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

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: LiNi_1/3Mn_1/3Co_1/3O₂ [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 LiNi_1/3Mn_1/3Co_1/3O₂ 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 LiNi_1/3Mn_1/3Co_1/3O₂/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.