Resolving Charge Distribution for Compositionally Heterogeneous Layered Cathode Materials
Authors: Mu Linqin , Zhang Jin , Liu Yijin , Lin Feng 
Published: November 2022

Rechargeable batteries are built on the foundation of redox-active compounds. Thus, understanding charge compensation mechanisms represents one of the most critical research directions in developing battery materials. Due to the compositional versatility of Li-containing layered cathode, i.e., LiNi1-x-yMnxCoyO2 (NMC), one can tune the TMs (transition metals) composition to satisfy various demands. As the Ni/Mn/Co stoichiometry moves across the compositional space, the charge compensation mechanism also gradually evolves due to the changing of TM 3d orbital energy levels. Thus, tailoring TMs stoichiometry has become effective to fine-tune battery performance. In a previous study, we found that NMCs with a highly heterogeneous TM distribution still exhibited a globally layered structure.1Another study on dopant distribution also suggests that the heterogeneous dopant can positively impact battery performance.2 Our previous studies show that the charge heterogeneity can result in local stress hotspots and exacerbate the chemomechanical breakdown of NMC particles.3 When the compositional heterogeneity prevails in individual NMC particles, charge distribution likely exhibits a heterogeneous pattern due to distinct redox reactions in domains with different TM stoichiometries. Thus, we believe that understanding the interplay between compositional heterogeneity and its impact on charge distribution may provide insights into engineering compositional heterogeneity to achieve desired charge distribution. Herein, in this work, we designed the compositionally heterogeneous NMC cathode with the ensemble-average layered structure as the platform to investigate the charge distribution with single-particle spectroscopic imaging measurements.4 We establish the relationship between charge distribution and local compositional heterogeneity at the single-particle level. The local Mn and Ni concentrations in individual NMC particles are positively and negatively correlated with the electrochemically induced Ni oxidation, respectively, whereas the Co concentration does not impose a clear effect on the Ni oxidation. The resulting material delivers excellent reversible capacity, rate capability, and cycle life at high operating voltages. Engineering Ni/Mn/Co distribution in NMC particles may provide a path toward controlling the charge distribution and thus chemomechanical properties of polycrystalline battery particles.

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