Alternative Electrolytes for Li-Ion Batteries Using Glutaronitrile and 2-methylglutaronitrile with Lithium Bis(trifluoromethanesulfonyl) Imide

Solutions containing the lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI) in glutaronitrile (GLN) or in 2-methylglutaronitrile (MGLN) are investigated as electrolytes for Li4Ti5O12/LiNi1/3Co1/3Mn1/3O2 (LTO/NMC) battery. At 25°C, the highest conductivity, obtained for a LiTFSI concentration of 1 mol.L−1, is 2.31 and 1.90 mS.cm−1 for GLN- and MGLN-based electrolytes, respectively. According to Raman spectroscopy and DFT calculations, the mean solvation number of Li+ in these solvents decreases from 3.6 to 2.4 in GLN and from 2.9 to 2.1 in MGLN by increasing the LiTFSI concentration from 1 to 4 mol.L−1. DSC and TGA measurements indicate that these electrolytes remain liquid in a large range of temperatures suitable for energy storage applications. Using galvanostatic cycling, the discharge capacity of LTO/NMC cells is 140 mAh.g−1 at C/5 rate but decreases to 125 mAh.g−1 for MGLN and to less than 100 mAh.g−1 for GLN at C/2. Extended cycling reveals that 98% of the initial capacity is retained after 200 cycles at C/2 in MGLN, while only 50% is maintained in GLN. Interestingly, no modification of the surface of the electrodes is observed by scanning electron microscopy (SEM) after cycling while electrochemical impedance spectroscopy highlights a sharp increase in contact resistances by using the GLN-electrolyte.