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Hydrolysis of LiPF in Carbonate-Based Electrolytes for Lithium-Ion Batteries and in Aqueous Media Michael Stich, Mara Göttlinger, Mario Kurniawan, Udo Schmidt, and Andreas Bund J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02080 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018
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The Journal of Physical Chemistry
Hydrolysis of LiPF6 in Carbonate-Based Electrolytes for Lithium-Ion Batteries and in Aqueous Media Michael Stich, Mara Göttlinger, Mario Kurniawan, Udo Schmidt, Andreas Bund* Electrochemistry and Electroplating Group, Technische Universität Ilmenau, GustavKirchhoff-Straße 6, D-98693 Ilmenau, Germany
*Corresponding author E-mail:
[email protected], Phone: +49 3677 69 3107
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Abstract The conducting salt in lithium–ion batteries, LiPF6, can react with water contaminations in the battery electrolyte, releasing HF and further potentially harmful species, which decrease the battery performance and can become a health hazard in case of a leakage. In order to quantify the hydrolysis products of LiPF6 in a water contaminated battery electrolyte (1 mol l-1 LiPF6 in EC/DEC) and in aqueous solution, ion chromatography (IC), coulometric Karl-Fischer titration (cKFT) and acid-base titration were used on a timescale of several weeks. The results show that the nature of the hydrolysis products and the kinetics of the LiPF6 hydrolysis strongly depend on the solvent, with the main reaction products in the battery electrolyte being HF and HPO2F2. From the concentration development of reactants and products, we could gain valuable insight into the mechanism of hydrolysis and its kinetics. Since the observed kinetics do not follow simple rate laws, we develop a kinetic model based on a simplified hydrolysis process, which is able to explain the experimentally observed kinetics.
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1. Introduction The electrolytes of commercial lithium-ion batteries (LIBs) usually consist of a lithium salt dissolved in a carbonate-based solvent. Those organic electrolytes are able to withstand the large operating voltages of LIBs, which can reach, depending on the electrode combination, more than 4 V. The most commonly used lithium salt, lithium hexafluorophosphate, is not an ideal choice as a conducting salt, but it has the overall best combination of the key requirements - conductivity, dissociation constant, ionic mobility and stability – compared to alternative candidates (LiAsF6, LiBF4 or LiClO4)
1–3
stability
4–10
. One of the biggest problems with LiPF6, however, is its limited thermal and its disposition to hydrolytic decomposition
8,11–18
. The hydrolysis of
LiPF6 can not only lead to a decreased battery performance due to the reactive decomposition products and a lower electrolyte conductivity, but it will become a health hazard in case of the battery electrolyte being exposed to a humid environment 19. Since LIBs have become an everyday power source in most people’s lives, be it in mobile devices or electric vehicles, it is crucial to not only produce safe and long lasting batteries but also to be able to reliably determine the health risks associated with batteries that came in contact with water, may it be due to a lacking quality control during manufacturing (poor sealing of LIB cells, insufficient drying of hygroscopic electrode materials
20
) or due to fractured and leaking LIBs. Therefore it
is very important to understand the hydrolytic decomposition of the battery electrolyte better and to identify and quantify the reaction products as well as to determine the kinetics of these reactions. There is only a limited amount of publications on the hydrolysis of LiPF6 in aqueous
14,15,21
and organic
5,11,12,14–18
solvents. The hydrolysis
of LiPF6 has previously been experimentally investigated by Karl-Fischer titration
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16,18
, NMR
5,11
, GC-MS
5,18
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, IC (including ESI-MS and ICP-OES)
15,21
, acid-base
titration 18 or theoretically by DFT and MD studies 22. Literature findings show that in both types of solvents, LiPF6 is to a great extent dissociated into its ions (Eq. (1)) and is also in equilibrium with its molecular dissociation compounds LiF and PF5 (Eq. (2)), even at room temperature 11,15,23.
LiPF ⇌ Li + PF
(1)
LiPF ⇌ LiF + PF
(2)
The strong Lewis acid PF5 has been shown to react fast in the presence of water according to Eq. (3) 14. (3)
PF + H O → POF + 2 HF
It has to be mentioned that the hydrolysis of LiPF6 is strongly dependent on the solvent. It has consistently been reported that LiPF6 is hydrolyzing very slowly in aqueous solutions, while it is hydrolyzing faster in organic solutions with water additions
14,21
. This intriguing phenomenon has been related to the finding that the
dissociating power for LiPF6 is much stronger in water than in the organic medium (i.e. ethylene carbonate, EC)
14
and thus inhibits the production of PF5. It has been
shown, that the concentration of the intermediates PF5 and POF3 in the carbonatebased electrolyte is very small, (~9 ppm POF3 in 1 mol l-1 LiPF6 in EC/EMC)
11,14
.
Besides HF, HPO2F2 appears to be one of the main reaction products of the hydrolysis in water as well as carbonate electrolytes
11,21
. The further reaction of
HPO2F2 with water leads to H2PO3F which can be found in aqueous solutions
21
, but
the literature findings on the presence in the organic carbonate electrolyte are contradictory
11,15
(Eq. (5)). The last hydrolysis step in aqueous media yields H3PO4
(Eq. (6)). 4 ACS Paragon Plus Environment
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POF + H O → HPO F + HF
(4)
HPO F + H O → H PO F + HF
(5)
H PO F + H O → H PO + HF
(6)
2. Experimental At the start of the hydrolysis experiments, 1000 ppmw (≙ 70 mmol l-1) of H2O were added to the electrolyte 1.0 mol l-1 LiPF6 in EC/DEC=50/50 (v/v) (from Sigma-Aldrich,
