From Trigonal to Cubic LiVO2: A High-Energy Phase Transition toward Disordered Rock Salt Materials

Facile and reversible insertion and extraction of Li+ was recently demonstrated to be possible with a new cation-disordered rock salt (DRS) LiVO2 compound (Space Group (SG): Fm (3) over barm). Further structural analyses describing the links between the well-known layered trigonal structure of LiVO2 (space group:R (3) over barm) and DRS LiVO2 synthesized by mechanical ball milling are presented. Rietveld refinements of the X-ray diffraction patterns confirm the formation of LiVO2 in the disordered rock salt phase. We observed the phase transition from the trigonal to cubic structure and the evolution of cell parameters during the ball-milling process using ex situ X-ray diffraction. The reverse phase transition from the cubic to trigonal structure is achieved via thermal treatment, which was confirmed via differential scanning calorimetry and in situ X-ray diffraction experiments. The phase transition to the trigonal structure suggests the metastability of the cubic structure. An exhaustive computational analysis based on a cluster expansion model trained by density functional theory calculations is conducted to confirm the metastability. The presented methods and results show that different (meta)stable phases can be achieved depending on the synthesis method, which is an important aspect to consider in investigating the properties of the new DRS cathode materials for Li-ion batteries.