Organic Analysis of Electrolyte Reactions Containing Additives

LiPF₆ based organic carbonates are primary solvents widely used in lithium-ion battery electrolytes. Degraded compounds of electrolytes are generated by electrochemical reaction in lithium ion batteries¹⁾.

  With regard to anode surface reactions, some degraded compounds formed a functional film that prevents continuous reactions on the electrode surface, the other degraded compounds cause battery performance decrement¹⁾. To reduce this decrement, it is known that some reagents (e.g. vinylene carbonate, fluoroethylene carbonate, etc.) are added into electrolytes^{2), 3)}. However, it has been not clarified in terms of the cathode. Understanding the mechanisms of organic carbonates reactions containing those additives is essential for the development of better lithium-ion batteries.

  In this work, we report organic compositional change of electrolyte and surface deposition on cathode active materials (solvent extract) caused by containing propane sultone as additives, with organic analysis techniques (¹H NMR, GC/MS, IC, LC/MS/MS).

  Cathode active material samples (NCM) for various potentials (3.0V, 4.1V, 4.4V) were prepared by the chemical delithiation. The chemical delithiation were carried out by immersing the powder sample with oxidant (NO₂BF₄) in acetonitrile under an inert atmosphere_. The reaction proceeds as follows.

    LiMO₂ + NO₂BF₄ → Li_xMO₂ + (1-x) LiBF₄ + (1-x) NO₂↑(M: Ni, Co, Mn etc.)

  Obtained cathode active samples contained Li amount corresponding with SOC (Fig. 1). Using these chemically treated active materials, immersion tests were carried out in some solvents [a: EC/DEC=1/1 (without PS), b: 5% propane sultone in EC/DEC=1/1 (with PS)] for 2 days at 40^oC. We analyzed recovered solvents and D₂O extracts of cathode by ¹H NMR, GC/MS, IC, LC/MS/MS methods.

  Fig. 2 shows analysis results of D₂O cathode extracts from ¹H-NMR, IC. Acetate salt is thought of as oxidative degradation of DEC, and the amount of acetate in samples of “without PS” increased along with the cathode potential. On the other hand that in samples of “with PS” did not increase along with the cathode potential, and were detected much lower than that of “without PS”. It is considered that oxidative reactions proceed on the charged cathode and the presence of PS suppressed oxidative decomposition.

  Fig. 3 shows analysis results of “with PS” cathode extracts (D₂O) from LC/MS/MS (example MS and MS/MS spectra of compounds which were characteristically detected in 3.0V or 4.4V acquired from multivariate analysis of a large number of detected compounds). The m/z 261.01 negative ion (C₆H₁₄O₇S₂) was detected as the PS ring-opening product. The detectable amount in 3.0V was greater than 4.1V, 4.4V. The m/z 274.99 negative ion (C₆H₁₂O₈S₂) was detected as the oxidative decomposition product. The detectable amount in 4.4V was greater than 3.0V, 4.1V. These data will be further discussed in this presentation.

  Reaction analysis between cathode active materials and electrolyte is difficult due to various effects in the full cell (e.g. anode-reductive degradation, Li salts, binder, etc.). We performed to analyze these reactions only between electrolyte and cathode active materials without various effects described above by chemical Li-delithiation sample preparation and organic analysis techniques.

1) V. Agubra, J. Fergus, J. Power Sources, 268, 153 (2014).

2) V. Agubra, J. Fergus, Materials, 6 (4), 1310 (2013).

3) W. J. Zhang, J. Power Sources, 196, 13 (2011).