A Calculation Model to Assess Two Irreversible Capacities Evolved in Silicon Negative Electrodes

Abstract

A novel calculation model is devised to quantitatively assess two irreversible capacities evolved in Si negative electrodes: electrolyte decomposition and Li trapping. In this model, the capacity of the electrode reaction (Li-Si alloy formation, Qnalloy), which is the only implicit value on the galvanostatic charge/discharge voltage profiles, is calculated with the data obtained from GITT (galvanostatic intermittent titration technique) experiment. When the calculation model is applied to two Si electrodes of different particle sizes, the particle size is found to significantly affect the nature of the irreversible reactions. In the bulk-sized Si electrode, Li trapping is dominant over electrolyte decomposition. This feature must be due to an electrical contact loss that is caused by crack formation, which is more vulnerable to bulk-sized Si particles. The hump behavior in the Coulombic efficiency profiles is also explained by the Li trapping. In the nano-sized Si electrode, electrolyte decomposition is the major irreversible reaction because of its larger surface area. Because of a stronger endurance against mechanical stress, crack formation and subsequently Li trapping are less severe than that of the bulk-sized one.