Abstract
Rechargeable Li-metal batteries have the potential to more than double the specific energy of the state-of-the-art rechargeable Li-ion batteries, making Li-metal batteries a prime candidate for next-generation high-energy battery technology1,2,3. However, current Li-metal batteries suffer from fast cycle degradation compared with their Li-ion battery counterparts2,3, preventing their practical adoption. A main contributor to capacity degradation is the disconnection of Li from the electrochemical circuit, forming isolated Li4,5,6,7,8. Calendar ageing studies have shown that resting in the charged state promotes further reaction of active Li with the surrounding electrolyte9,10,11,12. Here we discover that calendar ageing in the discharged state improves capacity retention through isolated Li recovery, which is in contrast with the well-known phenomenon of capacity degradation observed during the charged state calendar ageing. Inactive capacity recovery is verified through observation of Coulombic efficiency greater than 100% on both Li||Cu half-cells and anode-free cells using a hybrid continuous–resting cycling protocol and with titration gas chromatography. An operando optical setup further confirms excess isolated Li reactivation as the predominant contributor to the increased capacity recovery. These insights into a previously unknown pathway for capacity recovery through discharged state resting emphasize the marked impact of cycling strategies on Li-metal battery performance.
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Data availability
All the data that support the findings of this study are available from the corresponding author upon request. Source data are provided with this paper.
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Acknowledgements
We acknowledge support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy under the Battery Materials Research (BMR) Program and Battery 500 Consortium. D.T.B. acknowledges support from the National Science Foundation Graduate Research Fellowship Program. S.T.O. acknowledges support from the Knight Hennessy scholarship for graduate studies at Stanford University. R.A.V. acknowledges support from the National Academy of Sciences Ford Foundation Fellowship, the National Science Foundation Graduate Research Fellowship Program (NSF GRFP, grant no. DGE-1656518) and the Enhancing Diversity in Graduate Education (EDGE) Doctoral Fellowship Program at Stanford University. We also acknowledge T. Sogade for coin-cell material preparation. Part of this work was performed at the Stanford Nanofabrication Facility (SNF).
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Contributions
W.Z., P.S. and Y.C. conceived the idea and created the cycling protocol. W.Z. designed the optical cell and performed the optical mapping analysis. X.X. performed TGC experiments. W.Z., D.L. and P.S. designed full cell experiments. W.Z. and P.S. analysed and interpreted the results. S.B.S. performed and analysed the XPS experiments. S.T.O., R.A.V., D.T.B., S.C.K., M.S.K., S.E.H., S.F.B. and Y.Y. assisted in result interpretation and feedback. D.L. assisted with figure creation. W.Z. and P.S. wrote the paper.
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Extended data figures and tables
Extended Data Fig. 1 Hybrid cycling protocol and corresponding CE.
CE of Li | |Cu half-cells running on the hybrid cycling protocol for a, Current profile of Li | |Cu half-cell running the hybrid cycle protocol. The first 10 cycles are continuous. 11th cycle starts with a 12-hour rest period in the discharged state followed by 11th cycle charge and discharge. Discharged state rest is applied during every subsequent cycle. b, 1 M LiPF6 with EC/DEC (ethylene carbonate/ diethyl carbonate) + 10% FEC (fluoroethylene carbonate) and c, 4 M LiFSI DME (dimethoxyethane). Cells cycled with 1 M LiPF6 EC/DEC + 10% FEC demonstrated an average ΔCE increase of greater than 5% between the 10th and 11th cycle. However, the 11th cycle CE does not exceed 100%. This may be due to the dendritic nature of Li morphology formed in carbonate electrolytes leading to faster corrosion, and increased native SEI formation on the i-Li. Nevertheless, the 11th cycle CE is greater than that of every prior continuous cycle, strongly suggesting capacity recovery is taking place. The cells cycled with 4 M LiFSI DME demonstrate greater than 100% CE for the 11th cycle with an average ΔCE increase of greater than 3% between the 10th and 11th cycle.
Extended Data Fig. 2 Various charging capacity hybrid cycle performance.
CE of Li | |Cu half-cells running with LHCE (LiFSI: 1.2 DME: 3 TTE) on the hybrid cycling protocol (10 continuous cycles followed by discharged rest for subsequent cycles) plated to capacities of a, 0.3mAh. b, 1mAh. c, 3mAh. The average CEs of the continuous cycles increase with increasing capacities, which can be attributed to lower surface area to volume ratio, and higher stack pressure. The post discharged rest ΔCE increase brings the 11th cycle CE to greater than 100% for all capacities cycled. d, the average ΔCE between the 10th and 11th cycle decreases with increase capacity due to more efficient cycling during the continuous cycles leading to a smaller fraction of active Li becoming isolated Li. The effect of lower average CE during continuous cycling leading to a higher ΔCE is most prominently demonstrated in cells cycled to a capacity of 0.3mAh. This is likely due to a higher percentage of plated capacity lost during the initial 10 cycles compared to cells cycled to 1mAh and 3mAh. The difference between the ΔCE of 1mAh and 3mAh are less significant. e, plot of Δ capacity (difference between 10th and 11th cycle discharged capacity) for all three capacity conditions after discharged state calendar ageing. Error bars in d and e represent s.d.
Extended Data Fig. 3 Various discharge current density hybrid cycle performance.
CE of Li | |Cu half-cells running with LHCE on the hybrid cycling protocol (10 continuous cycles at 1 mA cm−2 and 1mAh followed by discharged rest for subsequent cycles) with various 11th cycle discharge current densities of a, 0.25 mA cm−2. b, 1 mA cm−2. c, 2 mA cm−2. d. Average ΔCE increase between the 10th and 11th cycle does not vary considerably from discharge current densities of 0.25 mA cm−2 to 2 mA cm−2. This result indicates that quantity of capacity recovered after discharged calendar ageing has little dependence on the discharge current density. Error bars in d represent s.d.
Extended Data Fig. 4 Various rest time hybrid cycle performance.
CE of Li | |Cu half-cells running with LHCE on the hybrid cycling protocol (10 continuous cycles followed by discharged rest for subsequent cycles) with various 11th cycle rest times of a, 5 min. b, 1 h. c, 6 h. d, 12 h. e, 24 h. f, Average ΔCE for all discharged state rests times between 5 min and 24 h. The increase in ΔCE from 5 min to 12 h suggests increased time for r-SEI dissolution and more favorable environment for reconnection. The decrease in ΔCE seen at 24 h of rest compared to at 12 h of rest can be driven by the larger role Li metal corrosion plays at longer rest times. Corrosion of isolated Li and dissolution of residual SEI both occur concurrently during discharged rest. The corrosion of isolated Li can reduce the quantity of metallic isolated Li available for recovery as well as thicken the native SEI on isolated Li. These factors can reduce the efficacy of Li reconnection, decreasing the ΔCE recovered for longer rest periods. However, the data for the 24 h discharged rests still shows 11th cycle CE greater than 100%. This strongly suggests that isolated Li recovery, not isolated Li corrosion, remains the dominant factor driving cycle performance, even after longer discharged state resting periods. Error bars in f represent s.d.
Extended Data Fig. 5 Various rest cycle number hybrid cycle performance.
a, The relationship between the rest cycle number and ΔCE (the CE difference on the last continuous cycle and the first rested cycle) is investigated. Tests were conducted varying the first discharge rest cycle from the 3rd to 40th cycle while keeping the 12-hour rest period constant. The results show that capacity recovery increases with cycle number, however, the data follows a power law trend rather than a linear trend. This is most likely due to SEI and isolated Li buildup limiting mass transport in later cycles, causing isolated Li from early cycles to become increasingly inaccessible for electrical reconnection. However, once the top layers of isolated Li are recovered after rest, there is less material impeding the recovery of prior layers of isolated Li in subsequent cycles, allowing for additional capacity recovery. b, This phenomenon is observed in the three cells which ran 40 continuous cycles hybrid protocol revealing greater than 100% CE in the four cycles following the start of resting. c, Zoomed in view of CE for cycle 35–45. All cells tested were cycled with LHCE.
Extended Data Fig. 6 Optical cell assembly and cycle performance.
Images of the modified positive casing and current collector of the in-operando cell. a, shows the hole punched along with the glass window epoxied on. b, 200 nm of Cu is sputtered on with a 1.5 mm strip left un-sputtered across the window. c, a Cu mesh TEM grid is placed in the center of the window. d, assembled cell with the optical window facing up. Plan view though optical window of the Li | |Cu optical half-cell at the: e, pristine state prior to cycling, and f, with Li plated after first cycle plating at 1 mA cm−2 1mAh. g, Comparison of the CE between optical cells, standard coin cells, and pressure-free coin cells. All cells were cycled at a current density of 1 mA cm−2 and a capacity of 1mAh with LHCE. The optical cell and the standard coin cell with similar stack pressure have comparable CEs. The “no pressure” coin cells are assembled with the Li metal counter and the Cu current collector separated by a gap of 2 mm. The average CE of the “no pressure” cells is 15% lower than that cells with stack pressure, emphasizing the large effect pressure has on cycle performance. h, Comparison of the CE between optical cells run at 1 mA cm−2 and 0.67 mA cm−2. The Cu mesh has 2/3 the frontal surface area compared to bare Cu current collector, enabling local current densities of 1.5 mA cm−2. on the Cu mesh grid when cycling at 1 mA. Reducing the current density from 1 mA to 0.67 mA normalizes the local current density on the Cu grid to 1 mA cm−2. The similarities in CE of optical cells running at 1 mA and 0.67 mA demonstrates that variation between 1 mA cm−2 and 1.5 mA cm−2 does not greatly affect cycle performance. Error bars in g and h represent s.d.
Extended Data Fig. 7 Optical cell colored areal maps of isolated and recovered Li.
Colored areal maps of i-Li and recovered Li (r-Li) for each cycle comparing cells cycled with and without discharged rest after cycle 2 at a capacity of 1mAh for all cycles. Maroon, red, and orange correspond to i-Li formed from cycle 1, 2 and 3 respectively. Blue and cyan correspond to recovered Li from cycles 2 and 3 respectively. a-c Optical images (top) and threshold colormaps (bottom) of i-Li and r-Li in each cycle of Cell-1 running the continuous cycling protocol and current density of 1 mA cm−2 (a.i-a.iii), Cell-2 running the hybrid cycling protocol and current density of 1 mA cm−2 (b.i-b.iii), Cell-3 running the hybrid cycling protocol and current density of 0.67 mA cm−2 (ci-c.iii). Plots of the areas occupied by i-Li and r-Li for Cell-1 (a.iv), Cell-2 (b.iv), and Cell-3 (c.iv). CE for Cell-3 (c.iv). The i-Li area is normalized to cycle 1 i-Li. Scale bar 100μm. The r-Li to i-Li ratio (R/I) is calculated by dividing the area of r-Li by the area of i-Li for that specific cycle. 2nd cycle R/I is calculated by dividing the blue area by the red area. 3rd cycle R/I is calculated by dividing the cyan area by the orange area. Since the areal map data cannot incorporate the volume and movement in Li deposits, the R/I ratio is an approximation for the quantity of r-Li and i-Li rather than the exact amount. However, the stack pressure and Cu mesh help to mitigate thickness variability and Li movement. In the instances i-Li deposits are displaced, the previously i-Li location is marked as “recovered” at the pre-displacement location and as “isolated” at a new location. The R/I for this Li deposit would be approx. 1 and therefore would not be counted toward i-Li or r-Li. Cells were cycled with LHCE.
Extended Data Fig. 8 LFP | |Cu pouch cell and long cycle Li | |Cu half-cells cycle performance.
a, CE of LiFePO4 (LFP) anode-free pouch cells running on a hybrid cycle with rest between the 10th and 11th cycle. The cells have a loading of approx. 200mAh and an electrolyte to capacity ratio of 3 μl mAh−1. Cells were set to a pressure of 150 kPa and are charged and discharged at 1 C between 3 V and 4 V. No formation cycle is run so all the capacity lost and recovered during cycling is accounted for in the CE plot. The average 11th cycle post discharge rest CE is 101%, demonstrating that i-Li recovery though discharged state calendar ageing is viable in high capacity, lean electrolyte anode-free LMBs. b, charge and discharge capacity of the LFP anode-free pouch cell running on a hybrid cycle. A substantial increase in capacity of approx. 3.6mAh can be observed on the 11th cycle discharged which is maintained for the subsequent rested cycles. c, CE of Li | |Cu half-cells cycling with 1-hour discharged rest every cycle and continuous protocols. The beneficial effect of discharged rest is most apparent in the first 40 cycles where cells rested during discharged rest have an average CE 0.74% greater than continuously cycled cells. This CE gap between the two cycling conditions diminishes with increasing cycle numbers and converge around the 100th cycle. The saturation of the electrolyte with r-SEI species potentially decreases r-SEI dissolution and isolated Li recovery. This is likely a major driver of the decrease in CE gap. Cells cycled with LHCE. Error bars in c represent s.d.
Extended Data Fig. 9 SEI dissolution.
a. SEI dissolution was quantified using electrochemical quartz crystal microbalance (EQCM). SEI is formed on the surface of the EQCM and then allowed to rest at open circuit voltage. This creates a similar environment to resting a cell in the discharged state. The results show a substantial drop in the normalized SEI mass during rest. 4 M LiFSI in DME is represented in blue, LHCE in green and LP40 in orange. Detailed experimental method can be found at https://pubs.acs.org/doi/10.1021/jacs.3c03195. Since organic SEI components are generally more soluble (Extended Data Fig. 10), we believe that electrolyte formulations have a considerable effect on how much i-Li is recovered. SEIs derived from anion-rich Li solvation structure have more insoluble and inorganic SEIs, likely decreasing Li recovery during discharged state rest. Electrolyte formulations also affect Li plating morphology, and likely also affect the probability of Li reconnection. Dendritic Li morphologies can have a lower likelihood of i-Li reconnection because of increased favorability of elastic deformation when a pressure is acted upon it. On the other hand, non-dendritic morphologies likely have higher probability of recovery due to increased favorability of plastic deformation under pressure, enabling reconnection. Reprinted (adapted) with permission from J. Am. Chem. Soc. 2023, 145, 22, 12342–12350. Copyright 2023 American Chemical Society. b, SEI degradation test: SEI was formed on the current collector of five Li | |Cu half-cells with LHCE. A constant current of −0.1 mA was applied until a voltage of 10 mV above plating potential followed by a constant voltage hold of 10 mV until the current of −0.5uA (CC-CV). The cells were then rested for different time periods (x-axis) before running a second CC-CV cycle identical to the first. The SEI degradation over rest can be related to the capacity (y-axis) used during the second CC-CV cycle.
Extended Data Fig. 10 NMR and XPS analysis of discharge rested SEI and electrolyte.
a, XPS atomic ratios of the SEI on discharged Cu current collector of Li | |Cu cell with and without rest. The atomic ratio of F/C, O/C, N/C and S/C all increased after rest, suggesting the dissolution of SEI favors organic compounds. XPS spectra of Li | |Cu cells in the stripped state for F 1 s with no rest (b) and with rest (c) and O 1 s with no rest (d) and with rest (e). The increase in LiF to C-F signal ratio and the appearance of Li2O after rest supports the notion of r-SEI dissolution favoring organics compounds over inorganic compounds. Two cells and six sample locations were run for each cycle condition. Cells were run for 1 complete cycle at a current density of 1 mA cm−2 and a capacity of 1mAh with LHCE. f, NMR analysis of pristine LHCE electrolyte and aged electrolyte in Li metal. Additional peaks can be seen in the in the aged electrolyte due to soluble organic SEI components. g, NMR analysis of pristine LP40 electrolyte and aged electrolyte in Li metal. Additional peaks can be seen in the in the aged electrolyte due to soluble organic SEI components. h, NMR analysis of pristine 4 M LiFSI in DME electrolyte and aged electrolyte in Li metal. Additional peaks can be seen in the in the aged electrolyte due to soluble organic SEI components. The chemical shifts of the additional species suggest that they are derived from solvents (carbonate-based for LP40 and ether-based for the rest) used in the electrolyte. However, full NMR characterization of SEI soluble species is not possible for our experimental setup, due to the low concentration of the SEI species. Error bars in a represent s.d. Reprinted (adapted) with permission from J. Am. Chem. Soc. 2023, 145, 22, 12342–12350. Copyright 2023 American Chemical Society.
Supplementary information
Supplementary Video 1
Optical time lapse corresponding with Fig. 2d–h showing isolated Li being formed during cycle 1 and subsequently stripped away after cycle 2.
Supplementary Video 2
Magnified optical time lapse corresponding with Fig. 3a–c depicting optical cell 1 running on the continuous cycling protocol for three cycles.
Supplementary Video 3
Magnified optical time lapse corresponding with Fig. 3d–f depicting optical cell 2 running on the hybrid cycling protocol (two continuous cycles followed by 12-h discharged rest and a final third cycle).
Supplementary Video 4
Magnified view of a first location in Supplementary Video 3 showing the formation of isolated Li after the first cycle and subsequent recovery of Li after discharged rest.
Supplementary Video 5
Magnified view of a second location in Supplementary Video 3 showing the formation of isolated Li after the first cycle and subsequent recovery of Li after discharged rest.
Supplementary Video 6
Optical time lapse corresponding with Fig. 4d–h showing the residual SEI fading in contrast with the background strongly suggesting its dissolution in the electrolyte. The deposit of isolated Li is subsequently stripped away in the cycle following discharged state calendar ageing.
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Zhang, W., Sayavong, P., Xiao, X. et al. Recovery of isolated lithium through discharged state calendar ageing. Nature 626, 306–312 (2024). https://doi.org/10.1038/s41586-023-06992-8
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DOI: https://doi.org/10.1038/s41586-023-06992-8
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