In-Situ TEM Observation of Random Solid Solution Zone in LiFePO4 Electrode

Olivine structured LiFePO₄ (LFP) as electrode material has attracted great interest in the Li-ion battery community. Recently there are debates on the presence and roles of solid solution zone (SSZ) in lithiation/delithiation, originating from the apparent high rate capability of LFP battery despite poor electronic/ionic conductivities of bulk LiFePO₄ and FePO₄. A potentiostatic in-situ transmission electron microscopy (TEM) technique was used to study the LiFePO₄ electrode kinetics during delithiation. On the basis of real time high-resolution TEM analysis, SSZ forms quickly and reaches 10-25 nm × 20-40 nm on ac face that directly contacts the electrolyte, in lieu of sharp LiFePO₄→FePO₄ interfaces observed in some other situations. This 20 nm scale SSZ was apparently quite stable and persisted for hundreds of seconds duration of our experiment. In contrast to atomically sharp LiFePO₄→FePO₄ interface, the wide SSZ seen here contains no dislocations, so reduced fatigue and enhanced cycle life can be expected along with enhanced rate capability. These findings suggest that SSZ can dominate electrode transformation when the particle size reaches sub-100 nm; for larger particles, SSZ could still be important during fast charging, as it provides out-of-equilibrium but atomically wide avenues for Li⁺/e^- transport (Nano Letters, 14 2014 4005).