An alternative route to single ion conductivity using multi-ionic salts

Abstract

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 t^PP₊ = 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 (t^PP₊ = 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.