Stability of LiF Deposited by ALD on High-Voltage Spinel/Polyimide Composite Electrodes

Abstract

Li–Ni–Mn-O spinel cathode materials operating at ∼5 V vs Li⁺/Li appear to be very interesting alternatives to Co-containing layered materials in terms of rate capability, energy and power densities, and sustainability of material resources. Nevertheless, their high operating voltage, which has been an asset to date, does not allow them to be used with conventional carbonate-based electrolytes. The latter undergoes spontaneous oxidation when in contact with the charged electrode, resulting in a reduction of the cathode material, an imbalance in the Li-ion system, and a subsequent rapid loss of capacity. This incompatibility could be overcome by creating a stable, electronically insulating solid interphase at the surface of the composite electrode. Here, we report the direct deposition of lithium fluoride (LiF) on LNMO electrodes by atomic layer deposition (ALD). LiF prepared with a specific combination of precursors (lithium bis(trimethylsilyl)amide and titanium tetrafluoride) has a total impurity content of less than 2% in the bulk. In addition, to enable direct coating by ALD on the positive electrode, a commonly used binder (polyvinylidene fluoride) was replaced with polyimide (PI), a more thermally stable and nonfluorinated polymer. Using X-ray photoelectron spectroscopy (XPS) and electrochemical analysis, we demonstrate the excellent thermal stability of this LNMO/PI electrode up to 300 °C as well as its electrochemical and chemical stability in a standard carbonate electrolyte. Electrochemical data show that LiF extends the cycle life of the LNMO/PI half-cell at a high C-rate (1C). The LiF layer has been proven to be stable on the pristine electrode upon prolonged exposure to the electrolyte. However, when charged at a low C-rate, the layer exhibits a tendency to disappear. The reasons for this behavior are not yet clear but could be linked to the degradation reactions in the electrolyte or to the local concentration changes.