Dinitrile–Mononitrile-Based Electrolyte System for Lithium-Ion Battery Application with the Mechanism of Reductive Decomposition of Mononitriles

The development of electrolytes capable
of performing at a high
voltage (>5 V) is essential for the advancement of lithium-ion
batteries.
In the present work, we have investigated a dinitrile–mononitrile-based
electrolyte system that can offer electrochemical stability up to
5.5 V at room temperature. The electrolytes consist of 1.0 M lithium
bis­(trifluoromethane)­sulfonamide in various volume proportions of
glutaronitrile, a dinitrile, and butyronitrile, a mononitrile (10/0;
8/2; 6/4; 4/6; 2/8; 10/0). The ionic conductivity of the electrolytes
was found to be 3.1 × 10–3–10.6 ×
10–3 S cm–1 at 30 °C, comparable
with commercially used carbonate-based electrolytes. However, butyronitrile
reacts with Li metal to give 3-amino-2-ethylhex-2-ene-nitrile, 2,6-dipropyl-5-ethylpyrimidin-4-amine,
and oligomers/polymers. These compounds have been characterized by
nuclear magnetic resonance techniques, and based on these findings,
a plausible mechanism of reactivity of mononitriles toward Li metal
has been proposed. Finally, 5 wt % of vinylene carbonate is added
to the glutaronitrile/butyronitrile (6/4 ratio) system to inhibit
the reductive decomposition of butyronitrile. The resultant electrolyte
system is used in the assembly of several coin cells consisting of
a LiFePO4 composite cathode and a Li metal anode. The cells
perform up to 3 C charge/discharge rate with reasonably good discharge
capacity and also display a cycle life of more than 100 cycles at
a 0.5 C rate with capacity retention above 95% at room temperature.