Substituting Na for Excess Li in Li_1+x(Ni_0.6Mn_0.4)_1−xO₂ Materials

Abstract

NMC640, a series of Li1+x(Ni0.6Mn0.4)1-xO2 materials, are important Co-free mid-Ni cathode materials for Li-ion batteries, offering high energy density and better cost-efficiency than Ni-rich counterparts. These materials require excess Li compared to stoichiometric composition to improve the electrochemical performance in terms of rate capability and cycling stability. Although lithium-to-transition metal ratios up to 1.15 can be used to optimize the performance, less than 80% of this lithium is electrochemically active during cycling up to a 4.4 V upper cut off. This study explores whether some percentage of the inactive Li can be replaced by sodium to make these materials more cost-effective and bring potential improvements in electrochemical performance. Various amounts of excess Li were substituted by sodium in the structure. The results show that sodium can be integrated into the layered oxide structure without forming any impurity phases and effectively decreases the cation mixing observed in these layered structures. However, this does compromise cycling stability and rate capability. Na tends to occupy Li sites rather than transition metal sites, resulting in electrochemical instability and capacity loss. Even though excess Li is not electrochemically active, it cannot be effectively replaced by sodium without compromising battery performance of Li1+x(Ni0.6Mn0.4)1-xO2 materials.