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_yFe_1-yPO₄, 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 LiMn_0.4Fe_0.6PO ₄ 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.