Tailoring Electronic Structure to Achieve Maximum Utilization of Transition Metal Redox for High-Entropy Na Layered Oxide Cathodes

Abstract

Charge compensation from cationic and anionic redox couples accompanying Na⁺ (de)intercalation in layered oxide cathodes contributes to high specific capacity. However, the engagement level of different redox couples remains unclear and their relationship with Na⁺ content is less studied. Here we discover that it is possible to take full advantage of the high-voltage transition metal (TM) redox reaction through low-valence cation substitution to tailor the electronic structure, which involves an increased ratio of Na⁺ content to available charge transfer number of TMs. Taking Na_xCu_0.11Ni_0.11Fe_0.3Mn_0.48O₂ as the example, the Li⁺ substitution increases the ratio to facilitate the high-voltage TM redox activity, and further F-ion substitution decreases the covalency of the TM–O bond to relieve structural changes. As a consequence, the final high-entropy Na_0.95Li_0.07Cu_0.11Ni_0.11Fe_0.3Mn_0.41O_1.97F_0.03 cathode demonstrates ∼29% capacity increase contributed by the high-voltage TMs and exhibits excellent long-term cycling stability due to the improved structural reversibility. This work provides a paradigm for the design of high-energy-density electrodes by simultaneous electronic and crystal structure modulation.