Ion Transport Properties in Glyme-Based Highly Concentrated Electrolyte with Asymmetric Li Imide Salts

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The spread of electric vehicles is being expanded around the world with the aim of reducing carbon dioxide emissions toward carbon neutrality by 2050. This trend requires even higher performance for Li secondary batteries. Previous research has focused on increasing ionic conductivity of the electrolyte. It is now clear that the Li ion transference number is equally important to further improve the performance of batteries for automotive applications[1]. However, the relationship between the structure of Li salts and ion transport properties is not understood well, and there is no established method to systematically increase the Li ion transference number. Here we show how the structure of the Li salt affects the ion transport properties for an equimolar mixture of a novel Li salt and tetraglyme (G4). We have synthesized a new Li imide salt, Li (N-(trifluoromethyl)sulfonyl)acetamide) (Li[TfNAc]), having both sulfonyl and carbonyl groups. For the novel highly concentrated electrolyte [Li(G4)][TfNAc], we observed a high Li ion transference number of 0.65 at the cost of a low ionic conductivity of 0.21 mS cm−1. This trade-off between ionic conductivity and transference number is in line with previous empirical observations made in our group[2][3]. Ab initio molecular orbital calculations of lithium affinity in comparison with anions in established electrolytes revealed the order [TFSA]− < [BF4]− < [TfNAc]− < [TFA]−, consistent with the qualitative order observed in ionic transport properties. Thus, ion transport is affected significantly by small changes to the anion structure. We observed that the solvate structure of [Li(G4)][TfNAc] is dominated by correlated motion in ion pairs and larger aggregates, in line with previous studies.[3] Based on the concepts of ionicity (inverse Haven ratio) and Onsager transport coefficients[4], we were able to quantify the impact of ion interactions on the macroscopic transport properties.[1]K. M. Diederichsen et al, ACS Energy Lett., 2017, 2, 2563.[2]K. Shigenobu, et al., Phys. Chem. Chem. Phys., 2020, 22, 15214.[3] K. Shigenobu, et al., Phys. Chem. Chem. Phys., 2021, 23, 2622.[4] Dong, D.; Sälzer, F.; Roling, B.; Bedrov, D., Phy. Chem. Chem. Phy., 2018, 20, 29174.