LiMn2O4/Graphite Cell Degradation Mechanisms Studying How Mn Deposition Accelerates Lithiated Graphite Reactivity with Electrolyte

Abstract

In this work, pouch cells of spinel LiMn2O4 (LMO) / artificial graphite (AG) were tested under varying conditions to investigate degradation mechanisms. Mn dissolution from the positive electrode and deposition onto the graphite negative electrodes immediately after formation was found to be significantly suppressed by operating cells at −10 °C during the formation cycle and limiting the upper cutoff voltage. A formation cycle at an elevated temperature of 70 °C greatly increases the Mn deposition and gas generation from electrolyte reduction after just a single cycle and had long term effects at increasing lifetime gassing and Mn deposition. The cold formation advantage disappeared once all cells were cycled at 40 °C, with similarly terrible cycle life (60–200 cycles) regardless of formation conditions. Studying fully lithiated graphite (LiC6) extracted from LMO cells in an isothermal microcalorimeter (IMC), we found that the parasitic heat flow associated with electrolyte reduction and graphite delithiation increased for anodes with significant Mn deposition. Therefore, Mn deposition on the negative electrode causes cell failure by compromising the anode passivation, increasing lithiated graphite-electrolyte reactivity and thereby accelerating the lithium inventory loss due to electrolyte reduction on the Mn sites on the negative electrode.