Understanding the Relationship between State-of-Charge and Structural Fragility in Ni-Rich Layered Materials for Lithium Batteries

Abstract

Lithium batteries are at the frontier of energy storage. Co-free, Ni-rich layered transition metal oxide (TMO) materials have gained interest within the past decade due to their low cost and increased practical capacity. The challenge with employing these layered oxide cathodes is their inherent structural fragility, facilitated by metal cation migration and lattice oxygen loss. Upon charging, the redox center becomes oxidized, generating thermodynamically favorable electron transfer pathways between the electrolyte and the layered oxide. This transfer pathway is mainly due to the TM3d-O2p orbital hybridization perturbation during battery charging.Surface reconstruction and the structural reordering that accompanies it can undesirably influence the cathode – electrolyte interphase, leading to enhanced electrolyte degradation pathways that can minimize Coulombic efficiency. Electrolyte decomposition occurs at the oxide cathode surface, but can lead to bulk electronic and structural changes of the layered oxide, chemomechanical breakdown, and irreversible phase transformations.In this study, we focus on understanding the interplay between bulk phase transitions and surface instability with respect to electrolyte chemical decomposition. We use novel galvanostatic cycling parameters, extensive electron microscopy, and synchrotron X-ray techniques to decouple chemical and electrochemical electrolyte decomposition pathways to understand their role in Co-free, Ni-rich oxide cathode degradation. This work allows us to decouple surface and bulk chemistries at the cathode – electrolyte interphase to inform further insight into optimal Ni-rich oxide cathode design.