(Invited) Self-Discharge of Layered Oxide Cathodes Induced by Carbonate-Mediated Hydrogenation

Abstract

Significant efforts have been made to improve the energy and power characteristics of cathodes in Li-ion batteries, while the research into cathode self-discharge has lagged behind. Self-discharge is the voltage drop experienced by all rechargeable batteries while stored in the charged state. The voltage drop becomes more problematic with increased cathode voltage and temperature. The physicochemical nature of internal reactions leading to cathode self-discharge is hardly understood and seldom discussed.To tackle this challenge, the structure and redox evolution of commercial LiNi0.5Mn0.3Co0.2O2 electrodes and single crystal cathode thin films upon the self-discharge in carbonate-based electrolyte is revealed by surface-sensitive X-ray scattering, spectrometric and electrochemical characterizations in conjunction with thermodynamic analysis. As evidenced by the interfacial toolkits and theoretical calculations, there is an evolution and growth of cathode surface reduction layer in different types of electrolyte after self-discharged from different potentials. Structural and chemical chacterizations as well as the elemental depth profiles within cathode thin film confirms proton-insertion-induced layered cathode hydrogenation. Calculation shows that such process is both thermodynamically and kinetically favorable and is triggered by the interfacial hydrogen atom abstraction of methylene group in carbonate solvent on delithiated cathode surface. A combination of experimental and theoretical studies reveals the carbonate-mediated cathode hydrogenation mechanism accounting for the voltage drop in the self-discharge. This offers additional understanding regarding defect generation and the degradation mechanism in layered cathodes beyond the traditional behaviors of lithium-diffusion-induced self-discharge and rock-salt phase induced cathode degradation. This study offers foundational knowledge of the interfacial degradation of cathode that can be translated to the rational design of improved cathodes and electrolytes, and the self-discharge mechanism studies in other electrochemical ion insertion materials and devices.