Transition-metal oxide and phosphate materials, commonly used for lithium battery devices, are active as oxygen evolution reaction (OER) catalysts under alkaline and neutral solution conditions. Electrodes composed of LiCoO(2) and LiCoPO(4) exhibit progressive deactivation and activation for OER catalysis, respectively, upon potential cycling at neutral pH. The deactivation of LiCoO(2) and activation of LiCoPO(4) are coincident with changes in surface morphology and composition giving rise to spinel-like and amorphous surface structures, respectively. The amorphous surface structure of the activated LiCoPO(4) is compositionally similar to that obtained from the electrodeposition of cobalt oxide materials from phosphate-buffered electrolyte solutions. These results highlight the importance of a combined structural and electrochemical analysis of the materials surface when assessing the true nature of the OER catalyst.

The lithium-ion battery is the most advanced energy storage system, which utilizes an electrode reaction involving reversible lithium intercalation into a solid matrix. The structural and transport properties of these battery materials have been extensively studied as a function of lithium content, and structural/electronic phase diagrams have been revealed for a wide variety of lithium intercalation compounds. Here, we focused on the electrochromic response upon lithium intercalation and discovered distinctive color changes of Li x FePO4, which is recognized as a promising cathode material for large-scale lithium-ion batteries. The emergence of a broad optical absorption band over the visible spectrum with coloring from pale gray to dark green was observed in accordance with the increase of the Fe3+/Fe2+ mixed-valence state and hence the solid–solution compositional domain in the phase diagram of Li x FePO4. The color changes were analyzed using ab initio computational methods and rationalized to the intervalence charge transfer (IVCT) transition and its kinetic activation energy based on Marcus–Hush theory.

A variety of cathode materials in lithium ion batteries exhibit phase separations during electrochemical reactions, where two phases with different Li compositions are in equilibrium across the phase interface. Because of the lattice mismatch between these phases, large structural distortions are introduced around the interface region. To characterize their potential effect upon the Li migration behavior, the phase interface structure should be determined accurately. In this study, we perform sophisticated structural analyses for phase interfaces in the well-known cathode material Li x FePO4, using atomic resolution scanning transmission electron microscopy. The lattice deformation behavior and Li composition gradient are separately measured across the interface and superimposed after spatial calibrations. The combined result reveals that their relationship significantly deviates from simple models, such as Vegard’s law or other higher order interpolations. Notably, the interface region has small lattice sizes comparable to the FePO4 phase, while having intermediate Li compositions. The origin of observed structure is discussed considering the local phase stability by estimating the pair distance variations of dominant attractive/repulsive ionic couples. Because of the nonlinear variations of each structural parameter, well-optimized experiments with high spatial resolutions and sufficient accuracies are required to correctly understand the phase interface structures.

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