Quantifying the Thermochemistry of Kinetic Transitions in a Li/Llzo/Layered Metal Oxide Cathode Solid-State Battery with Varying Cathode States of Charge

Abstract

The safety of solid-state battery is multifaceted and requires analysis of the thermal runaway onset temperature, total heat released, maximum heat release rate, the amount and toxicity of any gases released, and other factors1. The onset temperature of strong self-heating is particularly important, and hence quantitative analysis of exothermic reactions as a function of temperature is required. We use differential scanning calorimetry to measure heat flow for solid-state cells with a Li metal anode, an LLZO separator, and both LiCoO2 and NMC cathode sheets (which also contain a conductive additive and a binder), and quantitatively analyze the data to develop a thermochemical reaction map consisting of all major reactions that take place upon heating. This quantitative thermochemistry map can serve as a basis to assess cell safety, and in particular the rate and extent of temperature rise. We find that primary reaction that determines the total heat released is lithium reaction with oxygen from delithiated cathode decomposition (i.e., 4Li+O2→2Li2O), but other components in the cell such as conductive carbon, PVDF binder and current collectors also have significant participation in the reaction pathway. As previous studies for graphite/liquid electrolyte/metal oxide cathode cells have shown, reducing the state of charge also reduces the amount of heat released upon heating2, with implications for safe storage and operation of future cells.