Ionic liquids (ILs) have been studied as a unique electrolyte for a lithium battery for a past two decades ¹⁾.  The thermal stability and less-volatility have been expected to improve safety of a conventional lithium ion battery system using flammable non-aqueous electrolytes. For example, using bis(trifluoromethylsulfonyl)imide (TFSA-) –ILs as a electrolyte, LiFePO₄ half cell could be charged and discharge over 100 °C ^2).

 In this study, we will report that another unique and important features of a zero-solvent electrolyte like ILs was indicated at a high voltage (> 4.7 V) charging and discharging cycle using a transition metal oxide cathode as a LCO and NMC. A modification of NMC by coating with Al₂O₃ particle successfully improved the high voltage operation over 4.2 V as already reported ³⁾. However, we found that a LCO and also NMC cathode without any coating could be stable over 4.7 V charging and discharging cycle even in 1-ethyl-3-methylimidezolium bis(fluoromethylsulfonyl)amide ([EMI][FSA]) containing a certain amount of Li[FSA]. This finding indicate that a catalytic site for oxidation decomposition of a conventional organic electrolyte on a transition metal oxide, which was deactivate by a surface coating with Al₂O₃, did not affect the oxidation of the ionic liquids electrolyte, which did not contain any organic solvents. On the other hand, a cycle stability at a high temperature over 45 °C was effectively stabilized by a surface coating on NMC with ZrO₂. The result implies that the surface modification of a transition metal cathode effectively protected the structural disorder of a cathode material by a high temperature condition.

These results show that using zero-solvent electrolyte like ILs might be a useful tool to elucidate the effect of surface modification in an organic solvent electrolyte.

Acknowledgment

This work was supported by the “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING)” project of the New Energy and Industrial Technology Development Organization (NEDO).

1)    H. Matsumoto, in Electrolytes for Lithium-ion Batteries, eds. T. R. Jow, K. Xu, O. Borodin and M. Ue, Springer, 2014, ch.4. 

2)    H. Matsumoto and K. Kubota, ECS Transactions, 64(4), 425 (2014).

3)    K. Araki, N. Taguchi, H. Sakaebe, K. tatsumi, Z. Ogumi, J. Power Sources, 269 (2014) 236.