Chemo-Mechanical Deformations in Lithium Titanate Composite Electrode Upon Over Lithiation

Abstract

Lithium titanate (Li[Li1/3] Ti5/3]O4, LTO) electrodes have received significant attention as an anode for Li-ion batteries due to their “zero-strain” feature, associated with their negligible volumetric changes to form Li2[Li1/3] Ti5/3]O4 during cycling during cycling. However, there is not much known about chemo-mechanical instabilities in the LTO electrode when it is over-lithiated ( Li2+x[Li1/3] Ti5/3]O4) at lower voltages (<1.0 V vs Li). Here, chemo-mechanical deformations in the LTO composite electrode were investigated by conducting operando strain and stress measurements at various voltage windows, supported by ex-situ XPS. The LTO experienced a reversible deformation with a negligible amount of strain when cycled between 1.0 – 3.0 V voltage window. When polarized to 0.35V vs Li, the LTO electrode experienced large strains (0.8%) and compressive stress generation, with a significant amount of irreversible deformation. The Li-intercalation-induced strains were predicted using a mathematical model. Predicted strains match well with the strain generation at higher voltages (1.0 – 3.0V); however, there is a large misalignment between predicted strains and experimentally measured strains at lower voltages. XPS measurements exhibited the formation of thick SEI layers on the LTO electrodes when cycled at lower voltages. The combination of experimental and predicted strains indicates large deformations between 0.5 and 0.8 V during lithiation, and it was attributed to the formation of SEI layers. Our study provides a comprehensive analysis of the chemo-mechanical deformations in the LTO electrode at over-lithiated state, and we foresee that similar approach can be utilized to understand the deformations mechanisms of other zero-strain electrodes at higher state-of-(dis)charge.