Research: Lithium Plasticity
Future anodes for battery electric vehicles favor metallic lithium which provides the highest energy density among the various materials available. However, due to its extreme reactivity, low melting temperature (180 oC), low modulus and yield strength, there are many challenges associated with its processing (via rolling, etc.), surface protection, and battery cycling performance. Dendrite propagation leading to capacity fade and degradation are additional challenges that need solutions for the ubiquitous use of lithium for EV batteries. The mechanical deformation and combined chemo-mechanic phenomena unique to Li metal, are currently not well understood. In particular, plasticity at small length scales is critically important. To date there is limited evidence showing much higher yield stress at submicron dimensions. The proposed research will consist of both experiments and modeling that explores both plasticity at small length scales, and the relationship these effects have on the cycling performance of Li metal anodes.
Reactive molecular dynamics and continuum modeling in combination with unique nano-mechanical experiments will provide a baseline for electrochemical studies that will be designed to address the following questions:
- Does plasticity in a dendrite impact penetration through a polymer separator. For example, can dendrite penetration be inhibited if the modulus of the separator is significantly larger than the yield stress of the dendrite? To what extent do surface passivation layers (SEI and “artificial” SEI films) mechanically confine surface perturbations in the underlying Li metal electrode? This may involve the competition between the strain rate of Li with the formation rate of surface passivation layers.
- Can alloying strategies be employed to improve dendrite resistance (i.e., by altering deformation mechanisms that contribute to the phenomena mentioned above)?
Several rate dependent events, such as diffusion, creep, stripping/plating, can jointly control the Li dendrite resistance. The insights gained through these fundamental studies will help to design Li metal electrodes via alloying, surface protection, and rolling to eliminate defects. These improvements will provide a basis for delivering long cycle life and high coulombic efficiency.