Structural Changes and Reversibility Upon Deintercalation of Li from LiCoPO₄ Derivatives

In an effort to improve the cycle life and rate capability of olivine LiCoPO4, Cr, Fe, and Si were added to produce nominal Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025. This cathode material has an energy density comparable to LiCoPO4, 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 LiCo0.75Fe0.25PO4 and LiCo0.75Fe0.25PO4. Throughout charging, the extent of solid-solution domains was significantly increased in Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 and LiCo0.75Fe0.25PO4 compared to LiCoPO4. These domains reduce the mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 operated at notably higher average potential than LiCo0.75Fe0.25PO4, which would increase the energy density of the cell. Ex situ 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 LiCoPO4 electrodes to maximize the cycle life while preserving high energy density, especially compared to LiFePO4.