Ionic liquids and oligomer electrolytes based on the B(CN)4− anion; ion association, physical and electrochemical properties

Abstract

The role of B(CN)(4)(-) (Bison) as a component of battery electrolytes is addressed by investigating the ionic conductivity and phase behaviour of ionic liquids (ILs), ion association mechanisms, and the electrochemical stability and cycling properties of LiBison based electrochemical cells. For C(4)mpyrBison and C(2)mimBison ILs, and mixtures thereof, high ionic conductivities (3.4 ≤σ(ion)≤ 18 mS cm(-1)) are measured, which together with the glass transition temperatures (-80 ≤T(g)≤-76 °C) are found to shift systematically for most compositions. Unfortunately, poor solubility of LiBison in these ILs hinders their use as solvents for lithium salts, although good NaBison solubility offers an alternative application in Na(+) conducting electrolytes. The poor IL solubility of LiBison is predicted to be a result of a preferred monodentate ion association, according to first principles modelling, supported by Raman spectroscopy. The solubility is much improved in strongly Li(+) coordinating oligomers, for example polyethylene glycol dimethyl ether (PEGDME), with the practical performance tested in electrochemical cells. The electrolyte is found to be stable in Li/LiFePO(4) coin cells up to 4 V vs. Li and shows promising cycling performance, with a capacity retention of 99% over 22 cycles.

Mixtures of ionic liquids

“…49 Hence, it is often difficult to obtain a full phase diagram. 36,[50][51][52][53][54] Also, it has been pointed out that thermograms may arise from samples that are not in thermodynamic equilibrium. 35 found the glass transition temperature of the mixture to be a weighted average of the values for pure components.…”

“…, though no glass transition was found for low imidazolium concentrations. 53 Most salt mixtures display a eutectic (a composition that freezes at a single temperature, without separation of the pure components, or a change in composition, and at the lowest temperature of any composition of the mixture) in their phase diagram. 10,56 One way of exploiting this to get around the well-known problem of preparing ionic liquids in high purity has been offered by Dunstan and Caja, by making two ionic liquids that melt above room temperature, using recrystallization to purify these and then mixing them at their eutectic composition to create a highly pure ionic liquid mixture.…”

Lithium electrochemistry and cycling behaviour of ionic liquids using cyano based anions

“…We have tried to solubilise a range of lithium salts (e.g., LiDCA, LiTFSI, LiFSI, LiPF 6 and LiBF 4 ) into C 4 mpyr TCB with no success above a concentration of 0.3 mol.kg À1 , which limits the viability of C 4 mpyr TCB for use as a lithium battery electrolyte. Scheers et al 31 have reported the same issue for TCB based ILs; in order to overcome this, they introduced glycol dimethyl ether to dissolve the Li salts in the mixture, which is similar to the approach of Watanabe and co-workers. [32][33][34][35] For C 4 mpyr DCA, we have chosen to use a salt concentration of 0.5 mol.kg À1 LiDCA as this was found to exhibit the optimum Li|Li + behaviour in solution.…”

Dielectric study on mixtures of ionic liquids

“…refs. 14,16,20,21,22,23,24) and solely Stoppa et al 15 performed dielectric spectroscopy. In the present work, we thoroughly investigate two binary mixtures of ionic liquids using broadband dielectric spectroscopy and differential scanning calorimetry (DSC).…”