Trimethylboroxine Vs. Lithium Tetrafluoroborate As Electrolyte Additives for High Voltage LiCoPO4 Cathode Materials: Key Role of Boron Trifluoride

High voltage cathode materials are promising technologies for increasing the energy density and thus increasing the range of electric vehicles. LiCoPO₄ (LCP) 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 performance on cycling in commonly used electrolytes containing LiPF₆.

Recently, Sharabi et al. ^[2] have used an electrolyte containing LiPF₆ with trimethylboroxine (TMB) and fluoroethylene carbonate (FEC) that allows maintaining a capacity of 100 mAh/g after 100 cycles for a LCP electrode. In our previous work ^[3], we have shown that i) TMB is oxidized to boron trifluoride (BF₃), phosphorylfluoride (POF₃) and carboanion (CH₃^-) and ii) a high amount of additive (>0.5%_wt) in a cell results in a very bad stability. However, the role of TMB in the performance of LCP was not clearly identified.

Lithium tetrafluoroborate (LiBF₄) was investigated as unique lithium salt ^[4] and as additive in LiPF₆ containing electrolyte ^[5]. It was found that LiBF₄ has good Aluminum passivation properties and improves the electrochemical performance of LiNi_0.5Co_0.2Mn_0.3O₂. In the presence of LiPF₆ salt, LiBF₄ is expected to form BF₃ and lithium fluoride (LiF) ^[5].

In the present work, the effect of BF₃ gas in the LCP performance is investigated through a comparison between TMB and LiBF₄ as additive in LP30. Considering the amount of BF₃ produced from the decomposition of both additives, the electrolyte containing 0.2%_wt TMB, which presented the best electrochemical behavior ^[3], and an electrolyte with 0.4%_wt LiBF₄ are compared in Figure 1. Interestingly, for a theoretical similar amount of BF₃ in the cell, LCP exhibits the same performance in both electrolytes.

An optimization of the amount of LiBF₄ additive in LP30 electrolyte for LCP electrodes will be presented. The best results are obtained with a low content of LiBF₄ (<3%wt). Secondly, OEMS ^[6] and CV experiments on LiBF₄-containing electrolyte will confirm the decomposition of LiBF₄ and the presence of BF₃ during the oxidation scan until 5.2 V. Additionally, impedance experiments (EIS) will be shown and results will be compared to the data obtained in the case of TMB-containing electrolyte.

Finally, as LiBF₄ is known to passivate the Aluminum well, the effect of TMB and LiBF₄ on Aluminum and stainless steel used as current collector in Swagelok cells will be studied by SEM-EDX, XPS and ICP. In order to distinguish between the beneficial effect brought by a better current collector passivation and a possible layer formed on LCP, similar experiments will be performed in coin cells, which utilize a higher area of stainless steel in contact with the electrolyte.

Figure 1: Discharge capacity (in mAh/g_LCP) vs. Cycle number of LCP electrode (4 mg/cm²) in different electrolytes (120 µL electrolyte, 3 glass fiber separator, Swagelok cell) with the following cycling procedure: 2 cycles at C/15 and subsequent 150 cycles at C/2.

References:

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

[2] 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.

[3] A. Freiberg, M. Metzger, D. Haering, S. Bretzke, S. Puravankara, T. Nilges, C. Stinner, C. Marino, H. A. Gasteiger - J.Electrochem. Soc. 161, A2255 (2014)

[4] S. S. Zhang, K. Xu, T. R. Jow – J.Electrochem. Soc. 149, A586 (2002).

[5] X. Zuo, C. Fan, J. Liu, X. Xiao, J. Wu, J. Nan – J.Electrochem. Soc. 160, A1199 (2013).

[6] N. Tsiouvaras, S. Meini, I. Buchberger, H. A. Gasteiger – J.Electrochem. Soc. 160 (3) (2013) A471.

Acknowledgements:

This work was supported by BMW AG and we would like to thank S. Bretzke, S. Puravankara and T. Nilges (TUM) for the synthesis of the LCP material.