New Process of Synthesizing LiPF6 in Organic Solvent for Electrolyte
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 PFx 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 PFx, 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.