Lithium tracer diffusion in ion-beam sputtered nano-crystalline and amorphous LiNi0.33Mn0.33Co0.33O2 films

Abstract

The LiNi_0.33Mn_0.33Co_0.33O₂ compound is a cathode material for Li-ion batteries. Li diffusion in this material directly influences charging/discharging times, power densities, maximum capacities, stress formation and possible side reactions. In the present study, Li tracer self-diffusion is investigated on ion-beam sputtered films after deposition (amorphous) and after crystallization at 700 °C. For the experiments, ⁷Li isotope enriched films with about 1.5 μm thickness were combined with a 50–90 nm thick ⁶Li tracer layer with the same chemical composition. Afterwards, the films were diffusion annealed between 100 and 300 °C. For analysis secondary ion mass spectrometry in depth profile mode was applied. The diffusivities of the crystalline films are identical to those of sintered bulk samples within error limits as known from literature and show an activation enthalpy of diffusion about 0.9 eV. In contrast, the diffusivities of the amorphous films are about one order of magnitude lower at 100 °C due to a higher activation enthalpy of diffusion of 1.1 eV. We attribute this higher activation enthalpy to a hindered diffusion in the amorphous state of the two-dimensional ion conductor.