Capacity Recovery via Rest Induced Revival of Isolated Lithium in Lithium Metal Batteries

Li metal is a promising high energy anode material for future generation rechargeable batteries. However, Li metal anodes face the obstacle of rapid capacity decay, which must be overcome before Li metal battery (LMB) technology can be commercially embraced. The heterogeneity of Li metal morphology¹ and solid electrolyte interphase (SEI)² are two factors which contribute to capacity decay by promoting uneven stripping, leading to the eventual disconnection and isolation of Li metal. Electronically isolated Li cannot partake in electrochemical reactions and therefore contributes to lost capacity. A recent work has demonstrated the recoverability of isolated Li using an electric field to drive spatial progression towards the electrode³, opening a new path to fight capacity decay in LMBs. Hence, a better mechanistic understanding of isolated Li revival through in-operando studies is essential for future work toward improved inactive capacity recovery. However, the phenomenon of isolated Li recovery has not been documented in prior in-operando optical studies most likely due to the large differences in conditions between optical setups and standard cell architecture.<br/><br/>Here we present a novel in-operando optical cell setup which maintains the pressurized environment and performance of standard coin cells, enabling the first-time imaging of Li metal isolation, reconnection, and recovery. Furthermore, this new optical cell platform is used to demonstrate the degradation of residual SEI during rest and how this evolution improves the recoverability of the isolated Li in subsequent cycles. These observations of enhanced lithium recovery after discharged state resting are corroborated by improved cycling performance using a hybrid continuous/rest cycling protocol. Coulombic efficiencies (CE) greater than 100% are achieved in the first rested cycle following continuous cycling. Additionally, titration gas chromatography results also show decreased quantities of isolated Li on current collectors after the rest cycle compared to those which ran continuous cycles, verifying the recovery of isolated Li from prior cycles. By connecting the observations from this novel optical cell with the unique cycling protocol, we reveal not only new insights on the mechanics of isolated Li reactivation, but also inform how residual SEI degradation can enhance inactive capacity recovery.<br/><br/>1. K. N. Wood, E. Kazyak, A. F. Chadwick, K. H. Chen, J. G. Zhang, K. Thornton, N. P. Dasgupta, <i>ACS Cent. Sci.</i> <b>2</b>, 790–801 (2016).<br/>2. Y. Li, W. Huang, Y. Li, A. Pei, D. T. Boyle, Y. Cui, <i>Joule</i>. <b>2</b>, 2167–2177 (2018).<br/>3. F. Liu, R. Xu, Y. Wu, D. T. Boyle, A. Yang, J. Xu, Y. Zhu, Y. Ye, Z. Yu, Z. Zhang, X. Xiao, W. Huang, H. Wang, H. Chen, Y. Cui, <i>Nature</i>. <b>600</b>, 659–663 (2021).