Electrochemical Properties of Three Li₂Ni₂TeO₆ Structural Polymorphs

Three different structures of the composition Li₂Ni₂TeO₆ have been synthesized and their electrochemical properties characterized: a disordered orthorhombic phase with tellurium ordering and strong Li/Ni cation mixing, a T^#2-layered phase, and an O3-layered phase; each have a distinct cycling performance. The disordered orthorhombic structure is the most stable at room temperature and has the highest capacity with the lowest overpotential. The T^#2 phase is the least stable and undergoes a phase transformation once lithium is extracted. Density functional theory calculations reconcile the relative structural stabilities and physical properties of each phase with their electrochemical performances. Calculations also confirmed experimental observations regarding the synthesis of each structure and indicated the mechanism for capacity fade of each Li₂Ni₂TeO₆ cathode. Strong tellurium-oxygen covalent bonding increases the voltage of the Ni^2+/3+ redox couple in each of these materials over what is normally observed in layered oxides.