Three-dimensional phase evolution and stress-induced non-uniform Li intercalation behavior in lithium iron phosphate†
A lithium iron phosphate (LiFePO4) olivine cathode displays rich phase transition behavior during charging/discharging. Despite extensive studies, the evolution of the LiFePO4/FePO4 two-phase coexistence structure and the underlying mechanism are still a point of debate due to the intricate and often disparate phase morphologies observed. Here we apply phase-field simulations to provide detailed insights into the three-dimensional phase evolution process in LiFePO4 upon delithiation under a large driving force. The simulation reveals that the coherency stress arising from the LiFePO4/FePO4 lattice mismatch destabilizes the initially flat delithiation front and induces filamentary growth of the FePO4 phase along , which is in excellent agreement with a recent operando X-ray imaging study [Ohmer et al. Nature Communications 6, 6045 (2015)]. The combined simulation and experimental results provide the first illustration of the phenomenon of stress-induced instability of the lithium (de)intercalation front in battery electrode compounds. We show that this phenomenon is facilitated by the surface mode of coherent spinodal decomposition in LiFePO4 and influenced by the anisotropies of misfit strain, elasticity and lithium diffusion. It is detrimental to battery performance and life by causing stress concentration and reducing the lithium (de)intercalation kinetics and thus should be mitigated.