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[Arghya Dutta](https://orcid.org/0000-0002-3769-7820), Emiko Mizuki, Yuka Tomori, [Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404)

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[Optimizing Discharge Rate for Li Metal Stability in Rechargeable Li|NMC Batteries under Lean Electrolyte Condition](https://mdr.nims.go.jp/datasets/1df1b994-5580-48fc-a4dc-002e3600d1e6)

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Optimizing Discharge Rate for Li Metal Stability in Rechargeable Li|NMC Batteries under Lean Electrolyte ConditionOptimizing Discharge Rate for Li Metal Stability in RechargeableLi|NMC Batteries under Lean Electrolyte ConditionArghya Dutta,* Emiko Mizuki, Yuka Tomori, and Shoichi Matsuda*Cite This: ACS Appl. Energy Mater. 2024, 7, 3824−3830 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Recent studies have highlighted the impressive performance oflithium metal batteries (LMBs), showcasing cell-level energy densities surpassing350 Wh kg−1. However, the intricate mechanisms leading to cell degradation inthese batteries remain elusive, impeding their widespread utilization as energystorage devices. Specifically, the influence of the discharge rate on thedeterioration of lithium metal electrodes remains poorly understood. In thisstudy, pouch-type Li|NMC811 cells were fabricated employing a lean electrolyte,and a comprehensive exploration was conducted into the effects of the dischargerate on the battery performance. Intriguingly, our findings illustrate a positivecorrelation between the increase in discharge rate within the range of 0.4−1.6 mAcm−2 and an improvement in the cycle life of LMBs. In-depth analyses indicate that increasing the discharge current density to 1.6mA cm−2 effectively suppresses irreversible volume expansion in lithium metal electrodes. Consequently, this suppression in volumeexpansion is identified as a significant factor contributing to the enhanced cycle life at increased discharge rates. Conversely, whenthe discharge rate surpasses 1.6 mA cm−2, a detrimental impact on the cycle life is observed due to kinetic limitations experienced bythe NMC electrode. These findings elucidate the operational principles governing LMBs, offering insights into achieving both high-power density and extended cycle life.KEYWORDS: lithium metal battery, high-capacity battery, failure analysis, discharge rate, battery diagnosis■ INTRODUCTIONLithium metal batteries (LMBs) are a promising next-generation energy storage technology due to their high energydensity.1 However, their practical implementation is hinderedby critical challenges, such as inefficient lithium (Li) plating/stripping, low Coulombic efficiency (CE), short cycle life, anddendrite formation.2,3 Recent research efforts focusing onelectrolyte optimization, particularly highly concentratedelectrolytes, have shown significant progress in promotingstable, nondendritic Li deposition/dissolution, leading toimproved CE, cycle life, and safety.4−13 As a result, superiorperformance of LMBs with cell-level energy density higherthan 350 Wh kg−1 with stable operation over 200 cycles couldbe achieved.14 Despite these achievements, the intricatedegradation mechanism of Li metal electrodes, distinct fromthat of conventional graphite electrodes, remains obscure,limiting their widespread practical application. Particularly,understanding the impact of charge/discharge rates on thedegradation of Li metal is vital given the varied rates requiredin many practical applications. In this regard, several studieshave extensively probed the influence of charging rates onLMB performance, revealing an inverse correlation withstability and cycle life.15,16 High charging rates lead to issuessuch as uneven Li plating, dendritic morphologies, volumeexpansion, and cell short-circuits due to long-range diffusionlimitations and inhomogeneous current density distribution. Incontrast, the effect of the discharging rate on the performanceof LMBs remains unclear. Recent studies have observed thatLMBs exhibit enhanced cycle life due to the improved stabilityof the Li electrode when discharged at relatively higher ratesthan the charging rates.17,18 However, these studies employed amaximum discharge current density of 3 mA cm−2 (0.5−1 Cdepending on the positive electrode loading), leaving thepotential effects of higher rates unexplored. As a result, the trueimpact of a wide range of discharge current densities on thestability of the Li electrode and the overall performance ofLMBs remains elusive. Thus, to develop LMBs capable ofdelivering high-power discharge, a comprehensive under-standing of the effects of high discharge rates on the stabilityof high-energy LMBs is crucial. This necessitates a systematicinvestigation of the Li electrode cycled under a broad range ofdischarge current densities coupled with an analysis of theunderlying kinetic factors.Based on these research backgrounds, in this study, weinvestigated the impact of discharging rates ranging from 0.4Received: January 23, 2024Revised: March 20, 2024Accepted: April 8, 2024Published: April 19, 2024Articlewww.acsaem.org© 2024 The Authors. Published byAmerican Chemical Society3824https://doi.org/10.1021/acsaem.4c00180ACS Appl. Energy Mater. 2024, 7, 3824−3830This article is licensed under CC-BY-NC-ND 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on June 19, 2024 at 02:55:11 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arghya+Dutta"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Emiko+Mizuki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuka+Tomori"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shoichi+Matsuda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsaem.4c00180&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=&ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/9?ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/9?ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/9?ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/9?ref=pdfwww.acsaem.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00180?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsaem.org?ref=pdfhttps://www.acsaem.org?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/mA cm−2 up to a very high value of 8.0 mA cm−2 on thedegradation of the Li metal electrode and the kineticlimitations in pouch-type Li|NMC811 cells with a leanelectrolyte. To ensure a comparable assessment, we uniformlymaintained the charge current density and the cycled capacityat 0.4 mA cm−2 and 4 mAh cm−2, respectively, while thedischarge current density was varied across all cells. Notably,our findings indicate a favorable impact of increased dischargerates within the 0.4−1.6 mA cm−2 range on improving thecycle life of LMBs. Comprehensive analytical assessmentsindicate that the effective suppression of irreversible volumeexpansion in Li metal electrodes under high discharge rates canbe attributed to one of the major reasons behind thisnoteworthy improvement in cycle life. In contrast, dischargerates exceeding 1.6 mA cm−2 exhibit a detrimental effect oncycle life, attributed to kinetic limitations observed at theNMC electrode. These revelations significantly contribute tocomprehending the interplay between discharge currentdensity and the behavior of pouch-type Li|NMC811 cellsfeaturing lean electrolytes, bearing profound implications forthe practical advancement of high-energy-density and high-power LMBs.■ EXPERIMENTAL METHODSElectrolyte. A solution of lithium bis(fluorosulfonyl)imide(LiFSI) in 1,2-dimethoxyethane (DME) was prepared with a Li+concentration of 4 M. Both LiFSI and DME were obtained fromKishida Chemical Co., Ltd., with purity >99.0 and >99.5%,respectively.Positive Electrode. A mixture comprising lithium nickelmanganese cobalt oxide (LiNi0.8Mn0.1Co0.1O2 or NMC811) activematerial (94 wt %), acetylene black (Denka Black HS100; obtainedfrom DENKA Co.; 3 wt %), and polyvinylidene fluoride (PVDF;sourced from KUREHA Co.; 3 wt %) binder dissolved in N-methyl-1,2-pyrrolidone (NMP; super-dehydrated; obtained from FUJIFILMWako Pure Chemical Co.) was spread onto an aluminum (Al) currentcollector (with a thickness of 20 μm). The NMP solvent wasevaporated by heating at 230 °C in a nitrogen atmosphere for 30 min,resulting in the production of electrode sheets with a loading amountof the active materials of approximately 30 mg cm−2.Negative Electrode. The negative electrode employed in thestudy consisted of a 50 μm thick Li on a 10 μm thick copper (Cu)current collector (Honjo Metal Co., Ltd.).Cell Fabrication. A laminated pouch cell was assembled with apositive electrode (40 mm × 30 mm), a separator (Teijin, 46 mm ×36 mm), and a negative electrode (42 mm × 32 mm) stacked inside.Three sides of the pouch were sealed, and 120 μL of electrolyte (10μL cm−2, approximately 2 g Ah−1 considering the NMC811 loading)was injected before the remaining side was sealed under vacuum. Allcells were assembled inside a dry room with the dew point below −50°C, and electrolyte injection was performed in a fume hood with thedew point below −85 °C.Charge/Discharge Protocol. The cells were charged anddischarged at 25 °C using a Hokuto Denko HJ1001SD8 batterycycler. All cells were cycled at a constant charge current density of 0.4mA cm−2 with a limited capacity of 4.0 mAh cm−2 in the voltage range2−4.2 V versus Li/Li+. Five different discharge current densities wereapplied: 0.4, 0.8, 1.6, 4.0, and 8.0 mA cm−2. For the constant currentconstant voltage (CCCV) discharge protocols, the cells underwentdischarge at various constant current densities of up to 2.0 V versusLi/Li+. Subsequently, a constant voltage step was implemented at 2.0V versus Li/Li+ until the current value fell below 0.04 mA cm−2.Electrochemical Impedance Spectroscopy (EIS). EIS measure-ments were performed on the cells using a Biologic VMP3potentiostat/galvanostat in the frequency range of 100 kHz−10mHz with a potential amplitude of 10 mV. The Ohmic resistance ofthe cells was determined from the intersection of the Nyquist plotwith the real axis in the high-frequency limit. The interphasialimpedance corresponding to the negative electrode was determinedfrom the diameter of the semicircle in the high-to-midfrequency rangeof the Nyquist plots.X-ray CT Measurement. X-ray computed tomography (XCT)analysis of pouch cells was performed using an Xradia 520 Versainstrument (ZEISS, Germany) with a source voltage of 140 kV andpower of 10 W. Following a certain number of charge/dischargecycles, cells were mounted on the sample holder and rotated 360° for4501 scans, each with an exposure time of 10 s. The XCT images hada pixel resolution of 3.385 μm.■ RESULTS AND DISCUSSIONIn our present research, we fabricated LMBs featuring a high-nickel-content lithium nickel manganese cobalt oxide (Li-Ni0.8Mn0.1Co0.1O2 or NMC811) positive electrode, a thin Limetal negative electrode (50 μm), and a lean electrolytecondition (10 μL cm−2 or around 2 g Ah−1). The electrolyteused was 4 M lithium bis(fluorosulfonyl)imide (LiFSI) indimethoxy ethane (DME). A schematic representation of thepouch cell is shown in Figure 1a. We first examined theinfluence of the discharging rate on the LMB cycle perform-ance. All the fabricated LMB cells were charged at a fixedcurrent density (0.4 mA cm−2) and subsequently discharged atvarying current densities: 0.4, 0.8, 1.6, 4.0, and 8.0 mA cm−2.During these charge/discharge processes, we limited the cyclecapacity to 4 mAh cm−2 to avoid any undesired overchargingphenomenon. Notably, a fixed charge current density andlimited cycle capacity enable delineating the influence ofdischarge current density on the cell performance.The galvanostatic charge/discharge curves for selected cyclesare depicted in Figure 1b−f, where the highest cycle numberFigure 1. (a) Schematic representation of the pouch cell used in thisstudy. (b−f) Galvanostatic charge/discharge curves of selected cyclesfor the cells charged at 0.4 mA cm−2 and discharged at 0.4, 0.8, 1.6,4.0, and 8.0 mA cm−2, respectively, with the fixed capacity set at 4mAh cm−2. (g) Discharge capacity (Q) vs cycle number plots for thesame cells as in (b−f). (h) Bar diagram showing the number of cyclesbefore the discharge capacity drops below 80% of the initially setcapacity of 4 mAh cm−2.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.4c00180ACS Appl. Energy Mater. 2024, 7, 3824−38303825https://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig1&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00180?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asdisplayed signifies the cycle at which the capacity (Q) of thecell drops below 3.2 mAh cm−2, which is 80% of its setcapacity. The results reveal an intriguing pattern of capacityretention, which becomes even more evident in Figure 1g,h.With the increase in the discharge current density from 0.4 to0.8 and 1.6 mA cm−2, cycling stability demonstrates anenhancement, reaching 168, 292, and 313 cycles, respectively.However, further increasing the discharge current density to4.0 and 8.0 mA cm−2 diminishes the cycling stability to 274and 118 cycles, respectively. The voltage−time curves depictedin Figure S1a−e indicate that cell failure cannot be ascribed toa short circuit in any of these cases. Instead, a distinct rise involtage polarization becomes evident from Figures S2 and S3as the cycles progress across all instances. This voltagepolarization, when prematurely reaching the cutoff potential,results in a lower capacity compared to the predeterminedvalue. Therefore, to elucidate the correlation betweendischarge rate and cycle life, it is crucial to meticulouslyexamine the evolution of discharge-voltage polarization and itsdependence on discharge rates throughout the cycles.Since Li electrode degradation is the most significant reasonfor cell failure in LMBs, to attain an intricate insight into thestructural changes of Li electrodes, we performed XCT analysison the pouch cells after 50 cycles. One remarkable advantageof utilizing XCT lies in its nondestructive characteristic,enabling analysis without causing damage to either the cell orelectrodes during measurement.17,19 Notably, XCT analysisfurnishes valuable information regarding the physical trans-formation of Li metal electrodes and facilitates the estimationof changes in their thickness. The cross-sectional XCT imagesof the pouch cells after 50 cycles are depicted in Figure 2a−e.Noteworthy observations emerge from Figure 2a, revealing asubstantial expansion of the Li electrode with notable porosityin the cycled portion when discharged at 0.4 mA cm−2. On theother hand, increasing the discharge current density to 0.8 and1.6 mA cm−2 effectively suppresses the thickness growth of theLi electrode, as evident in Figure 2b,c, respectively. However,the findings presented in Figure 2d,e demonstrate that afurther increase in discharge current density to 4.0 and 8.0 mAcm−2 does not provide additional advantageous effects insuppressing the expansion of the Li electrode. A quantitativecomparison of Li electrode swelling after 50 cycles, exhibited inFigure 2f, underscores the extent of these changes. Cycling at0.4 mA cm−2 leads to an expansion of the Li electrodethickness from an initial 50 μm to an astonishing 202 μm.Increasing the discharge current density to 0.8 and 1.6 mAcm−2 limits this thickness to 124 and 92 μm, respectively.Interestingly, when discharge current densities of 4.0 and 8.0mA cm−2 are implemented, the Li electrode thicknessmeasures to be 86 and 91 μm, respectively. These resultsclearly reveal that the increase in discharge rate is beneficial forsuppressing the undesired irreversible volume expansion of theLi metal electrodes.We also conducted electrochemical impedance spectro-scopic analysis of the cells to acquire quantitative informationabout the internal resistance. Details of the EIS analyses areprovided in Experimental Methods, and the Nyquist plots arepresented in Figure 3a−e. Figure 3f offers insights into thechanges in Ohmic resistance (ROhmic), representing the contactand electrolyte resistances of the cells. Notably, all cellsdischarged at different current densities exhibited a gradualincrease in ROhmic throughout the cycling. However, intrigu-ingly, the cells discharged at a rate of 0.4 mA cm−2 showed arapid increase in ROhmic beyond 70 cycles, reaching over 3 Ωafter 100 cycles. In contrast, the cells with a discharge rate of0.8 mA cm−2 or higher exhibited a limited increase in ROhmic. InFigure 3g, the changes in interphasial resistance (RInt) over thecycles are also depicted, revealing an increasing trend of RInt asFigure 2. (a−e) Cross-section XCT images of the pouch cells after 50 cycles at a charge current density of 0.4 mA cm−2 and a discharge currentdensity of 0.4, 0.8, 1.6, 4.0, and 8.0 mA cm−2, respectively. (f) Bar diagram showing the thickness of the Li metal electrode after 50 cycles.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.4c00180ACS Appl. Energy Mater. 2024, 7, 3824−38303826https://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00180/suppl_file/ae4c00180_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00180/suppl_file/ae4c00180_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig2&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00180?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe cycles progress for all cells. Even for RInt, trends similar tothose of ROhmic are observed. The cell discharged at a rate of0.4 mA cm−2 exhibited a significant increase in RInt as thecycles progressed, and the value of RInt exceeded 10 Ω on the100th cycle. In sharp contrast, in the cells with a discharge ratehigher than 0.4 mA cm−2, the value of RInt remained less than 2Ω after 100 cycles. These results clearly highlight thesubstantial impact of the discharge rate on the increase ininternal resistance.Based on the results of XCT and impedance analyses, let ussummarize the impact of the discharge rate on the performanceof the Li|NMC811 cell. XCT analysis reveals that increasingthe discharge rate in the range of 0.4−1.6 mA cm−2significantly suppresses the volume expansion of the lithiummetal electrode. Under high-rate discharge conditions, thestripping of Li particles primarily occurs at the tips and cornersof the deposit, where equipotential lines are concentrated. Thishigh-rate Li stripping reduces inhomogeneity and suppressesthe formation of dead Li. Additionally, this reducedinhomogeneity decreases the surface area of Li exposed tothe electrolyte, leading to a decrease in the level ofaccumulation of SEI compounds. Consequently, the volumeexpansion of the electrode is constrained under high-ratedischarge conditions. Figure 4 schematically shows a proposedmechanism of Li dissolution under high and low rates ofdischarge. According to this mechanism, the suppressedvolume expansion indicates less depletion of active Li andreduced accumulation of SEI products, collectively benefitingcell performance. Due to the relatively low electrolyte content(approximately 2 g Ah−1) in the cells, the entrapped electrolytewithin the expanded porous electrode results in an electrolyteshortage. This electrolyte deficiency, coupled with the growthof the insulating SEI layer, exacerbates impedance growthwithin the cell at low discharge rates. Consequently, increasingthe current density within the range of 0.4−1.6 mA cm−2 has abeneficial effect on enhancing the cycle life of the cell. Whileimpedance growth is also expected on the NMC811 electrodeside during cycling, the correlation between discharge rate-Figure 3. (a−e) Nyquist plots after selected cycles for the cellscharged at 0.4 mA cm−2 and discharged at 0.4, 0.8, 1.6, 4.0, and 8.0mA cm−2, respectively. (f) Changes in Ohmic resistance (ROhmic) and(g) interphasial resistance (RInt) of the cells over the cycles.Figure 4. (a) Schematic representation of the Li plating−stripping cycles at different rates of discharge. (b) Explanation of Li-stripping processes atfast and slow rates.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.4c00180ACS Appl. Energy Mater. 2024, 7, 3824−38303827https://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig4&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00180?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asdependent Li electrode expansion and increasing impedance isevident.Next, we turned our attention to investigating the reason forthe decreasing cycle life of Li|NMC811 cells at dischargecurrent densities above 1.6 mA cm−2. Apart from the factorsoriginating from the Li negative electrode, various detrimentalmechanisms from the NMC811 positive electrode side alsocontribute collectively to the termination of cell life.20−25 Onesignificant aspect of this electrode degradation is the parasiticreaction of the electrolyte on the surface of NMC811 underhighly delithiated conditions (high state of charge).23−25 Thisenvironment promotes chemical reactions between Ni4+ ionsand the electrolyte, leading to the dissolution of transitionmetals and the formation of an electrically insulatinginterphasial layer. Other degradation processes experiencedby NMC811 include increased Li/Ni mixing, structuraltransformations that give rise to the formation of spinel androck-salt structures on the surface, and particle cracking,among others.20−22 These degradation processes hinder Li+diffusion transport between the bulk lattice and electrolyte andcontribute to irreversible capacity loss. Since all the cells in ourexperiments were charged uniformly at the same currentdensity of 0.4 mA cm−2 to a consistent cutoff potential of 4.2 Vversus Li/Li+ for a fixed capacity of 4.0 mAh cm−2, it can beassumed that the extent of electrolyte decomposition and itssubsequent impact on the positive electrode during thecharging process is relatively similar across all cells.Based on the above considerations, we performed theCCCV discharge test to deconvolute the reversible andirreversible capacity losses originating from the NMC811positive electrode. During the constant voltage (CV) dischargeprocess at 2.0 V, the capacity loss due to kinetic limitationsduring variable rate constant current discharge can berecovered.26 The corresponding charge−discharge voltageprofiles for the first cycle are depicted in Figure 5a. At theend of galvanostatic discharge (CC step), all cells exhibited CEvalues significantly below 100%. Notably, as the dischargecurrent density increased, the first-cycle CE graduallydiminished, indicating greater capacity loss at higher rates.For example, as illustrated in Figure 5b, the cell discharged at0.4 mA cm−2 demonstrated a first-cycle CE of 76%. In contrast,a mere 60% CE was attained when the cell underwent adischarge at 8.0 mA cm−2. Remarkably, after the CV discharge,all cells recovered a certain amount of capacity, with cellsdischarged at higher discharge current densities showing moresubstantial recovery. As a result of the CV discharge, all cellsexhibited comparable first-cycle CE values within the range of81−84%. These results provide clear insights into thesignificant influence of kinetic limitations (Li+ diffusion inboth electrolyte and NMC811) on capacity loss during high-rate discharge.Observing a significant capacity loss in the first cycle due tokinetic constraints, we also performed a similar CCCVdischarge protocol for long cycles in the case of 8.0 mAcm−2 discharge current density to clarify the origin of capacityfading during cycling at high discharge rates. Figure 5cillustrates the voltage profile during the 50th cycle under theCCCV discharge protocol at a rate of 8.0 mA cm−2. Thefindings reveal that at the end of the CC step the dischargecapacity reached 78% of the set value of 4 mAh cm−2.However, when the CV step was applied, 100% of the capacitywas recovered. Additionally, in Figure 5d, we compared thecapacity retention trends between CC and CCCV discharges at8.0 mA cm−2. While the capacity under CC discharge exhibitedfluctuations above 50 cycles and dropped below 80% of the setcapacity within 118 cycles, the CCCV discharge retained 100%capacity for at least 200 cycles. Thus, it becomes evident thatin the case of CC discharge at 8.0 mA cm−2, the capacitydegradation primarily occurred due to kinetic limitationswithin the cell.■ CONCLUSIONSIn summary, we conducted an in-depth analysis to investigatethe effects of discharge current densities, reaching an ultrahighvalue of 8.0 mA cm−2, in Li|NMC811 pouch cells utilizing alean electrolyte. Employing a nondestructive ex situ XCTtechnique, we delved into the evolution of Li electrodes, whichexperience volume expansion during cycling, attributed to theaccumulation of both electrically isolated dead Li and SEIcompounds. Our XCT findings provide compelling evidencethat increasing the discharge current density up to a certainthreshold of 1.6 mA cm−2 effectively curtails the volumeexpansion of the Li electrode. However, any further rise indischarge current density does not yield additional benefits,nor do we observe any detrimental consequences. The capacityretention and cycling stability trends of the cells discharged atvarying rates reveal intriguing observations. Increasing thedischarge current density from 0.4 to 1.6 mA cm−2 enhancescycling stability, directly aligning with the suppressed volumeexpansion of the Li electrode at higher discharge rates.Although the gradual deterioration of the NMC811 electrodeand impedance growth at its interface may also affect capacityfading and require further analysis, the correlation between thecycle life and discharge rate-dependent evolution of the Lielectrode up to a current density of 1.6 mA cm−2 is evident.These observations unequivocally demonstrate the beneficialeffect of increasing the discharge current density up to 1.6 mAcm−2 for a higher stability of the Li electrode, resulting in animproved cycle life. Conversely, further increasing the rate to4.0 and 8.0 mA cm−2 exhibits a declining trend in capacityretention. While the negative electrode is not adversely affectedby a high discharge rate, our kinetic analysis has establishedFigure 5. (a) Charge/discharge voltage profiles of the first cycle forthe cells charged at a constant current density of 0.4 mA cm−2 anddischarged under a CCCV protocol with varied discharge currentdensities, followed by a constant voltage hold at 2.0 V. (b) Bardiagram showing the first cycle CE of the cells before and after theemployment of the CV step. (c) Charge/discharge voltage profile ofthe 50th cycle for the cell discharged at a current density of 8.0 mAcm−2 under the CCCV protocol. (d) Comparison of dischargecapacity (Q) retention between cells discharged under CC and CCCVprotocols at a current density of 8.0 mA cm−2.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.4c00180ACS Appl. Energy Mater. 2024, 7, 3824−38303828https://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00180?fig=fig5&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00180?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthat a high discharge current density triggers capacity loss dueto inefficient Li kinetics within the cell. Kinetic limitations athigh discharge rates increase the discharge overpotential.Consequently, this leads to premature reaching of cutoffconditions with higher discharge overpotential and lowercapacity, ultimately resulting in a shorter cycle life. Therefore,the combined effects of the stability of the Li electrode andkinetic constraints have identified an intermediate dischargerate, offering a superior cycle life. The findings presentedcontribute to a deeper understanding of Li metal electrodestabilization and kinetic constraints in a high-energy-densityLMB under ultrahigh discharge current rates. Theseunprecedented insights hold significant implications foradvancing high-power and high-energy LMBs for practicalapplications.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsaem.4c00180.Voltage−time curves of the cells discharged at differentcurrent rates, average discharge, and charge voltageversus cycle number plots (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsArghya Dutta − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience, Tsukuba 305-0044 Ibaraki, Japan; orcid.org/0000-0002-3769-7820; Email: DUTTA.Arghya@nims.go.jpShoichi Matsuda − Center for Green Research on Energy andEnvironmental Materials, NIMS-SoftBank AdvancedTechnologies Development Center, and Center for AdvancedBattery Collaboration, Center for Green Research on Energyand Environmental Materials, National Institute forMaterials Science, Tsukuba 305-0044 Ibaraki, Japan;orcid.org/0000-0002-0640-3404;Email: MATSUDA.Shoichi@nims.go.jpAuthorsEmiko Mizuki − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience, Tsukuba 305-0044 Ibaraki, JapanYuka Tomori − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience, Tsukuba 305-0044 Ibaraki, JapanComplete contact information is available at:https://pubs.acs.org/10.1021/acsaem.4c00180FundingJST COI-NEXT Grant Number JPMJPF2016.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe present work was partially supported by JST COI-NEXTGrant Number JPMJPF2016. 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