Design of Porous Solid Electrolytes for Rechargeable Metal Batteries

Abstract

Recently there has been interest in porous lithium metal electrodes, which have demonstrated improvements in cycle life and charge-rate capability compared to planar lithium metal for rechargeable batteries. Here, the porous electrode consists of a rigid porous solid electrolyte with void space in the discharged state. During charge, metal plates onto the surface of the solid electrolyte, filling the pores. The rigid electrolyte's shape remains fixed. Here, we present an electrochemical model for a cell with a porous anolyte based on porous-electrode theory, and apply the model to optimize the design of the porous anolyte for energy density and mitigation of dendrites. For example, 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 agents are not necessary. Furthermore, if the charge-transfer resistance of the porous electrode is too low, the reaction rate can be very high next to the separator, which can have deleterious consequences for cycle life and safety. We evaluate design options for avoiding the build up of stress next to the separator. Lithium metal offers the possibility of increasing the volumetric energy density of rechargeable batteries by over 30%. However, it has not been commercialized because of the risk of lithium metal growing through the separator (" dendrite "), leading to catastrophic internal short. Many separators have been researched over the past half century to suppress dendrites, including glass, ceramic, polymer, liquid additives, and composites. 1 However, none of them have been sufficiently robust in the presence of unavoidable defects and manu-facturing tolerances. Lithium dendrites push through grain boundaries of ceramics and cracks in glass. 2 Impurities, thickness nonuniformity, and/or temperature gradients cause nonuniform deposition that cre-ates shorts even through high-strength polymers. 3 There is a com-mon failure mechanism to previously proposed separator designs: 1) lithium deposits adjacent to the separator, 2) nonuniformity and/or defects lead to nonuniform deposition, 3) ohmic potential gradients in the separator create a larger driving force for reaction closer to the counter-electrode which creates a driving force for dendrite propaga-tion, 4) the deposited lithium pushes against the separator, 5) over the course of repeated cyclic stress, eventually lithium pushes through a weak location. Designs that enable lithium to deposit in a way that does not push against the separator can therefore reduce the risk of dendrites causing cracks or creep. Because nonuniform deposition is a major factor for dendrite ini-tiation, it is important to understand which factors have the largest effect on deposition uniformity. Factors that affect the deposition dis-tribution include ionic conductivity, charge-transfer resistance, ge-ometry, and mechanical stress. When considering robustness, both the average values and the distribution of these properties must be considered. In lithium batteries, the charge-transfer resistance at the lithium metal is often the largest component of the cell resistance. Therefore, a variation in the local charge-transfer resistance of even a few percent results in a low-resistance region, which creates a lo-cally high reaction rate, which initiates a protrusion, which can even-tually grow across the separator. Charge-transfer resistance can be decreased by increasing the surface area available for reaction. De-signs which increase the surface area therefore can reduce the magni-tude of the driving force (surface overpotential) which drives dendrite protrusion. However, another failure mechanism with lithium metal is that very few electrolytes are thermodynamically stable with lithium metal. The electrolyte reacts with the lithium metal to form a pas-sivation layer (often referred to as solid-electrolyte interphase, SEI), which is often only metastable and decomposes at elevated temper-ature. If depositing lithium results in change in the shape of the