The initiation of void growth during stripping of Li electrodes in solid electrolyte cells
We analyse the initiation of void growth in the Li electrode during the stripping phase of an Li-ion cell with a solid (ceramic) electrolyte. We first show that standard Butler-Volmer kinetics fails to predict the observed void formation. This motivated us to recognise that void initiation/growth involves power-law creep of the Li electrode that is linked to the motion of dislocations. We show, via thermodynamic considerations, that dislocations significantly affect the interface kinetics and use variational principles to develop a modified form of Butler-Volmer kinetics for the interface flux that is associated with a deforming Li electrode. Numerical solutions are presented for the coupled flux of Li+ in a single-ion conductor solid electrolyte and the associated creep deformation of the Li electrode for an imposed stripping current. This involves solution of a Laplace equation for flux in the electrolyte and the nonlinear Stokes flow equations for a power-law creeping solid in the electrode. These two domains are coupled together via the modified Butler-Volmer relation. The calculations predict that an increasing stack pressure needs to be exerted with increasing cell current to avoid the initiation of void growth and are in excellent quantitative agreement with measurements for an Li/LLZO/Li cell.