Litdi As Electrolyte Salt for Li-Ion Batteries: Transport Properties in EC/Dmc and Cyclability of Li/Graphite and Li/LiFePO4 Half Cells

The lithium-ion battery (LiB) technology has become over the years one of the most popular type of rechargeable batteries for portable electronics [1]. Now in a world knowing a progressive electrification of transport systems the LiBs are growing in popularity for large-scale applications such as electric vehicles. Therefore the continuing success and progress of the LiB technology depends on the careful choice of the battery components such as the electrolyte [2-4].

It is needless to say that the electrolyte is a very important component in batteries as it allows ionic movements, and should maintain its functions over its life-span without presenting safety risks, especially when damaged. The most commonly used electrolytes are lithium hexafluorophosphate (LiPF₆) dissolved in mixtures of alkylcarbonate solvents. LiPF₆ is actually the reference Li-salt for LiB due to its numerous convenient properties [3, 5, 6]. However its thermal instability and moisture sensitivity, which induce high risks of releasing HF via the hydrolysis of the PF₆^- anion, have been highly criticized justifying the importance in finding a replacement.

The new 4,5-dicyano-2-trifluoromethyl-imidazolide (TDI^-) anion is a promising anion for lithium salt which will be used in  future energy storage systems such as lithium-ion batteries. Here we will compare LiTDI solutions in EC/DMC (50:50 wt %) to competing salts like LiTFSI, LiFAP and the commonly used LiPF₆, known for its numerous advantageous properties, in the same solvent mixture. LiTDI in EC/DMC exhibits the lowest viscosity among the investigated electrolytes, and a fair conductivity of 6.84 mS.cm^-1 at 25°C (1mol.L^-1). LiTDI also displays in EC/DMC a higher transference number than LiPF₆ in the same solvent mixture which is interesting since only the current carried by the lithium ions is useful.  LiTDI, as many other lithium salts, is not completely dissociated in EC/DMC and an estimation of the ion-pair dissociation coefficient was achieved by using the Walden rule. An estimation was also calculated for LiTFSI, LiFAP and LiPF₆.

In order to evaluate the performance of the LiTDI-EC/DMC electrolyte for use in Li-ion batteries, Li/graphite and Li/LiFePO₄ half-cell systems have been investigated. Despite an important irreversible capacity within the lithium/graphite half-cell during the first charge-discharge cycle, a reasonable average reversible capacity has been obtained at a low cycling-rate (C/24) and easily repeated after 30 cycles. The obtained capacity was also higher than for LiTFSI and LiFAP based electrolytes in the same conditions. Nevertheless, at higher cycling-rates the performance of the LiTDI based electrolyte is not as good as LiPF₆, even though the coulombic efficiencies range from 99.5% and 100%. A more resistive or/and thicker solid electrolyte interface (SEI) was identified as the cause of capacity loss in LiTDI half-cell. Tests are on the way (XPS, EDX, EIS and EQCM) to investigate the composition and characteristics of the SEI formed in the presence of LiTDI. Several additives have also been tested by addition to the electrolyte mixture and their impact on the SEI observed.  Lithium/LiFePO₄ half-cells exhibit good capacity retention over 100 cycles at low rate (150 mA.h.g^-1 at C/24) which does not drop too much when cycling at a C/2 (128 mA.h.g^-1).

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