Difluorophosphoric Acid Generation and Crossover Reactions in LiNixCoyMnzO2 Cathodes

Abstract

Layered LiNixCoyMnzO2 (NCM) materials are one of the most widely utilized cathode materials for Lithium-ion batteries (LIB) with their high theoretical capacity and relatively high voltage window. Increasing the Nickel content and the cycling voltage can increase the capacity of these cells at a lower cost. But these high Nickel cathodes cycled to a higher upper cut off voltage have poor cycling stability. The cause of this rapid capacity decrease has been attributed to decomposition of the anode solid electrolyte interphase (SEI) from transition metal deposition on the anode surface and impedance growth on the cathode surface. Recent studies have also attributed this to acidic species generated from oxidative decomposition of the electrolyte and their crossover reactions degrading the SEI, particularly the Difluorophosphoric acid.In this study the role of transition metal dissolution and Difluorophosphoric acid generation has been investigated through various quantitative analysis techniques including, ICP-MS, XPS, quantitative solution NMR, and EIS. Full cells were constructed using graphite as the negative electrode and NCM cathodes with different Nickel contents as the positive electrode. These cells contain the same base electrolyte formulation (1.2M LiPF6 and EC: EMC, 3:7) and they were cycled at different temperatures and voltages to investigate the correlation between acid and transition metal concentration, cell voltage and capacity fade.