Capillary Electrophoresis as Analysis Technique for Battery Electrolytes: (i) Monitoring Stability of Anions in Ionic Liquids and (ii) Determination of Organophosphate-Based Decomposition Products in LiPF6-Based Lithium Ion Battery Electrolytes

Abstract

Abstract:In this work, a method for capillary electrophoresis (CE) hyphenated to a high-resolution mass spectrometer was presented for monitoring the stability of anions in ionic liquids (ILs) and in commonly used lithium ion battery (LIB) electrolytes. The investigated ILs were 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 13 TFSI) and 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (PYR 13 FSI). The method development was conducted by adjusting the following parameters: buffer compositions, buffer concentrations, and the pH value. Also the temperature and the voltage applied on the capillary were optimized. The ILs were aged at room temperature and at 60 • C for 16 months each. At both temperatures, no anionic decomposition products of the FSI − and TFSI − anions were detected. Accordingly, the FSI − and TFSI − anions were thermally stable at these conditions. This method was also applied for the investigation of LIB electrolyte samples, which were aged at 60 • C for one month. The LP30 (50/50 wt. % dimethyl carbonate/ethylene carbonate and 1 M lithium hexafluorophosphate) electrolyte was mixed with the additive 1,3-propane sultone (PS) and with one of the following organophosphates (OP): dimethyl phosphate (DMP), diethyl phosphate (DEP), and triethyl phosphate (TEP), to investigate the influence of these compounds on the formation of OPs.

Quantification of PF5 and POF3 from Side Reactions of LiPF6 in Li-Ion Batteries

“…Starting at 4.95 V vs. Li + /Li, a significant oxidation current (black line in Figure 6a) and the simultaneous evolution of CO 2 (dark blue line in Figure 6b) are observed, as previously reported from the oxidation of EC-based electrolytes, 35,42,43 along with an enhanced formation of H 2 and "POF 3 ". Density functional theory calculations by Borodin et al 24 and Li et al 25 have pointed out that upon EC oxidation (i.e., after the first electron transfer), the abstraction of a proton by a neighboring PF 6 − anion would occur in LiPF 6 -based electrolytes, leading to HF and PF 5 formation, and ultimately to the release of CO 2 and a vinyl alcoholate radical (see reaction pathway (1) in Scheme 1). The produced PF 5 would then detected as "POF 3 " in our OEMS setup, while the reduction of HF at the Li metal counter electrode 21 would result in the evolution of H 2 : 2 HF + 2 Li → LiF + H 2 [6] However, contrary to this mechanisms, the evolution of H 2 and "POF 3 " between 4.2-4.95 V occurs without the simultaneous formation of CO 2 , suggesting that a different process is taking place in this potential range.…”

“…Density functional theory calculations by Borodin et al 24 and Li et al 25 have pointed out that upon EC oxidation (i.e., after the first electron transfer), the abstraction of a proton by a neighboring PF 6 − anion would occur in LiPF 6 -based electrolytes, leading to HF and PF 5 formation, and ultimately to the release of CO 2 and a vinyl alcoholate radical (see reaction pathway (1) in Scheme 1). The produced PF 5 would then detected as "POF 3 " in our OEMS setup, while the reduction of HF at the Li metal counter electrode 21 would result in the evolution of H 2 : 2 HF + 2 Li → LiF + H 2 [6] However, contrary to this mechanisms, the evolution of H 2 and "POF 3 " between 4.2-4.95 V occurs without the simultaneous formation of CO 2 , suggesting that a different process is taking place in this potential range. As trace amounts of water gradually react with EC to form ethylene glycol, 22,38 the latter is a likely impurity in EC present at ppm levels (unfortunately below the NMR detection level).…”

“…A quantitative thermal dissociation of dry LiPF 6 occurs between 100 and 200 • C, depending on the experimental conditions (i.e., sealed or open containers, 2 sample size), 3,4 according to the following equilibrium: [2][3][4][5] LiPF 6 ↔ LiF + PF 5 [1] Common Li-ion battery electrolytes, namely anhydrous solutions of LiPF 6 in a mixture of ethylene carbonate (EC) and dialkyl carbonates, show negligible thermal aging up to 60 • C. 6,7 If storage temperatures of 80 • C are exceeded, the electrolyte solution darkens and forms large amounts of gas within days. 2,[8][9][10][11] Next to PF 5 , 2 also alkyl fluorophosphates, 2,7,[9][10][11][12][13][14] oligomers of the carbonate solvents, 8,10 fluorinated hydrocarbons, 2,10,11 POF 3 , 13 and HF 13,14 have been found in thermally aged (80-100 • C) electrolyte solutions, indicating that not only LiPF 6 , but also the organic carbonate solvent itself is decomposed.…”

“…19,23 LiPF 6 + H 2 O ↔ POF 3 + 2 HF + LiF [2] POF 3 + H 2 O ↔ HF + HPO 2 F 2 [3] Besides these two major decomposition pathways, LiPF 6 also affects the stability of the electrolyte solvents at high potentials. Density functional theory (DFT) calculations indicate that the oxidation potential of EC-PF 6 − complexes is lower than for isolated EC, and that HF and PF 5 can be formed from the oxidized complexes at room temperature. 24,25 Tebbe et al 26 suggested that coordination to PF 5 reduces the activation energy for EC dimer formation or ring opening.…”

Mn2+ or Mn3+? Investigating transition metal dissolution of manganese species in lithium ion battery electrolytes by capillary electrophoresis

“…However, the investigation of Mn 2+/3+ was not performed with a CE before. It is obvious that the CE is getting more common in LIB research, for example, for the investigation of electrolyte decomposition products or for the quantification of TMs in cathode materials [45–50]. The investigation of the oxidation states of dissolved TMs is a new topic in LIB research and the CE might play a major role for this.…”