Stress relaxation through interdiffusion in amorphous lithium alloy electrodes

At high guest (lithium) atom concentrations, the diffusion of host (e.g., silicon) atoms may become significant in amorphous Li-alloy-based solid electrodes. The effect of this diffusion mechanism on stress development is in addition to guest atom diffusion, stress-induced enhancement of guest atom diffusion and plasticity. The effect of the diffusive migration of host atoms in amorphous Li-alloy-based electrodes is investigated using a continuum model. A mixed-form finite element framework is developed to simulate the full coupling between stress development and interdiffusion. This framework overcomes the challenges associated with the numerical evaluation of the hydrostatic stress gradient. The analysis focuses on the relative importance of the mechanical driving force and chemical driving force for host migration. Calculations show that host migration can cause stress reductions of up to ∼20% in Li–Si electrodes at stress levels below the yield threshold of the material. Analyses also show that the long-term steady state of stress distribution is independent of the host diffusivity and the thermodynamic factor of diffusion which quantifies the tendency of the two species of atoms to chemically mix, even though the transient behavior (in particular, the peak stresses during charging being important quantities) does depend on the thermodynamic factor and the host diffusivity. The diffusion of Si (host) introduces a time scale which, along with the time scale for Li (guest) diffusion, controls the diffusional response of electrodes.