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Introduction of bis(fluorosulfonyl)imide (FSI) as an anion to an ambient-temperature ionic liquid electrolyte based on 1-ethyl-3-methylimidazolium and lithium (Li) as cations can provide a reversible Li intercalation into a graphitized electrode, while the intercalation is completely irreversible without FSI. Laser desorption/ionization mass spectroscopy and ac impedance spectroscopy suggest that FSI-coordinated Li + is facilely mobile even without any solvents and is reversibly accessible to a graphite interior. IntroductionIonic liquids at an ambient temperature have attracted increasing attention in many fields including batteries, because of their diverse properties such as a wide electrochemical potential window, acceptable ionic conductivity, high thermal stability and negligible vapor pressure. In particular, the application of these ionic liquids in lithium (Li)-ion secondary batteries has been considered in order to ensure safety by taking advantage of their lower flammability and lower reactivity than conventional organic electrolytes. However, Li-ion batteries with ionic liquid electrolytes have some difficulties in their charge-discharge performance; especially a fatal problem has been their irreversibility at a carbon negative electrode (1-13).Some organic additives, typically vinylene carbonate (VC), have been introduced into ionic liquid electrolytes to stabilize and protect an interface between a carbon negative electrode and an ionic liquid phase against an undesirable irreversible reaction of ionic liquid components (1-3). In spite of many reports concerning this strategy, however, there had been no report on room-temperature ionic liquids that can provide the reversibility of a graphitized negative electrode without both a halide ion and a solvent.Our group (6, 11 and 13) as well as Matsumoto et al. (5), Guerfi et al. (8) and Seki et al. (12) reported on ionic liquids based on bis(fluorosulfonyl)imide (FSI) for rechargeable Li batteries. The ionic liquids were found to have quite low viscosity and high ionic conductivity when compared to those based on bis(trifluoromethylsulfonyl)imide (TFSI). Especially, FSI-based electrolytes containing Li ions exhibited practical ionic conductivity, and a graphite/Li cell with the Li-containing electrolytes showed excellent cycleability without any solvents (6, 12). 67 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.215.174.8 Downloaded on 2015-06-25 to IPIn this report, we analyzed the interfacial physicochemistry of a graphitized carbon electrode cycled in ionic liquid electrolytes with and without FSI and in a typical solvent electrolyte by using electrochemical ac impedance spectroscopy (EIS) and laser desorption/ionization mass spectroscopy (LDI-MS). In this context, recently, electrospray ionization-MS (ESI-MS) experiments were carried out for binary ionic liquids mixtures and pure ionic liquids to develop qualitative scales of the interaction between cations and anio...

We evaluated the electrochemical behavior of a LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) electrode in a neat ionic liquid, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI), electrolyte. According to charge/discharge measurements, the discharge capacity of the NMC cathode in EMImFSI at 1 C in a voltage range of 3.0-4.5 (vs. Li/Li + ) shows 165 mAh g -1 , which is higher than that in a conventional electrolyte, EC+DMC. The rate capability of a cell with EMImFSI exceeds that with EC+DMC, especially at high rates, probably due to a low resistance at the electrode/electrolyte interface. These results suggest that EMImFSI is a suitable electrolyte for Li-ion batteries utilizing a NMC cathode. Also, we applied fluorine-containing solvents (FCSs) to an electrolyte for Li-ion batteries with a cobalt oxide (LiCoO 2 ) cathode to enhance the upper voltage limit and investigated its electrochemical properties in a proposed FCS-based electrolyte. A test cell containing a FCS-based electrolyte exhibits high and stable discharge capacity for 30 cycles even with a high upper cutoff voltage of 4.5 V when compared with a cell containing a conventional electrolyte. IntroductionAmbient temperature ionic liquids have attracted increasing attention in batteries because of their wide potential window, acceptable conductivity, high thermal stability and negligible vapor pressure. However, Li-ion batteries with ionic liquid electrolytes have some difficulties in their charge-discharge performance. Decomposition of ionic liquids on a negative electrode and intercalation of an organic cation, which lead to irreversible Li + insertion/extraction, are serious problems of losing their charge-discharge capacity.We successfully applied bis(fluorosulfonyl)imide anion (FSI)-based ionic liquids to electrolytes for Li-ion secondary batteries, which provide reversible Li-ion insertion/extraction and high charge-discharge efficiency for a graphitized negative electrode without any additives (1-5). We have also investigated electrochemical behavior of various electrodes in these ionic liquids, e.g., Ni-Si-C composite or La-Ni-Sn alloy (6). We found these electrodes exhibit reversibility in our various ionic liquid electrolytes. Furthermore, a LiNi 1/3 Mn 1/3 Co 1/3 O 2 positive electrode showed excellent reversibility even at 4.5V in an EMIFSI-based electrolyte with an initial capacity of 171 ECS Transactions, 33 (28) 29-36 (2011) 10.1149/1.3563087 © The Electrochemical Society 29 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 141.211.4.224 Downloaded on 2015-07-12 to IPmAh g -1 and 98% of this capacity at the 50th cycle. Elexcel Co. tested Li-ion batteries containing FSI ionic liquids. A Li-ion battery with LiMn 1/3 Ni 1/3 Co 1/3 O 2 as cathode material maintained over 80% of its initial capacity in 2000 cycles at 100% DOD. The power density of the cell reached to 2050W kg -1 at 50% SOC.On the other hand, recently, a nonflammable fluorine-containing solvent (FCS) has ...