Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

Abstract

The electrolyte additive fluoroethylene carbonate (FEC) is known to significantly improve the lifetime of Li-ion batteries with silicon anodes. In this work, we show that FEC can indeed improve the lifetime of silicon-carbon composite anodes but is continuously consumed during electrochemical cycling. By the use of 19 F-NMR spectroscopy and charge/discharge cycling we demonstrate that FEC is only capable to stabilize the cell performance as long as FEC is still remaining in the cell. Its total consumption causes a significant increase of the cell polarization leading to a rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of μmol FEC /mg electrode ) in the cell. The latter quantity therefore determines the lifetime of silicon anodes rather than the concentration of FEC in the electrolyte. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in literature, a new reductive decomposition mechanism for FEC is proposed yielding CO 2 , LiF, In the emerging market of electric vehicles (EVs), the development of batteries with higher energy density and improved cycle-life is essential.1 However, their penetration of the mass market significantly depends on cost and the available driving range.2 The US Advanced Battery Consortium (USABC) defined the target value of 235 Wh/kg (at a C/3 rate) on a battery level until 2020.3 As outlined in the recent review by Andre et al., 4 reaching this goal requires an increase of the energy density of today's batteries by a factor of roughly 2 to 2.5 and can only be achieved by the development and integration of novel anode and cathode active materials. A critical element to reach this goal is the implementation of anode active materials with much higher specific capacity than currently used graphite anodes (372 mAh/g 1,5,6 ), with silicon being considered as the most likely next generation anode material due to its high natural abundance and very high theoretical specific capacity of roughly 3600 mAh/g (corresponding to the Li 15 Si 4 phase 7 ). The alloying of silicon with lithium is accompanied by large structural changes, resulting in a volume increase by 310% upon full lithiation.5,7-11 These huge volumetric changes upon lithium insertion and extraction are responsible for the generally shorter cycle-life of silicon electrode materials comp...

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

“…As more remaining FEC is found in the cells with lithium oxalate (green symbols) compared to their counterparts without lithium oxalate that underwent the same number of cycles (black symbols), it is apparent that the FEC consumption per cycle for cells containing both FEC and lithium oxalate is lower. Previous studies have shown that the amount of consumed FEC correlates linearly with the cumulative irreversible discharge capacity (i.e., the sum of the differences between discharge and charge capacity over a certain amount of cycles), 22 which has recently also been demonstrated for SiG anodes with identical composition. 25 Therefore, the cumulative irreversible discharge capacity for each of the analyzed cells is shown on the y-axis of Figure 5, while the amount of consumed FEC in μmol is shown on the lower x-axis.…”

“…As electrolyte solution, we use LP57 + 5 wt% fluoroethylene carbonate (FEC), as this additive is known to improve the capacity retention of silicon-based electrodes. [19][20][21][22][23][24][25] The capacity retention and the coulombic efficiency of the LNMO/SiG cells with 5 wt% (green symbols) and without lithium oxalate (black symbols) in the cathode are shown in Figures 4a and 4b, respectively. The first charge capacities are 145 mAh/g LNMO for cells without lithium oxalate and 173 mAh/g LNMO for cells with 5 wt% lithium oxalate (open symbols), similar to the corresponding cells with graphite anodes.…”

“…FEC consumption in LNMO/SiG full cells with and without lithium oxalate.-To understand to which extent CO 2 participates in SEI formation when FEC is present, we conducted a post-mortem analysis of the electrolyte solution of aged LNMO/SiG cells without lithium oxalate and with 5 wt% lithium oxalate after 50 and 250 cycles, quantifying the amount of residual FEC by 19 F-NMR (for more details see Jung et al 22 ). The upper x-axis of Figure 5 shows the amount of residual FEC found in the electrolyte solution.…”

“…As the obtained 19 F-NMR spectra show only peaks that can either be ascribed to PF 6 − or FEC, and assuming that changes in the PF 6 − concentration in the electrolyte solution during cycling are negligible, the PF 6 − anion can be used as an internal standard. 22 The amount of FEC remaining in the electrolyte solution can thus be determined by the ratio of PF 6 − and FEC peak integrals. A more detailed description of this method can be found in a recent publication by Jung et al 22 …”

“…22 The amount of FEC remaining in the electrolyte solution can thus be determined by the ratio of PF 6 − and FEC peak integrals. A more detailed description of this method can be found in a recent publication by Jung et al 22 …”