Investigations on Electrochemical Performance As Well As Thermal Stability of Two New Lithium Electrolyte Salts Compared to LiPF6

The demand for lithium-ion batteries has increased tremendously in the last few decades.[1] Since many applications require a wide working temperature range, new cell materials, especially regarding the conducting electrolyte salt, are necessary. Besides the temperature stability, the alternating salt should yield comparable results regarding the electrochemical cycling, the conductivity, the ability to form a stable layer on the graphite anode and the specification to avoid aluminum dissolution of the current collector on the cathode side.[2] Additionally, a low price, decent availability and a low toxicity are desirable. Another very important aspect, regularly highlighted by the media, is the safety of the battery.

All these points have to be taken into consideration when testing alternatives for the currently used electrolyte components.[3, 4]

In this work we investigated electrolyte solutions, containing two lithium salts lithium-cyclo-difluoromethane-1,1-bis(sulfonyl)imide (LiDMSI) and lithium-cyclo-hexafluoropropane-1,1-bis(sulfonyl)imide (LiHPSI), dissolved in organic carbonate solvents.[5] These electrolytes were electrochemically investigated on graphite (Figure 1) and LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) electrodes and compared to the electrolyte salt LiPF₆with regard to conductivity, the electrochemical stability window, the anodic dissolution behavior vs. aluminum as well as the thermal stability behavior at 60 °C (Figure 2). Furthermore, XPS studies were carried out to investigate the influence of the salt on the composition and the thickness of the solid electrolyte interphase (SEI). Constant current cycling experiments proved the potential applicability of the investigated salts for lithium ion batteries.

Figure1:

First 23 charge/discharge cycles of graphite in a EC:DEC (1:1) (by weight %)  electrolyte containing 1 M of the electrolyte salts LiDMSI, LiHPSI or LiPF₆, cycled at 20°C with Li as CE and RE. The rate of the first three cycles was C/5 and for the following 20 cycles 1C. A 1 h constant potential step at 0.025 V vs. Li/Li⁺ was implemented into the discharge step. The potential range values from 1.5 V to 0.025 V vs. Li/Li⁺.  Both, the de-intercalation capacity (discharge capacity) and the coulombic efficiency are plotted versus the number of cycles. The graphite electrodes had an average mass loading of about 2.5 mg.

[1] J.B. Goodenough, Accounts Chem Res, (2012).
[2] E. Kramer, T. Schedlbauer, B. Hoffmann, L. Terborg, S. Nowak, H.J. Gores, S. Passerini, M. Winter, J Electrochem Soc, 160 (2013) A356-A360.
[3] K. Xu, Chem Rev, 104 (2004) 4303-4417.
[4] R. Wagner, N. Preschitschek, S. Passerini, J. Leker, M. Winter, J Appl Electrochem, 43 (2013) 481-496.
[5] L.H. Pohl, Volker; Sartori, Peter; Juschke, Ralf, PCT Int. Appl. (1997), WO 9731909 A1 19970904.