In-Situ TEM Electrochemical Processes in Conversion-Based Li-Ion Battery Electrodes

Intercalation-based cathode materials such as LiCoO2 (~140 mAh/g) and LiFePO4 (~160 mAh/g) are predominantly used in lithium-ion batteries because they provide exceptional cycling reversibility [1]. However, these materials have low capacity as they are limited to single electron charge transfer reaction, and thus are unlikely candidates for use in electric vehicles (EVs). Conversion compounds such as iron fluorides (FeF2) are promising because of their high capacities (~570 mAh/g), 2-4 times higher than intercalation-based cathodes, by utilizing more than one Li ion per transition metals [2]. However, these materials suffer from issues such as slow kinetics, large cycling hysteresis (i.e. voltage gap between charge/discharge at certain charge-of-state), and insufficient charge reversibility preventing them from further commercial viability. We perform in-situ transmission electron microscopy (TEM) study to understand some of the key questions pertaining to conversion reaction mechanisms such as phase nucleation and evolution, electron and ion transport, reaction kinetics and reversibility during electrochemical charge and discharge.