Abstract
Charge/discharge of lithium-ion battery cathode material LiFePO4 is mediated by the structure and properties of the interface between delithiated and lithiated phases. Direct observations of the interface in a partially delithiated single crystal as a function of time using scanning transmission electron microscopy and electron energy-loss spectroscopy help clarify these complex phenomena. At the nano-scale, the interface comprises a thin multiphase layer whose composition varies monotonically between those of the two end-member phases. After partial delithiation, the interface does not remain static, but changes gradually in terms of orientation, morphology and position, as Li ions from the crystal bulk diffuse back into the delithiated regions. First-principles calculations of a monoclinic crystal of composition Li2/3FePO4 suggest that the interface exhibits higher electronic conductivity than either of the end-member phases. These observations highlight the importance of the interface in enabling LiFePO4 particles to retain structural integrity during high-rate charging and discharging.
Highlights
Charge/discharge of lithium-ion battery cathode material LiFePO4 is mediated by the structure and properties of the interface between delithiated and lithiated phases
Charge performance is typically limited by the rate of charge/ discharge that can be sustained by the positive electrode without serious degradation over many battery cycles, so the development of structurally and chemically stable positive electrode materials is crucial for achieving this goal
Because the (010) surface of LiFePO4 is perpendicular to the main Li-ion migration pathway in the olivine structure, it is considered to be the most pertinent in terms of electrochemical behaviour[31,32], and this is the surface that we focus on here
Summary
Charge/discharge of lithium-ion battery cathode material LiFePO4 is mediated by the structure and properties of the interface between delithiated and lithiated phases. Detailed STEM and EELS observations of the interface between Li1–αFePO4 and LiβFePO4 phases at different times after chemical delithiation of this surface reveal that LiFePO4 comprises a thin multiphase layer that does not remain static but relaxes gradually outwards to the crystal surface as Li ions diffuse from the crystal bulk back into the delithiated regions.
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