Unraveling the Origin of Performance Degradation in High-Capacity Cation-Disordered Oxide Cathodes

Abstract

Due to the substantial energy gap between state-of-the-art cathodes and anodes, development of new-generation Li ion batteries (LIBs) critically depends on cathode material discovery and improvement. In recent years, a combination of reversible redox activities of both transition-metal cations and oxygen anions were found to be feasible in a number of transition-metal (TM) oxides, such as the layered Li- and Mn-rich, lithium nickel manganese cobalt oxides (LMR-NMCs) and Li-rich rock-salt compounds. Such combination allows a significant enhancement in the charge storage capacity of LIB cathodes. However, challenges exist in these LIB cathodes with both cationic and anionic redox activities. After extensive cycling, performance issues such as capacity fade, voltage decay, hysteresis, and impedance rise often occur. Understanding the chemical origin of these issues in LMR-NMC has been on-going for well over a decade. Various degradation mechanisms, such as TM migrations, have been proposed. However, the effect of oxygen redox in cation-disordered cathode is largely unknown. In this work, we performed detailed studies on one of the most promising cation-disordered rock-salt cathodes, Li1.3Nb0.3Mn0.4O2 (LNMO). Systematic evaluation on the electrochemical behavior as a function of oxygen redox revealed that the performance deterioration is directly correlated to the extent of oxygen redox involvement. On the fundamental side, we performed a series of hard and soft X-ray absorption spectroscopic (XAS) studies to analyze the changes in Mn and oxygen activities due to cycling, and illustrated the chemical and structural origin for the observed voltage decay and capacity fade. It was found that electrochemical cycling with large contribution of oxygen redox significantly decrease the redox activity of oxygen itself as well as Mn redox activity. In contrast to what was reported on layered LMR oxides, extensive TM reduction was observed but phase transition resulting from cation site migration was not detected in the cycled oxide. A densification/degradation mechanism was proposed accordingly which elucidates how a unique combination of extensive chemical reduction of TM and reduced quality of the Li percolation network in cation-disordered rock-salts can lead to performance degradation in these newer cathodes with 3-dimentional Li migration pathways. On the basis of these studies, cation site chemistry of Li-Nb-Mn rock salt system was systematically tailored to tune the contribution of oxygen redox. Opportunities in balancing capacity and cycling stability will be discussed.