Currently employed consumer batteries for high energy density applications rely on layered transition metal oxides as cathode active materials due to their high theoretical capacity and excellent capacity retention over prolonged cycling. Today’s research focus lies on Ni- and Li-rich materials (Li_wNi_xCo_yMn_zO₂ materials like NCM811 with w=1, x=0.8, y=z=0.1 or Li-rich high energy NCM (HE-NCM) with w+x+y+z=2 and w>1), motivated by a drive towards lower battery cost in $/kWh (1).

However, today’s battery systems can only utilize roughly 70% of the maximum state of charge (SOC) range of NCMs (i.e., Δw = 0.0-0.7), as severe capacity fading is observed for higher SOC values, i.e., for higher cut-off potentials (2). On the other hand, Li-rich HE‑NCM materials require activation to 100% SOC in the first cycle, and capacity and voltage fading are a challenge with these materials. Interestingly, molecular O₂ release initiating at roughly 80% SOC has been observed for all these materials, which is accompanied by electrolyte oxidation (evident from CO₂ and CO evolution) (3). As these coupled reactions always occur at roughly 80% SOC (i.e., at different potentials), they do not seem to originate from an electrochemical oxidation mechanism, for which one would expect a clear potential dependency. It was therefore concluded that the oxygen released must be highly reactive, and singlet oxygen (excited state ¹Δ_g, furtheron referred to as ¹O₂) was suggested (4). Recently, experimental evidence for the validity has been found (5). The question remains how ¹O₂ can react with the electrolyte and why battery life is reduced so drastically once ¹O₂ release occurs.

To examine these questions, an experimental set‑up was designed for the on-line detection of the reaction products of carbonate solvents/electrolytes with ¹O₂. Artefacts arising from electrochemical processes can thus be excluded, and even the effect of conducting salts can be examined independently. The measurement is based on the formation of ¹O₂ by photoexcitation of the Rose Bengal dye in presence of ground-state triplet oxygen (6). Gaseous decomposition products are analyzed by on-line mass spectrometry, and solution species after illumination can be analyzed post-mortem by standard analytical methods. It was found that the reactivity of different carbonate solvents with ¹O₂ can vary substantially.

To gain a mechanistic understanding on the reactivity of carbonates with ¹O₂ and to gather further insight into potentially detrimental reaction products, ab initio calculations to characterize the energy profile as well as dynamic on-the-fly simulations to explore the configuration space were performed.

The results of gassing analysis and ab initio predicted transitions states and reaction products for the standard electrolyte solvent ethylene carbonate (EC) are shown in Fig. 1a and b, respectively. The on-line gassing analysis shows a clear CO₂ evolution during excitation of the Rose Bengal dye which is accompanied by O₂ consumption, proving the reactivity of ¹O₂ with EC. The only gaseous decomposition product is CO₂, which is also found as product in the calculations shown in Fig. 1b. Analysis of the solution after reaction with ¹O₂ proves the formation of H₂O₂, which confirms the theoretically predicted reaction mechanism (Fig. 1b).

This study provides experimental tools as well as computational concepts to understand the chemical reaction of ¹O₂ formed at the cathode/electrolyte interface with the electrolyte solvent and allows quantitative comparison of a large variety of solvents, starting from cyclic EC over linear diethyl and dimethyl carbonate to F-substituted carbonates like mono- and difluoro EC (FEC and DiFEC). Further evaluation of the effect of conducting salt and ¹O₂ scavengers can therewith be undertaken to get fundamental insights, with important implications for high-energy Li-Ion batteries.

References

1. G. E. Blomgren, J. Electrochem. Soc., 164, A5019 (2016).
2. I. Buchberger, S. Seidlmayer, A. Pokharel, M. Piana, J. Hattendorff, P. Kudejova, R. Gilles and H. A. Gasteiger, J. Electrochem. Soc., 162, A2737 (2015).
3. B. Strehle, K. Kleiner, R. Jung, F. Chesneau, M. Mendez, H. A. Gasteiger and M. Piana, J. Electrochem. Soc., 164, A400 (2017).
4. R. Jung, M. Metzger, F. Maglia, C. Stinner and H. A. Gasteiger, J. Electrochem. Soc., 164, A1361 (2017).
5. J. Wandt, A. T. S. Freiberg, A. Ogrodnik and H. A. Gasteiger, submitted (2018).
6. M. DeRosa, Coord. Chem. Rev., 233-234, 351 (2002).

Acknowledgements

We thank Dr. Alexander Ogrodnik and Sophie Solchenbach from TUM for fruitful discussions.

Fig.1: Reaction of EC with ¹O₂. a) On-line gassing analysis of decomposition products and oxygen consumption during excitation of the Rose Bengal dye to produce ¹O₂ (t = 0-1 h). b) Reaction mechanism suggesting a stoichiometry of O₂ consumed to CO₂ formed of 2:1, whereby initiation of carbonate decomposition is caused by elimination of H₂O₂.