Li Coordination of a Novel Asymmetric Anion in Ionic Liquid-in-Li Salt Electrolytes

Abstract

We analyze the influence of the asymmetry of the anion on coordination and transport processes in a Li salt/ionic liquid system. The relatively new asymmetric 2,2,2-trifluoromethylsulfonyl-N-cyanamide (TFSAM) anion was investigated in Pyr14TFSAM(1–x)LiTFSAM x over a broad concentration range (up to x = 0.7 Li salt) and was compared to the well-known bis­(trifluoromethanesulfonyl)­amide (TFSA) anion. In contrast to the TFSA-based system, the system with TFSAM has no phase transition over the whole concentration range. Raman spectroscopy and NMR chemical shifts elucidate the Li coordination in detail. Up to x = 0.3, the asymmetric anion coordinates to Li+ only via the cyano group. With increasing Li salt fraction, the contribution of Li–oxygen coordination increases. This coordination effects influence the transport properties of the system, as examined via pulsed-field-gradient NMR (PFG-NMR). Although the overall diffusivity of both systems is decreasing because of viscosity effects, the relative diffusivity of the Li cation is increasing with x. This suggests a change in the transport mechanism depending on the Li salt fraction. Interestingly, the contribution of structural diffusion at high Li salt concentrations (x ≥ 0.6) seems to be higher in the TFSAM system, influenced by the nonsymmetric coordination, while in the TFSA system, the vehicular transport seems to be still predominant at x ≥ 0.6.

Ionic Liquid in Li Salt Electrolyte: Modifying the Li+ Transport Mechanism by Coordination to an Asymmetric Anion

“…[ 44 ] In this context, a comparison to the corresponding system with the asymmetric anion TFSAM is interesting. [ 13 ] There, Li + coordination with the strongly interacting CN group is preferred, whereas coordination to the more weakly interacting TFSI group does not occur at all up to a Li salt fraction above 0.3. This more pronounced asymmetry is arising, as in TFSAM, the smaller functional group provides stronger coordination, whereas in FTFSI, the bulkier functional group has a stronger Li + coordination.…”

“…One example is the 2,2,2‐trifluoromethylsulfonyl‐ N ‐cyanoamide (TFSAM) anion with asymmetric coordination properties, combining the functional groups of bis(trifluoromethanesulfonyl)imide (TFSI) and dicyanamide (DCA). [ 12,13 ] Furthermore, Giffin et al introduced the FTFSI anion, which combines the terminal groups of TFSI and FSI, in IL‐in‐salt systems and demonstrated that it hinders the crystallization at sample concentrations of up to 65 mol% Li salt at room temperature. [ 14 ]…”

Pyrrolidinium Containing Ionic Liquid Electrolytes for Li-Based Batteries

“…Utilization of different anions will allow for more electrolyte tunability. For example, the asymmetric TFSAM anion promotes single Li + hopping due to higher structural diffusion and is capable of inhibiting crystallization over a broader temperature range when compared to TFSI [180]. In addition, alternative anions can be utilized to synthesize ILs suitable for electrodeposition, such as C 4 mpyrDCA, which has been utilized to deposit Ge thin films [199].…”

“…This study highlighted the effectiveness of Li + mobility within C 4 mpyr-based ILs, which is why extensive research has been carried out using C 4 mpyr-based electrolytes with those and trifluoromethanesulfonyl-N-cyanoamide (TFSAM) anions. Recently, TFSAM has been utilized as an anion due to its ability to inhibit crystallization over a broader temperature range when compared to TFSI [180]. The asymmetric TFSAM anion, promotes single Li + hopping due to higher structural diffusion that occurs as the LiTFSAM salt concentration increases [180].…”

“…Recently, TFSAM has been utilized as an anion due to its ability to inhibit crystallization over a broader temperature range when compared to TFSI [180]. The asymmetric TFSAM anion, promotes single Li + hopping due to higher structural diffusion that occurs as the LiTFSAM salt concentration increases [180]. Electrolytes containing TFSAM anions, have not been widely investigated, and present a potential area for analysis, however, C 4 mpyrTFSAM compatibility with NCM electrodes has been investigated by Hoffknecht et al [181].…”