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 Li_1.025Co_0.84Fe_0.10Cr_0.05Si_0.01(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 LiCo_0.75Fe_0.25PO₄ and LiCo_0.75Fe_0.25PO₄. Throughout charging, the extent of solid-solution domains was significantly increased in Li_1.025Co_0.84Fe_0.10Cr_0.05Si_0.01(PO₄)_1.025 and LiCo_0.75Fe_0.25PO₄ compared to LiCoPO₄. These domains reduce the mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. Li_1.025Co_0.84Fe_0.10Cr_0.05Si_0.01(PO₄)_1.025 operated at notably higher average potential than LiCo_0.75Fe_0.25PO₄, 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.

Extended solid solutions and coherent transformations in nanoscale olivine cathodes

Nanoparticle LiFePO4, the basis for an entire class of high power Li-ion batteries, has recently been shown to exist in binary lithiated/delithiated states at intermediate states of charge. The Mn-bearing version, LiMn(y)Fe(1-y)PO4, exhibits even higher rate capability as a lithium battery cathode than LiFePO4 of comparable particle size. To gain insight into the cause(s) of this desirable performance, the electrochemically driven phase transformation during battery charge and discharge of nanoscale LiMn0.4Fe0.6PO4 of three different average particle sizes, 52, 106, and 152 nm, is investigated by operando synchrotron radiation powder X-ray diffraction. In stark contrast to the binary lithiation states of pure LiFePO4 revealed in recent investigations, the formations of metastable solid solutions covering a remarkable wide compositional range, including while in two-phase coexistence, are observed. Detailed analysis correlates this behavior with small elastic misfits between phases compared to either pure LiFePO4 or LiMnPO4. On the basis of time- and state-of-charge dependence of the olivine structure parameters, we propose a coherent transformation mechanism. These findings illustrate a second, completely different phase transformation mode for pure well-ordered nanoscale olivines compared to the well-studied case of LiFePO4.