Singlet Oxygen Reactivity with Standard Li-Ion Battery Electrolyte Carbonate Solvents
Authors: Freiberg Anna Teresa Sophie , Roos Matthias K. , Wandt Johannes , de Vivie-Riedle Regina , Gasteiger Hubert A. 
Published: February 2020

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 (LiwNixCoyMnzO2 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 1Δg, 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

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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.

Figure 1