Sodium-Ion Battery Cathode with Dominating Copper and Oxygen Redox Chemistry

Sodium-ion batteries offer low-cost energy storage solutions for the grid and electric vehicles, leveraging the established “rocking-chair” Li-ion design and the natural abundance of sodium. However, SIBs face challenges such as relatively lower voltage and capacity than lithium-ion batteries as well as dependence on nickel resources. In this work, a new nickel-free cathode material, Na_0.75Li_0.08Cu_0.25Mn_0.66O₂, was designed and synthesized. This material has a capacity of ∼125 mAh/g and an average discharge voltage of 3.5 V. Notably, more than one-third of the capacity arises from lithium substitution of Cu (∼8 mol %) and high voltage activation to 4.6 V. Multimodal synchrotron X-ray characterization combining spectroscopy, microscopy, and scattering reveal the capacity is primarily from the redox of copper and oxygen, with a minor contribution from the manganese redox. Lithium substitution alters the phase transition mechanism from a two-phase transition in P3–Na_2/3Cu_1/3Mn_2/3O₂ to a solid-solution in Na_0.75Li_0.08Cu_0.25Mn_0.66O₂, enhancing the reversibility of this material.