Quantitative spatiotemporal Li profiling using nanoindentation

The rate capability and lifetime of Li-ion batteries is largely dictated by the composition dynamics of the electrodes. We set forth a nanoindentation approach to probe the spatiotemporal Li profile using the functional dependence of the mechanical properties on Li composition. This mechanics-informed material dynamics allows us to measure the composition-dependent diffusivity, assess the rate-limiting process in Li reactions, and quantitatively evaluate the stress regulation on Li transport. The experiments show that Li diffusivity in amorphous Si varies exponentially by three orders of magnitude from the pristine to the fully lithiated state. Lithiation in amorphous Si is limited by diffusion at the micron scale. We further evaluate the thermodynamic driving force for Li diffusion by including the material non-ideality and mechanical stresses. Through computational modeling, we find that the composition dependence of the Li diffusivity in general creates an asymmetry on the rate capability during lithiation versus delithiation. In Si, the exponential dependence results in a fast lithiation that proceeds via a steep concentration gradient compared to a slow and relatively smooth delithiation. This asymmetric behavior appears to be a root cause of Li trapping and loss of the deliverable capacity in Si. This work sheds light on the thermodynamics of Li transport and the lithiation kinetics of amorphous Si. It demonstrates the potential of operando nanoindentation in the mechanics-informed understanding of Li chemistry and aiding battery research beyond mechanical measurement.