Phase Stability of Li-rich Layered Cathodes: Insight into the Debate over Solid Solutions vs Phase Separation

Li-rich layered transition metal oxides (Li_1+xM_1-xO₂ or mLi₂MnO₃-nLiMO₂) have been widely studied as cathode materials for Li-ion batteries recently due to their enhanced capacity of larger than 250 mAh g^-1. However, even the qualitative nature of the phase stability of these materials, whether they form a solid solution or are phase separated, has been the subject of intense debate. In this work, we use density functional theory calculations to investigate the phase stability of these Li-rich layered transition metal oxides (Li₂MnO₃-LiMO₂, M = Co, Ni, Mn). We calculate the mixing enthalpy and coherency strain energy between Li₂MnO₃ and LiMO₂ for two distinct cases: (1) mixing of M on the Li and Mn sites respectively in the transition metal layer of Li₂MnO₃, resulting in a solid solution with C2/m symmetry, and (2) mixing of Li and Mn on the M sites of LiMO₂, resulting in a solid solution with R3̅m symmetry. We show that phase separation is energetically preferred relative to a solid solution at T = 0 K, and the coherency strain energy has little influence on phase stability. Results also display that a solid solution with R3̅m symmetry has a larger mixing enthalpy than that with C2/m symmetry at T = 0 K. Furthermore, we use the mixing enthalpies along with mean-field mixing entropies to calculate free energies and phase diagrams. At low temperature, the system exhibits phase separation between the C2/m and R3̅m phases, with appreciable solubility in each phase, and at high temperature, there is a transformation to the single-phase R3̅m solid solution. For high Li content compositions, the phase diagram shows a region of stability for the single phase C2/m solid solution. Our calculations support one possible explanation for the discrepancies between various reports of the structure of these Li-rich layered materials; the compositions and temperatures of these synthesized materials could be close to phase boundaries separating the regions of solid solution vs phase-separation. The calculated phase diagrams also indicate that the phase stability of Li-rich layered materials largely depends on the synthesis temperature, the amount of excess Li, and the combination of transition metals.