A New Family of Anion-Binding-Agent (ABA) Based Stable Salts for Li and Li-Ion Batteries

Introduction:

Thermal instability of LiPF₆ at elevated temperatures is well known. Thus there is an imminent need to develop thermally stable salt to replace LiPF₆. A variety of new salts have been studied in Li-ion batteries without much success.  One of the salts we are currently evaluating is LiF. This is a low molecular weight salt that has high voltage and thermal stability. However, it doesn’t dissolve in common battery solvents.  This limitation can be solved with the addition of anion binding agents (ABAs). For example, Mehta and Fujinami¹ proposed boroxine (tri coordinate boron ring) which is a Lewis Acid, to solubilize salt in common battery electrolyte.  This compound traps the anion of the salt and frees up the Li+ for conduction. A similar idea was developed by McBreen etal^2-4 where they synthesized boron containing additives to dissolve LiF in organic solvents. However, most of the ABAs investigated are heavy with molecular weight exceeding 500 g/mole. Bulky ABAs impede access of the anion of the salt to the binding site (boron center) thus leading to poor dissolution of the salt^{5, 6}.

                Inspired by McBreen’s approach we launched a program and developed lower molecular weight (~250 g/mole) ABAs. We quickly learned that the as prepared ABAs do have DMSO solvent bound to the boron (this is designated as impure ABA). We purified the impure ABA by reacting with LiF in acetone which yielded a pure ABA+LiF salt. The purified salt gave a clear solution whereas the impure ABA+LiF salt gave a cloudy solution in EC:EMC (Photo-1).

We have prepared a variety of ABAs and tested them for electrochemical performance. Most of them could solubilize LiF only poorly and showed very low conductivity. However, two of the ABAs are promising and the results on one of them will be described here.  

 

Electrochemical Performance:

                Figure 2 compares conductivity for the pure and impure Oxalic-ABA+LiF in EC:EMC

We have prepared a variety of ABAs and tested them for electrochemical performance. Most of them could solubilize LiF only poorly and showed very low conductivity. However, two of the ABAs are promising and the results on one of them will be described here.  

 

Electrochemical Performance:

                Figure 2 compares conductivity for the pure and impure Oxalic-ABA+LiF in EC:EMC solvent blend at different temperatures.  The pure ABA shows higher conductivity than the impure.

Electrochemical measurements and cell building was performed with the purified salt. This showed stability up to 4.5V vs. Li⁺/Li but unstable close to Li voltage, which was eliminated by adding low concentration of LiPF₆ to the solution.

Figure 3 shows formation of an 18650 cell containing carbon anode, NMC (523) cathode in EC:EMC(3:7 w%)-ABA+LiF_10mM LiPF₆. The reversible capacity is ~ 1.03Ahrs.