A New Thermally Stable and Conducting Electrolyte Based on Litdi As Lithium Salt for Li-Ion Batteries

Abstract

Most of the LiBs used in electrical devices currently available contain lithium hexafluorophosphate (LiPF6) dissolved in a mixture of alkylcarbonates as the electrolyte. The main reasons for this [1] are the high conductivity of LiPF6 solutions, the formation of a protective SEI layer on graphite and its ability to passivate aluminum, a low-cost metal which can be used as a current collector. Other advantageous properties, which should also be observed by all LiB electrolytes, include low viscosity and a large life-span without safety risks, especially when damaged. However, the thermodynamic instability and moisture sensitivity of LiPF6 induce high risks of releasing HF and/or PF5 via its thermal decomposition or hydrolysis in the presence of traces of water. These drawbacks have been highly criticized justifying the importance in finding a replacement. Owing to (i) its high thermal stability, (ii) low sensitivity to the presence of water and (iii) its electrochemical stability [2], the lithium 4,5-dicyano-2-trifluoromethyl-imidazole (LiTDI) is seen as a promising substitute for LiPF6in LiBs. In a previous study [3] the poor dissociation of ion pairs (IP) of LiTDI in electrolyte solutions was discussed and the IP dissociation coefficient was found to be around 30 % in an ethylene carbonate/dimethyl carbonate (EC/DMC) mixture. The power capability of LiTDI in EC/DMC with an added 2% FEC in half-cells, using either a NMC or a graphite electrode, and in full NMC/graphite systems was investigated and compared to that of LiPF6 in the same solvent mixture. It was noticed that both lithium salts performed equally within graphite half-cells but not in the NMC half-cells where a greater capacity loss was observed in the case of the LiTDI based electrolyte when increasing the NMC lithiation speed. The reason was attributed to the lower IP dissociation of LiTDI; LiPF6 being 85% dissociated in EC/DMC. Taking into account this observation a new solvent mixtures is proposed. We present here the results obtained using this new solvent mixture allowing a better dissociation of LiTDI (37 %), a higher conductivity (8.5 mS.cm-1 instead of 6.1 mS.cm-1in EC/DMC at 25°C) and a lower viscosity (2.4 mPa.s instead of 2.8 mPa.s in EC/DMC at 25°C). In addition a better power capability is achieved at higher current rates. 1. Eshetu, G.G., et al., LiFSI vs. LiPF6 electrolytes in contact with lithiated graphite: Comparing thermal stabilities and identification of specific SEI-reinforcing additives. Electrochimica Acta, 2013. 102(0): p. 133-141. 2. Niedzicki, L., et al., New covalent salts of the 4V class for Li batteries. Journal of Power Sources, 2011. 196(20): p. 8696-8700. 3. Berhaut, C.L., et al., LiTDI as electrolyte salt for Li-ion batteries: transport properties in EC/DMC. Electrochimica Acta, 2015. 180: p. 778-787.