Hard carbon: a promising lithium-ion battery anode for high temperature applications with ionic electrolyte

Abstract

Electrochemical behavior of a hard carbon plate in an electrolyte based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion was investigated. Hard carbon is found to be less prone to passivation due to the high electrochemical stability of the ionic liquid. Lithiation and de-lithiation of the carbon anode is strongly affected by temperature. At room temperature, the hard carbon is difficult to lithiate and a high potential hysteresis is observed between charge and discharge curves. Increasing temperature contributes to higher reversible capacity, higher coulombic efficiency, and lower potential hysteresis. At 80 °C, a reversible capacity of 16.2 mAh cm⁻² (equivalent to 675.0 mAh g⁻¹) was obtained with 73.6% of the first cycle coulombic efficiency. Considering that graphitic anodes do not work at this temperature due to the decomposition of the solid electrolyte interphase (SEI), the combination of hard carbon with ionic electrolyte can meet some special requirements for high temperature applications with improved safety. The activation energy for Li-ion transfer across the hard carbon/ionic electrolyte interface was measured by ac impedance spectroscopy; a value of 80 ± 5 kJ mol⁻¹ was obtained. The large activation energy implies a high barrier of activation at the electrode/electrolyte interface, which accounts for the poor rate performance of the hard carbon anode in the ionic electrolyte.