Coexistence of conversion and intercalation mechanisms in lithium ion batteries: Consequences for microstructure and interaction between the active material and electrolyte

Abstract

Abstract Conversion-type lithium ion batteries experience severe and partly irreversible phase transitions during operation. Such phase transitions reduce the crystallite size and therefore enhance the exchange of the Li ions. Concurrently, the irreversible nature of the phase transitions may deteriorate the cycling stability and the long-term capacity of conversion-type batteries. In this contribution, the observed correlations between the crystal structures of compounds which are employed as anodes in conversion-type Li ion cells, the capacity and the long-term stability of these cells are discussed. The central characteristics affecting the performance of conversion-type Li ion cells seem to be the similarity of crystal structures of intermediately forming phases during the charge/discharge process, which facilitates strong local preferred orientation of nanocrystallites of neighboring phases and for the formation of local strain fields at partially coherent phase boundaries. The effect of the above-mentioned phenomena on capacity and cycle stability is argued from the point of view of a possibly impeded ion exchange. Equilibrium open circuit potentials are calculated using the CALPHAD method. However, it is shown that in order to better reproduce the experimentally determined plateau voltages, thermodynamic descriptions of the non-equilibrium intermediate phases have to be included. In addition, the stabilization of the conversion reaction by the electrolyte is pointed out.