Evaluation of Litdi Vs. LiPF6 As Electrolytes in Contact with Several Cathodes and Anodes Materials
Authors: Schmidt Gregory , Cayrefourcq Ian , Paillet Sabrina , Fréchette Joël , Barray Françis , Clément Daniel , Hovington Pierre , Guerfi Abdelbast , Zaghib Karim 
Published: February 2020

Introduction

Lithium-ion batteries supplying energy for electric cars and other devices are mostly based on electrolytes using LiPF6. 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 LiPF6 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 LiPF6 by salts with improved thermal, chemical and electrochemical properties. Unfortunately, most attempts to replace LiPF6 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 LiPF6. 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 LiMn2O4 at operational voltage range of 3 - 4.3 V.
In the present study, we are interested to compare LiPF6 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: LiNi1/3Mn1/3Co1/3O2 [3-4.2V], LiMn2O4 [3–4.4V], LiFePO4 [2-4V], LiCoO2 [3- 4.2V] and also with two type of anodes materials: graphite [0-2.5V], and Li4Ti5O12 [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 LiPF6.
During the formation, we found that:
- for positive electrodes, results with and without FEC gave similar discharged and irreversible capacities compared to LiPF6.
- for graphite, results showed that it’s necessary to use FEC to form a thin and stable SEI (validated by SEM analysis).
- for Li4Ti5O12 , the use of additive increases the irreversible capacity.
For all materials, the ragone analysis showed competitive results between LiTDI and LiPF6, as it can be seen in the Figure 1, for LiNi1/3Mn1/3Co1/3O2 and LiMn2O4 .
Moreover, thanks to its high chemical stability, LiTDI has showed better performances than LiPF6 in term of safety. Indeed, burning experiments have been realized and clearly showed a decrease of generated dangerous gas.
Pouch cells batteries with LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12 [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.