Solvation structures of potassium bis(trifluoromethylsulfonyl)imide–glyme highly concentrated electrolytes and cycling on organic cathodes

Abstract

The physicochemical properties of highly concentrated electrolytes consisting of potassium bis(trifluoromethylsulfonyl)imide in oligo(ethyleneglycol) dimethyl ethers (glymes) are reported. The solvation structures were analyzed by Raman spectroscopy, single crystal X-ray diffraction, and theoretical calculations revealing significant ion-pairing at high concentrations especially for the shorter glymes, leading to them being classified as poor solvate ionic liquids with the general formula [K(L)_n][Tf₂N] (L = G1, G2, G3, or G4; n = 2, 3 or 4). [K(G1)₂][Tf₂N] and [K(G2)₂][Tf₂N] had a sufficiently high ionic conductivity and were assessed as potential electrolytes in next-generation potassium-ion batteries. They were subjected to galvanostatic cycling on the new organic cathode material K₂–Co–PTtSA (PTtSA = benzene-1,2,4,5-tetra-methylsulfonamide). The results show a significantly improved capacity retention with these electrolytes, especially [K(G2)₂][Tf₂N] (91% after 200 cycles), compared to a benchmark electrolyte consisting of 1 mol L⁻¹ KPF₆ in a mixture of ethylene carbonate and propylene carbonate (19% after 200 cycles). However, the performance at high C-rates was worse due to the high viscosities of highly concentrated electrolytes. This work shows that highly concentrated electrolytes and solvate ionic liquids can be promising electrolytes for next-generation potassium-ion batteries but can be further improved to reduce their viscosity and increase their ionic conductivity.