A study of room-temperature Li_xMn_1.5Ni_0.5O₄ solid solutions

Understanding the kinetic implication of solid-solution vs. biphasic reaction pathways is critical for the development of advanced intercalation electrode materials. Yet this has been a long-standing challenge in materials science due to the elusive metastable nature of solid solution phases. The present study reports the synthesis, isolation and characterization of room-temperature Li_xMn_1.5Ni_0.5O₄ solid solutions. In situ XRD studies performed on pristine and chemically-delithiated, micron-sized single crystals reveal the thermal behavior of Li_xMn_1.5Ni_0.5O₄ (0 ≤ x ≤ 1) cathode material consisting of three cubic phases: LiMn_1.5Ni_0.5O₄ (Phase I), Li_0.5Mn_1.5Ni_0.5O₄ (Phase II) and Mn_1.5Ni_0.5O₄ (Phase III). A phase diagram capturing the structural changes as functions of both temperature and Li content was established. The work not only demonstrates the possibility of synthesizing alternative electrode materials that are metastable in nature, but also enables in-depth evaluation on the physical, electrochemical and kinetic properties of transient intermediate phases and their role in battery electrode performance.