An alternative route to single ion conductivity using multi-ionic salts†
Multi-ionic lithium salts comprised of polyoligomeric silsesquioxanes (POSS) functionalized with eight – (LiNSO2CF3) groups, referred to as POSS-(LiNSO2CF3)8, can be dissolved at very high loadings into tetraglyme (G4), where they can be considered solvent-in-salt electrolytes. With increasing dilution, colloidal solutions are formed. Two systems were investigated, neat POSS-(LiNSO2CF3)8 in G4 and mixtures of POSS-(LiNSO2CF3)8 with LiPF6 or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). PFG-NMR indicates that Li can be un-dissociated, completely dissociated and surrounded by G4 molecules, or as contact ion pairs (in which there are 3–4 ether oxygen contacts and one contact with the oxygen from the anion). Equilibria exist between the species of POSS-(LiNSO2CF3)8 and if there is rapid equilibration between the Li states, and close enough proximity between the POSS-(LiNSO2CF3), then the Li+ ions can migrate by a Grotthus-type coordinated hopping mechanism, as well as by a purely diffusive motion. Unlike polymer single ion conductors, where the backbone flexibility permits cluster/aggregate formation, which inhibits escape and mobility of the Li+ ions, the rigid POSS cube and its colloidal structure in G4 prevents formation of POSS-(NSO2CF3−)⋯Li+⋯(−CF3NSO2)-POSS triplets. Instead, the solvated Li+ in POSS-(NSO2CF3−)⋯Li+⋯G4 can be more easily removed to form conductive G4⋯Li+−⋯G4. Good ionic conductivities (∼10−4 S cm−1) and lithium ion transference numbers of tPP+ = 0.65 can be achieved in these systems. Mixtures of 80 wt% LiTFSI and 20 wt% POSS-(LiNSO2CF3)8 in G4 at an O/Li ratio of 20/1, yield both high conductivity (σ = 3.3 × 10−3 S cm−1) and high (tPP+ = 0.65) transference number. Stable cycling between C/2 and 2C with high capacity retention was achieved using Li/[G4/80 wt% LiTFSI/20 wt% POSS-(LiNSO2CF3)8]/LiFePO4 half-cells.
- This article is part of the themed collection: International Year of the Periodic Table: Elements for Next Generation Batteries