The Effect of CO2 on Alkyl Carbonate Trans-Esterification during Formation of Graphite Electrodes in Li-Ion Batteries

Graphite is the most widely used anode material in Li-ion batteries due to its excellent lithium storage capability as well as long-term and cycle stability. Carbonate-based electrolytes are reduced on the graphite surface and form a solid electrolyte interface (SEI) within the first few cycles. The passivating layer inhibits continuous electrolyte decomposition by blocking the electron transport while allowing Li-ions to pass through. Additives are often used to influence the SEI chemistry [1]. Most studies focus on the composition of the SEI and investigate the insoluble products deposited onto the graphite surface by FTIR and XPS [2].

In this work, we employ on-line electrochemical mass spectrometry (OEMS) to analyze in a complementary manner the gas evolution and change of volatile electrolyte components during the formation of graphite electrodes, using a sealed 2-compartment cell [3]. Figure 1 shows the electrochemical and mass spectrometric data for the first CV cycle of a SLP30 graphite electrode vs. metallic lithium in LP57 electrolyte (1M LiPF₆ in EC:EMC, 3:7 by weight). It is apparent that C₂H₄ is the main gas evolved during SEI formation, according to the reduction of EC in Eq. (1) [4]. However, the gases H₂, CO, and CO₂can also be quantified and reveal the important processes at the electrode-electrolyte interface.

The evolution of CO is accompanied by the release of lithium alkoxides LiOR which initiate the conversion of EMC into DMC and DEC, as illustrated in Eq. (2) [5,6]. The trans-esterification causes the change of many different mass signals such as m/z = 15, 31, 77 related to the fragmentation patterns of the linear carbonates (EMC, DMC, DEC). OEMS data suggest that the reaction starts in parallel to the reduction of EC and proceeds quantitatively in LP57 electrolyte.

In contrast, the addition of VC decreases the released amount of C₂H₄ by more than half and prevents successfully the trans-esterification [7]. Its positive effect seems not only be based on the incorporation of reduction products such as poly(VC) into the SEI but also on the evolution of CO₂ which may trap lithium alkoxides, as proposed in Ref. [1]. Based on this assumption, the influence of CO₂ in the head space of the cell is investigated in pure LP57 electrolyte, focusing on the extent of trans-esterification. To gain a mechanistic insight into the reaction, the formation of graphite electrodes is repeated in the model electrolyte 1M LiPF₆ in EMC which reflects solely the reactivity of EMC without the interaction of EC, accelerating its conversion. The results may contribute to elucidate in general the role of CO₂ in CO₂-forming additives such as VC and FEC.

References:

[1]     S. S. Zhang, J. Power Sources, 162, 1379 (2006).

[2]     P. Verma et al., Electrochim. Acta, 55, 6332 (2010).

[3]     M. Metzger et al., J. Electrochem. Soc., 162, A1123 (2015).

[4]     P. Novák et al., J. Power Sources, 81-82, 212, (1999).

[5]     E. S. Takeuchi et al., J. Electrochem. Soc., 144, 1944 (1997).

[6]     H. Kim et al., Electrochim. Acta, 136, 157 (2014).

[7]     R. Petibon et al., J. Electrochem. Soc., 161, A1167 (2014).

Acknowledgement:

We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry and batteries.