An Advanced Tool for the Selection of Electrolyte Components for Rechargeable Lithium Batteries

Abstract

The solid electrolyte interphase (SEI) plays a key role in lithium‐metal, lithium‐alloy, and lithium‐ion batteries. The SEI on both lithium and carbonaceous electrodes consists of many different materials including LiF, Li 2 CO 3 , LiCO 2 ‐ R , Li 2 O , lithium alkoxides, nonconductive polymers, and more. Close to the lithium or Li x C 6 the SEI consists of the thermodynamically stable anions, such as O 2 − , S 2 − , and halides. Close to the solution the SEI also contains partially reduced materials such as polyolefins, semicarbonates, etc. These materials form simultaneously and precipitate on the electrode as a mosaic of microphases. These phases may, under certain conditions, form separate layers, but in general it is more appropriate to treat them as heteropolymicrophases. The SEI composition and properties depend strongly on electrolyte composition and on other factors. Rapid formation of the SEI is important in all lithium batteries, especially in the case of lithium‐ion cells with graphite anodes. Hence SEI precursors must be selected from a group of materials that have high exchange current density ( i 0 ) for reduction. As it is difficult to measure i 0 on solid electrodes and only limited data, if any, are available, it is suggested to use the data bank for the rate constant for the reduction of electrolyte components by hydrated electrons ( e aq − ) . We have demonstrated that the rate constants of the reactions of solvated electrons and electrolyte components and impurities correlate well with the formation voltage of SEI and with SEI composition. Thus, the rate constants ( k e ) for these reactions are proposed as a tool for the first screening of electrolyte components when a new electrolyte is designed.