Understanding the Failure Mechanisms of LiMn2O4/Graphite Pouch Cells Using Isothermal Microcalorimetry

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LiMn2O4 (LMO) cells are known to suffer from the electrochemical oxidation and dissolution of manganese from the positive electrode into the electrolyte and subsequent transport to the negative electrode, where the metals are deposited on the solid electrolyte interphase (SEI)1-3. Pouch cells of polycrystalline and single crystal spinel LMO / artificial graphite (AG) were tested under varying conditions of temperature and upper cutoff voltage 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 reducing 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.Figure 1: Correlating variables critical to the degradation mechanism of LMO/AG cells for polycrystalline (PC) and single crystal (SC) LMO cells. (a) Arrhenius plot of Mn deposition on the graphite electrode with exponential dependence on formation temperature. (b) Formation gas from electrolyte reduction vs Mn deposition. (c) Parasitic heat flow from lithiated graphite reacting with electrolyte in pouch bags measured by isothermal microcalorimetry as a function of Mn deposition. (d) Cell capacity loss and heat energy dissipation during voltage hold experiments as a function of Mn deposition.