Insight into the Structural Evolution of a High-Voltage Spinel for Lithium-Ion Batteries

With a high operating voltage and three-dimensional lithium (Li)-ion diffusion pathways, Li x Ni 0.5 Mn 1.5 O 4 (high-voltage [HV] spinel) is considered to be a promising high-energy and high-power density cathode material for Li-ion batteries. Here, we extensively investigate the structural dependence of the spinel cathode on the stoichiometry of the cation/anion ratio through an unprecedented overstoichiometric Li intercalation. This material undergoes the well-known cubic phase transition with one Li insertion from a fully delithiated state. The further overstoichiometric Li intercalation results in a cubic-to-tetragonal phase transition when x reaches 3. When x is electrochemically pushed to ∼4, the coexistence of a rock-salt structure with a layered component is observed. The parent spinel structure is reformed upon complete deintercalation. This reversibility underscores the fact that the HV spinel has a distinct memory of its original form. The resultant phases and morphologies are identified by X-ray diffraction (XRD) and microscopy methods.