Electrolyte Formulations Based on Dinitrile Solvents for High Voltage Li-Ion Batteries

In this work, we have investigated the suitability of aliphatic dinitrile solvents with the chemical formula NC-(CH 2 ) n -CNwhere n varies from 3 to 8 in single, binary (mixed with ethylene carbonate, EC) or ternary (mixed with EC and dimethyl carbonate, DMC) electrolyte solutions for the high voltage (4.7 V) LiMn 1.5 Ni 0.5 O 4 cathode material in lithium batteries. We report that the conductivity of all the electrolyte solutions (with LiTFSI or LiBF 4 as salt) decreases as a function of "n", i.e. as the alkane chain become longer while the viscosity increases. The electrochemical stability window is about 7 V for the single electrolyte solutions and drops to 6-6.5 V for the binary and ternary ones. ATR IR spectra of all the electrolyte solutions indicate the presence of a strong interaction between Li ions and the different solvents. Li/LiMn 1.5 Ni 0.5 O 4 half cell batteries assembled using dinitriles as the main solvent (50% by volume), LiBF 4 salt and LiBOB co-salt show good performance only in the ternary solutions. Those with shorter alkane dinitriles with n = 4 and 5 retain the capacity better after 50 cycles than the longer ones with n = 6 and 8. Investigation of the surface of the cycled electrode by XPS reveals that DMC plays a great role in surface passivation at high voltages by preventing salt decomposition in ternary solutions.