A Baseline Kinetic Study of Co-Free Layered Li1+x(Ni0.5Mn0.5)1−xO2 Positive Electrode Materials for Lithium-Ion Batteries

Abstract

Variations of Li chemical diffusion coefficient () with voltage in a series of Co-free Li1+x(Ni0.5Mn0.5)1−xO2, 0 ≤ x ≤ 0.12, materials were systematically investigated using the recently developed “Atlung Method for Intercalant Diffusion”. The effects of primary and secondary particle sizes, excess Li content, heating temperature, and synthesis atmospheres on were measured. Li-ion kinetics can be enhanced by an order of magnitude by lowering the amount of Ni atoms in the Li layers (cation mixing) from 10% to 4%. Decreasing cation mixing can be accomplished by either increasing the excess Li content or heating temperature. When cycled to 4.6 V, higher specific capacities were obtained, but with a penalty to due to transition metal migration to the Li layers. The primary particles control the Li diffusion length in these materials, regardless of the secondary particle size indicating that grain boundary diffusion must be very rapid. The general trends observed in this work are of great value for the development of higher Mn-containing, Co-free materials. It should be possible to increase energy/power density by making large secondary particles, composed of small primary particles to minimize the solid-state diffusion length while maximizing grain boundary diffusion.