Using In-Operando Raman to Understand Interface Evolution in Anode Free Solid and Gel Electrolyte Systems

Advanced ceramic and polymer electrolyte systems offer many advantages over their liquid counterparts, promising higher energy densities, and safer performance. Gel polymer electrolytes consisting of a polymer matrix mixed with lithium salts possess excellent processability, flexibility, safety, and good interfacial contact with electrodes by forming robust electrode electrolyte interfaces. However, they present inferior thermal and electrochemical stability, and unsatisfactory ability to suppress lithium dendrite growth. Inorganic ceramic electrolytes require no supporting solvent, possess a broad electrochemical window, and high mechanical strength, but have poor interfacial contact with electrodes driven by imperfect surface contact, and the formation of adverse degradation products on the lithium metal surface.<br/>Herin we directly observe and compare the evolution of the interfacial chemistry of a gel polymer electrolyte and a sulfide ceramic electrolyte in an “anodeless” configuration using <i>in-operando </i>Raman spectroscopy. For the gel polymer electrolyte we used 1:1 dimethoxyethane (DME) and dioxolane (DOL) with lithium bistrifluoromethanosulfonyl imide (LiTFSI) and lithium nitrate entrapped in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). This gel-polymer system exhibited excellent stability in symmetric cell cycling demonstrating stable performance for over 500 hours. The ceramic counterpart we compared it to is sulfide based argyrodite, Li₆PS₅Cl_0.5Br_0.5. To understand the fundamental interfacial phenomenon governing lithium deposition and growth in these systems an anode less Cu|Li cell was constructed to perform <i>in-operando </i>Raman. Copper was sputtered onto the polymer and sulfide electrolyte before assembly with a thickness of 20nm, providing an optically transparent electrode through which the Raman laser can pass through to detect interfacial species. In doing so we not only observe initial interface species forming on the copper surface but the growth and evolution of the lithium metal and associated interphase during the plating and stripping cycle.