in Solid‐State Li‐O2 BatteriesResearch ArticleHow to cite: Angew. Chem. Int. Ed. 2025, 64, e202507967doi.org/10.1002/anie.202507967Li-O2 Batteries Hot PaperDischarge Rate-Driven Li2O2 Growth Exhibits UnconventionalMorphology Trends in Solid-State Li-O2 BatteriesXiaozhou Huang, Matthew Li,* Yanan Gao, Moon Gyu Park, Shoichi Matsuda,and Khalil Amine*Abstract: Solid-state lithium oxygen batteries (LOBs) are known for their enhanced safety, higher electrochemical stability,and improved energy density compared to liquid-state LOBs. However, the investigation of solid-state LOBs is limitedwith little understanding of their discharge and charge processes. In this work, a polymer-based solid-state LOB is used toinvestigate the effect of discharge rate on lithium peroxide (Li2O2) formation, the oxygen evolution reaction (OER), andcycle performance. Notably, we observe a counterintuitive trend: Li2O2 particle size increases with increasing dischargecurrent density, in contrast to liquid systems. This behavior arises from inherent space charge layers that restrict Li+transport under high current, and spatially heterogeneous active sites at the solid electrolyte–cathode interface, directlyevidenced by small angle X-ray scattering (SAXS), which govern nucleation accessibility and promote site-selective Li2O2growth. Furthermore, higher current densities improve ORR and OER efficiency but accelerate anode degradation, whilelower currents promote side reactions. These opposing effects result in a trade-off that defines an optimal discharge rate(0.1 mA cm−2) for maximizing cycle life. This study provides a new mechanistic perspective on discharge-driven processesin solid-state LOBs and offers practical guidelines for performance optimization in future high-energy battery systems.IntroductionLithium-oxygen batteries (LOBs) have garnered significantattention as a next-generation energy storage technologydue to their remarkable theoretical energy density(3.5 kWh kg−1), making them critical for high-energyapplications such as aviation, where mass and specificenergy requirements exceed the capabilities of traditionallithium-ion batteries.[1–6] This remarkable energy density[*] X. Huang, M. Li, K. AmineChemical Sciences and Engineering Division, Argonne NationalLaboratory, 9700 S Cass Ave, Lemont, IL 60439, USAE-mail: matthew.li@anl.govamine@anl.govY. Gao, S. MatsudaCenter for Green Research on Energy and Environmental Materials,National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki305-0044, JapanM. G. ParkMaterials Science Division, Argonne National Laboratory, 9700 SCass Ave, Lemont, IL 60439, USAS. MatsudaSoftBank-NIMS Advanced Technologies Development Center,National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki305-0044, JapanAdditional supporting information can be found online in theSupporting Information section© 2025 The Author(s). Angewandte Chemie International Editionpublished by Wiley-VCH GmbH. This is an open access article underthe terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in anymedium, provided the original work is properly cited, the use isnon-commercial and no modifications or adaptations are made.arises from the electrochemical reaction between lithiumand oxygen gas, producing lithium peroxide (Li2O2) as themain discharge products.[7] Though promising, the currentLOBs still suffer from multiple challenges that hinder theirpractical application, such as large overpotentials, electrolyteinstability, and poor cyclability.[8–10]One of the critical factors affecting the performanceand reversibility of LOBs is the morphology of Li2O2discharge product. The formation and decomposition ofLi2O2 at the cathode determine the round-trip efficiency, ratecapability, and long-term stability. In liquid-state systems, therelationship between discharge rate and Li2O2 morphologyhas been extensively studied. Griffith et al. demonstratedthat higher discharge current densities lead to smaller Li2O2particles, accompanied by a decrease in capacity.[11] Therelationship between capacity and rate can be explained bythe reduced penetration depth of O2 into the cathode atelevated discharge rates, which can be associated with thesmaller particles blocking O2 transport pathways, limitingLi2O2 growth. Horstmann et al. conducted nanoscale con-tinuum modelling of the discharge process in LOB and alsofound that an increase in discharge rate tended to increasecoverage of Li2O2 on cathode surface and with smallerparticles.[12] The morphologies of Li2O2 is strongly affectedby discharge rates. In some cases, at low discharge rates,Li2O2 is predominantly found with a toroidal morphology,maintaining a relatively constant average radius. At thehigher rates, the Li2O2 particles transition into a needle-like shape.[11] Li et al. observed that as the discharge rateincreases, the Li2O2 morphology transitions from nanorodsto toroidal structures composed of aggregated nanosheets.[13]Xia et al. found that excessive current (−1 mA cm−2) couldsuppress Li2O2 formation entirely due to rapid carbon surfaceAngew. Chem. Int. Ed. 2025, 64, e202507967 (1 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbHhttps://orcid.org/0000-0001-9206-3719mailto:matthew.li@anl.govmailto:amine@anl.govhttp://creativecommons.org/licenses/by-nc-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fanie.202507967&domain=pdf&date_stamp=2025-08-01Research Articlepassivation.[14] Mitchell et al. revealed that low dischargerates lead to Li2O2 particles initially forming as stacked thinplates that evolve into toroids through secondary nucleation,whereas high discharge rates promote dense nucleation ofequiaxed particles, resulting in less-defined disc and toroidstructures.[15] More recent studies often involve complex cath-ode structures or catalytic surfaces, which though improvingperformance, introduce additional variables that can obscurethe intrinsic relationship between discharge rate and Li2O2morphology. A comprehensive review by Liu et al. furthersummarizes how current density and depth of dischargeaffect Li2O2 growth modes, emphasizing that morphologicalcontrol is key to optimizing battery reversibility and energyefficiency.[16] It is important to note that Li2O2 morphologyis also influenced by the electrolyte composition and thecathode’s surface conditions.[17]Although this relationship has been extensively exploredin liquid-state Li-O2 batteries (LOBs), the correlationbetween discharge rate and Li2O2 morphology remainsrather unexplored in solid-state LOB systems.[18,19] It remainsunclear how the varying morphological or dimensional char-acteristics of Li2O2 impact the oxygen evolution reaction(OER) and how they influence the cycling performance of thesolid-state LOB.[20–24] Compared with liquid-state LOB sys-tems, solid-state LOB systems exhibit distinct characteristicsin terms of interfacial impedance, Li+-ion conductivity andLi+-ion diffusion pathway.[25] These differences may result inan unique influence of the discharge rate on the morphologyof Li2O2, discharge-charge profile, and the cycle performanceof LOBs. Understanding the interplay between these metricsis crucial towards optimizing LOB battery cycled in solid-stateelectrolytes.Among the different types of solid-state electrolytes,polymer-based systems have emerged as the dominant choicefor solid-state LOBs in recent years. In our recent comprehen-sive review of the literature from 2009 to 2024,[20] we foundthat polymer electrolytes have been increasingly adoptedsince 2016, and now constitute the majority of reportedsolid-state LOB systems. This growing preference reflectsnot only their favorable electrochemical and mechanicalproperties but also their better compatibility with the uniqueelectrode architecture of LOBs. Specifically, the cathodes inLOBs are typically composed of porous carbon materialsto facilitate oxygen diffusion and Li2O2 deposition. Polymerelectrolytes, due to their soft, conformable nature, can achieveintimate interfacial contact with such porous structures—an essential factor for stable electrochemical performanceand reproducible morphology studies. In contrast, rigidceramic electrolytes often suffer from poor contact withporous electrodes, leading to high interfacial resistance andless controlled nucleation behavior. These considerationsmake polymer-based solid electrolytes a particularly suitableplatform for investigating discharge-rate-dependent processesin solid-state LOBs.To enable mechanistic insights that are broadly applicable,this study employs a composite SSE composed of PVDF-HFP, LiTFSI, and LLZTO. These materials have been widelyadopted and thoroughly studied in the field. Our recent com-prehensive review of literature from 2009 to 2024 confirmsthat these components are among the most frequently usedin solid-state Li-O2 systems.[20] This choice not only ensuresgeneral relevance and reproducibility of the results but alsoavoids complications associated with the instability of lessestablished materials in Li-O2 system. Specifically, PVDF-HFP is an excellent matrix material due to its relatively highionic conductivity at room temperature and wide electro-chemical stability window (exceeding 5 V versus Li/Li+).[26]In contrast, other commonly used polymer, such as PEO, isonly stable below 4.5 V,[27] which may limit its applicabilityin high-voltage systems like LOBs. In addition, PVDF-HFPexhibits sufficient mechanical strength, which is particularlyimportant in solid-state LOBs. During cell assembly, a metalmesh is typically placed on top of the cathode to applylocalized pressure, ensuring that the cathode region servesas the primary active area. Without this configuration, theapplied pressure may distribute unevenly across the cell,reducing performance. The robust mechanical properties ofPVDF-HFP help the electrolyte membrane withstand theapplied pressure without being penetrated or damaged bythe metal mesh, thereby preventing short-circuiting andimproving structural integrity under compression. LiTFSIhas also been widely used in LOBs.[20] While LiClO4 is aviable alternative, its discharge capacity is generally lowerthan that of LiTFSI.[28] LLZTO plays a key role in thecomposite by enhancing the ionic conductivity, mechanicalstability, and processability of the SSE membrane.[29] LLZTOcan act as Li+ carriers and serve as interfacial and internalchannels for Li+ transportation.[30,31] La atoms in LLZTOcan coordinate with N atoms and C═O groups in typicalsolvent molecules such as DMF, where the N atoms possesshigh electron density. Acting as Lewis bases, these speciescan form complexes that promote partial dehydrofluorina-tion within CPEs. This interaction strengthens the bondingbetween the PVDF matrix, lithium salt, and LLZTO particles.The resulting structural modification significantly enhancesthe mechanical strength of the SSE membrane, therebyimproving its processability and reproducibility.In this work, we report a counterintuitive correlationbetween discharge rate and Li2O2 morphology in solid-stateLOBs: the particle size of Li2O2 increases with increasingdischarge current density. This trend stands in sharp contrastto the well-established behavior in liquid electrolyte systemsand indicates that discharge product formation in solid-state LOBs follows fundamentally different transport andnucleation dynamics. We attribute this divergence to theformation of space charge layers (SCLs) at the cathode–electrolyte interface, which restrict Li+ transport at lessaccessible regions under high-current conditions. As a result,the number of electrochemically active nucleation sitesdecreases at high discharge current densities, favoring thegrowth of existing Li2O2 particles over the formation ofnew ones—opposite to the behavior observed at low currentdensities. This structural and morphological heterogeneity issupported by SAXS and SEM analyses, which qualitativelyalign with the proposed mechanism. In addition, we findthat the extent of parasitic reactions also depends on thedischarge rate, likely due to differences in LiO2 intermediatebehavior. Low current densities promote more uniformAngew. Chem. Int. Ed. 2025, 64, e202507967 (2 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch Articlenucleation but lead to increased side-product formation,whereas high-current densities improve OER/ORR efficiencyyet accelerate anode degradation. These competing effectsdefine a unique trade-off in solid-state LOBs, allowing us toidentify an optimal discharge current density (0.1 mA cm−2)that balances capacity, efficiency, and long-term stability.Altogether, this work establishes a new mechanisticframework for understanding current-dependent behavior insolid-state LOBs, and opens up new possibilities for rationalinterface engineering and performance optimization in futuresolid-state energy storage systems.Results and DiscussionImpact of Discharge Rates on the Formation of Li2O2Discharge current densities directly influence the morphol-ogy of Li2O2 formed in LOBs, which in turn determinesthe discharge capacity, reversibility, overpotential, and ratecapability of the cells. The morphology of Li2O2 formed insolid-state LOB was evaluated as a function of differentdischarge current densities. To ensure the conclusions of thisstudy are broadly applicable, a typical composite polymerelectrolyte (CPE) film, composed of PVDF-HFP, LiTFSI,and LLZTO, is chosen for investigation. The front andback optical images of CPE are shown in Figure S1a,b.The SEM images of the front and back sides are shownin Figure S2a. The back side exhibits a smoother surface,making it more suitable for contact with the lithium metalanode to ensure better interfacial contact. In contrast, thefront side is rougher, which is advantageous for interfacingwith the porous carbon surface of the GDL (Gas diffusionlayer), enhancing the connection and minimizing interfacialimpedance. When the membrane orientation is reversed, thecell performance is significantly degraded and becomes highlyunstable, with large fluctuations and poor reproducibility(Figure S2b). For example, DEMS shows that O2 evolutionduring the charge process drops to only −20% of thetheoretical yield, indicating poor reversibility (Figure S2c).These results clearly demonstrate that reversing the mem-brane orientation leads to unreliable performance due toimproper interfacial contact. Figure S1c shows the cross-section of pristine CPE. The structure appears dense anduniform, with a continuous and compact polymer network.The surface exhibits a relatively smooth texture with finegranular features, indicating excellent electrolyte integrity.Figure S1d shows a cycled GDL pressed with CPE. The mor-phology is more heterogeneous, with visible fibrous structuresfrom the GDL interspersed within the CPE. The polymerelectrolyte appears to have infiltrated the GDL, but thereare noticeable voids and rougher textures, suggesting a moreporous and interconnected network compared to the pristineCPE. Atomic force microscopy (AFM) was conducted underthe protection of Ar atmosphere in a glovebox. This wasto prevent water absorption by the LiTFSI in the CPE.AFM images in Figure S3 reveal that both CPE and carbonsurfaces of GDL are rough, which enhances interconnectivityand electrolyte infiltration. The minimal contrast in phase(related to elasticity of the sample) suggests uniform materialdistribution, ensuring stable interfaces. This rough surfacemorphology is beneficial for reducing interfacial impedanceand improving overall cell performance. The basic characteri-zations of SSE are provided and discussed in Figures S4–S7,including structure determination by FTIR, liquid contentanalysis by TGA (Thermal gravimetric analysis), ionic con-ductivity measurement by EIS, and rate capability evaluationthrough a Li | CPE | Li symmetric cell test. The CPE showssufficient thermal stability for room-temperature operation,with negligible weight loss below 100 °C as indicated byTGA. The addition of LiTFSI and LLZTO slightly reducesthe thermal decomposition onset compared to pristine PVDF-HFP, which is consistent with previous literature. However,the overall thermal behavior remains well within the safemargin for Li-O2 battery operation at ambient conditions. Italso exhibits high ionic conductivity (5.34 × 10−4 S cm−1) andexcellent rate capability from 0.05 to 0.4 mA cm−2.The cells were deep discharged at 0.05, 0.1, and0.2 mA cm−2 up to 8.4 mAh cm−2. All three cells showstable discharge plateaus up to 6 mAh cm−2. The dischargepotential decreases with discharge currents, following thevoltage relationship of 0.05 > 0.1 > 0.2 mA cm−2 during theearly stages of discharge (Figure 1a). However, the voltagerelationship shifts to 0.1 > 0.05 > 0.2 mA cm−2 at the endof discharge, suggesting that cells discharged at 0.1 mA cm−2maintain a better balance between Li2O2 formation andelectron transfer, leading to improved discharge performance.After deep discharge, the discharge product was examinedby SEM. As shown in Figure 1c,d, spherical Li2O2 particlescould be clearly observed from all three cells. Similar toprevious observations, the SEM beam was destructive to befor Li2O2 particles and could be degraded by SEM beam,and the degradation of small Li2O2 was quicker.[32] However,unlike the case in liquid-state LOBs, where toroidal Li2O2particles are observed,[17] the Li2O2 particles formed in thesolid-state LOBs are more irregularly shaped. Interestingly,it has been observed that the size of Li2O2 particles increaseswith higher discharge rates. Specifically, the average diametersof Li2O2 particles are measured to be 0.45, 0.74, and 1.2 µm(bimodal distribution centered at around 0.6–0.7 and 1.5 µm)as shown in Figure 1b–d (SEM) and Figure 1e–g (histograms),at discharge rates of 0.05, 0.1, and 0.2 mA cm−2, respectively.This trend is opposite to the widely accepted behaviorin liquid-state cells, where Li2O2 particles size typicallydecreases as the discharge current increases.[11] Griffith et al.indicated that the particle size of Li2O2 decreases at higherdischarge rates due to the rapid supersaturation of dischargeproduct in the electrolyte promoting nucleation rather thangrowth.[11] Johnson et al indicated that a higher donor numberof the solvent increase the Li2O2 particle size due to the higherstability of LiO2 against disproportionation into Li2O2.[33]However, compared to liquid-state LOBs, several fac-tors differ in solid-state LOBs, leading to differing Li2O2nucleation and growth behaviors. In liquid-state LOB, O2and Li+ ions are dissolved in the electrolyte while electronsare transported through the carbon. This configuration onlyrequires a two-phase boundary. In solid-state LOBs, wherefree solvent does not readily exist, O2 cannot diffuse freelyAngew. Chem. Int. Ed. 2025, 64, e202507967 (3 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleFigure 1. Correlation between discharge current densities and morphology of Li2O2. a) discharge profiles at different rates. (b-d) Scanning electronmicroscopy of the morphology of Li2O2 particle formed at 0.05, 0.1, and 0.2 mA cm−2. (e-g) their respective histograms.in the polymer electrolyte and must rely open pore structurefor transport. Therefore, electrochemically active site mustinvolve three phases: O2, Li+ ions and electron transportmedia. Accordingly, the degree of tortuosity in O2, Li+,and electron transport pathways are likely different fromone another suspect that oxygen diffusion pathways mightbe more selective with a large portion being of lowerquality, resulting in a current focusing effect where highlyO2 transporting pathways are drastically favored. This leadsto the formation of distinct active sites at the SSE–cathodeinterface, with varying favorability for Li2O2 nucleation andgrowth. The presence of such heterogeneous sites is supportedby synchrotron small-angle X-ray scattering (SAXS) results,which will be discussed in detail later.While a similar phenomenon can be claimed in a liquidsystem as well, a key distinction in solid-state electrolyteslies in the more severe blockage of O2 transport pathways.This is primarily attributed to the formation of a spacecharge layer (SCL) (Scheme 1), which results from thelimited ability of lithium salts to neutralize local chargeaccumulation, thereby hindering Li-ion transport, especiallyunder high-current densities.[34] At high discharge rates, moreLi+ ions accumulate near the interface, strengthening theSCL.[10] The enhanced SCL increases interfacial impedanceand hinders the further transport of Li+, O2, and electronsto the active sites. As a result, only a few highly active siteswith good transport pathways can sustain growth, leadingto the enlargement of existing Li2O2 particles. Less activeAngew. Chem. Int. Ed. 2025, 64, e202507967 (4 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleScheme 1. Growth mechanism of Li2O2 at different discharge current affected by space-charge layer. a) At low current density the transport of bothLi-ion and O2 can be sustained. b) At higher current densities, the mismatch between the O2 and Li-ion transport likely creates a space chargelayer-like interface that cannot be neutralized by the polymer electrolyte resulting in Li-ion transport pathways becoming focused to regions where O2and Li-ion transport can be sustained.sites suffer from a mismatch of excess Li+ and limited O2,causing passivation and blocking. Therefore, solid-state LOBsunder high current tend to favor the growth of existingLi2O2 rather than the nucleation of new particles. At lowrates, due to the lower current, the Li+ repulsion effectfrom the SCL is weaker, allowing more sites to remainaccessible for nucleation. Thus, Li2O2 formation proceedsthrough nucleation at multiple sites rather than growth ofexisting deposits.This mechanism can be supported by two lines of evidence.First, SEM images reveal that Li2O2 particles formed underhigh-discharge-current densities are fewer in number butsignificantly larger in size, whereas those formed at lowercurrents are more numerous yet smaller. This trend alignswell with the impact of space charge layers (SCLs), whichsuppress nucleation at less active sites and promote growth atlimited favorable regions. Second, synchrotron SAXS results,which provide direct structural evidence for the existenceof distinct favorable sites at the SSE–cathode interface thatfacilitate or hinder the nucleation and growth of Li2O2. SAXStechnique is a powerful technique for probing the extentand manner in which porous structures are filled or alteredby other materials.[35] In our studies, ex situ SAXS data ofpristine cathode, pressed cathode and SSE, and cathode filledwith DMF solvent were shown in Figure S8. The pore volumedistribution as a function of radius is shown in Figure S8b.The scattering vector q is defined as, q = 4λsinθwhere λ is theX-ray wavelength and θ is the scattering angle. The Pore sizeD can be estimated by the equation D = a ∗ 2πq , where a is anempirically determined instrument-specific constant derivedfrom standard calibration samples. Accordingly, smaller qvalues correspond to larger pore features. The low-q region(q < 0.03 Å−1) corresponds to large pores, the mid-q region(0.03 Å−1 < q < 0.07 Å−1) corresponds to medium-sizedpores. high-q region (q > 0.07 Å−1) is attributed to smallpores. In addition, the scattering intensity I is proportionalto electron density contrast �ρ difference between matrixmaterial and any substances present within the pores, I ∝ |ρmatrix−ρfill |. In our case, the matrix material is porous carbon,while the pore-filling materials include gas (when the poresare empty), solid-state electrolyte (SSE), and DMF liquidsolvent.Compared with pristine cathode (black curve, Figure S8c),the SAXS intensities of cathode/SSE sample (red curve) arelower in both low-q and high-q regions, but remain similarin mid-q region. This indicates that large and small poresAngew. Chem. Int. Ed. 2025, 64, e202507967 (5 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch Articleare partially or fully filled by the SSE layer, leading toreduced electron density contrast and hence lower scatteringintensity. In contrast, the mid-sized pores, particularly aroundq = 0.04 Å−1 where the two curves overlap, appear toremain unfilled. The unchanged intensity in this regionsuggests that the pores are still occupied by argon gas,resulting in no significant change in electron density contrast(�ρ) and thus no change in scattering intensity. This resultsuggests that pores of different sizes are filled by the SSEto varying degrees: some pores are fully filled, others arepartially filled, and some remain unfilled (Figure S8d). Thevolume distribution for the cathode/SSE sample supports theproposed mechanism. The observed decrease in both smalland large pore volumes suggests that these pores are occupiedby the SSE membrane (Figure S8b). In contrast, the volumedistribution of mid-sized pores increases, which may indicatethat some large pores are only partially filled by the SSEand now fall within the mid-size range. This partial fillingcould result in a modest increase in the fraction of mid-sizedpores. Additionally, as the volume fractions of small and largepores decrease, the relative contribution of mid-sized poresnaturally becomes more significant.A fundamental design challenge in solid-state LOBs arisesfrom the opposing requirements of achieving intimate SSE–cathode contact and maintaining porous pathways for oxygentransport. On the one hand, high-pressure crimping is oftennecessary to reduce interfacial impedance by ensuring closecontact between the SSE and cathode. On the other hand,excessive pressure can collapse the porous structure requiredfor O2 diffusion. Our SAXS results provide direct structuralevidence for this trade-off: pores within the cathode exhibitheterogeneous filling behavior, with some fully filled by theSSE, others partially filled, and some remaining open. Thisleads to a distribution of interfacial regions with varyingaccessibility to Li+ and O2, corresponding to active and lessactive reaction sites. The presence of such spatial heterogene-ity supports the proposed mechanism in which SCL furtherrestricts transport at less accessible regions, especially underhigh discharge currents. The SAXS data, in conjunction withSEM evidence, confirms that solid-state LOBs batteries oper-ate under a site-selective mechanism where SCL strength,pore accessibility, and transport heterogeneity jointly governthe Li2O2 formation pathway.Moreover, TGA reveals the presence of residual DMFwithin the membrane; however, the amount is minimal anddoes not extend into or fill the porous cathode structure, asconfirmed by SAXS results. As shown in Figure S8e, whenthe porous carbon cathode is fully filled with DMF liquid,the scattering intensities in both the low-q and mid-q regionsdecrease significantly, indicating that large and medium-sizedpores are occupied by the solvent. In contrast, the high-qregion remains unchanged, suggesting that small pores arenot filled—likely due to the surface tension of DMF, whichprevents it from penetrating pores below a certain size. Com-pared to the cathode/SSE sample, the cathode/DMF sampleexhibits much lower scattering intensity, further supportingthat in the actual integrated cathode/SSE structure, DMFdoes not overflow into the cathode to fill all pores. Instead, itremains primarily confined within the membrane. The volumedistribution shows a decrease in mid- and large-sized pores,indicating that these pores are filled by the DMF solvent. Incontrast, pores with radius smaller than 30 Å remain unfilled,likely due to the inability of DMF to penetrate such smallpores as a result of surface tension limitations (Figure S8b).While the structural and interfacial analyses aboveexplain the site-selective behavior of Li2O2 formation atthe mesoscale, further mechanistic insight can be gainedby considering the behavior of LiO2 intermediates at themolecular level. The difference in LiO2 mobility betweenliquid and solid-state systems introduces an additional layerof control over where and how Li2O2 forms and grows.The role of LiO2 intermediates provides key insight intothe contrasting behaviors of liquid and solid-state Li-O2systems. In solid-state systems, LiO2 is essentially insolubleand immobile due to the absence of a liquid phase. Itstays fixed at the site where Li+, O2, and electrons meeta true three-phase boundary. The number and quality ofsuch active sites depend on how well the SSE physicallycontacts the porous cathode, as shown by SAXS. Underlow discharge rates, the space charge layer (SCL) effect isrelatively weak, allowing Li+ to reach even fewer active sites.This leads to dispersed Li2O2 nucleation and the formationof small particles at many locations. At high discharge rates,Li+ and O2 fluxes increase, but the SCL effect becomesstronger, limiting Li+ transport to less accessible regions.Since LiO2 cannot diffuse away, it reacts immediately at thefew remaining active sites. As the total discharge capacityremains fixed, these sites receive more reactants, resulting infaster local growth and the formation of larger Li2O2 particles.This morphology shift arises directly from the low solubilityand immobility of LiO2, which prevents it from redistributingacross the electrode. This rate-dependent Li2O2 morphologyin which low rates yield many small particles while highrates produce fewer but larger ones can be directly confirmedby SEM analysis. In contrast, in liquid-state systems, LiO2is soluble and mobile. At low current densities, it has timeto diffuse before undergoing disproportionation or electrontransfer. During this time, LiO2 may be absorbed by pre-existing Li2O2 deposits, leading to the formation of largerparticles. Therefore, low rates in liquid systems tend to yieldfewer but larger Li2O2 particles. At high rates, however, LiO2has insufficient time to diffuse and instead reacts quickly nearits origin, forming localized Li2O2 deposits that may blockthe site. Simultaneously, rapid supersaturation promoteswidespread nucleation across multiple sites, resulting in a highdensity of smaller particles.Investigation of the Overpotential in Oxygen Evolution ReactionThe OER process of Li2O2 deposited at different dischargerates were investigated, all three cells were then charged atthe same current density of 0.1 mA cm−2. The discharge-charge profile is shown in Figure 2a. All cells exhibit astable plateau during the oxygen reduction reaction (ORR)process. The voltage profiles of all three cells during thecharging process exhibit an initial sloped regime followed bya plateaued regime. Overall, the higher the discharge currentAngew. Chem. Int. Ed. 2025, 64, e202507967 (6 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleFigure 2. The OER overpotential investigation. a) Discharge-charge profile of solid-state LOBs at different discharge rates and same charge rates. b)Bar graph of voltage tuning point of charge step and discharge overpotential of cells operating at different discharge current densities. c) Nyquistplot, and d) Impedance magnitude bode plot of EIS of solid-state LOB cells after discharging at 0.05, 0.1, and 0.2 mA cm−2, respectively, up to acapacity of 8 mAh cm−2.density, the lower the charge potential. The capacity uponwith the turning points between the initial potential slopedregime and the potential plateau regime are approximately0.04, 0.06, and 0.08 mAh cm−2 for the cells discharged at0.05, 0.1, and 0.2 mA cm−2, respectively (Figure 2b). Thedischarge capacity was increased to 0.5 mAh cm−2, but thecharge voltage still follows the same trend as before: higherdischarge rate results in lower charge voltage (Figure S9).These discharge-charge profiles do not align with previousstudies on liquid-state LOB systems. Li2O2 that can beoxidized at lower voltages have been previously associated tofilm-like or nanosized Li2O2,[36] which is not the case in thisstudy. To understand this unexpected trend, the dQ/dV wascalculated to further study the trends in the voltage profile(Figure S10). Two major peaks are observed for all samples,one at low potential (<3.8 V) and the other at a relativelyhigher potential (>3.8 V), which corresponds to the twodifferent charging regimes in (Figure 2a). The lower potentialpeak appears to be further composed of two peaks for the celldischarge at 0.1 mA cm−2. The cell discharged at 0.2 mA cm−2exhibited the largest peak magnitude followed by 0.1 andthen 0.05 mA cm−2. However, the potential position of thepeak was the lowest for 0.05 mA cm−2 (−3.55 V) versus 3.65and 3.75 V for 0.2 and 0.1 mA cm−2, respectively. The celldischarged at 0.05 and 0.1 mA cm−2 do indeed appear tohave lower first peak position, which could indicate that thesmaller particle sizes of Li2O2 that have been formed at lowercurrent discharge (as found in the SEM in Figure 1b,c) possessa lower charge potential initially and suggests that initially,the charge trends in observed in the liquid system[36] stillhold true to some degree. However, the 2nd peak for 0.05 and0.1 mA cm−2 occurs at a higher potential than the 1st peakof 0.2 mA cm−2, possibly indicating different Li2O2 oxidationmodes depending on the particle size or impurities.As the overpotential is associated with both the impedanceof the anode and cathode interfaces,[37] investigated it iscrucial to first deconvolute the source of the large overpo-tential. Initial EIS measurements were conducted on all cells.Subsequently, the cells were subjected to deep discharge atcurrent densities of 0.05, 0.1, and 0.2 mA cm−2, respectively,up to a capacity of 8 mAh cm−2. Following the deep discharge,post-discharge EIS measurements were performed on allcells.The Nyquist plot and Bode-like plot of three cells areshown in Figure 2c,d, respectively. Three distinct impedancesources, labeled as Z1, Z2, and Z3 from high frequency tolow frequency, were identified based on fitting the data to theequivalent circuit shown in in Figure 2c. This circuit consists ofa bulk resistor and three parallel combinations of a constantphase element (CPE) and a resistor. Following studies byAdams et al., Landa–Medrano et al, and Højberg et al.’ stud-ies, we associate Z1 with the anode and Z2 and Z3 attributedto the cathode.[38–40] Z2 is believed to be a cathode-specificprocess that is irrelevant to the oxygen reduction, as it couldbe present in the argon atmosphere.[40] Z3 represents theresistance of the electronic transport through the dischargeproducts, such as Li2O2.[40] The decrease of impedancesassociated with the increase of discharge current densities areAngew. Chem. Int. Ed. 2025, 64, e202507967 (7 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleTable 1: EIS fitting results for solid-state LOB cells discharged at differentrates.mA cm−2 R bulk Z1 Z2 Z30.05 51.4 76.04 244 94820.1 53.37 96.86 241.1 39970.2 54.73 101 159.4 1416consistent with the charge profile where the charge voltageare also lower in the cell that is discharged at higher currentdensities (Table 1). Clearly, the impedance of dischargeproducts on cathode surface is higher when they are pro-duced at lower discharge current densities (discharged up to8.4 mAh cm−2) where Z3 was fitted to be 9482, 3997, and 1416from 0.05, 0.1, and 0.2 mA cm−2, respectively and corrobo-rates the overpotentials trends with discharge current density.From the SEM results, a high discharge rate tends toproduce larger Li2O2 particles, suggesting that larger Li2O2particles are more easily oxidized during charging as theyexhibit a lower overpotential charge (dQ/dV analysis) andlower impedance. Typically, smaller Li2O2 particles have ahigher surface area relative to their volume (comparativelyhigher contact area with conductive support and shortertransport path length for Li-ion and electrons than largerparticles) and in theory, should facilitate oxidation, leadingto a lower overpotential charge.[14,36] The fact that our resultindicates larger Li2O2 particle yield lower OER potentialsuggest something else is possibly dominating the OERpotential. dQ/dV analysis suggests there exist at least threeoxidation events stemming from either sufficiently distinctgroups of Li2O2 particle sizes (with differing impedance) ordiffering degree of impurities arising from parasitic reactionsduring ORR. Given that the particle size distribution appearsto be relatively monomodal (Figure 1d,e) for both 0.05 and0.1 mA cm−2, it is unlikely there is a secondary distribution ofLi2O2 that exists unless they are on order of nanometers andnot easily visible by SEM.Whether the cause of the observed trend in OER overpo-tential and discharge current density is truly the results of anunseen (by SEM) distribution of Li2O2 or differences in para-sitic reactions can be probed by simply measuring the purity ofthe discharge product. The yield of Li2O2 from three carbonpaper cathodes discharged at different rates was titratedusing a Ti(IV)OSO4 solution. The results indicate that theLi2O2 yield was 79.8%, 82.3%, and 83.3% for cells dischargedat 0.05, 0.1, and 0.2 mA cm−2, respectively (Figure S11).This trend suggests that a lower discharge rate leads to alower Li2O2 yield, accompanied by an increased side productformation. The side products on cathodes were dissolvedin D2O solution, and the resulting solutions were analyzedusing NMR spectroscopy. It is well-established that the mainbyproducts in LOB systems are lithium formate and lithiumcarbonate.[41] Lithium formate exhibits a characteristic peakat approximately 8.28 ppm in the 1H NMR spectrum, whilelithium carbonate shows a peak at around 168 ppm in the 13CNMR spectrum. These peak positions were confirmed usingcommercial lithium formate and lithium carbonate powdersas reference standards (Figure S12c,f). Figure S12a,d show the1H and 13C NMR spectra of the cathode extracts from cellsdischarged at different current densities. All samples exhibitpeaks at −8.28 ppm in the 1H NMR and −168 ppm in the 13CNMR, indicating the presence of lithium formate and lithiumcarbonate, respectively. These peaks match those observed forthe commercial reference standards shown in Figure S12c,f.Since all samples were dissolved in the same volume of D2Oand measured under identical conditions and parameters, theintensity of the characteristic peaks can be correlated with theconcentration of lithium formate and lithium carbonate. Theresults show that as the discharge current density increases,the peak intensity at 8.28 ppm in the 1H NMR and at 168 ppmin the 13C NMR progressively decreases (Figure S12b,d),confirming that the overall concentration of these byproductsdecreases with increasing discharge current density.′′Consequently, we believe that the highimpedance/overpotential during OER observed in celldischarged under low current densities can be primarilyattributed to these byproducts and less likely to befrom a multimodal particle size distribution. Differentialelectrochemical mass spectrometry (DEMS) conductedduring OER was also carried out to study the OER efficiency.Cells were discharged to 0.5 mAh cm−2 at current densitiesof 0.05, 0.1, and 0.2 mA cm−2 and all charged at 0.1 mA cm−2with the O2 and CO2 quantified by DEMS. The DEMSresults of cell discharged at different current densitiesto 0.5 mAh cm−2 are presented in Figure 3. The chargeprofiles follow the same trend as previously discussed: cellsdischarged at higher current densities exhibit lower chargevoltages, consistent with the earlier observations.The low OER rate observed at the initial stage of chargingis a well-known challenge in LOBs, including solid-statesystems. This phenomenon is often attributed to the oxidativedecomposition of parasitic byproducts during the dischargeprocess. In our system, lithium formate and lithium carbonatewere identified as discharge byproducts (characterizationdetails provided later). Notably, lithium formate decomposesaround 3.4 V versus Li/Li+,[42] which aligns with the onsetof the charging process. The oxidation of these byproductsat the early stage of charging contributes to the suppressedOER rate. Additionally, if the lithium anode is not properlypassivated, oxygen consumption from side reactions can fur-ther inhibit OER activity. The absence of a liquid electrolytein solid-state systems exacerbates these effects, leading toa delayed onset of oxygen evolution and a persistently lowOER rate in the initial charging phase. Similar behavior hasbeen reported in previous studies, such as Liang et al.,[43]where stronger parasitic reactions in the absence of redoxmediators led to significant charging instability and delayedO2 evolution.Since the capacity is 0.5 mAh cm−2 for all cells, whichis equivalent to −9.32*10−6 mole cm−2 of O2 generationbased on the reaction 2Li+ + 2e− + O2→Li2O2 can beused to quantify the OER efficiency of each cell. The Li2O2DEMS yield rates are 72.7%, 80.7%, and 85.8% for cellsdischarged at 0.05, 0.1, and 0.2 mA cm−2, respectively. Clearly,cells discharged at higher current density have both a higherLi2O2 yield during ORR (titration result, Figure S11) anda corresponding OER efficiency. Furthermore, the higherAngew. Chem. Int. Ed. 2025, 64, e202507967 (8 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleFigure 3. OER DEMS results (at 0.1 mA cm−2) for cells discharged at different rates. a) 0.05 mA cm−2, b) 0.1 mA cm−2, and c) 0.2 mA cm−2.OER yield is not simply due to a higher Li2O2 yield duringORR. At 0.05 mA cm−2 the Li2O2 yield is 79.8% butthe OER O2 yield was only 72.7%, a loss of 7.1% fromOER alone. With increasing current density this inefficiencydecreases. At an ORR current density of 0.1 mA cm−2, thecorresponding loss from OER is 1.6%, which is then furtherreduced to 0.2 mA cm−2 to < 0% (i.e., within measurementof titration/DEMS and/or formation) of non Li2O2 dischargeproducts. The increasing OER efficiency with increasing ORRdischarge rate is likely due to a lowering of the chargepotential, reducing side reactions. From the DEMS results,these cells evolved 0.651, 0.559, and 0.483 µmol of CO2 gasfor discharge rates of 0.05, 0.1, and 0.2 mA cm−2, respectively.This confirms that cells discharged at a higher rate tend togenerate fewer byproducts, benefiting both ORR, formingmore Li2O2 and the subsequent OER for a more efficientcharging process.To further understand the origin of this improvedreversibility and suppressed side reaction behavior, weexamined the role of LiO2 intermediates and their nucle-ation dynamics under different discharge current densities.Discharge current density significantly influences both theactivation of nucleation sites and the behavior of LiO2intermediates at these sites. At low current densities, LiO2is generated slowly and can disproportionate across a widerrange of surface sites due to the weaker electrostatic repulsionfrom the SCL. This spatially dispersed nucleation leads tolower local LiO2 concentrations and slower disproportion-ation kinetics. As a result, LiO2 remains at the interfacefor longer durations and has increased opportunities toparticipate in parasitic reactions with the surrounding solidelectrolyte or cathode components, compromising reversibil-ity. In contrast, at higher current densities, the SCL becomesmore intensified and restricts Li+ transport to less favorableregions, effectively localizing electrochemical activity to alimited number of highly active sites where transport remainssufficient. LiO2 accumulates more rapidly at these sites, pro-moting aggregation and disproportionation into Li2O2 beforesignificant side reactions can occur. This site-concentratedgrowth pathway favors the formation of denser and purerLi2O2 deposits, thereby reducing side product formation.These trends are supported by DEMS results, which showenhanced oxygen evolution at higher current densities, and bySEM observations of fewer but larger Li2O2 particles underthe same conditions.To further confirm the impact of discharge current densityon OER efficiency, we employed a constant voltage dischargewhere the current versus time profile that the system cansustain is used to test the extreme case of maximally high(at 2.3 V) current density. Cells were first discharged atconstant current (1 mA cm−2) to 2.3 V in an attempt toeliminate capacitive capacity contribution. Subsequently, cellswere held at a reducing potential of 2.3 V versus Li+/Liand the current versus time profile was collected (shownin Figure 4a). During the initial stage of discharge, thecurrent drops rapidly from a very high value, followed by asmaller and stable current. The cells were discharged untilcapacity reached either 0.3 mAh cm−2 (high-current region)or 1.5 mAh cm−2 (low current region). High-current regionwas selected to ensure most of the Li2O2 capacity stemsfrom high-current density discharge while the low currentregion has the majority of its Li2O2 deposition occurring atlower current density (−0.42 mA cm−2). Subsequently, bothcells underwent OER with gas evolution being monitoredby DEMS. The total evolution of O2 gas was quantified andcompared with the expected amount based on the capacityobtained during constant voltage discharge (Figure 4b,c). Theefficiency of the high-current region was determined to be85.6%. To isolate the efficiency of the low-current region, thehigh-current region’s capacity and its corresponding higherefficiency contribution were subtracted using Equation S1 andcalculated to be 81.6%, indicating a higher OER efficiencythan the high-current region and aligning with the constant-current density result (Figure 3). It should be noted that theyield from the potentiostatic discharge appears to be generallylower than the galvanostatic discharge, which we believestems from a lower discharge potential (2.3 V versus > 2.6 V).Nevertheless, this result further indicates that the tendency fora higher discharge rate to result in less byproducts holds trueeven at the extreme case of constant voltage discharge withcurrent densities reaching well over 1.8 mA cm−2 during theearly stages of discharge.Impact of Discharge Rates on Cycle PerformanceTo evaluate the impact of discharge rate on cycle performanceof solid-state LOBs, cells were made and cycled at dischargedcurrent densities of 0.05, 0.1, and 0.2 mA cm−2, while main-taining a consistent charge current density of 0.1 mA cm−2.Angew. Chem. Int. Ed. 2025, 64, e202507967 (9 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleFigure 4. Effect of discharge current on OER efficiency during constant-voltage discharge. a) Discharge profile showing an initial constant-current(CC) step used to remove capacitive contributions, followed by a constant-voltage (CV) step for faradaic discharge. The profile is divided intohigh-current and low-current regions based on the current response. DEMS results of b) High-current discharge region and c) Low current dischargeregion. The current density of DEMS charge process is 0.1 mA cm−2.All cells were cycled with voltage cut-off limits set between2.3 and 4.7 V (Figure S13). The average charging voltageand end discharging voltage versus cycle number are plottedhere (Figure 5a). As a result, the 0.1-cell (the cell dischargedat 0.1 mA cm−2) exhibits the longest cycle life up to 245cycles. In comparison, the 0.05-cell had 235 cycles and 0.2-cell had 153 cycles (Figure 5b–d). Moreover, while the 0.2-cellexhibited the lowest OER potential in the initial few cycles,after six cycles, the difference was nullified with the 0.2-cellbecoming the cell with the high OER potential at the 36thcycle.Based on previous results, we believe that the cycle per-formance is affected by several factors. When the dischargevoltage of the 0.2 mA cm−2 cell drops below 2.30 V at the163rd cycle (last cycle of 0.2-cell), its corresponding chargevoltage is already at 4.56 V whereas 0.05 and 0.1-cells exhibitonly −4.52 V (for both). As the OER/ORR efficiency at0.2 mA cm−2 was found to be high, the culprit for thedecrease cycling performance likely stems from the anoderather than the cathode. The impedance element Z1 (whichwe attribute to the Li metal) increases with discharge rates.This indicates that high-rate discharge may lead to increasedimpedance at the anode. While a high discharge rate canreduce the overpotential of OER at the cathode side, itappears to increase the impedance of the anode, adverselyimpacting the long-term cycling performance of the cells.High-rate discharge accelerates the increase in impedance atthe anode-SSE interface. During cycling, the solid electrolyteinterface (SEI) formed from side reactions deteriorates thecontact between the anode and SSE, increasing impedance.Additionally, the interface between anode and SSE can bedegraded due to an unstable SEI. Such damage on the anodeside significantly affects the cycle performance of solid-stateLOBs. The 0.05-cell exhibited a sharp discharge voltage decayat the −225th cycle accompanied with a drop-in chargingvoltage, suggest some degree of shorting from dendriticlithium, whereas the 0.1-cell exhibited a more gradual celldeath. As discussed earlier, DEMS yield of low-discharge-rate cells is also lower, accompanied by parasitic reactions thatproduces byproducts. These side products can be formed fromthe degradation of SSE. These accumulate within the batteryduring cycling, eventually leading to its failure.Angew. Chem. Int. Ed. 2025, 64, e202507967 (10 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch ArticleFigure 5. Cycle performance. a) Plot of terminal average charge/discharge voltage versus cycle number of solid-state LOBs discharged at differentrates of 0.05, 0.1, and 0.2 mA cm−2, and charged at same rate of 0.1 mA cm−2. All capacities are 0.1 mAh cm−2. b)–d) Voltage profiles of solid-stateLOBs discharged at different rates at every 50 cycles until the end of life.At the beginning of the cycle, high-rate discharge resultsin a lower charge voltage and overall higher coulombicefficiency based on OER/ORR yields, which is expected tocontribute to longer cycle life. However, this trend cannot bemaintained throughout cycling due to instability at the anode.Additionally, while our findings align with previous studieson liquid electrolyte system showing that high-rate dischargeleads to lower charge voltage, Lu et al. reported a contrastingobservation: during the first 75% of the charge process, high-rate discharge resulted in a higher charge voltage, whereas inthe final 25%, the trend was consistent with our results.[44]This discrepancy remains unexplored. Clearly, charge voltageis significantly influenced by capacity, byproduct formation,and nucleation or growth phase of Li2O2. In our work, high-rate discharge is beneficial for having lower charge voltageand higher overall ORR/OER efficiency. However, it causesmore damage to the anode surface, resulting in a rapid decayof discharge voltage and significantly shortening the cycle life.While the difference between 0.2 and 0.1 mA cm−2 is nottypically considered significant for lithium metal battery, wewant to point out that such lithium metal cycling conditionsin LOB are different than those of a traditional lithium metalanode cell due to the formation dissolution of numerous gasspecies existing in a LOB cell. Work by Matsuda et al. hasshown that cathode/anode crossovers of O2 and CO2/H2Oformed from side reactions unique to LOB are detrimentalto the cycling of the lithium metal anode and performancedegradation occurs at current densities of a similar order ofmagnitude (0.4 mA cm−2) to our work.[45] Low-rate dischargeis advantageous to having lower discharge overpotential andmilder effect on the anode but suffers from lower ORR/OERefficiency, resulting in the formation of more side products.These side products degrade the interfaces of each batterycomponent and weaken the contact between the electrodesand the SSE, finally leading to battery failure. In our analysis,a discharge rate of 0.1 mA cm−2 is identified as the optimalrate for extending cycle performance.To evaluate the impact of residual casting solvent (DMF)on the observation and conclusion of this study, two additionalSSE films containing different amounts of residual solventwere prepared by varying the drying time. TGA analysisrevealed that the residual DMF contents in these SSEfilms were 18.3% and 30.9%, respectively (Figure S14a,b).Using the two SSE films, deep discharge experiments wereperformed, and the discharged cathodes were analyzed bySEM to observe the size of the Li2O2 particles. As shownin Figure S14e,f, a lower discharge rate produced smallerLi2O2 particles when using the SSE film containing 18.3%residual DMF. A similar trend was observed with the SSE filmcontaining 30.9% residual DMF, as shown in Figure S14g,h.These results confirm that the Li2O2 particle size still increasewith the increase of discharge rate regardless of residualsolvent in the SSE film.In addition, the first-cycle discharge/charge voltage pro-files show that cells discharged at higher current densitiesexhibit lower charge voltages (Figure S15a,e). DEMS analysisAngew. Chem. Int. Ed. 2025, 64, e202507967 (11 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 LicenseResearch Articlerevealed that the OER efficiencies for cells using the SSE filmwith 18.3% residual DMF were 76.1% and 81.1% at dischargecurrent densities of 0.1 and 0.2 mA cm−2, respectively(Figure S15c). Similarly, the OER efficiencies for cells usingthe SSE film with 30.9% residual DMF were 82.0% and86.2% at 0.1 and 0.2 mA cm−2, respectively (Figure S15g).These results demonstrate that the OER efficiency improveswith increasing discharge current density, regardless of theresidual DMF content in the SSE film. CO2 evolution alsoincreases with higher discharge rates, suggesting that lowerdischarge rates lead to the formation of more byproducts(Figure S15d,h), which is consistent with our previousobservations. For cycling performance, the cell dischargedat 0.1 mA cm−2 exhibited a longer cycle life than the celldischarged at 0.2 mA cm−2, regardless of the residual DMFcontent in the SSE (Figure S15i,j). This trend is also consistentwith our previous observations. All these results exhibit thesame trend across SSE films containing different amounts ofresidual DMF, 18.3%, 23.3%, and 30.9%, confirming that thepresence of residual casting solvent (DMF) does not affect thekey observations or conclusions of this study.ConclusionThis work reveals the impact of discharge rate on the mor-phology of discharge product, resulting in varying ORR/OERefficiencies and ultimately, different electrochemical per-formance. It is worth noting that the difference betweensolid-state LOBs and liquid-state LOBs may change thebehavior of electrochemical reactions. Li2O2 particle sizeswere found to increase with increasing current density whichcontrasts with commonly accepted trends in pure liquidelectrolyte system. Compared to liquid-state LOBs, solid-state LOBs have lower ionic conductivity and homogeneityin Li-ion transport and O2 transport. Such limitations mightimpose a degree of difficulty of Li-ion to nucleate new Li2O2rather than simply contributing to the growth of alreadyexisting Li2O2 in established Li-ion conducting paths, unlikethe uniform distribution observed in liquid-state LOBs. Wepropose that the formation of space charge layers leads to thedeactivation of less accessible interfacial sites, where limitedLi+ and O2 transport prevents effective electrochemicalreactions. As a result, only a subset of highly active sitesremains electrochemically functional under high-current den-sities, giving rise to site-selective Li2O2 growth. This selectivebehavior is consistent with the spatial heterogeneity at theSSE–cathode interface, as revealed by synchrotron SAXSanalysis. The solid-state LOB system consistently exhibitslower charge voltage at high discharge rates like previousstudies in liquid-state LOB systems. However, in the solid-state LOB system, the charge voltage is primarily affectedby byproducts formed during low-rate discharge, rather thanthe dimensional size of Li2O2 as seen in liquid-state LOBsystems. In addition, it is found that the efficiencies of both theOER and ORR improve with increasing discharge rate. As aresult, in our benchmark, a discharge rate of 0.1 mA cm−2 isidentified as the optimal rate for extending cycle performance.A low discharge rate promotes the formation of byproducts,while a high discharge rate accelerates the passivation of theanode interface.Supporting InformationSupporting information is available. The authors have citedadditional references within the Supporting Information.[46,47]AcknowledgementsThe authors gratefully acknowledge support of this workfrom the U.S. Department of Energy (DOE), Office ofEnergy Efficiency and Renewable Energy, Vehicle Technolo-gies Office. Argonne National Laboratory is operated forthe U.S. DOE, Office of Science, by UChicago Argonne,LLC, under Contract No. DE-AC02-06CH11357. Use of theCenter for Nanoscale Materials, an Office of Science userfacility, was supported by the U.S. DOE, Office of Science,Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Shoichi Matsuda acknowledges supportby Japan Science and Technology Agency (JST), AdoptingSustainable Partnerships for Innovative Research Ecosystem(ASPIRE), under Contract No. JPMJAP2309. This researchused resources of the Advanced Photon Source (12-ID-B), a U.S. Department of Energy (DOE) Office of Scienceuser facility operated for the DOE Office of Science byArgonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors gratefully acknowledge access tothe nGauge atomic force microscope loaned by ICSPI Corp.(Waterloo, Ontario, Canada). [Correction added on 6 August2025, after first online publication: Acknowledge to the ICSPICorp. has been insert in the Acknowledgement Section.]Conflict of InterestsThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.Keywords: Discharge current density • Lithium oxygen battery •Morphology • Oxygen evolution reaction • Solid-state battery[1] W.-J. Kwak, D. S. Rosy, C. Xia, H. Kim, L. R. Johnson, P. G.Bruce, L. F. Nazar, Y.-K. Sun, A. A. Frimer, M. Noked, S. A.Freunberger, D. Aurbach, Chem. Rev. 2020, 120, 6626–6683.[2] M. Balaish, E. Peled, D. Golodnitsky, Y. Ein-Eli, Angew. Chem.Int. Ed. 2015, 54, 436–440.[3] H.-D. Lim, H. Song, J. Kim, H. Gwon, Y. Bae, K.-Y. Park, J.Hong, H. Kim, T. Kim, Y. H. Kim, X. Lepró, R. Ovalle-Robles,R. H. Baughman, K. Kang, Angew. Chem. Int. Ed. 2014, 53,3926–3931.[4] J. Chen, A. Gutierrez, M. T. Saray, R. S. 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Ed. 2025, 64, e202507967 (13 of 13) © 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2025, 37, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202507967 by National Institute For, Wiley Online Library on [22/09/2025]. 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 Discharge Rate-Driven Li2O2 Growth Exhibits Unconventional Morphology Trends in Solid-State Li-O2 Batteries Introduction Results and Discussion Impact of Discharge Rates on the Formation of Li2O2 Investigation of the Overpotential in Oxygen Evolution Reaction Impact of Discharge Rates on Cycle Performance Conclusion Supporting Information Acknowledgements Conflict of Interests Data Availability Statement