Preparation of BF3 Carbonates and their Electrochemical Investigation as Additives in Lithium Ion Batteries

This study reports the behavior of the compounds BF3·D (with D = dimethyl carbonate, ethylene carbonate, ethyl methyl carbonate, propylene carbonate) as electrolyte additives in Li[Ni0.33Co0.33Mn0.33O2]/graphite cells with LiPF6 (1 M) in EC:EMC (3:7 wt%) as electrolyte. The adducts were prepared from gaseous BF3 with the corresponding carbonates in a simple Lewis acid base reaction and fully characterized (FT–IR and NMR spectroscopy, single crystal X-ray diffraction). The electrochemical behavior of the additives was examined at concentrations of 1 wt% and 0.25 wt% added to the electrolyte. Electrochemical impedance spectroscopy of cells with 1 wt% additive, display a significantly lower cell impedance than cells without additive, but also a slightly decreased capacity. To combine the high capacity retention with a low cell impedance the concentration of the additive was reduced. Thus, cell testing with 0.25 wt% additive incorporated with the electrolyte showed no difference in capacity retention after 55 cycles at 25 °C, but had lower cell impedance. The rate-performance test shows a higher capacity retention for cells with additive at high C–rates. The surface chemistry of the electrodes was studied by X-ray photoelectron spectroscopy and suggests that the additives react on both electrodes (Li[Ni0.33Co0.33Mn0.33O2] and graphite) and improve the electrode interphases.