Lithium Thin Films Unveil the Influence of Electrode/Electrolyte Degradation Processes on Cycle Life

The use of thin Li metal negative electrodes, below 20 μm, is a prerequisite to reach significantly higher energy densities compared to Li-ion batteries. However, the large-scale production of such thin electrodes is highly challenging. Additionally, chemical/electrochemical reactions at the Li metal/electrolyte interface can cause uneven plating/stripping of Li ions, leading to quick cell failure. Here, we demonstrate the synthesis by thermal evaporation of highly dense and smooth Li thin films featuring closely packed grains, deposited on copper foil. The electrochemical responses of various Li electrodes, ranging from anode-free (Cu foil) to 135-μm-thick Li foil with a focus on thin films between 2 and 30 μm, were investigated in a liquid electrolyte optimized for a Li-free cell paired with a high-loaded NMC811 positive electrode. We illustrate three different mechanisms of cell failure regarding the Li electrode thickness: (i) rapid consumption of the entire Li metal for electrodes thinner than 20 μm, (ii) too high interfacial polarization arising from solid electrolyte interphase (SEI) and dead Li for Li thicker than 50 μm, and (iii) a combination of both phenomena for intermediate thicknesses. Further rationalized by detailed post-mortem analyses, these observations for thin and intermediate thickness arise from discrepancies in terms of the rate of Li moss formation down to the Cu foil and its further propagation. These insights show the necessity to use thinner Li (<20 μm) and to find novel protective layers.