Predicting High-Temperature Decomposition of Lithiated Graphite: Part II. Passivation Layer Evolution and the Role of Surface Area

Abstract

The surface area dependence of the decomposition reaction between lithiated graphites and electrolytes for temperatures above 100°C up to ∼200°C is explored through comparison of model predictions to published calorimetry data. The initial rate of the reaction is found to scale super-linearly with the particle surface area. Initial reaction rates are suggested to scale with edge area, which has also been measured to scale super-linearly with particle area. As in previous modeling studies, this work assumes that electron tunneling through the solid electrolyte interphase (SEI) limits the rate of the reaction between lithium and electrolyte. Comparison of model predictions to calorimetry data indicates that the development of the tunneling barrier is not linear with BET surface area; rather, the tunneling barrier correlates best with the square root of specific surface area. This result suggests that tunneling though the SEI may be controlled by defects with linear characteristics. The effect of activation energy on the tunneling-limited reaction is also investigated. The modified area dependence results in a model that predicts with reasonable accuracy the range of observed heat-release rates in the important temperature range from 100°C to 200°C where transition to thermal runaway typically occurs at the cell level.