Hierarchical nickel valence gradient stabilizes high-nickel content layered cathode materials

TLDR

High‑nickel cathodes deliver high energy density but suffer from structural and surface instability that degrades capacity retention and thermal stability, and the role of a nickel concentration gradient in stabilizing these materials remains unclear. The study aims to isolate the effect of a nickel valence gradient on stabilization by designing a compositionally uniform LiNi₀.₈Mn₀.₁Co₀.₁O₂ with a hierarchical valence gradient. The authors synthesize this material and employ a nickel concentration‑gradient approach to create a hierarchical oxidation‑state gradient that separates the more stable Ni⁴⁺ from the less stable Ni³⁺ at particle surfaces. The valence‑gradient material exhibits markedly improved cycling and thermal stability compared to conventional cathodes, demonstrating that positioning Ni³⁺ away from secondary particle surfaces is an effective strategy for high‑nickel cathode optimization.

Abstract

High-nickel content cathode materials offer high energy density. However, the structural and surface instability may cause poor capacity retention and thermal stability of them. To circumvent this problem, nickel concentration-gradient materials have been developed to enhance high-nickel content cathode materials' thermal and cycling stability. Even though promising, the fundamental mechanism of the nickel concentration gradient's stabilization effect remains elusive because it is inseparable from nickel's valence gradient effect. To isolate nickel's valence gradient effect and understand its fundamental stabilization mechanism, we design and synthesize a LiNi0.8Mn0.1Co0.1O2 material that is compositionally uniform and has a hierarchical valence gradient. The nickel valence gradient material shows superior cycling and thermal stability than the conventional one. The result suggests creating an oxidation state gradient that hides the more capacitive but less stable Ni3+ away from the secondary particle surfaces is a viable principle towards the optimization of high-nickel content cathode materials.