Guide to water free lithium bis(oxalate) borate (LiBOB)

Abstract

Lithium bis(oxalate) borate, LiB(C2O4)(2) (LiBOB), is one of the most important electrolyte additives for Li-ion batteries (LIBs) due to its numerous advantages such as thermal stability, good solubility in organic solvents, high conductivity, and low cost as well as providing safer operations with superior electrochemical performance compared to conventional electrolyte combinations. However, the use of LiBOB is limited due to slight instability issues under ambient conditions that might require extra purification steps and result in poorer performances in real systems. Here, we address some of these issues and report a high purity water free LiBOB synthesized with fewer processing steps, employing lithium carbonate, oxalic acid, and boric acid as low-cost starting materials, and via ceramic processing methods under protective atmosphere. The physical and chemical characterizations of both anhydrous and monohydrate phases are performed with X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM) analyses to determine the degree of the purity and the formation of impurities, such as LiBOB center dot H2O, HBO2, and Li2C2O4, as a result of the aging investigations where the as-synthesized salt was exposed to ambient conditions for different durations. Differential thermal analysis (DTA) is applied to determine the optimum synthesis conditions for anhydrous LiBOB and to analyze the water loss and the decomposition of LiBOB center dot H2O. Aging experiments with the water free LiBOB are carried out to evaluate the effect of humidity on the phase changes and resulting impurities under various conditions. The detrimental effect of even slightest humidity conditions is shown, and protective measures during and after the synthesis of LiBOB are discussed. Anhydrous LiBOB could be widely used as an electrolyte additive to improve the overall electrochemical performances for LIBs through development of a protective solid electrolyte interface (SEI) on the surface of high voltage cathodes and by bringing about superior electrochemical properties with increased cycling stability, rate capability, and Coulombic efficiency, if synthesized, purified, and handled properly before use in real electrochemical systems.