Crystal Chemistry and Electrochemistry of Li_xMn_1.5Ni_0.5O₄ Solid Solution Cathode Materials

For ordered high-voltage spinel LiMn_1.5Ni_0.5O₄ (LMNO) with the P4₃2₁ symmetry, the two consecutive two-phase transformations at ∼4.7 V (vs Li⁺/Li), involving three cubic phases of LMNO, Li_0.5Mn_1.5Ni_0.5O₄ (L_0.5MNO), and Mn_1.5Ni_0.5O₄ (MNO), have been well-established. Such a mechanism is traditionally associated with poor kinetics due to the slow movement of the phase boundaries and the large mechanical strain resulting from the volume changes among the phases, yet ordered LMNO has been shown to have excellent rate capability. In this study, we show the ability of the phases to dissolve into each other and determine their solubility limit. We characterized the properties of the formed solid solutions and investigated the role of non-equilibrium single-phase redox processes during the charge and discharge of LMNO. By using an array of advanced analytical techniques, such as soft and hard X-ray spectroscopy, transmission X-ray microscopy, and neutron/X-ray diffraction, as well as bond valence sum analysis, the present study examines the metastable nature of solid-solution phases and provides new insights in enabling cathode materials that are thermodynamically unstable.