In Situ Observation of Random Solid Solution Zone
in LiFePO<sub>4</sub> Electrode
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Abstract
Nanostructured LiFePO<sub>4</sub> (LFP) electrodes have attracted
great interest in the Li-ion battery field. Recently there have been
debates on the presence and role of metastable phases during lithiation/delithiation,
originating from the apparent high rate capability of LFP batteries
despite poor electronic/ionic conductivities of bulk LFP and FePO<sub>4</sub> (FP) phases. Here we report a potentiostatic in situ transmission
electron microscopy (TEM) study of LFP electrode kinetics during delithiation.
Using in situ high-resolution TEM, a Li-sublattice disordered solid
solution zone (SSZ) is observed to form quickly and reach 10–25
nm × 20–40 nm in size, different from the sharp LFP|FP
interface observed under other conditions. This 20 nm scale SSZ is
quite stable and persists for hundreds of seconds at room temperature
during our experiments. In contrast to the nanoscopically sharp LFP|FP
interface, the wider SSZ seen here contains no dislocations, so reduced
fatigue and enhanced cycle life can be expected along with enhanced
rate capability. Our findings suggest that the disordered SSZ could
dominate phase transformation behavior at nonequilibrium condition
when high current/voltage is applied; for larger particles, the SSZ
could still be important as it provides out-of-equilibrium but atomically
wide avenues for Li<sup>+</sup>/e<sup>–</sup> transport