Comparison of Failure Mechanisms in Lithium Manganese Oxide and Lithium Nickel Manganese Oxide Spinel Cathodes

Abstract

LiNi0.5Mn1.5O4 (LNMO), with an operating voltage of 4.9 V vs Li/Li+ and a theoretical capacity of 147 mAhg-1 and LiMn2O4 (LMO), with an operating voltage of 4.1 V vs Li/Li+ and a theoretical capacity of 133 mAhg-1 are both highly attractive cathode material for secondary lithium ion batteries (LIB) owing to their low material cost, and excellent rate capability due to their spinel structure. Over the last two decades, much research effort has been focused on gaining a fundamental understanding of the failure mechanisms of these two electrodes. Dissolution and deposition of manganese was long thought to be the principle failure mechanism of LMO electrodes while for LNMO, because of its high operating voltage, electrolyte decomposition and concurrent degradative reactions at the electrode/electrolyte interfaces were thought to be the main culprit. In this study we have systematically investigated the failure mechanisms of LMO & LNMO electrodes. Full cells were built using graphite as the negative electrode and 1.2 M LiPF6 in EC: EMC, 3:7 as the electrolyte and were cycled at room temperature and at elevated temperature. Several quantitative and qualitative analysis techniques were conducted to investigate the correlation between capacity loss and Mn ion deposition, acidic species generation, solid electrolyte interphase (SEI) degradation, impedance growth as these cells cycled at different temperatures.