Design of Porous Anolytes for Rechargeable Metal Batteries

Abstract

Recently there has been great interest in porous lithium metal electrodes for rechargeable batteries, which have demonstrated remarkable improvements in cycle life and charge-rate capability compared to planar lithium metal [1]. In this type of cell design, the porous electrode consists of a porous solid electrolyte with void space in the discharged state. During charge, the metal plates onto the surface of the solid electrolyte, filling the pores. During discharge, the pores empty as lithium reacts at the surface of the electrolyte to produce lithium ions which travel through the porous electrolyte. The electrolyte is rigid, so its shape remains fixed over the life of the cell. This design is advantageous because the solid-electrolyte interphase (SEI) is stationary on the surface of the electrolyte, and therefore there is little capacity fade over cycling, because there is no volume change to cause cracks in the SEI. The design also allows space for the metal to deposit into, thereby reducing the cyclic stress of depositing metal pushing against the separator. By reducing the stress of the lithium pushing against the separator, the risk of a dendrite growing through a defect in the separator is reduced. Here we present an electrochemical model for a cell with a porous anolyte based on porous-electrode theory [2], and apply the model to optimize the design of the porous anolyte to maximize cycle life and safety. For example, we find that, because the electronic conductivity of lithium metal is over 7 orders of magnitude higher than the ionic conductivity of present state-of-the-art solid electrolytes, electronically conductive additives provide no benefit for the porous electrode. As the lithium plates onto the electrolyte surface (initially at the interface with the current collector), the plated lithium itself provides a sufficiently electronically conductive network. Furthermore, if the area-specific resistance of the porous electrode is too low, the current distribution can be highly nonuniform, which can have deleterious consequences for cycle life and safety. Finally, we discuss the implications of the model for the necessary mechanical properties of the solid electrolyte to prevent dendrites even in the presence of manufacturing tolerances.