Understanding the Failure Mechanisms of LiNixCoyMnzO2 Cathodes through Surface Reactions with the Electrolyte

Abstract

Layered LiNixCoyMnzO2 (NCM) cathodes are one of the most widely used cathode materials for Lithium-ion batteries (LIB) with a high theoretical capacity and relatively high voltage window. Increasing the Nickel content and the cycling voltage can increase the capacity and the energy density at a lower cost, but these high Nickel cathodes cycled to a higher cut off voltage have poor cycling stability. The cause of this rapid capacity decrease has been attributed to continuous growth of the anode solid electrolyte interphase (SEI) due to transition metal deposition on the anode surface and impedance growth on the cathode surface. Recent studies have also attributed acid mediated electrolyte degradation reactions that degrade the SEI with extended cycling for this rapid capacity fade.In this study merits of these 3 theories are being investigated through various quantitative analysis techniques including, ICP-MS, solution NMR, XPS and EIS. Full cells were constructed using graphite as the negative electrode and NCM cathodes with different Nickel contents. 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 compare the correlation between nickel content, voltage and capacity fade.