In order to nearly triple the driving range of electric vehicles (EV), the US Department of Energy has set a goal of achieving an energy density of 500 Wh kg^-1 in EV batteries, and the use of a lithium metal anode is essential to reaching this goal.(1) A key consideration in reducing the weight (and thereby increasing the energy density) of a lithium metal battery is its electrolyte, with the volume of electrolyte used and its salt concentration both affecting the overall battery weight. Unlike with the currently-used graphite anode, the highly reactive surface of a lithium anode varies widely from cycle to cycle, which irreversibly consumes the finite amount of electrolyte and causes lithium metal to suffer from low Coulombic efficiency. To account for Coulombic inefficiency caused by electrolyte depletion, many researchers "flood" their cells with electrolyte to prolong cycle life. However, commercial batteries will require smaller amounts of electrolyte in order to achieve weight and energy density goals.

Additionally, the components that make up the solid electrolyte interphase (SEI) layer on the surface of the lithium metal are soluble in the electrolyte to varying degrees. As the more soluble components selectively dissolve into the electrolyte, reparative SEI grows in its place. During non-uniform lithium plating, new SEI also forms over the freshly-exposed surface area. Both of these processes consume more of the limited electrolyte. Factors such as the volume of electrolyte used and its salt concentration affect the stability of the SEI against dissolution while also altering the composition of the SEI that forms, which can have further impacts on cell performance.

Here, three electrochemically-measurable factors are used to examine the stability of the SEI: formation rate (the rate at which the SEI forms or repairs itself each cycle), degradation rate (the rate at which the SEI dissolves or degrades during rest periods), and formation capacity (the total amount of charge dedicated to forming a stable SEI). Each of these factors will affect the consumption of electrolyte and ultimately dictate the amount of excess electrolyte needed to account for these losses.

Three electrolyte volumes and three salt concentrations were chosen to examine the impact that these parameters have on SEI stability and Coulombic efficiency in the standard 3:7 ethylene carbonate/ethyl methyl carbonate with 2 % vinylene carbonate electrolyte with LiPF₆ salt. Higher volumes of electrolyte resulted in increased SEI formation rates and dissolution rates. Differences in these factors at low and high salt concentrations were found to result from a change in the type of SEI that formed.

References

1. Battery500 Consortium to Spark EV Innovations: Pacific Northwest National Laboratory-led, 5-year $50M effort seeks to almost triple energy stored in electric car batteries, in Office of Technology Transitions Energy.gov (2016).