The thermodynamic stability of intermediate solid solutions in LiFePO4 nanoparticles

Theoretical predictions from first principles and recent advances in in situ electrochemical characterization techniques have confirmed the presence of solid-solution states during electrochemical (de) lithiation of LiFePO4 nanoparticles. Surprisingly, however, such thermodynamically unfavorable solid solution states have been observed at rates as low as 0.1C. Given the high diffusivity of Li in LiFePO4 and the thermodynamic instability of homogeneous solid solution states, spinodal decomposition to a thermodynamically favorable two-phase state is expected to occur on time scales as rapid as 1-100 ms. In this paper, we resolve this apparent paradox by demonstrating that, given the symmetry of the low-energy solid-solution Li/Va orderings and the 1D character of Li diffusion, spinodal decomposition from a solid solution preferentially leads to the formation of a diffuse ac interface with a large intermediate solid-solution region, as opposed to the commonly assumed bc interface. Our first principles predictions not only rationalize the persistence of solid-solution states at low-to-moderate C-rates in high-rate LiFePO4 electrodes, but also explain the observations of large intermediate solid-solution regions at an ac interface in single LixFePO4 particles quenched from a high-temperature solid solution.