Mitigating Storage-Induced Degradation of LiNi0.8Co0.1Mn0.1O2 for Lithium-Ion Batteries Via Surface Tuning with Phosphate

Abstract

Driven by strong demands for long-range electric vehicles, extensive efforts have been devoted to design and develop advanced cathode materials with high capacity and high operating voltages. Given the commercialization possibility on an industrial scale, Ni-rich layered transition metal oxides, such as LiNi0.8Co0.1Mn0.1O2 (NCM811), with a high specific capacity, may be a reasonable choice. However, due to its inherent structural/chemical/electrochemical instability, NCM811 with high Ni content suffers from significant performance degradation upon storage even in ambient atmospheres as well as during charge–discharge cycling. Herein, we demonstrate a simple but effective surface-tuning approach to mitigate storage-induced degradation of NCM811, which is based on the conversion of undesirable Li residues to a protective Li3PO4 nanolayer via phosphate treatment. The accelerated storage stability test (ASST) shows that phosphate-modified NCM811 exhibits remarkably improved electrochemical performance (capacity, cycle life, and rate capability) over the pristine one after being stored under harsh environmental conditions. Based on the ASST studies combined with material characterizations (SEM, TEM, and XPS), the enhanced cycling and storage stability of phosphate-modified NCM811 is explained as follows. First, phosphate treatment eliminates Li residues that can easily be transformed to undesirable impurity phases such as Li2CO3 and LiOH. Second, a thin Li3PO4 nanolayer produced by the chemical reaction between residual Li and (NH4)2HPO4 effectively mitigates the parasitic electrolyte decomposition by preventing the direct contact between NCM811 and electrolyte. Third, the formation of Li3PO4 contributes to the increased ordering of the layered structure (i.e., reduced cation mixing). Finally, Li3PO4 serves as an effective protecting layer for H2O and CO2 infiltration during storage, thereby suppressing the deterioration of particle integrity.