Novel Electrochemical Methodology for the Removal of HF from Carbonate-Based LiPF6-Containing Li-Ion Battery Electrolytes
Authors: Ge Xiaokun , Kuhlmann Andreas , Xu Xiaodan , Steinruck Hans-Georg 
Published: August 2024

Baseline Li-ion battery (LIB) electrolytes typically consist of LiPF6 salt dissolved in carbonate solvents. Due to trace amounts of water and the hydrolysis of PF6
- in carbonate solvents, these electrolytes always contain trace amounts of HF. [1] On the cathode, HF has negative effects, including transition metal dissolution and capacity fading. [2] On the anode, HF is involved in the formation and evolution of the solid electrolyte interphase (SEI). [3] The SEI originates in the thermodynamic instability of electrolyte moieties (salt, solvent, impurities) on the anode surface. As a result, electrolyte moieties are reduced and may then react with Li+ ions to form solid, insoluble products on the anode surface. This limits electron and solvent transport while permitting Li+ transport, kinetically stabilizing the interface. [4] The mechanism of nucleation and growth remains poorly understood despite their significance. The formation of LiF is one example. Although it is believed that direct PF6
- anion reduction is important, it is difficult to observe this reaction unambiguously because it frequently coexists with electrochemical signatures of electrocatalytic HF reduction or even acts in concert with one another. [5]

The objective of this work is to remove ppm-levels of HF from carbonate-based LiPF6-containing electrolytes to enable foundational studies of SEI nucleation (e.g., direct PF6
- anionic direction) without interfering HF effects. We propose an electrochemical purification approach, which is complementary to conventional approaches using scavenging molecules. [6] Utilizing the knowledge that all other electrolyte moieties in baseline carbonate LiPF6-containing LIB electrolyte remain intact at relatively high potentials (significantly > 1.5 V vs. Li/Li+), we postulate to solely and selectively electrocatalytically reduce HF on metal electrodes. Therefore, our basic hypothesis is that all the HF in the electrolyte will be electrocatalytically reduced when an appropriate potential is applied to a high-surface area catalytically active electrode, while all other moieties essentially stay unchanged. To achieve this, we employed a porous Cu-foam working electrode that is integrated into a Teflon cone-type cell that has a second Pt working electrode. This allows us to use cyclic voltammetry (CV) to electrochemically test the HF concentration in the electrolyte. We report on the successful removal of HF to levels below a few ppm. Additionally, we observe CV features which we tentatively assign to direct anion reduction. Investigations are underway to gain deeper insight into the specifics of this reaction. We anticipate that the developed electrochemical method could lead to new opportunities for HF removal and basic research on the SEI reaction in electrolytes containing LiPF6.

Literatur:

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[6] J.-G. Han, M.-Y. Jeong, K. Kim, C. Park, C.H. Sung, D.W. Bak, K.H. Kim, K.-M. Jeong, N.-S. Choi, An electrolyte additive capable of scavenging HF and PF5 enables fast charging of lithium-ion batteries in LiPF6-based electrolytes, Journal of Power Sources, 446 (2020) 227366.