The Effect of Shape in Porous Electrodes on Cell Resistance and Chemo-Mechanical Stresses

Abstract

In contrast to conventional planar electrodes, the use of electrodes with a three-dimensional (3D) shape has been shown to be an effective way to increase both the power and energy densities of a Li-ion battery. These 3D shapes allow batteries to overcome some of the capacity and rate performance trade-offs. For instance, they enable higher active-material loading, while keeping the transport pathways fixed and allowing for better material utilization. There are a variety of ways electrodes can be shaped: each electrode can be individually shaped and separated by a planar separator, or they can be completely intertwined with one another. In this talk, we consider both cases: a half cell with an electrode shape described by a sine wave and a full cell following an interdigitated design, where alternating planar fins protruding from the cathode and anode sandwich the electrolyte. First, we seek to understand how the electrode shape affects the electrical resistance of the cell based on a simple electrostatics model with a linearized Tafel expression. We identify the regimes of electronic to ionic conductivity ratios, Wagner numbers, and porosities that benefit most from electrode shaping, with a particular focus on low temperature electrolytes with reduced conductivity. Next, battery cycling is known to cause volumetric expansion and contraction of the electrodes. Therefore, we explore the chemo-mechanical stresses that arise in the same shaped electrodes with a solid electrolyte. To do so, we solve for the mechanical equilibrium for stress and deformation in the presence of an applied isotropic strain. By examining the stress concentrations at the electrode/electrolyte interface, we elucidate the likely failure mechanisms – interface delamination, crack propagation – of shaped electrodes in comparison to planar electrodes. Overall, we aim to provide design guidelines for how electrode shapes can be chosen to provide both low cell resistance and acceptable mechanical resilience.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL release number: LLNL-ABS-857536.