High-Performance Cells Containing Lithium Metal Anodes, LiNi_0.6Co_0.2Mn_0.2O₂ (NCM 622) Cathodes, and Fluoroethylene...

We report on the highly stable lithium metal|LiNi_0.6Co_0.2Mn_0.2O₂ (NCM 622) cells with practical electrodes' loading of 3.3 mA h g^-1, which can undergo many hundreds of stable cycles, demonstrating high rate capability. A key issue was the use of fluoroethylene carbonate (FEC)-based electrolyte solutions (1 M LiPF₆ in FEC/dimethyl carbonate). Li|NCM 622 cells can be cycled at 1.5 mA cm^-2 for more than 600 cycles, whereas symmetric Li|Li cells demonstrate stable performance for more than 1000 cycles even at higher areal capacity and current density. We attribute the excellent performance of both Li|NCM and Li|Li cells to the formation of a stable and efficient solid electrolyte interphase (SEI) on the surface of the Li metal electrodes cycled in FEC-based electrolyte solutions. The composition of the SEI on the Li and the NCM electrodes is analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A drastic capacity fading of Li|NCM cells is observed, followed by spontaneous capacity recovery during prolonged cycling. This phenomenon depends on the current density and the amount of the electrolyte solution and relates to kinetic limitations because of SEI formation on the Li anodes in the FEC-based electrolyte solution.