Unveiling the High-valence Oxygen Degradation Across the Delithiated Cathode Surface

Charge compensation on anionic redox reaction (ARR) has been promising to realize extra capacity beyond transition metal redox in battery cathodes. The practical development of ARR capacity has been hindered by high-valence oxygen instability, particularly at cathode surfaces. However, the direct probe of surface oxygen behavior has been challenging. Here, the electronic states of surface oxygen are investigated by combining mapping of resonant Auger electronic spectroscopy (mRAS) and ambient pressure X-ray photoelectron spectroscopy (APXPS) on a model LiCoO₂ cathode. The mRAS verified that no high-valence oxygen can sustain at cathode surfaces, while APXPS proves that cathode electrolyte interphase (CEI) layer evolves and oxidizes upon oxygen gas contact. This work provides valuable insights into the high-valence oxygen degradation mode across the interface. Oxygen stabilization from surface architecture is proven a prerequisite to the practical development of ARR active cathodes.

Insight into the Coprecipitation-Controlled Crystallization Reaction for Preparing Lithium-Layered Oxide Cathodes

The nucleation and growth of spherical Ni_0.6Co_0.2Mn_0.2(OH)₂ agglomerates using the hydroxide coprecipitation (HCP) method in the presence of ammonia is investigated through chemical equilibrium calculations and experiments. In the nucleation stage, the transition metal ions in the salt solution gradually complete the nucleation reaction in the diffusion process from pH 5.4 to 11 after dropping into the continuously stirred tank reactor, and then Me(NH₃)_n²⁺ and Me(OH)₂(s) (Me: Ni, Co, and Mn) reach a dynamic precipitation dissolution equilibrium. In the growth stage, the concentration ratio of Me(NH₃)_n²⁺ and OH^- (complexation and precipitation, R_c/p) in the solution has an important influence on obtaining high-quality materials, which is further confirmed using the first principles density functional theory calculations on surface energy and adsorption energy. Then, the HCP reaction could be divided into three parts through experiments: incomplete precipitation area (R_c/p > 10.1); time-dependent area (R_c/p = 0.1-10.1); and hard-to-control area (R_c/p <0.1). According to the optimal ratio (R_c/p = 3.4), a prediction formula for the optimal synthesis conditions of the materials is proposed (y = 0.7731 × ln(x + 0.0312) + 11.6708, the optimal pH value (y) corresponds to different ammonia concentrations (x)). The results obtained for the growth reaction mechanism and the prediction scheme would help the modification research of the materials and obtain the desired lithium-layered transition metal oxide cathode material with excellent performance in the shortest time.