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[Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404), [Eiki Yasukawa](https://orcid.org/0000-0003-3709-9473), Shin Kimura, Shoji Yamaguchi, [Kohei Uosaki](https://orcid.org/0000-0001-8886-3270)

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[Evaluation of performance metrics for high energy density rechargeable lithium–oxygen batteries](https://mdr.nims.go.jp/datasets/b332bcb4-b4b0-4e27-aa38-7f46d0a2368d)

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Evaluation of performance metrics for high energy density rechargeable lithium–oxygen batteriesFaraday DiscussionsCite this: Faraday Discuss., 2024, 248, 341PAPEROpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueEvaluation of performance metrics for highenergy density rechargeable lithium–oxygen batteries†Shoichi Matsuda, *ab Eiki Yasukawa,ab Shin Kimura,abShoji Yamaguchiab and Kohei Uosaki abReceived 24th April 2023, Accepted 29th June 2023DOI: 10.1039/d3fd00082fThe demand for practical implementation of rechargeable lithium–oxygen batteries(LOBs) has grown owing to their extremely high theoretical energy density. However,the factors determining the performance of cell-level high energy density LOBs remainunclear. In this study, LOBs with a stacked-cell configuration were fabricated and theirperformance evaluated under different experimental conditions to clarify the uniquedegradation phenomenon under lean-electrolyte and high areal capacity conditions.First, the effect of the electrolyte amount against areal capacity ratio (E/C) on thebattery performance was evaluated, revealing a complicated voltage profile for an LOBcell operated under high areal capacity conditions. Second, the impact of different kindsof gas-diffusion layer materials on the “sudden death” phenomenon during the chargingprocess was investigated. The results obtained in the present study reveal theimportance of these factors when evaluating the performance metrics of LOBs,including cycle life, and round-trip energy efficiency. We believe that adoptinga suitable experimental setup with appropriate technological parameters is crucial foraccurately interpreting the complicated phenomenon in LOBs with cell-level highenergy density.IntroductionLithium–oxygen batteries (LOBs) are potential candidates for the next-generationof rechargeable batteries because of their extremely high theoretical energydensity.1,2 In LOBs, the oxygen reduction reaction proceeds at the positive oxygenelectrode during the discharge process, thereby resulting in the formation ofLi2O2, which accumulates in the pores of the carbon electrode. During charging,the electrochemical decomposition of Li2O2 proceeds and generates oxygen. OnaCenter for Green Research on Energy and Environmental Materials, National Institute for Material Science,1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail: matsuda.shoichi@nims.go.jpbNIMS-SoBank Advanced Technologies Development Center, National Institute for Materials Science, 1-1Namiki, Tsukuba, Ibaraki 305-0044, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082fThis journal is © The Royal Society of Chemistry 2024 Faraday Discuss., 2024, 248, 341–354 | 341http://orcid.org/0000-0002-0640-3404http://orcid.org/0000-0001-8886-3270https://doi.org/10.1039/d3fd00082fhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fhttps://pubs.rsc.org/en/journals/journal/FDhttps://pubs.rsc.org/en/journals/journal/FD?issueid=FD024248Faraday Discussions PaperOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethe negative electrode side, the reversible lithium dissolution/deposition processshould proceed in association with the progress of the discharge/charge process.Thus, the electrochemical reaction characterising LOBs can be written as follows:O2 + 2Li+ + 2e− = Li2O2 (positive electrode)Li = Li+ + e− (negative electrode)Based on the weight of active materials (oxygen and lithium), a theoretical LOBenergy density is over 3500 W h kg−1. However, in practice, the weights of addi-tional components, such as carbon electrodes, electrolytes, gas-diffusion layers,and current collectors, should be considered for the calculation of the energydensity of LOB cells.Recent studies on the design of practical LOB cells revealed the importance ofthe ratio of electrolyte amount against areal capacity (E/C, g A−1 h−1) for deter-mining the performance of LOBs.3–5 The E/C value has been used as an empiricalparameter representing the electrolyte amount when studying lithium-ion battery(LIB) compounds. However, a recent study on lithium-metal-based rechargeablebatteries demonstrated that E/C is a crucial parameter for performance evaluationin practical cell design conditions (i.e., lean electrolyte conditions).6,7 Addition-ally, the importance of E/C has also been reported in the eld of LOBs.3–5 Notably,it was reported that E/C is a parameter that can be used as an indicator of energydensity for practical LOB cell design.3 In particular, to obtain LOBs with an energydensity above 300W h kg−1, the value of E/C should be maintained below 10 g A−1h−1. Although the relationship between E/C and LOB energy density has beeninvestigated, the effects of E/C values on other battery performance indicators,such as cycle life, round-trip energy efficiency, etc., are unknown.In addition to the E/C-based cell design strategy, understanding the effects ofcell congurations on the performance of LOBs is also crucial for practicalimplementation of cell-level high energy density LOBs. In particular, an appro-priate cell conguration should be adopted that ensures an effective oxygen owacross the entire porous carbon-based positive electrode through the gas-diffusion layer. For example, in coin and Swagelok cells, oxygen is transportedfrom the entire gas-diffusion layer in the vertical direction to the porous carbon-based positive electrode. In contrast, in practical stacked cells, oxygen can only betransported through the gas-diffusion layer in the horizontal direction. In addi-tion, according to cell-level energy density calculations, the gas-diffusion layeraccounts for more than 10% of the total weight of an LOB cell,8 which corre-sponds to the weight of the porous carbon electrode. Thus, the reduction of theweight of gas-diffusion layer materials without sacricing the oxygen transportproperty is required. Actually, a recent study demonstrated the use of a gas-diffusible current collector that combines the functions of oxygen mass trans-port and electron transfer.9 The application of such a novel concept for thepractical design of LOBs may benet from a detailed analysis of the cell perfor-mance at low E/C values.Based on these research backgrounds, in the present study, the relationshipbetween E/C and LOB cell performance was experimentally investigated. For thispurpose, a series of identical LOB cells were fabricated, and their performances342 | Faraday Discuss., 2024, 248, 341–354 This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fPaper Faraday DiscussionsOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinewere evaluated under different conditions (i.e., different limiting capacities anddifferent thickness of the carbon electrode). The obtained results were comparedand the effects of E/C on the cycle performance of LOBs were discussed. Subse-quently, the performance of an LOB cell equipped with different kinds of gas-diffusion layer materials was evaluated at low E/C values (E/C < 5 g A−1 h−1). Asa result, we observed a unique “sudden-death” phenomenon during the chargingprocess and its mechanism was discussed.ExperimentalPreparation of a carbon-powder-based self-standing carbon membraneA slurry mixture was prepared using 65 wt% of Ketjenblack (KB) (Lion SpecialtyChemicals, EC600J), 12 wt% of carbon ber (average ber diameter of 6 mm andaverage length of 3 mm), 23 wt% PAN, and NMP as a solvent for uniform disper-sion. The slurry mixture was formed into a sheet by moulding to a uniformthickness by a wet lm-forming method using a doctor blade. Aer moulding, thesample was immersed in methanol and converted to a porous lm by a non-solvent-induced phase separation method. Furthermore, the volatile solvent wasremoved by drying the sample at 80 °C for 10 h; subsequently, heat treatment wasperformed at 230 °C for 3 h in air using a box-type furnace (Denken High Dental).Battery assembly and testingA tetraethylene glycol dimethyl ether (TEGDME) solution containing 0.5 Mlithium bis(triuoromethanesulfonyl)imide (LiTFSI; Kishida Chemical, purity>99.9%), 0.5 M LiNO3 (Sigma-Aldrich, purity >99.9%), and 0.2 M LiBr (Sigma-Aldrich, purity >99.9%) was used as the electrolyte. LiNO3 and LiBr were driedat 120 °C under vacuum before use. A self-standing KB-based carbon membranewas used as the positive electrode. The carbon electrode was dried at 100 °C undervacuum for 12 h. A carbon ber membrane (200 mm thickness, TGP-H060, Toray)or a polytetrauoroethylene (PTFE) ber membrane (100 mm thickness,POREFLON™ PTFE membrane, WP-500-100) were utilized as the gas diffusionlayer. Ni-coated polyethylene terephthalate (PET) ber mesh (SEIREN) or SUSber mesh were utilized as the gas-diffusible current collector and SUS ber meshas the current collector. LOB cells were assembled in a dry room (<10 ppm water)by sequential stacking of a Li foil (20mm square, 100 mm thickness, HonjoMetal),polyolen-based separator (22 mm square, 20 mm thickness), porous carbonelectrode (20 mm square), gas-diffusion layer, and current collector. For electro-lyte injection into the porous carbon electrode, the stampingmethod was adoptedusing a PTFE membrane (Advantec Toyo Co., Ltd, diameter = 90 mm, diameter =1 mm) as the hydrophilic lter. The porous carbon electrode was sandwichedbetween two hydrophilic lters impregnated with a suitable electrolyte amount(via drop-casting), and the electrode was kept under vacuum for more than 3 min.For the fabrication of an LOB cell with protection of the lithium metal electrode,a ceramic-based, solid-state separator (90 mm thickness, LICGC, Ohara) was used.The ceramic-based solid-state separator was sandwiched between polyolenlayers, and the same electrolyte was used on the positive and negative sides of thecell. A pressure of 100 kPa was applied to the cell by a spring coil. Electrochemicalexperiments were conducted with TOSCAT (Toyo Systems) battery-test equipment.This journal is © The Royal Society of Chemistry 2024 Faraday Discuss., 2024, 248, 341–354 | 343http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fFaraday Discussions PaperOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineResults & discussionEvaluation of the effects of E/C on the performance of LOBsWe investigate the effect of E/C on the performance of LOBs by following two typesof experiments: (i) changing the capacity limiting condition using an LOB cellwith a xed amount of electrolyte, or (ii) changing the amount of electrolyte in theLOB cell by controlling the thickness of the carbon electrode and evaluating thexed capacity limiting condition. In our experiments herein, stacked-type LOBcells were utilized.10 A KB-based self-standing membrane4 with a mass loading of5.4 mg and a 100 mm-thick lithium foil were adopted as the positive and negativeelectrodes, respectively. A solution of 0.5 M LiTFSI, 0.5 M LiNO3, and 0.2 M LiBrdissolved in TEGDME was the electrolyte.11,12 The details of the components andcell conguration are presented in Table S1 and Fig. S1.† The amount of elec-trolyte in the electrode was controlled at 22 mg cm−2 using the stampingmethod.10 Fig. 1a shows the voltage prole of the LOB cell at a current density andareal capacity of 0.05 mA cm−2 and 0.5 mA h cm−2, respectively. During thedischarge process, the cell exhibited stable voltage plateaux at 2.75 V. Duringcharging, the cell initially exhibited a voltage of 3.5 V. The voltage graduallyincreased with charging progress and nally reached 3.8 V. As the cycle pro-gressed, a gradual increase in the overpotential during both discharge and chargewas observed. Even at the 100th cycle, the cell exhibited a stable voltage prole(red curve in Fig. 1a). The purple curve in Fig. 1a shows the voltage prole at the180th cycle, revealing a large voltage hysteresis above 1.5 V. At the 189th cycle, thedischarge voltage reached the cut-off value (pink curve in Fig. 1a).Similar discharge/charge cycle tests on the LOB cells were performed byvarying the current density and areal capacity conditions while the C-rate wasxed at 0.1 C (i.e., the duration of the discharge and charge processes was 10 heach). The resulting discharge/charge proles at current density values of 0.1, 0.2,Fig. 1 Discharge/charge profile of LOB cells with different capacity limitation and currentdensity conditions. (a) 0.05mA cm−2, 0.5 mA h cm−2, (b) 0.1 mA cm−2, 1 mA h cm−2, (c) 0.2mA cm−2, 2 mA h cm−2, (d and f) 0.4 mA h cm−2, 4 mA h cm−2 and (e) 0.1 mA cm−2,4 mA h cm−2. (f) 0.4 mA h cm−2, 4 mA h cm−2 and a ceramic-based solid-state separatorsandwiched between two pieces of a PO-based separator were adopted as the protectivelayer for the lithium metal electrode.344 | Faraday Discuss., 2024, 248, 341–354 This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fPaper Faraday DiscussionsOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineand 0.4 mA cm−2 and areal capacities of 1, 2, and 4 mA h cm−2 are shown inFig. 1b–d. The LOB cell operated at an areal capacity of 1 mA h cm−2 exhibiteda discharge voltage of 2.75 V at the 10th cycle (black curve in Fig. 1b). As thenumber of cycles increased, the discharge voltage decreased. At the 70th dischargeprocess, the voltage reached 2.48 V (red curve in Fig. 1b). During the chargingprocess, an increase of overpotential with increasing number of cycles can beseen. Even in the cell operated at an areal capacity of 1 mA h cm−2, a similardegradation phenomenon was observed (Fig. 1c). At the 39th cycle, the dischargevoltage reached a cut-off condition (blue curve in Fig. 1c). In contrast, in the caseof the cell operated at 4 mA h cm−2, the overpotential quickly increased in boththe discharge and charge processes (Fig. 1d). As a result, the cell stopped at onlythe 11th cycle (blue curve in Fig. 1d).In Fig. 2a, the values of the round-trip energy efficiency of the LOB cellsoperating under different conditions were plotted. In the case of the cell operatedat 0.5 mA h cm−2, the round-trip energy efficiency was over 85% (black data pointsin Fig. 2a) at the 10th cycle. As the number of cycles increased, the round-tripenergy efficiency gradually decreased and showed a sharp drop at around the170th cycle, falling below 70%. By increasing the current density and arealcapacity, the round-trip energy efficiency largely decreased (Fig. 2a), revealing thatE/C signicantly affects the performance of LOBs, not only for cycle life, but alsoround-trip energy efficiency. In Fig. 2b and c, the value of the average voltageduring the discharge/charge process was plotted against cycle life. As the prole ofthe average discharge voltage resembles the prole of the round-trip energyefficiency, the discharge reaction is considered as the main factor in determiningthe round-trip energy efficiency. Notably, in Fig. 2b, it can be seen that the LOBcell operated at 0.5 mA h cm−2 exhibited an average discharge voltage that washigher than the theoretical reaction voltage of LOBs, 2.96 V, at the beginning ofthe cycle.The magnied voltage prole of the LOB cell operated at 0.5 mA h cm−2 isshown in Fig. S2a.† It can be seen that there is a high-voltage region of around3.7–3.0 V at the beginning of the discharge process. It is considered that theoxidized form of the redox meditator generated during the charge processremains without reacting during the decomposition reaction of Li2O2 and isreduced during the discharging process, resulting in a high-potential dischargephenomenon. It should be noted that such an abnormal discharge process atpotentials >2.96 V was also reported for the LOBs containing RM (redoxmediator).11,13–17 The capacity during this high-potential discharge process can beassigned to re-reduction of the oxidized form of RM. At the 10th cycle, the capacityduring this high-potential discharge process is approximately 0.2 mA h cm−2,which corresponds with 40% of the limited capacity of 0.5 mA h cm−2. In thissense, it can be said that 40% of the current in this LOB cell is operated by theredox reaction of 3Br−/Br3− and/or NO2−/NO2−. With an increasing number ofcycles, a gradual decrease of this capacity was observed and it reached0.1 mA h cm−2 at the 150th cycle (blue curve in Fig. S2a†). We considered that thedecomposition of RM during cycling results in a decreased amount of RM in theelectrolyte,12 inducing the decrease of capacity during the high-potentialdischarge process. The magnied voltage prole of the LOB cells operated at 1or 2 mA h cm−2 are also shown in Fig. S2b and c.† In both cases, the capacityThis journal is © The Royal Society of Chemistry 2024 Faraday Discuss., 2024, 248, 341–354 | 345http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fFig. 2 (a) Round-trip energy efficiency, (b and c) average discharge/charge voltage, and(d) capacity retention of LOB cells with different capacity limitation and current densityconditions were plotted against cycle number. Black data points: 0.05 mA cm−2,0.5 mA h cm−2. Green data points: 0.1 mA cm−2, 1 mA h cm−2. Red data points: 0.2 mAcm−2, 2 mA h cm−2. Blue data points: 0.4 mA cm−2, 4 mA h cm−2. (e) Cumulative capacityof LOB cells under different capacity limitation and current density conditions.Faraday Discussions PaperOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineduring the high-potential discharge process is 0.2 mA h cm−2 and graduallydecreases with increasing cycle number.In Fig. 2d, the capacity retention of each LOB cell is plotted against the cyclenumber. Evidently, the number of cycles decreased signicantly as currentdensity and capacity increased. The cell operated at a current density and arealcapacity of 0.05 mA cm−2 and 0.5 mA h cm−2, respectively, achieved stabledischarge/charge reactions for more than 180 cycles. However, the cycle numberof the cell operating at a current density and areal capacity of 0.4 mA cm−2 and4 mA h cm−2, respectively, reached only 11 cycles.We also summarize the obtained series of results from the viewpoint ofcumulative capacity (Fig. 2e). In the cells operated at current densities of 0.05 and346 | Faraday Discuss., 2024, 248, 341–354 This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fPaper Faraday DiscussionsOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online0.1 mA cm−2, and areal capacities of 0.5 and 1 mA h cm−2, the cumulativecapacities were 94 and 90mA cm−2, respectively. By increasing the current densityand areal capacity, the cumulative capacity signicantly decreased. The celloperated at a current density of 0.4 mA cm−2 and areal capacity of 4 mA h cm−2exhibited an extremely low cumulative capacity of 44 mA h cm−2. This low valueoriginated either from the high current density and/or from the high arealcapacity.To distinguish the effects of these two factors on the cumulative capacity, theperformance of an LOB cell operated at a current density and areal capacity of 0.1mA cm−2 and 4 mA h cm−2, respectively, was evaluated. In this case, the cumu-lative capacity increased up to 80 mA h cm−2, thus conrming that the contri-bution of the current density to the cumulative capacity is the most relevant.Fig. 1e showed the voltage prole of the cell operated at a current density andareal capacity of 0.1 mA cm−2 and 4 mA h cm−2, respectively. By comparison withFig. 1e, the following three points were conrmed in Fig. 1d: (i) The voltagedecreased during the initial stage of the discharge process (blue arrow), (ii) a peakwas present during the initial stage of the charging process (red arrow), and (iii)a peak was present during the nal stage of the charging process (black arrows).Recently, our study using a three-electrode experimental setup revealed that thesefeatures originate from the change of reaction prole of the negative lithiumelectrode, not from the positive oxygen electrode.5 In addition, it was alsodemonstrated that chemical crossover from the positive to negative electroderesults in such a complicated reaction prole at the negative lithium electrode.5Actually, such a unique feature in the voltage prole of the discharge/charge cyclewas also observed in the LOB cell operated at 2 mA h cm−2 (blue, red and blackarrows in Fig. 1c). In addition, the peak present during the nal stage of thecharging process was also detected in the LOB cell operated at 2 mA cm−2 duringthe latter part of the cycle life (black arrows in Fig. 1a and b). These results suggestthat the deterioration of the negative lithium electrode is the main factor fordetermining the cycle life of the LOB investigated in the present study.To further investigate the effects of the chemical crossover between the elec-trodes on battery performance, an LOB cell with a protected lithium metal elec-trode was fabricated. The protective layer was a ceramic-based solid-stateseparator sandwiched between two pieces of a PO-based separator. Fig. 1fshows the discharge/charge prole of the LOB cell with the protected lithiummetal electrode, revealing a stable discharge/charge process up to the 10th cyclewithout visible cell degradation. During the 19th cycle, the discharge voltagesuddenly decreased, reaching the cut-off voltage. The cumulative capacitysignicantly improved, reaching 76 mA h cm−2 (Fig. 2e), thus conrming thebenecial effect of lithium electrode protection on the performance of LOB cells.Notably, three unique features originating from chemical crossover are notobserved in the voltage prole of the LOB cell with a protected lithium metalelectrode (Fig. 1f). By introducing a protective layer for the lithiummetal electrodeand suppressing chemical crossover, the LOB cell operated at a current densityand areal capacity of 0.4 mA cm−2 and 4 mA h cm−2, respectively, exhibiteda performance equivalent to the cell operated at 0.1 mA cm−2 and 4 mA h cm−2.In the LOB cells used in the above experiments, the amount of electrolyte wascontrolled at 22 mg cm−2. Thus, in the case of the LOB cell operated at4.0 mA h cm−2, the value of E/C = 5.5 g A−1 h−1. Under such low E/C conditions,This journal is © The Royal Society of Chemistry 2024 Faraday Discuss., 2024, 248, 341–354 | 347http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fFaraday Discussions PaperOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethe LOB can be expected to have a cell-level energy density of over 300 W h kg−1.3Under such conditions, the degradation of the negative lithium metal electrodehas a large impact on the voltage prole and cycle life. The result of the presentstudy reveals that lowering the current density or introducing a protective layer forthe lithium metal electrodes are effective at improving the performance of LOBs.Next, we turn our attention to the investigation of a second type of experiment:(ii) changing the amount of electrolyte in the LOB cell by controlling the thicknessof the carbon electrode and evaluating the xed capacity limiting condition. Forthis experiment, four types of KB-based self-standing membranes with the sameporosity but with different thicknesses (450 mm, 380 mm, 280 mm, and 220 mm)were utilized as positive electrodes. Here, the electrolyte injection ratio into thecarbon electrodes was adjusted to be approximately 80% for all the LOB cells byusing the stamping method.10 As a result, the amounts of electrolyte used in theLOB cells were 32 mg cm−2, 27 mg cm−2, 20 mg cm−2, and 17 mg cm−2.Fig. 3a–d show the voltage prole of the LOB cells with different electrolyteamounts. For all cells, the ceramic-based solid-state separator sandwiched by twopieces of a PO-based separator were introduced in order to minimize the degra-dation of the lithium metal electrodes. In this experiment, the current densityduring the discharge and charge processes was set to be 0.4 and 0.2 mA cm−2,respectively, and the areal capacity was set to be 4 mA h cm−2. It should be notedthat increasing the thickness of the positive electrode under the same capacitylimiting condition corresponds with decreasing the depth of discharge. Here, thevalues of E/C for the LOB cells were 8.0 A h g−1, 6.75 A h g−1, 5.0 A h g−1, and 4.25A h g−1. With increasing number of cycles, a gradual increase of overpotential inboth the discharge and charge processes can be seen. Aer a certain number ofcycles, the discharge voltage reached the cut-off condition and the cells stopped.In Fig. 3e, the number of cycles is plotted against the electrolyte amount in theFig. 3 Discharge/charge profile of LOB cells with different electrolyte amounts using thecarbon electrodes with different thicknesses. (a) Electrolyte amount of 32 mg cm−2,carbon electrode thickness of 450 mm, (b) electrolyte amount of 27 mg cm−2, carbonelectrode thickness of 380 mm, (c) electrolyte amount of 20 mg cm−2, carbon electrodethickness of 280 mm, and (d) electrolyte amount of 17 mg cm−2, carbon electrodethickness of 220 mm. (e) Relationship between the electrolyte amount and cycle number ofthe LOB cells. (f) Relationship between themass of the carbon electrode and cycle numberof the LOB cells.348 | Faraday Discuss., 2024, 248, 341–354 This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fPaper Faraday DiscussionsOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineLOB cells, revealing the clear correlation between the two factors. These resultsclearly indicate that E/C has a large impact on cycle life. Also, the relationshipbetween the cycle number of each LOB cell and the mass loading of the positiveelectrodes are summarized in Fig. 3f, revealing the high correlation between thesetwo factors. This is a reasonable result because there is a linear correlationbetween the mass loading of the positive electrode, the thickness of positiveelectrode, and the electrolyte amount (Fig. S3†). Notably, the linear correlationbetween cycle life and thickness of the positive electrode suggests that all parts ofthe positive electrode are fully utilized for the discharge/charge reaction.So, what are the physicochemical factors that determine the cycle life of LOBcells? In this experimental system, it is conrmed that the stable cycle of thelithium metal symmetric cell progresses for more than 40 cycles. Therefore, it isconsidered that the main factor of cell deterioration caused by overvoltage rise isthe positive electrode reaction, not the negative electrode reaction. The followingfactors are considered as possible origins of cell failure: electrolyte depletion,clogging of pores due to the accumulation of solid by-products on the carbonelectrode, and deterioration of the carbon electrode itself.18–28Evaluation of the effects of different gas-diffusion layers on the performance ofLOBsNext, we focused our attention on the investigation of the effects of different gas-diffusion layers on the performance of LOB cells under low E/C conditions. Forthis, three types of LOB cell were fabricated: cell A presented a carbon bermembrane as the gas-diffusion layer and SUS-ber mesh as the current collector;cell B was built using an Ni-coated polyethylene terephthalate (PET) ber mesh asa gas-diffusible current collector; and cell C had the same gas-diffusible currentcollector as that of cell B and a polytetrauoroethylene (PTFE) membrane as thegas-diffusion layer. The physical properties of the above cell components arelisted in Table S2.† It should be noted that oxygen transport through the gas-diffusion layer was only allowed in the horizontal direction, not the verticaldirection, in the stacked-cell conguration. In these experiments, a KB-based self-standing membrane with a mass loading of 3.2 mg cm−2 was utilized as thepositive electrode. The amount of electrolyte in the electrode was controlled at14 mg cm−2 (electrolyte injection ratio of 70%) using the stamping method.10 Byoperating these cells at an areal capacity of 4 mA h cm−2, the E/C value of the LOBcells was 3.5 g A−1 h−1. For suppressing the undesired chemical crossover reac-tion, a ceramic-based solid-state separator sandwiched by two pieces of a PO-based separator was adopted as the protective layer for the lithium metalelectrode.Fig. 4a shows the discharge/charge prole of cell A, which presents the typicalvoltage prole of an LOB cell with a stable voltage plateau at 2.6 V during thedischarge process. During the charging process, the cell voltage graduallyincreased from 3.2 to 4.2 V. As the cycle progressed, the overpotential graduallyincreased during both the discharge and charge processes. At the 12th cycle, thecharging voltage sharply increased, reaching the cut-off voltage of 4.5 V. A similarphenomenon was reported in previous studies on LOB cells at low E/C values.10 Asshown by the discharge/charge prole in Fig. 4b, the voltage prole of cell B issimilar to that of cell A up to the 10th cycle. Aer the 12th cycle, cell B still exhibitsThis journal is © The Royal Society of Chemistry 2024 Faraday Discuss., 2024, 248, 341–354 | 349http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fFig. 4 Schematic illustration and discharge/charge profile of the LOB cells. (a) LOB cellequipped with a carbon fiber membrane as the gas-diffusion layer and an SUS fiber meshas the current collector. (b) LOB cell equipped with a Ni-coated PET fiber mesh as the gas-diffusible current collector. (c) LOB cell equipped with a Ni-coated PET fiber mesh as thegas-diffusible current collector and a PTFE fiber membrane as the gas-diffusion layer.Faraday Discussions PaperOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinea stable discharge/charge prole, in contrast to that of cell A, although a gradualincrease in the voltage was observed at the beginning of the charging processduring the 13th and 14th cycles. During the 15th cycle, the charging voltageincreased sharply, reaching the cut-off voltage.One possible explanation for the physicochemical origin of this sharp increasein charging voltage is the electrolyte shortage mechanism.10 In Fig. 5, the quan-titative information relating to the electrolyte movement phenomenon issummarized. As for the carbon ber membrane-based gas-diffusion layer, itsthickness and porosity are 190 mm and 90%, respectively. Thus, its pore volume is15.6 mL cm−2 (Fig. 5a). When the LOB cell is fabricated with an electrolyteinjection ratio of 100%, the part of the electrolyte in the carbon electrode canmove to the carbon ber membrane-based gas-diffusion layer due to the hydro-philic nature of the carbon ber membrane (Fig. 6a). In addition, during theFig. 5 Schematic illustration for quantitative information related to the electrolytemovement phenomenon. (a) Pore volume of the carbon electrode and gas-diffusion layermaterials. (b) Estimation of electrolyte amount in cell A. (c) Estimation of electrolyteamount in cell B.350 | Faraday Discuss., 2024, 248, 341–354 This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fPaper Faraday DiscussionsOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinedischarge process, the electrolyte in the carbon electrode is pushed out owing tothe formation and accumulation of Li2O2. When the pushed-out electrolyte isabsorbed in the gas-diffusion layer, it does not return to the carbon electrode;thus, the amount of electrolyte in the carbon electrode decreases (Fig. 5b). In thecase of an areal capacity of 4 mA h cm−2, 1.5 mL cm−2 of the electrolyte, whichcorresponds to the volume of the generated Li2O2, is pushed out from the carbonelectrode. In addition to the electrolyte-absorbing property of the gas-diffusionlayer itself, the electrolyte that is gradually pushed out from the carbon elec-trode moves toward the gas-diffusion layer due to the driving force caused byLi2O2 formation. In principle, the movement of the electrolyte continues untilthere is an equal amount of electrolyte in the void spaces of the carbon electrodeand the gas-diffusion layer. As the cycling progresses, the amount of electrolyte inthe carbon electrode gradually drops until it is insufficient to ensure the transportof Li ions, which supports the electrochemical decomposition reaction of Li2O2.At this point, the overpotential increases sharply.Based on these considerations, we experimentally investigated the electrolytemovement phenomenon. By disassembling the LOB cell at the selected condition,the amount of electrolyte in the gas-diffusion layer was evaluated by simplymeasuring its weight change. For this experiment, the electrolyte injection ratiowas set as 100%. In the case of the LOB just aer cell assembly, the weight of thegas-diffusion layer did not largely change, suggesting that most of the electrolyteremains in the carbon electrode. In sharp contrast, an increase of 0.43 mg cm−2for the gas-diffusion layer aer the 1st discharge process can be seen (Fig. S4†).The result suggests that part of the electrolyte was moved to the gas-diffusionlayer from the carbon electrode. Aer the 1st charging process, the weight ofthe gas-diffusion layer decreased to the initial level, suggesting that the electrolytein the gas-diffusion layer moved back to the carbon electrode. We also tried toperform similar experiments for cycled LOB cells. However, for the cell aerrepeated discharge/charge cycling, the gas-diffusion layer became stronglyattached to the carbon electrode, making it difficult to separate them. Thus,further details of the electrolyte movement phenomenon should be investigatedby use of operando techniques.29In our experiment, the carbon ber-based gas-diffusion layer used in cell Acontained a larger amount of void space compared with that in the Ni-coated PETber mesh-based gas-diffusible current collector used in cell B, mainly because ofthe difference in the thickness of these two layers. As for the Ni-coated PET bermesh-based gas-diffusible current collector, its thickness and porosity are 50 mmand 70%, respectively. Thus, its pore volume is 3.5 mL cm−2. In this case, even forFig. 6 Photographic images of the electrolyte droplets on a series of gas-diffusion layers.(a) Carbon fiber membrane, (b) Ni-coated PET fiber mesh, (c) PTFE membrane and (d) Ni-coated PET fiber mesh placed on a PTFE membrane.This journal is © The Royal Society of Chemistry 2024 Faraday Discuss., 2024, 248, 341–354 | 351http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fFaraday Discussions PaperOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethe condition where equal amounts of electrolyte exist in the void spaces of thecarbon electrode and the gas-diffusion layer, most of the electrolyte remains inthe carbon electrode (Fig. 5c).To further test the validity of the idea that the increase of voltage during theinitial charging process originates from the electrolyte movement phenomenon,a PTFE membrane, which is hydrophobic against the electrolyte, was added to anLOB cell and its effects on the cell performance were evaluated. In Fig. 6,photographic images of electrolyte droplets on a series of gas-diffusion layers areshown. Here we added 20 mL of electrolyte to each 2 cm2-sized gas-diffusion layer.In the case of the carbon ber membrane and the Ni-coated PET ber mesh, theelectrolyte quickly spread across the whole of the membrane. In sharp contrast, inthe case of the PTFEmembrane and the Ni-coated PET ber mesh placed on PTFEmembrane, the electrolyte remained as a droplet in the center of the membrane.These results clearly revealed the repelling effect of the PTFE membrane againstthe TEGDME-based electrolyte. Fig. 4c shows the voltage proles of the discharge/charge cycles of cell C, which is characterised by a Ni-coated PET bermesh-basedgas-diffusible current collector and a PTFE membrane. Up to the 10th cycle, thecell exhibited essentially the same voltage prole as that of cell B, which con-tained only the Ni-coated PET ber mesh-based gas-diffusible current collector.Between the 12th and 14th cycles, the charging voltage increased; however, theincrease was not as sharp as that in the voltage prole of cell B. Cell C exhibiteda stable discharge/charge process until the 16th cycle, although the overpotentialcontinued to increase. During the 17th cycle, the discharge voltage graduallydecreased, reaching the cut-off condition. These results clearly conrm that theintroduction of the PTFE membrane suppressed the sudden increase in thecharging voltage, which was observed in cells A and B. This supports the proposedhypothesis that the voltage increase is caused by the electrolyte depletionphenomenon. The obtained series of results in the present study suggest theimportance of considering the hydrophobic properties of the gas-diffusion layermaterials against the electrolyte as an essential factor for realizing LOBs witha high energy density and long cycle life.ConclusionsIn summary, in the present study, we investigated the following issues by fabri-cating stacked congurations of LOB cells. First, we investigated the effect of E/Con the performance of the LOBs by (i) changing the capacity limiting conditionusing an LOB cell with a xed amount of electrolyte or (ii) changing the amount ofelectrolyte in the LOB cell by controlling the thickness of the carbon electrode andevaluating the xed capacity limiting condition. As a result, we revealed the largeimpact of E/C on the performance of LOBs, including cycle life and round-tripenergy efficiency. We also extended our research interest to the effect of cellconguration on the performance of the LOBs. In particular, the impact of a gas-diffusion layer on the “sudden death” phenomenon during the charging processwas experimentally demonstrated. The results obtained in the present studyrevealed that LOB studies should be performed under appropriate technologicalparameters to accurately interpret the complicated phenomena in LOBs with cell-level high energy density. We believe the knowledge obtained in the present study352 | Faraday Discuss., 2024, 248, 341–354 This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3fd00082fPaper Faraday DiscussionsOpen Access Article. Published on 14 August 2023. Downloaded on 6/19/2024 3:58:56 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinecontributes toward accelerating materials development for realizing cell-levelhigh energy density LOBs with long cycle life.Conflicts of interestThere are no conicts to declare.Author contributionsS. M. and K. U. conceived the project. S. M., E. Y., S. K. and S. Y. designed theexperiments. E. Y. and S. K. performed the experiments. S. M., E. Y., S. K. and S. Y.analysed the results. S. M. wrote the paper with the help of all the authors. S. 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See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f Evaluation of performance metrics for high energy density rechargeable lithiumtnqh_x2013oxygen batteriesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fd00082f