Lithium Hexafluorophosphate

Lithium Hexafluorophosphate (LiPF₆) is the most used electrolyte salt in commercial lithium-ion batteries. A 1 mol/L solution of LiPF₆ in carbonate solvents is commonly used as the electrolyte in these batteries. Examples of commercially available electrolytes are 1 mol/L LiPF₆ in ethylene carbonate/dimethyl carbonate and in ethylene carbonate/ethyl methyl carbonate. Newer lithium salts, such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium 4,5-dicyano-2-(trifluoromethyl)imidazol-1-ide (LiTDI), have been compared with LiPF₆ as alternative electrolyte salts.

Ion pairing and transport. In ethylene carbonate/dimethyl carbonate, a study of ionic conductivity, viscosity and ⁷Li self-diffusion inferred that LiPF₆ is likely to form solvent-separated ion pairs, or a mixture of solvent-shared and solvent-separated ion pairs. In a study combining atomistic modelling and transport measurements, LiPF₆ in ethylene carbonate/ethyl methyl carbonate showed lower ion dissociation and faster Li⁺ self-diffusion than LiPF₆ in ethylene carbonate alone. The authors attributed this to the mixing of cyclic and linear carbonates; the total ionic conductivities of the two electrolytes were similar.

Electrode and current collector. In conventional electrolytes, the chemical instability of LiPF₆ accelerates transition metal dissolution from the electrode material, yet the salt suppresses oxidative dissolution of the aluminium current collector. In high-voltage cells, replacing LiPF₆ with more stable lithium salts may reduce transition metal dissolution, but it can lead to severe oxidation of the aluminium current collector.

Ether electrolytes. In glyme ether electrolytes, the commonly used salt LiFSI has poor anodic stability because of severe aluminium corrosion. LiPF₆ is compatible with aluminium at high voltages, but its solubility in these solvents is abnormally low. One study attributes this low solubility to association of solvated Li⁺ with PF₆⁻. In 1,2-dimethoxyethane (DME), the association gives the insoluble solvate crystal Li(DME)₂PF₆. The same study predicted, and confirmed by experiment, that the salt and glyme ethers can form a homogeneous solution again above a critical LiPF₆ concentration. This requires the eutectic point between the Li(ether)ₓPF₆ solvate crystal and LiPF₆ to lie below the operating temperature. In cyclic ether electrolytes for lithium metal anodes, the chemical instability of LiPF₆ and its tendency to polymerise the ether solvent present challenges.

Hydrolysis and decomposition. LiPF₆ is hygroscopic and has poor thermal and hydrolytic stability. It forms HF in the presence of water. In the electrolyte at elevated temperatures, it also undergoes ligand abstraction, which leads to the formation of HF.

Kerr-gated Raman spectroscopy identified POF₃ on the surface of LiPF₆ particles after trace water was added. One study developed a method coupling ion chromatography with electrospray ionisation mass spectrometry to separate and determine the thermal decomposition products of LiPF₆-based electrolytes. Besides the known hydrolysis products of LiPF₆, the method separated and identified several further organophosphates during ageing studies of two commercial electrolytes. Radiolysis of diethyl carbonate/LiPF₆ solutions has been used as a model for the species formed during electrolysis in lithium-ion batteries. In these solutions, radiolysis gives the strong Lewis acid PF₅ and fluoride anions, which can precipitate as LiF. PF₅ then gives P=O compounds whose substituents are mainly fluoro, hydroxy and ethoxy groups. Ageing of an electrochemical cell filled with one of these solutions gave similar results.

Interphase formation. In LiPF₆-based organic electrolytes, HF is one of the major impurities. It can react with basic components of the solid electrolyte interphase (SEI), such as lithium alkoxides and lithium carbonate, to give an SEI richer in LiF. In situ electrochemical scanning tunnelling microscopy of highly oriented pyrolytic graphite in a LiPF₆ ethylene carbonate/dimethyl carbonate electrolyte showed exfoliation and cluster growth during initial lithium uptake. The surface changes were recovered when the potential was switched back, and control experiments attributed them to electrochemical reduction of solvent molecules. Modelling of LiPF₆ carbonate electrolytes containing fluoroethylene carbonate (FEC) indicates that FEC coordinated to Li⁺ is reduced preferentially, which helps to passivate the anode and facilitates an improved SEI.