The Impact of CO₂ Evolved from VC and FEC during Formation of Graphite Anodes in Lithium-Ion Batteries

Abstract

Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are commonly added to lithium-ion battery electrolytes in order to form a solid electrolyte interphase (SEI) on the anode, suppressing continuous solvent reduction. In this work, we directly compare VC and FEC by analyzing the SEI with FTIR and XPS, and the evolved gases with on-line electrochemical mass spectrometry (OEMS) in different model systems. Since both additives evolve mainly CO 2 during formation, the effect of CO 2 as an additive is compared to the addition of VC and FEC. While Li 2 CO 3 is as expected the main SEI compound found due to the added CO 2 , surprisingly no CO was detected in the gas phase of such cells. Based on FTIR, NMR and OEMS analyses of cells filled with 13 C labeled CO 2 , we suggest a mechanism explaining the beneficial effects of CO 2 and hence also of CO 2 evolving additives in lithium-ion battery cells. While the generation of polycarbonate from FEC or VC reduction is observed, the generation of Li 2 CO 3 may be as important as the generation of polycarbonate.

Operando Electrochemical Atomic Force Microscopy of Solid–Electrolyte Interphase Formation on Graphite Anodes: The Evolution of SEI Morphology and Mechanical Properties

“…In addition, compared with EC/EMC/VC, the FEC-added electrolyte shows a lower sum resistance of R SEI and R ct , suggesting that ion transportation is improved by FEC due to the inorganic LiF SEI component. 45 …”

Influence of Water Contamination on the SEI Formation in Li-Ion Cells: An Operando EQCM-D Study

“…A possible reason could be that, due to the high content of water in the electrolyte, a large surplus of hydroxide ions is generated, which scavenges the CO2 gas forming from reactions 4 & 5, and precipitates as Li2CO3 on the anode surface: (6) This process would also explain the high concentration of Li2CO3 found on cycled anodes in the presence of CO2 gas and would produce the rigid early interphase detected by EQCM-D in LP47 + H2O electrolyte. [36][37][38] The anode mpe-value measured during this early interphase formation process starts at 83 g mol -1 (2.2 V) and then slowly decreases to 60 g mol -1 at 1.7 V when the interphase formation has slowed down significantly. The SEI mass is therefore increasing at surprisingly high rates considering that the two electron reduction pathway from H2O to Li2CO3 (reactions 2, 3, and 6) would only cause a mpe value of 37 g mol -1 .…”

“…The direct reduction of CO2 might also occur in this potential window. 38 These processes contribute to the continuous slow increase of SEI mass and electrolyte viscosity during cell cycling (Table 1).…”

Influence of VC and FEC Additives on Interphase Properties of Carbon in Li-Ion Cells Investigated by Combined EIS & EQCM-D

“…By employing FTIR, XPS, NMR, OEMS, and other techniques, researchers have identified polymeric species (often denoted poly(vinylene carbonate), i.e., poly(VC)), Li2CO3, and CO2 as the main VC reduction products. [8][9][10]18,19 The precise VC decomposition mechanism remains unclear, however. It is considered likely that the additive decomposes via an electrochemically triggered radical polymerization mechanism -2 -to form CO2 gas and a cross-linked polymeric product which precipitates at the anode-electrolyte interface.…”

“…It is considered likely that the additive decomposes via an electrochemically triggered radical polymerization mechanism -2 -to form CO2 gas and a cross-linked polymeric product which precipitates at the anode-electrolyte interface. 8,9,18 The Li2CO3 salt frequently detected alongside the organic VC decomposition products could originate from the reaction of CO2 with hydroxide ions present in the electrolyte 20 , or also from the direct reduction of either CO2 19,21 or poly(VC) 22 in contact with lithiated carbon.…”