Chemical Imaging of Nanoscale Interfacial Inhomogeneity in LiFePO 4 Composite Electrodes from a Cycled Large-Format Battery.

The nanoscale interfacial inhomogeneity in a cycled large-format LiFePO4 (LFP) composite electrode has been studied by X-ray photoemission electron microscopy at single particle spatial resolution with a probe depth of ∼5 nm. The loss of active lithium in cycled LFP causes the coexsitence of fully delithiated LFP (FePO4 ) and partially delithiated LFP (Li0.6 FePO4 or Li0.8 FePO4 ) as a function of the extent of lithium loss. The distribution of various lithium loss phases along with local agglomeration of LFP and degradation of binder and carbon black are correlatively visualized. This is the first experimental exploration of chemical interplay between components in the composite electrode from a large-format battery, and implications on the LFP degradation in this battery are discussed.