LiMOB, an Unsymmetrical Nonaromatic Orthoborate Salt for Nonaqueous Solution Electrochemical Applications

Synthesis, characterization, ionic conductivity, electrochemical stability, and lithium-ion transport number of an asymmetric version of the successful orthoborate lithium salt lithium bis(oxalato)borate (LiBOB), are described. The salt, lithium (malonato oxalato) borate (LiMOB), is thermally stable up to 273°C. It is poorly soluble in common organic solvents such as 1,2-dimethoxyethane, tetrahydrofuran, acetonitrile, dimethyl carbonate, and propylene carbonate, but has moderate solubility in γ-butyrolactone (GBL) and N,N-dimethylformamide (DMF), and high solubility in dimethyl sulfoxide (DMSO). The 0.5 M solutions of LiMOB in GBL, DMSO, and DMF show high conductivities (5.0 × 10^-3 S cm^-1, 5.1 × 10^-3 S cm^-1, and 11.8 × 10^-3 S cm ^-1 at 25°C, respectively) relative to most 0.5 M nonaqueous solutions. Even the room temperature conductivity of the 0.08 M LiMOB-PC solution is close to 10^-3 S cm^-1. The conductivities of LiMOB solutions are close to those of LiBOB solutions, which has been demonstrated in recent studies to be a successful substitute for LiPF ₆ in lithium-ion batteries. The cyclic voltammograms show that LiMOB solutions in PC and GBL have electrochemical stability as high as 4.3 V vs. Li⁺/Li. The lithium-ion transport number at room temperature for 0.5 M LiMOB in DMSO-d₆ was 0.36, obtained from self-diffusivity measurement by the pulsed field gradient spin echo nuclear magnetic resonance technique. The highest occupied molecular orbital energy, which is usually correlated with electrochemical stability has been calculated, and electrostatic potential maps for the new anion and its fluorinated derivatives are presented. They suggest that the fluorinated analogue of LiMOB should be more stable and more soluble, and may have unique oroperties.