PNNL

Abstract

Li metal anode has attracted wide attentions for high-energy cells, but there have been few reports of long term cycling and the degradation mechanism of realistic high-energy Li metal batteries (LMBs). A key issue inherent to high-energy LMBs is cell swelling due to continuous side reactions between Li and electrolyte. Herein, a prototypical 300 Wh kg-1 (1.0 Ah) pouch cell is developed by combining Li metal anode and LiNi0.6Mn0.2Co0.2O2 cathode. In a conventional carbonate electrolyte, the cell can only last 12 cycles because of the depletion of Li and electrolyte accompanied by a serious cell swelling of more than 110% thickening. Through the integration of a new compatible electrolyte and a uniform external pressure, over 200 stable cycles with 86% capacity retention and 83% energy retention have been demonstrated. Using this 300 Wh kg-1 pouch cell as a platform, two continuous but different Li anode structural evolution processes are revealed: (1) in the initial 50 cycles, flat Li foil converts into large Li particles entangled within the solid electrolyte interphase (SEI) layer, leading to a fast anode volume expansion and cell thickening. (2) As cycling continued, the external pressure maintains good contact between Li particles to ensure a percolation pathway for both ions and electrons thus the electrochemical reactions continue to occur through the entire anode. The individual solid Li particles evolve into a porous structure buffering the further anode volume expansion. Accordingly, Li anode thickening is significantly reduced with only 19% cell swelling in subsequent 150 cycles.