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