Lithium-ion battery fast charging: understanding the effects of transport properties and thermal heterogeneity on lithium plating

Abstract

Fast charging of Li-ion batteries is associated with a risk of lithium plating, a process that can lead to rapid capacity fade and safety concerns. The work presented in this thesis aims to improve the understanding of the impact of thermal conditions and cell transport properties on the lithium plating behaviour. The study focuses on two areas: the effects of the temperature and cell type on the performance of charging protocols, and the impacts of through-plane thermal gradients on the severity and distribution of plating. Following a literature review, the modelling and experimental methods are discussed. Through modelling study, it is found that the relative effectiveness of different fast charging protocols at minimising plating depends on the timescales of both solid and electrolyte phase diffusion. When these timescales are low, in power-optimised cells or at high temperatures, protocols that feature high current boost stages may reduce the amount of plating. In the latter part of the work, thermal gradients are found to increase the rate of degradation during fast charging through amplifying non-uniformity in lithium deposition. The deposits are studied through microscopy and quanti ed using titration gas chromatography. The plating morphology is found to affect the associated SEI layer growth and the rate of capacity fade. In colder cells, or in the colder layers of cells cycled under a thermal gradient, more dendritic morphologies and increased SEI growth are observed. In warmer cells, or warmer layers, the deposition is denser and more uniform, with little or no correlation between the amount of deposition and the amount of the SEI. Finally, cathode kinetics are found to influence the plating distribution. These findings highlight the key role of transport properties, affected by the temperature and thermal heterogeneity, in determining the plating behaviour as well as the severity of its consequences.