Revealing ionically isolated Li loss in practical rechargeable Li metal pouch cells.

Abstract

The degradation of rechargeable lithium (Li) metal batteries is primarily attributed to active Li loss, encompassing isolated Li, also known as "dead Li", and solid electrolyte interphase (SEI-Li). Comprehending the formation of dead Li is pivotal for devising strategies to mitigate Li loss. Herein, we reveal the existence of an alternative form of dead Li, termed ionically isolated Li (I-iLi), which diverges from the traditionally recognized electronically isolated Li (E-iLi). This phenomenon is elucidated through a quantitative analysis of electrolyte-dependent capacity recovery at a specific state of health (SOH) in high-energy pouch cells, which originates from disconnections within the Li-ion percolation network induced by electrolyte de-wetting. Furthermore, we propose the electrolyte infilling ratio (EFR), contingent upon the porosity of Li deposits and electrolyte retention, as a criterion to evaluate electrode wettability. To uphold a high EFR environment, we introduce stress modulation to compact the anode structure and mitigate electrolyte degradation, significantly reducing the I-iLi content from 21% at 0.1 MPa to 1 % at 1 MPa. Harnessing these insights, a prototype anode-free LiNi_0.95Mn_0.03Co_0.02O₂||Cu pouch cell (1.4 Ah) achieves an extraordinary cell-level energy density of 551 Wh kg^-1 and maintains 70% capacity after 100 cycles at 0.2 C.