Study on Trimethylboroxine and Fluoroethylene Carbonate As Electrolyte Additive for High Voltage LiCoPO4 Cathode Materials

High voltage cathode materials are promising technologies for increasing the energy density and thus increasing the range of electric vehicles. LiCoPO₄ is known as a possible high voltage material, which is electrochemically active at 4.8 V vs. Li/Li⁺ with a theoretical capacity of 167 mAh/g ^[1], corresponding to a theoretical energy density of ~800 Wh/kg. Unfortunately, this material exhibits poor performances on cycling in commonly used electrolytes containing LiPF₆.

Recently, Markevich et al. ^[2] explained that LiPF₆ reacts with LiCoPO₄ through a nucleophilic attack of the F^- anion in solution on the P atoms of LiCoPO₄ material, causing capacity fade of the battery. In their recent work, the use of an electrolyte containing LiPF₆ with trimethylboroxine (TMB) and fluoroethylene  carbonate (FEC) allows to maintain a capacity of 100 mAh/g after 100 cycles ^[3]. From XPS studies, they conclude that FEC creates a better SEI layer, but the effect of TMB is not yet clear.

In the present work, an optimization of the amounts of TMB and FEC additives in LP30 electrolyte for LiCoPO₄ electrodes is carried out. The best results are obtained with an electrolyte containing 0.5%_wt TMB on the one hand and with 5%_wt FEC on the other hand. For a better understanding of the effect of those additives, we used cyclic voltammetry (CV), impedance measurements (EIS) and on-line electrochemical mass spectrometry (OEMS) ^[4]. For each technique, carbon electrodes (Super C65, TIMCAL) and LiCoPO₄ electrodes with in-house synthesized LiCoPO₄are analyzed.

CV experiments with OEMS show that FEC and TMB decompose completely at high potential during the first charge of the cell. Impedance measurements on electrolytes containing only FEC confirm the assertion from Sharabi et al. ^[3]that this compound forms a better SEI layer. Moreover, an EIS study with different amounts of TMB will be presented in order to explain the low performance of an electrolyte with a high TMB content.

Finally, OEMS experiments on the initial charging of the battery until 5.5 V will be presented for optimized electrolytes with TMB and FEC. For example, figure 1 shows the OEMS results for the anodic decomposition of an electrolyte with 0.5%_wt TMB on a carbon electrode. During the decomposition of TMB at 4.8-4.9 V, a product with the mass/charge-ratio 49 is detected and identified as BF₃. A correlation between the XPS results from Sharabi et al. ^[3] and this experiment will be done.

References:

[1] N. Bramnik, K. Bramnik, T. Buhrmester, C. Baehtz, H. Ehrenberg, H. Fuess - J. Solid State Electrochem. 8 (2004) 558.

[2] E. Markevich, R. Sharabi, H. Gottlieb, V. Borgel, K. Fridman, G. Salitra, D. Aurbach, G. Semrau, M. A. Schmidt, N. Schall, C. Bruenig – Electrochem. Comm. 15 (2012) 22.

[3] R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach, G. Semrau, M. A. Schmidt, N. Schall, C. Bruenig – Electrochem. Comm. 28 (2013) 20.

[4] N. Tsiouvaras, S. Meini, I. Buchberger, H. A. Gasteiger – Journal of The Electrochemical Society 160 (3) (2013) A471.

Acknowledgements:

This work was supported by BMW AG.

Figure 1: OEMS analysis of the anodic decomposition of 1M LiPF₆ EC/DMC + 0.5%_wtTMB on a carbon electrode. Evolution of the mass/charge-signal 49 normalized by the Ar signal 36 (top), and evolution of current of the cell (bottom) as functions of potential.