Transformation Induced Plasticity Drives Stress Instability During Electrochemical Delithiation of LiCoO2

Abstract

Mechanical degradation is a major cause of electrochemical instability of “layered”/“cation-ordered” Li–transition metal (TM) oxides, but with the focus so far being directed mainly to deep delithiation states of Ni-containing Li-TM-oxides. By contrast, our operando stress measurements during galvanostatic cycling of LiCoO2 (viz., the base Li-TM-oxide) has revealed that mechanical instability can be initiated even during the initial stages of delithiation, especially upon initiation/propagation of “HI → HII” phase transformation, starting at ∼3.8 V vs Li/Li+ [as partly reported in, Malav et al., ECS Electrochem. Lett., 4(12), A148 (2015)]. “Flattening”/“plateauing” of overall/average in-plane stress build-up in the operando stress profiles is a signature of the same. In a bid to further understand and throw new insights onto such electro-chemo-mechanical responses, the present work develops a mathematical model that includes in-plane extension/bending to signify (de)lithiation-induced deformation, concentration-triggered phase transformation and, more importantly, concepts associated with transformation induced plasticity. Internal stress build-up during “HI → HII” phase transformation of LiCoO2 is predicted to be a likely cause for plastic deformation. Overall, the model is able to capture the initial monotonic stress build-up, followed by “plateauing” during phase transformation, and reaffirms a strong correlation between phase transformations, transformed phases and accompanying mechanical stresses during electrochemical delithiation/lithiation of Li-TM-oxide.