PNNL

Abstract

The role of nanoscale chemomechanical behavior in macroscopic performance of functional materials is well recognized. For lithium-ion battery cathodes, tremendous effort has been devoted to the development of new chemistry and structure, yet fundamental understanding in the correlation between redox processes and mechanical properties of the novel materials lags behind. In the present study, we prepare large discrete single-grains of Li-excess cation-disordered rocksalts (DRX) and investigate their chemomechanical behavior at the particle-level, using nano-resolution X-ray and electron-based spectro-imaging and chemical mapping techniques. While irregular cracking upon lithium extraction leads to the eventual breakdown of the baseline DRX oxide (Li1.2Ti0.3Mn0.4O2) particles at high delithiation state, the fluorinated-DRX (Li1.3Ti0.3Mn0.4O1.7F0.3) clearly displays directional cracking along the [001] direction. The resulting periodicity in cracking patterns enables the particles to retain their integrity and consequently, improved electrochemical stability. Our study reveals the unique role of fluorine in modulating nanoscale chemomechanics, which in turn influences the evolution of charge and strain heterogeneity at the particle level. These insights provide important design guidelines in further improving DRX cathode materials.