Optimizing the design of 3D-pillar microbatteries using finite element modelling

We here present finite element methodology simulations of current distribution, materials utilization and electrochemical cell behavior of a rechargeable Li-ion three dimensional microbattery (3D MB) with 'concentric' architecture, i.e., with one electrode material deposited through filling a 3D structured pillar template of the other electrode. The electrode materials modelled are LiCoO2 and lithiated graphite, respectively, where thin films of LiCoO2 are coated onto 3D-structured current collectors. Time dependent simulations of a range of template pillar heights (h) and interpillar distances (d) have been performed, and the ionic conduction pathways resulting from the different 3D MB concentric architecture design are analyzed. The dynamics of the discharge curves, Li-ion concentration and concentration gradient for each cell design are compared between the different structures and put into context of the state of charge in the electrodes during one full charge/discharge cycle. It is shown that the architecture with the shortest interpillar distance (d = 10 μm) provides higher capacity per footprint area without displaying underutilization of the active material. Higher pillars, on the other hand, result in higher areal capacity, and an optimum height is considered to be ca. 70 μm by balancing the active material utilization in the electrodes and the effective battery volumetric usage. Moreover, it is shown that there are significant differences in terms of cell capacity and power capabilities depending on if the anode or the cathode is coated onto the pillar current collectors.