The Effect of Trimethoxyboroxine on Carbonaceous Negative Electrodes for Li-Ion Batteries

The effects of trimethoxyboroxine (TMOBX), a useful electrolyte additive, on the electrochemical properties of carbonaceous negative electrode materials for Li-ion batteries (graphite and petroleum coke) were studied. TMOBX was added to electrolyte to examine the impact on parasitic reaction rate and electrode impedance. In order to test parasitic reaction rates with TMOBX containing electrolytes: 1) graphite/Li half cells were constructed and cycled on the High Precision Charger at Dalhousie University (HPC) to accurately measure the coulombic efficiency at different temperatures and 2) Coke/Li cells were constructed for storage experiments at different temperatures and potentials. The results of the HPC cycling showed that adding TMOBX decreases the coulombic efficiency compared to control cells. The storage experiments on the coke electrodes agree well with the HPC results in that the rate of voltage increase was accelerated by the addition of TMOBX. Electrochemical impedance spectroscopy measurements were conducted on coke/coke symmetric cells and while small concentrations (0.3 and 1%) of TMOBX may slightly decrease the cell impedance, higher concentrations (1.5 and 2%) of TMOBX in the electrolyte substantially increase impedance. This work suggests that TMOBX increases the rate of reactions on carbonaceous negative electrodes and at high concentrations can result in higher electrode impedance. These results help explain why TMOBX must be used in combination with another additive, like vinylene carbonate, to provide a significant benefit to Li-ion cells. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.010303jes] All rights reserved.