Recovery of Isolated Lithium through Discharged State Calendar Aging

Rechargeable Li-metal batteries (LMBs) have the potential to more than double the specific energy of the state-of-the-art rechargeable Li-ion batteries (LIBs), making Li metal a promising anode material for future generation rechargeable batteries. However, Li metal anodes suffer from greater cycle degradation compared to their Li-ion counterparts due to excess formation of capacity depleting byproducts.¹ Two major byproducts generated are: 1) the solid electrolyte interphase (SEI), a product of Li, salt and organic species reactions at the Li metal and the electrolyte interface to form a ‘passivation’ layer and 2) ‘dead’ or isolated Li, which traps capacity as Li metal detached from the electrochemical circuit. Recent studies have revealed that charged state calendar aging promotes further corrosion reactions between active Li and the surrounding electrolyte, leading to additional byproduct generation and further capacity loss.^2-3 Although these studies highlight the detrimental effects of calendar aging in the charged state, they have not reported on the effects of discharged state calendar aging on cell performance. In the discharged state, all electronically active Li has been stripped away leaving only isolated Li and SEI on the anode.⁴ In this environment absent of active Li corrosion, the effects of rest on isolated Li, SEI, and cycle performance have not been explicitly explored and remain unknown.<br/><br/>In this work, we report calendar aging in the discharged state improves capacity retention through isolated Li recovery, a surprising contrast to the well-known phenomenon of capacity degradation observed during aging in the charged state. Discharged state rested cells achieve a sustained greater than 1% average CE increase over both continuously cycled and charged state rested cells. Additionally, running cells on a hybrid continuous/discharged state rest cycling protocol in combination with titration gas chromatography (TGC) reveal isolated Li recovery as the main contributor to capacity gain. Furthermore, a novel operando optical setup in combination with electrochemical quartz crystal microbalance (EQCM) reveal that the process of SEI dissolution during discharged resting promotes excess isolated Li recovery. These insights into a new pathway for capacity recovery through discharged state calendar aging emphasize the significant effects cycling strategies have on LMB performance.<br/><br/>1. Han, B. et al. Conformal three-dimensional interphase of Li metal anode revealed by low-dose cryoelectron microscopy. Matter <b>4</b>, 3741- 3752 (2021).<br/><br/>2. Boyle, D.T. et al. Corrosion of lithium metal anodes during calendar ageing and its microscopic origins. Nat. Energy <b>6</b>, 487-494 (2021).<br/><br/>3. Merrill, L.C., Rosenberg, S.G., Jungjohann, K.L., Harrison, K.L. Uncovering the relationship between aging and cycling on lithium metal battery self-discharge. ACS Appl. Energy Mater. <b>4</b>, 7589-7598 (2021).<br/><br/>4. Xiao, J. et al. Understanding and applying coulombic efficiency in lithium metal batteries. Nat. Energy <b>5</b>, 561–568 (2020).