Lithium hexafluorophosphate(1-)

Hydrolysis

Description of key information

LiPF6 is hydrolytically unstable, reacting with water to release HF, LiF and phosphoric acid. The kinetics of the hydrolysis reaction have been tested by monitoring of pH and pF values (Unpublished stability and degradation report, 2011).  After initial investigations of temperature- and concentration- dependence of the reaction, hydrolysis rate was determined measuring pH and pF for up to 49 seconds after addition of 30g LiPF6 to 500 ml water at 20°C.  In a series of experiments, the rapid reaction phase (considered to be release of HF, LiF and H3PO4 as shown above) was effectively complete within 3 seconds.  The rate at which subsequent ionisation of LiF occurs will be limited primarily by the rate of LiF dissolution in water.  
A subsequent investigation of reaction time in moist air indicated a two-stage reaction, starting after 10 -20 minutes according to LiPF6 particle size and progressing slowly over 1 or 2 hours. High reactivity with water has also been reported in an expert review (Xu, 2004).

Key value for chemical safety assessment

Half-life for hydrolysis:

0.05 min

at the temperature of:

20 °C

Additional information

The following hydrolytic reactions are believed to occur:

LiPF6 → LiF + PF5, followed by PF5 + 4H2O → 5HF + H3PO4

or

LiPF6 + H2O → LiF + 2HF + POF3, followed by POF3 + 3H2O → 3HF + H3PO4.

Since it has been reported that, at room temperature, LiPF6 can be represented as an equilibrium, the former of these two equations might be considered the more representative, but given the speed of LiPF6 reaction with water and the instability and gaseous natures of POF3 and PF5, the overall reaction is best summarised as:

LiPF6 + 4H2O → 5HF + LiF + H3PO4.

Hence the hydrolysis products which would be present in biological systems following LiPF6 exposure are HF and the ionised species F(-), Li(+) and PO4(3 -).