A Comparative Study of Pyridine-Boron Trifluoride, Pyrazine-(BF 3 ) 2 and Triazine-(BF 3 ) 3 as Electrolyte Additives for Lithium-Ion Cells

Abstract

The novel electrolyte additives pyridine boron trifluoride, pyrazine di-boron trifluoride and triazine tri-boron trifluoride were compared in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite and Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 /graphite pouch cells. This series of additives allowed the Lewis base:BF 3 ratio to be systematically varied. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC) or prop-1-ene-1,3-sultone (PES). Increasing the BF 3 content on the additive can reduce the impedance of cells during charge-discharge cycling. However the coulombic efficiency, charge endpoint capacity slippage and long-term cycling behavior of these pouch cells was not improved by increasing the content of BF 3 in the additives. Li-ion batteries (LIBs) are widely used in applications from portable consumer electronics to electric vehicles (EVs). 1,2 Incorpo-rating electrolyte additives into the electrolyte is the most effective method for extending lifetime of LIBs. In a previous study, pyri-dine boron trifluoride (PBF) showed clear benefits as an additive for NMC-based lithium ion cells. 3 The two major functional groups pyridine (organic base) and BF 3 were combined together to try to solve issues in lithium-ion cells, including: 1) transition metal dis-solution from the positive electrode material at high potential; 4 2) electrolyte (salts and solvents) decomposition at high potential and elevated temperature; 5 and 3) impedance growth during high potential and long-term cycling. 6 However, the mechanism by which PBF-type additives function is not understood. When PBF-type additives are used there is a 1:1 addition of the pyridine derivative and BF 3 . This may not be the optimum ratio. Zuo et al. 7 reported that LiBF 4 added as an additive could reduce impedance during high potential cycling and that the beneficial ef-fects of LiBF 4 arise from the single BF 4 − ion. It is important to know which functional group, pyridine or BF 3 , has a greater impact in PBF-type additives and which functional group is more beneficial to Li-ion cell lifetime. By using the same simple synthesis method, 8 one, two or three BF 3 molecules can easily be added to pyridine, pyrazine or tri-azine, respectively, to learn about the impact of this series of additives with increased BF 3 content. In this work, additives with different amounts of BF 3 were suc-cessfully synthesized with 1:1 to 1:3 mole ratio of Lewis base:BF 3 . These additives were used to study the benefits of BF 3 as a functional group and to optimize the BF 3 content in these additives.