Performance of Si Anodes in Ionic Liquid Electrolytes with Added Carbonates

Abstract

Whether by sound political response to the climate challenge or by the eventual lack of resources, the world is set to transition from a fossil energy supply to renewable electric energy. As larger amounts of our energy will come from sources of intermittent nature, the demand for storage will inevitably rise. Batteries provide an efficient mode of electric energy storage and has a wide applicability both in stationary and mobile applications due to the scalability and high energy density of the technology. Improvements to the energy density and safety as well as lowering the price per kWh will be important in accelerating the implementation of batteries worldwide. The gradual replacement of graphite with silicon as the active material in the lithium ion battery (LIB) anode can increase the energy density, but large volume changes upon (de-)lithiation of the Si causes problems such as continuous electrolyte decomposition and excessive solid electrolyte interphase (SEI) growth [1]. Ionic liquid electrolytes may solve this problem by being stable at the operating potentials of the Si electrode or by forming a stable SEI, as well as improving the safety of LIBs due to their non-flammability. High viscosity and thus low ionic conductivity especially at low temperatures has been one of the problems hindering the use of ionic liquids in electrolytes. The addition of small amounts of low viscosity carbonates may provide an electrolyte with a good compromise between safety and performance [2]. Electrolytes consisting of the lithium bis(fluorosulfonyl)imide (LiFSI) salt and the room temperature ionic liquid N-Propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI) were made with and without the addition of standard linear and cyclic carbonates. The conductivity of the electrolytes were measured and the Li+ charge transfer kinetics were investigated by electrochemical impedance spectroscopy using the methods explained by Xu et al. [3] and Jow et al. [4]. Coin cells were assembled using a 73 wt% µ-Si electrode in half and full cell setup to determine the effect of the electrolytes on rate performance and cycling stability. Silicon electrodes were extracted from the cycled cells and the composition and growth of the SEI layer was analyzed using ATR-FTIR and XPS. Cross sectional SEM imaging was used to investigate the macroscopic change in electrode structure upon cycling in the different electrolytes. The combination of these techniques provides information on the differences in SEI formation and composition, initially and after cycling. Initial results show that mixing small amounts of carbonate into ionic liquids and carbonates in the electrolyte improves the stability of silicon electrodes in Li-ion batteries as compared to electrolytes of pure ionic liquids and carbonates only. Combined with knowledge of the SEI forming reactions, these results may help in designing electrolytes that enable increased amounts of silicon in battery electrodes while improving the overall safety of the Li-ion battery.