Coupled mechano-diffusional driving forces for fracture in electrode materials

Li/Si undergoes significant softening in the form of rapid decreases in elastic modulus and yield stress as lithium concentration increases. To investigate how this lithiation-induced softening affects the fracture behavior of electrodes, we formulate a J-integral for coupled mechanical deformation and mass diffusion processes. This measure is used to analyze mechano-diffusional driving forces for fracture through simulations using a mixed finite element framework. Calculations show that under tensile loading, Li accumulates in front of crack tips, leading to an anti-shielding effect on the energy release rate. For a pre-cracked Li/Si thin-film electrode, it is found that the driving force for fracture is significantly lower when the electrode is operated at higher Li concentrations – a result of more effective stress relaxation via global yielding. The results indicate that operation at higher concentrations is an effective means to minimize failure of thin-film Li/Si alloy electrodes. A design map for avoiding failure is developed.

► A theory of coupled mechano-diffusional driving forces for fracture (J-integral) is developed.
► Analysis shows that J and K are not uniquely related due to full deformation–diffusion coupling.
► Global yielding and lithiation-induced softening reduce energy release rate in thin-film Li/Si electrodes.
► Operation at higher Li concentrations mitigates fracture of thin-film Li/Si electrodes.
► A design map is developed for guiding the safe operation of thin-film Li/Si electrodes.