Evaluation of Litdi Vs. LiPF6 As Electrolytes in Contact with Several Cathodes and Anodes Materials (2014)

Introduction Lithium-ion batteries supplying energy for electric cars and other devices are mostly based on electrolytes using LiPF 6 . The popularity of this salt is mostly due to the formation of a passivation layer with aluminum which made possible the utilization of this low cost material as current collector. In counter part, the thermal instability of LiPF 6 and it’s reactivity with moisture or protic species release a toxic, highly aggressive and undesirable gaz, HF. These major drawbacks have prompt researchers to replace LiPF 6 by salts with improved thermal, chemical and electrochemical properties. Unfortunately, most attempts to replace LiPF 6 with hydrolytically-stable salts have been unsuccessful because of Al corrosion [1] . Niedzicki et al. [2] showed that lithium 4,5-dicyano-2-(trifluoromethyl) imidazolide, commonly known as LiTDI could be an interesting substitute to LiPF 6 . They showed LiTDI didn’t corrode Al, which are consistent with our results, and have a thermal stability up to 250°C. It has a satisfactory conductivity thereby showing good performances with LiMn 2 O 4 at operational voltage range of 3 - 4.3 V. In the present study, we are interested to compare LiPF 6 and LiTDI. This research is in collaboration between Arkema [3] , which has developed an industrial process to synthesize battery grade LiTDI, and Hydro-Québec. Electrochemical performances of LiTDI in EC-DEC (3-7 v-v) was evaluated in coin cells (vs. Li) with four different cathodes materials: LiNi 1/3 Mn 1/3 Co 1/3 O 2 [3-4.2V], LiMn 2 O 4 [3–4.4V], LiFePO 4 [2-4V], LiCoO 2 [3- 4.2V] and also with two type of anodes materials: graphite [0-2.5V], and Li 4 Ti 5 O 12 [1.2-2.5V]. Results The results obtained in this study with LiTDI are very promising. For all materials tested, the LiTDI has showed good compatibilities and results are comparable with those of LiPF 6 . During the formation, we found that: - for positive electrodes, results with and without FEC gave similar discharged and irreversible capacities compared to LiPF 6 . - for graphite, results showed that it’s necessary to use FEC to form a thin and stable SEI (validated by SEM analysis). - for Li 4 Ti 5 O 12 , the use of additive increases the irreversible capacity. For all materials, the ragone analysis showed competitive results between LiTDI and LiPF 6 , as it can be seen in the Figure 1, for LiNi 1/3 Mn 1/3 Co 1/3 O 2 and LiMn 2 O 4 . Moreover, thanks to its high chemical stability, LiTDI has showed better performances than LiPF 6 in term of safety. Indeed, burning experiments have been realized and clearly showed a decrease of generated dangerous gas. Pouch cells batteries with LiNi 1/3 Mn 1/3 Co 1/3 O 2 /Li 4 Ti 5 O 12 [1.5-2.8V] were also assembled for long-term cycling (C/4-D/4) at 25°C and 40°C. At the moment at 130 cycles, the lost capacity is only 2% for each temperature, which demonstrate the good stability of LiTDI. References 1. K. Krause et al , J. Power Sources, 68 ( 1997 ), 320-325 2. Niedzicki et al. , J . Power Sources , 196 ( 2011 ), 8696-8700 3. G. Schmidt et al ., FR2982610.