Designing Electrode Electrolyte Interphases for Stable Lithium Metal Batteries
Reactive metal anodes are known to electrodeposit in the form of irregular morphological features on planar substrates. Formed during the earliest stages of deposition, these features are thought to eventually result in non-planar, mossy structures that proliferate in the electrode spacing, hampering electrode reversibility. A growing body of work suggests that the mechanics, structure, ion transport properties, reductive stability, and interfacial energy of interphases formed spontaneously on the metal electrode play important, but separate roles in regulating nucleation, growth, and reversibility of these non-planar structures. In this study, we examine the effect of fluorinated thermosets on the early stages of lithium metal deposition and subsequent growth rate of the deposit front. By performing a theoretical linear stability analysis of metal electrodeposition across elastic interphases, we find that interphase thickness, mechanics, ion transport and interfacial properties all play precise and differentiated roles in setting the optimal interphase design. Motivated by this analysis, we focus specifically on thermosetting polymer interphases because their mechanical and chemical properties can be readily manipulated. Our goal is to develop design rules that can be implemented without adding substantially to the weight or volume of the metal electrode. Additionally, motivated by a growing body of work showing that fluoride-containing components in liquid electrolytes enhance Li reversibility by regulating the chemistry of the SEI formed on the Li electrode, we specifically focus on interphases created using fluorinated polymers that are held together by covalent cross-links. By tuning the chemistry of the backbone and sidechains of the network, we try to elucidate the effect of the physical properties of the polymeric interphase on the morphology of electrodeposited lithium. Using experimental characterization by techniques that include scanning electron microscopy and operando visualization of the metal deposition, we report that parameters like polymer thickness, metal-polymer interfacial energy and elasticity have a profound effect on the morphology of electrodeposited lithium and correlate that with the theoretical results. Specifically, it is found that an interplay between elasticity and diffusivity leads to an optimum thickness value of the polymeric interphase while higher interfacial energy augment elastic stresses at the metal surface in preventing out of plane growth of the deposited metal. These findings can potentially guide the design of artificial interphases as well as electrolyte components that lead to specific compositions of the SEI.