Lithium hexafluorophosphate explained

Lithium hexafluorophosphate is an inorganic compound with the formula LiPF₆. It is a white crystalline powder.

Production

LiPF₆ is manufactured by reacting phosphorus pentachloride with hydrogen fluoride and lithium fluoride^{[1] [2]}

PCl₅ + LiF + 5 HF → LiPF₆ + 5 HCl

Suppliers include Targray and Morita Chemical Industries Co., Ltd.

Chemistry

The salt is relatively stable thermally, but loses 50% weight at 200 °C (392 °F). It hydrolyzes near 70 °C (158 °F)^[3] according to the following equation forming highly toxic HF gas:

LiPF₆ + 4 H₂O → LiF + 5 HF + H₃PO₄

Owing to the Lewis acidity of the Li⁺ ions, LiPF₆ also catalyses the tetrahydropyranylation of tertiary alcohols.^[4]

In lithium-ion batteries, LiPF₆ reacts with Li₂CO₃, which may be catalysed by small amounts of HF:^[5]

LiPF₆ + Li₂CO₃ → POF₃ + CO₂ + 3 LiF

Application

The main use of LiPF₆ is in commercial secondary batteries, an application that exploits its high solubility in polar aprotic solvents. Specifically, solutions of lithium hexafluorophosphate in carbonate blends of ethylene carbonate, dimethyl carbonate, diethyl carbonate and/or ethyl methyl carbonate, with a small amount of one or many additives such as fluoroethylene carbonate and vinylene carbonate, serve as state-of-the-art electrolytes in lithium-ion batteries.^{[6] [7] [8]} This application takes advantage of the inertness of the hexafluorophosphate anion toward strong reducing agents, such as lithium metal, as well as of the ability of [PF6-] to passivate the positive aluminium current collector.^[9]

References

1. Web site: Dunn . JB . Gaines . L . Barnes . M . Sullivan . J . Wang M . Material and Energy Flows in the Materials Production, Assembly, and End-of-Life Stages of the Automotive Lithium-Ion Battery Life Cycle . 5 December 2020 . 28 . Sep 2014.
2. Web site: O'Leary . Brian . High-Volume Manufacturing of LiPF6, A Critical Lithium-ion Battery Material . 5 December 2020 . 5 . 11 May 2011.
3. Xu. Kang. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chemical Reviews. October 2004. 104. 10. 4303–4418. 10.1021/cr030203g. 15669157. 33074301.
4. 10.1055/s-2004-829550 . Lithium Hexafluorophosphate-Catalyzed Efficient Tetrahydropyranylation of Tertiary Alcohols under Mild Reaction Conditions . 2004 . Nao Hamada. Sato Tsuneo . Synlett . 1802–1804 . 10.
5. Bi. Yujing. Wang. Tao. Liu. Meng. Du. Rui. Yang. Wenchao. Liu. Zixuan. Peng. Zhe. Liu. Yang. Wang. Deyu. Sun. Xueliang. Stability of Li2CO3 in cathode of lithium ion battery and its influence on electrochemical performance. RSC Advances. 6. 23. 2016. 19233–19237. 2046-2069. 10.1039/C6RA00648E. 2016RSCAd...619233B.
6. Goodenough. John B.. Kim. Youngsik. Challenges for Rechargeable Li Batteries. Chemistry of Materials. 9 February 2010. 22. 3. 587–603. 10.1021/cm901452z.
7. Qian. Yunxian. Hu. Shiguang. Zou. Xianshuai. Deng. Zhaohui. Xu. Yuqun. Cao. Zongze. Kang. Yuanyuan. Deng. Yuanfu. Shi. Qiao. Xu. Kang. Deng. Yonghong. How electrolyte additives work in Li-ion batteries. Energy Storage Materials. 20. 2019. 208–215. 2405-8297. 10.1016/j.ensm.2018.11.015. 139865927.
8. Book: Jow . T. Richard . Borodin . Oleg . Ue . Makoto . Xu . Kang . Electrolytes for Lithium and Lithium-Ion Batteries . 2014 . Springer: New York . 9781493903023.
9. Corrosion inhibition of aluminum current collector with molybdate conversion coating in commercial LiPF6-esters electrolytes. 2021. Corrosion Sci. 190/11. S.L. Yang, S.M. Li, Y.B. Meng, M. Yu, J.H. Liu, B. Li. doi: 10.1016/j.corsci.2021.109632.