γ-Butyrolactone-ethylene carbonate based electrolytes for lithium-ion batteries

Abstract

γ-Butyrolactone-ethylene carbonate (BL-EC) mixtures have been investigated as electrolytes for Li-ion batteries using LiPF₆ and LiBF₄ as lithium salt. The thermal stability of the electrolytes in a large range of temperatures (−90 °C to 40 °C) have been studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). From the results of these experiments, the phase diagram of the BL-EC system has been determined. It is characterised by a eutectic point at −56.3 °C and a molar fraction in EC: x _EC = 0.1. A metastable compound has been demonstrated below −90 °C at x _EC = 0.4. Conductivity measurements of BL-EC solutions, in the presence of LiPF₆ and LiBF₄, indicate that LiPF₆ in the eutectic mixture is the most conducting electrolyte in the range of temperatures investigated (−30 °C to room temperature). Nevertheless, at low temperature, LiBF₄ based electrolytes compete well with LiPF₆, especially when the amount of EC in the mixture is as high as x _EC = 0.5. Moreover, recrystallisation of the salt below −20 °C is avoided when LiBF₄ is used as salt. A large increase in viscosity of the solvent mixture is observed when a salt is added, but the increase is lower for LiBF₄ than LiPF₆. When EC is added to BL at constant salt concentration (1 M), the conductivity of LiPF₆ solutions decreases more rapidly than LiBF₄ solutions. This has been attributed, at least partially, to the dissociating power of EC. The electrochemical windows of BL-EC (equimolar) mixtures in the presence of LiPF₆ and LiBF₄ are comparable but it is shown that the solvents oxidation rate at high potentials is lower when LiBF₄ is used.