Modeling of Metal Fiber-Enhanced Ion Transport in Lithium-Ion Batteries

Lithium-ion batteries employing a metal fiber fleece as a 3D current collector show an improved ion diffusivity in the electrolyte due to additional surface transport in the electrical double layer at the fiber surface. Since the metal fiber fleece is electrochemically inactive, the objective is to identify a fiber architecture where the enhanced diffusivity overcompensates for the inactive volume penalty. For optimization of the copper fiber architecture in a graphite half-cell, a reduced Poisson-Boltzmann model coupled with a surface diffusion mechanism is fitted against microfluidic channel experiments and integrated into a pseudo-two-dimensional (P2D) battery model. Full-cell reconstruction with a virtual NMC111 cathode enables evaluation of the volumetric energy and power density as optimization objectives. C-rate, electrode thickness, fiber radius and fiber volume fraction are varied to quantify their effect on energy and power. The optimization at fixed electrode thicknesses and given fiber radii identifies design guidelines for the optimal fiber volume fraction across power demands: At low power, fewer metal fibers minimize the inactive volume penalty. More metal fibers are required for high power applications to increase the specific metal fiber surface area and thus the surface transport in the electrical double layer. Compared to electrodes with current collector foils, the power density at SE=350WhL improves by up to 38% for a fixed fiber radius of rmetal=6.65μm.