Electrochemically Driven Phase Transitions in Insertion Electrodes for Lithium-Ion Batteries: Examples in Lithium Metal Phosphate Olivines

The thermodynamics and kinetics of phase transformations in electrochemical systems are reviewed. Phase transitions in LiMPO4 (M = Fe, Mn, Ni, Co) olivines are highlighted. The phase transformation phenomena in LiMPO4 are diverse and include thermodynamic effects of particle size and applied overpotential, the appearance of metastable phases, and the effects of defects from atomic disorder and aliovalent doping. Such phenomena also include kinetic effects such as interface motion and diffusion of Li-electron complexes. The nature of phase transitions directly influences electrode performance in battery applications. Reduced particle size and doping can reduce or eliminate room-temperature Li miscibility gaps, which in turn affect characteristics of state of charge versus voltage and the elastic energy due to volume mismatches between phases. Near the conditions for a phase transition, Li diffusion coefficients are reduced. Nucleation and growth kinetics produce a series of phase transition sequences, which can result in the accumulation of noncrystalline phases during electrochemical cycling.