An Investigation of the Impact of Protic Impurities, Different Housing Materials and Silicon on the Thermal Aging of Electrolytes Used in Lithium-Ion Batteries

Concerning prospective energy storage, lithium-ion batteries (LIBs) are one of the most outstanding technologies due to their high energy density as well as great cycle stability. To increase their operational lifetime, thermal degradation of the most common conductive salt lithium hexafluorophosphate LiPF₆ (1) should be avoided. Therefore, it’s crucial to minimize traces of protic impurities resulting from e.g. organic carbonates. In fact, the products of the endothermic equilibrium of LiPF₆ are preferably formed at higher temperatures, leading to the formation of hydrofluoric acid (HF) and subsequently to further degradation of electrolyte components. (2) 

LiPF₆ (s)  LiF (s) + PF₅(g)                          [1]

PF₅ (g) + H₂O (l) → O=PF₃(g) + 2 HF (l)           [2]

Lux et al. (3) followed the HF formation in LiPF₆ containing electrolytes by spectroscopic ellipsometry of SiO₂ layers. As we reported recently (4), SiO₂ probably promotes the degradation of LiPF₆ in organic carbonate based electrolytes.

Herein, we report about the decomposition of 1M LiPF₆ in a binary mixture of ethylene carbonate and diethylene carbonate in a ratio of 40:60 (w/w) at ambient and elevated temperature (cf. table 1). Degradation products are studied by nuclear magnetic resonance spectroscopy (NMR, cf. figure 1), gas chromatography mass spectrometry (GC-MS) and headspace GC-MS (HS-GC-MS). Acid-base and coulometric titration are used to determine the total amount of acid and water content upon aging, respectively. Ultraviolet-visible (UV-Vis) spectroscopy is used to follow the color change of the electrolyte (cf. figure 2). The influence of protic contamination, different housing materials and added active materials on the electrolyte aging is shown.

Acknowledgement

VOLKSWAGEN VARTA Microbattery Forschungsgesellschaft and Herbert Quandt Foundation are gratefully acknowledged for funding.

References

1. K. Xu, Chem. Rev., 104, 10 (2004).

2. C. L. Campion, W. Li, B. L. Lucht, J. Electrochem. Soc., 152, A2327 (2005).

3. S. F. Lux, I.T. Lucas, E. Pollak, S. Passerini, M. Winter, R. Kostecki, Electrochem. Commun., 14, 47  (2012)

4. P. Handel, G. Fauler, K. Kapper, M. Schmuck, C. Stangl, F. Fischer, F. Uhlig, S. Koller, J. Power Sources, 267, 255 (2014).