Abstract
In this work, four selected ionic liquids (ILs), in combination with lithium bis(fluorosulfonyl)imide salt (LiFSI) were explored as electrolytes for anodes made from micron-sized metallurgical grade silicon. The ionic liquids were based on the cations; pyrrolidinium (PYR13), imidazolium (EMI) and phosphonium (P111i4) and the anions; bis(fluorosulfonyl)imide (FSI) and bis(trifluorosulfonyl)imide (TFSI). The cycling stability and rate performance were investigated in pseudo-full cells with silicon anode and commercial LiFePO4, and compared to a carbonate electrolyte. Electrolytes based on PYR13FSI and P111i4FSI exhibit a decent rate performance up to C/5 and showed stable cycling over ∼100 cycles, maintaining a reversible capacity of >1200 mAh gSi-1 at cycle 100 (C/5 rate) and over 2000 mAh gSi-1 at C/20. These electrolytes also had the best oxidation stability (> 5.3 V vs Li/Li). Based on assessment of the limiting current density in symmetrical cells, the Li-ion mobility was determined to be slightly higher for EMIFSI, but differences in Li-ion mobility cannot account for the differences in cycling stability among these electrolytes. While the SEI formed in EMIFSI electrolyte is the most conductive, the highest coulombic efficiency was obtained for PYR13FSI, indicating that the best passivating SEI was formed in this electrolyte.
Highlights
X-ray photoelectron spectroscopy measurements.—X-ray photoelectron spectroscopy (XPS) measurements were performed on a pristine silicon electrode and on silicon anodes cycled for one full lithiation and delithiation in Si-LFP cells with the ionic liquid electrolytes
It should be mentioned that the potential found for the oxidative stability vs Au or reductive stability vs Cu will not accurately predict the stability against other electrode materials, but allows for a relative assessment of the electrochemical stability
EMI cations is influenced by the anion, and that solid electrolyte interphase (SEI) formed in ionic liquids (ILs) based on FSI anions might be dominated by FSI reduction product, both due to “shielding effects,” with Li+ and FSI ions dominating the inner electrode/electrolyte interphase,[88] as well as the fact that LiFSI anions are susceptible to decomposition.[89]
Summary
Evaluation of Selected Ionic Liquids as Electrolytes for Silicon Anodes in Li-Ion Batteries. The aim of this work was to identify the best performing ionic liquid electrolyte in combination with μMG-Si anodes among two pyrrolidinium ILs (PYR13FSI, PYR13TFSI), one imidazolium IL (EMIFSI) and one phosphonium IL (P111i4FSI). As well as rate performance was investigated for the electrodes in an LFP pseudo-full cell setup, in combination with electrolytes made with 0.74–0.79 m LiFSI mixed with each of the four ionic liquids, as well as one electrolyte with a conventional mix of carbonates as a reference (EC:DMC (1:2 w/w), 5 wt% FEC, 1 wt% VC). X-ray photoelectron spectroscopy measurements.—X-ray photoelectron spectroscopy (XPS) measurements were performed on a pristine silicon electrode and on silicon anodes cycled for one full lithiation and delithiation in Si-LFP cells with the ionic liquid electrolytes. Binding energies were calibrated based on the adventitious carbon C 1 s signal set to 285.0 eV
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