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 N C-(CH2)(n)-C N where 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) LiMn1.5Ni0.5O4 cathode material in lithium batteries. We report that the conductivity of all the electrolyte solutions (with LiTFSI or LiBF4 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/LiMn1.5Ni0.5O4 half cell batteries assembled using dinitriles as the main solvent (50% by volume), LiBF4 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. (C) 2013 The Electrochemical Society. All rights reserved.