Extended Tests of Pyrazolides As Lithium Salts for Lithium-Ion Cells

Lithium ion batteries nowadays dominate the market of high power energy storage devices [1]. One of the crucial points for next generation lithium batteries and dual-ion cells for stationary energy storage is the development of lithium salts with enhanced electrochemical stability and without corrosive impact at Al current collectors. Additionally, novel electrolytes for lithium batteries should have a high conductivity and should guarantee long term stability of anode and cathode interfaces as well as lead to stable SEI films. Here, we introduce first examples of a new generation of lithium salts based on pyrazolide anions [2] (cf. Fig. 1). The cyclic anion contains different perfluorinated side chains R_F at the 3- and 5-position. Further tests were done with additional modifications at the anionic structure.

Fig. 1: Structures and half-cell cycling with different discharge rates of the anionic pyrazolides and their modified types, R_F representing perfluorinated alkyl chains of different lengths.

Both, an efficient synthetic route as well as a thorough characterization of electrochemical stability and conductivity as well as cell tests were carried out. The salts showed an excellent electrochemical stability in non-aqueous solvents up to +5 V vs. Li/Li⁺. Moreover, cycling of LiFePO₄ half-cells revealed structure-conductivity relationships for the novel salts. Both salt-types allowed at least 200 full cycles. Thus, the novel class lithium salts exhibits promising candidates for novel liquid and polymeric electrolyte systems and future safe lithium ion batteries.

The authors acknowledge funding by the German federal ministry of education and research (BMBF) within the project “Insider”, No. 03EK3031A.

[1] M. Winter, R. J. Brodd, Chemical Reviews, 104, 4245, 2004.

[2] M. Grünebaum, H.-D. Wiemhöfer, M.M. Hiller, WO2013135824 A2, 2012.