Structural Changes and Reversibility Upon Deintercalation of Li from LiCoPO Derivatives.

In an effort to improve the cycle life and rate capability of olivine LiCoPO, Cr, Fe, and Si were added to produce nominal LiCoFeCrSi(PO). This cathode material has an energy density comparable to LiCoPO, with markedly improved electrochemical performance. Here, we apply operando X-ray diffraction to gain an understanding of the crystallographic delithiation mechanism of this new substituted electrode material, compared to both LiCoFePO and LiCoFePO. Throughout charging, the extent of solid-solution domains was significantly increased in LiCoFeCrSi(PO) and LiCoFePO compared to LiCoPO. These domains reduce the mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. LiCoFeCrSi(PO) operated at notably higher average potential than LiCoFePO, which would increase the energy density of the cell. measurements reveal the persistence of structural irreversibilities in the substituted phase after the first cycle, identifying avenues for further improvement in durability. This finding sheds light on the strategies for judicious cation substitution in LiCoPO electrodes to maximize the cycle life while preserving high energy density, especially compared to LiFePO.