Chemo-mechanical deformations in lithium titanate composite electrodes upon over-lithiation

Abstract

Lithium titanate (Li[Li_1/3Ti_5/3]O₄, LTO) electrodes have received significant attention as anodes for Li-ion batteries due to their “zero-strain” feature, associated with their negligible volumetric changes to form Li₂[Li_1/3Ti_5/3]O₄ during cycling. However, there is not much known about chemo-mechanical instabilities in the LTO electrode when it is over-lithiated to Li_2+x[Li_1/3Ti_5/3]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 in various voltage windows, supported by ex situ XPS. The LTO experienced a reversible deformation with a negligible amount of strain when cycled between the 1.0 and 3.0 V voltage window. When polarized to 0.35 V 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. The predicted strains match well with the strain generation at higher voltages (1.0–3.0 V); 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 electrodes in over-lithiated states, and we foresee that a similar approach can be utilized to understand the deformation mechanisms in other zero-strain electrodes in higher states of (dis)charge.