In situ method for stress measurements in film-substrate electrodes during electrochemical processes: key role of softening and stiffening

Electrode stress is one of the main driving forces of electrochemical degradation, which is directly related to battery cycle life, thus attracting great interest. Herein, we propose an in situ method to measure bilayer stresses in film-substrate electrodes during electrochemical processes. This method consists of two parts: stress models featuring Li-dependent material modulus and in situ deformation measurements, through which electrode bilayer stresses evolution accompanied by Li-dependent material modulus can be quantitatively characterized. As application of the method, typical silicon-composite and carbon-composite film-substrate electrodes are selected for in situ mechanical measurements and experimental analysis is performed. Results show that silicon material and carbon material exhibit significant, continuous softening and stiffening, respectively. In two film-substrate electrodes, electrode material films experience compressive stress and current collector substrates undergo a tensile-to-compressive conversion across the thickness. Besides, moduli and stresses in both electrodes vary nonlinearly with capacity, presenting non-overlapping paths between lithiation and delithiation. Based on experimental data, we further demonstrate the key role of Li-dependent modulus on electrode stresses, finding that silicon material softening decreases and carbon material stiffening increases electrode stresses. The deficiencies of current stress measurement method based on Stoney equation and the applicability of our method are discussed. Graphic Abstract