Delithiation-induced secondary phase formation in Li-rich cathode materials

Li-rich layered oxides can potentially provide high capacity, thereby enhancing energy density as cathode materials in Li-ion batteries. However, one of the main drawbacks is their low cycling stability. It has been proposed that the structural stability of a solid solution compound might be enhanced by exploiting the high-entropy concept. Here, we studied two Li-rich layered oxide cathode materials with multiple cations in their transition metal sites, categorized as medium or high entropy: Li(Li_0.2Co_0.18Ni_0.18Mn_0.44)O₂ and Li(Li_0.2Co_0.18Ni_0.18Mn_0.18Ti_0.26)O₂. The synthesized materials, however, experienced a large capacity loss during the first charge/discharge cycle. We performed first-principles calculations to understand the mechanism behind the capacity fading and discovered significant structural changes in both systems. Specifically, we observed extensive Li/Ni interchange, migration of transition metal ions to Li sites, and formation of secondary phases. For the Ti-containing material, which shows a larger capacity fade than the other system, we even observed the formation of a spinel phase. The computed enthalpies of secondary phase formation reactions exhibit large negative values. However, the estimated (maximum) configurational entropy contributions to the free energies of these reactions are much smaller and therefore not determining factors. This study provides crucial insights into degradation mechanisms in Li-rich high-entropy systems, aiding the future design and development of advanced cathode materials for next-generation lithium-ion batteries.