New Process of Synthesizing LiPF6 in Organic Solvent for Electrolyte

Lithium ion batteries have been used by mobile devices such as smartphones. Furthermore the demand for lithium ion batteries by electric vehicles market will dramatically increase in the next few years. When it does, the lithium ion batteries industry will transform from a relatively minor player to become a foundation of the global economy. It is necessary to supply the cost effective mass production materials for lithium ion batteries. The cause of such a background, new process of synthesizing LiPF₆ was developed by us.

Lithium ion batteries are composed of cathode, anode, separator, and electrolyte. The electrolyte is mixture of organic solvents and lithium salt. In the lithium salt, LiPF₆ is widely used because of superior characteristics such as high solubility to organic solvents, high stability under oxidation and reduction atmosphere, and high lithium ion conductivity.

LiPF₆ and electrolyte have been manufactured as follows. LiPF₆ is mainly synthesized from LiF and PF₅ in HF solution. Then LiPF₆ powder is obtained by crystallization process. And then the electrolyte is manufactured by dissolving LiPF₆ powder in organic solvent.

However, this method has several problems. The synthesis process of PF₅ gas has difficulty for removing heat of reaction and handling of raw materials (PCl₅). It is also difficult to scale up the crystallization process under HF solution. Furthermore, the dissolution process of LiPF₆ powder required a great deal of time because LiPF₆ powder was unstable and difficult to handle.

We studied a new process to synthesize LiPF₆ in organic solvent to solve these problems. We can obtain electrolyte directly without isolating LiPF₆ itself as powder in this process. As raw materials, LiCl, PCl₃ and solvents such as DMC, EMC or DEC were used. These raw materials were put in stirring tank and reacted with Cl₂ at first, and HF next. Finally LiPF₆ was obtained.

LiCl + PCl₃ + Cl₂ → LiPCl₆

LiPCl₆ + 6HF → LiPF₆ + 6HCl

This method has advantages. At first, PCl₃ is cheaper than PCl₅, and is liquid state which is easier to treat it than solid state PCl₅. And the second, we can save the number of reactors by this method, because the generation of PF₅ gas is not necessary in this method. These advantages reduce the processing time and facilities cost. In addition, the solvents used as a reaction solvent had no damage during this process and can directly be used as electrolyte solvent. So the reaction solvent is available as a final product.

In experiment, we tried to synthesize in laboratory scale (maximum a few liter). By-products of HCl and PF_x which reacted PCl₃ and HF were removed by simple distillation. Furthermore as for the electrolyte solvent removed of the simple distillation, it was reusable by neutralization and distillation.

For our new method, it was found that DMC, EMC and DEC are available as a reaction solvent. The reaction yield was more than 90% at these solvents. Among them, the highest yield was obtained by DMC, while the lowest yield was obtained by DEC. Phosphorus and fluorine were removed by the simple distillation as PF_x, and lithium remained as LiF when the yield decreased. This LiF were collected by filtration, and can reuse as a lithium source of the next reaction instead of LiCl.

Finally, we obtained LiPF₆/DMC, EMC or DEC electrolytes with low free-acid, low metal contamination and confirmed that the LiPF₆ produced by our new method has the equal quality to that of conventional process. We performed high volume production of both 100 liter scale and several thousand liters for a plant scale, and we concluded that the quality of the products was equal to a laboratory scale.