Effect of Anion Mass on Conductivity and Lithium-Ion Transference Number in the Isomorphic Cocrystals (Adpn)2LiXF6 (Adpn = Adiponitrile, X = P, As, Sb)

Abstract

The solid lithium-ion electrolytes (Adpn)₂LiXF₆ (Adpn = adiponitrile, X = P, As, Sb) are isomorphic salt-solvate cocrystals with slight differences in lattice spacing (<0.15 Å). The Li⁺ cations are coordinated by Adpn molecules and separated from the anions so that the diffusion of the anions and cations is decoupled. As shown previously for the hexafluorphosphate analogue, (Adpn)₂LiPF₆, the motion of Li⁺ ions is through a solvate-mediated hopping mechanism, which is expected to be similar in all of the cocrystals. The crystal grains are surrounded and connected by a fluid-like grain-boundary network. Pulsed-field gradient ⁷Li NMR, which measures diffusion in both the grains and the grain boundaries, indicated that the Li diffusion coefficients for the cocrystals were similar (<D_Li⁺> = 1.77 × 10^–6 cm²/s). The transference numbers for Li⁺ ions in Adpn₂LiPF₆ measured by PFG-NMR at 80 °C, t_Li⁺,PFG = 0.54, is in great agreement with t_Li⁺,MD = 0.54 - predicted by molecular dynamics simulations at 27 °C using a grain-boundary atomistic model. Lithium-ion transference numbers, t_Li⁺, calculated from steady-state impedance spectroscopy are 0.53, 0.63, and 0.83 for X = P, As, and Sb cocrystals, respectively, showing a lower contribution of anion charge carriers, with increasing mass of the anions, to the conductivity of these cocrystalline electrolytes. Diffusion coefficients for the AsF₆^– and SbF₆^– anions were calculated using measured values of σ and t_Li⁺ and decreased with increasing mass of the anion in the order D_PF6- > D_AsF6- > D_SbF6-. Conductivities of the cocrystals measured by EIS are in the order σ(Adpn₂LiPF₆) > σ(Adpn₂LiAsF₆) > σ(Adpn₂LiSbF₆), while conductivities of 0.04 M solutions of the salts in Adpn decreased slightly in the opposite order LiSbF₆ > LiAsF₆ > LiPF₆. The latter reflects better dissociation (and thus a greater number of free ions) of Li⁺ from the heavier, more polarizable anions in dilute solution, attributed to hard–soft acid–base theory. In contrast, in the solid cocrystal, all ions are separated, and so conductivity is governed by the hopping ability of the ions, where the heavier anions diffuse more slowly. Since the total conductivity decreases in the opposite order, MD simulations suggest that the cations and anions in the nanoconfined regions of the grain boundaries are more concentrated and are exchangeable with the bulk phase grains.