The concentration effect of LiPF6, EC (ethylene carbonate) DMC (dimethyl carbonate) electrolyte on the stability of residual lithium carbonate is investigated. Thus, such adverse effects of residual lithium carbonate are one of the widely researched issues. A detailed investigation of electrolytes, which play a crucial role in the stability and transformation dynamics of the lithium carbonate at the surface,13 has not been performed.
In this study, the effects of salt concentration on the stability of Li2CO3 remaining on the surface of Ni-rich layered oxides are reported. E0 = 3.82 V vs Li/Li+ Eq.1 The potential dependence in our experiments strongly supports this electrochemical decomposition reaction. Similarly, activation of Li2CO3 decomposition by electrochemical oxidation of electrolyte and electrolyte impurities reported in other papers cannot explain the different onset of evolution potentials that were within this experiment.4 Therefore, instead of a decomposition from impurities, electrochemical decomposition is likely.
The XPS spectra of the cathode after the initial cycles also reveal increased decomposition of residual Li2CO3 at low salt concentrations.
The interfacial impedance at the cathode was then measured by cycling at different salt concentrations to assess the effects of residual Li2CO3. The impedance of each cell was measured from formation to the 100th cycle shown in the figure. R1 is the solution resistance and R2 is the combination of the contact resistance and the charge transfer resistance of the CEI.33 Many reports have an additional pair of a resistance and a.
CPE to distinguish contact resistance and charge transfer resistance, but the circuit in this experiment is simplified to allow a better understanding of the interphase impedance without artificially adjusting the circuit value.33,34. This may be due to the reduced residual lithium carbonate, which has a better ionic conductivity compared to the electrolyte.35 This reduction in residual Li2CO3 would therefore have an increased resistance at the interfacial layer. This increase is consistent with the deleterious effects of the residual Li2CO3 degradation product.
Decomposition of Li2CO3 would result in less Li2CO3 on the surface, which has high ionic conductivity.35 Decomposition of Li2CO3 could therefore increase R2 values, which supports our data. Although none of our experiments alone can show direct evidence of the decomposition stability of residual Li2CO3 on Ni-rich layered oxides, a comprehensive understanding of each experiment leads to the conclusion that residual Li2CO3 exhibits enhanced decompositions when the concentration is low. The only source of CO2 within our experimental potential window is from residual Li2CO3.
Meanwhile, the detrimental effects of Li2CO3 decomposition on the performance of LIBS have been demonstrated in many reports.14,15 The strong increase in charge transfer resistance and lower retained capacity of our experiments are both consistent with existing reports on Li2CO3 decomposition .14, 15The byproducts shown at OEMS, the disappearance of existing Li2CO3 from XPS, and the comparison of the recyclability and EIS data to existing reports all support that there is increased degradation at low concentrations. This is caused by the increased surface adhesion of free EC on residual Li2CO3. It is hypothesized that free EC has lowering effects on the kinetic barrier and thereby increases the rate of degradation of residual Li2CO3.
１５3.Conclusion
１６4.Experimental
The HPR-40 OEMS device was purchased from Hiden Analytical and was modified to suit battery gas analysis as specified in the Supplementary Data 1,2. Differences in temperature and pressure in the cell were calibrated using a 3D contour plot using standard gases at and 2000 ppm of different gases with different inlet pressures, which were verified to be accurate within our experimental range shown in Supporting Information 4. Electrochemical data were collected with VSP-200 and the m/z signals were collected with Hiden analytical software.
All experiments with the OEMS had an OCV step for 5 hours before the experiment to stabilize the signals from the OEMS equipment. Then the cells and a fresh cathode sample were disassembled and rinsed with DMC and sealed in an argon environment. All XPS testing is conducted by the Korea Institute of Ceramic Engineering and Technology with an argon environment to ensure that the cathode-electrolyte interface (CEI) does not change.
For the cyclability test, the coin cells were charged with a current of 40 mA/g to 4.3 V and held at 4.3 V until the current dropped to 10 mA/g. Then they were discharged with the same current to 3.0 V and held until the current dropped to 10 mA/g. EIS spectroscopy from 1 MHz to 1.2 Hz was measured with Biologic VSP300 potentiostat right after the discharge step of the forming cycle, and at the discharge state after 100th cycles with the same coin cell.
１９5.Supplementary data
Fiberglass separators, insulating gasket and PTFE spacers electrically isolate the top of the cell from the bottom of the cell. A 40mm diameter lithium counter electrode is mounted in the center of the bottom of the cell, which is also SUS, and has a recess for the P55 main O-seal. The whole system is tightly bolted with 8 M3 screws and nuts, which are made of SUS, to ensure the gas cell is impervious.
The OEMS cell is attached to a standard leak that has a flow rate of 1 µl/s, assuming the cell pressure is 1 atm and 25 °C, and consists only of argon. To quantify this reduced flow rate with the actual amount of gaseous products, a contour of the dependence of mass spectrometer signals with both the amount of real gas and flow rate must be considered. Comparing the Coulombic efficiency with Supplementary Table 1, the Coulombic efficiency of the OEMS cell is ∼3% lower than the coin cell data.
Since the dimensions of the DEMS cell do not exactly match the token cells, there would be different quantities. The difference in specific filling capacity and discharge capacity is the result of measurements of the active gravity of the slurry. Assuming homogeneous sampling of gas species through the standard leak,39 the ppm value of gas products in the headspace will remain the same.
3, where t is the cell pressure, ppm are the standard gas concentrations and Torr are the actual signals from the mass spectrometer at secondary electron multipliers. Using this 3D plot, the mass spectrometer signals with the inlet pressure are calculated to the actual ppm values of the cell. The cell after 25 hours of sampling was 0.8 atm, which means that the decrease in flow rate affects the signal intensity of the mass spectrometry.
To test our 3D plot, an empty cell with 1 ml of 1M electrolyte is quantified, shown in the Supplementary Figure. The Nyquist plot of the actual data and the calculated circuit are shown in the supplementary figure.
３０ 6.OEMS development appendix
The DEMS applies a polytetrafluoroethylene (PTFE) membrane that separates the hydrophilic electrolyte from the high vacuum of the mass spectrometer. Gaseous species can pass through the pores of the PTFE membrane and enter directly into the sample of the mass spectrometer, hence the name "Differential". Therefore, instead of gaseous products entering the mass spectrometer, solvent species pass directly through the membrane causing serious damage to the vacuum system.
This made it difficult to quantify the gases produced, as well as unwanted reactions with the introduction of ambient air. The flow cell design and membrane had major problems of air entrainment and solvent introduction into the mass spectrometer. The introduction of this standard leak had partial success, and we were able to detect most of the gaseous species reported to develop in battery cells with this system.
The second cell design and interface allowed us to detect repetitive signals from gaseous products evolved at high voltages. The similarity of the current and oxygen intensities showed that our system provides proportional signals to the gases generated. However, the signal-to-noise ratio of the system was not yet sufficient to detect oxygen development.
Some correlation between 4.8 V and oxygen evolution is shown, although the fluctuating background makes it difficult to quantify the gas species evolved. Also, the correlation of oxygen evolution is observed at high voltages, although modification of the background signal is still required. All data are quantified by considering the total gas pressure and mass spectrometer signals and converting the signals within the dynamic range of the experiment.
３７7.Reference
Stability of Li2CO3 in the cathode of a lithium-ion battery and its influence on electrochemical performance. Editors' Choice – Washing of nickel-rich cathode materials for lithium-ion batteries: towards a mechanical understanding. Effects of a high-concentration LiPF6-based carbonate ester electrolyte on the electrochemical performance of a high-voltage layered LiNi0.6Co0.2Mn0.2O2 cathode.
Role of Li concentration and the SEI layer in enabling high performance Li metal electrodes using a phosphonium. Effect of LiFSI concentrations to form thickness- and modulus-controlled SEI layers on lithium metal anodes. A new online mass spectrometer design for the study of multiple charge cycles of a Li-O 2 battery.
Recharge of Li-air cathodes pre-charged with discharge products using an ether-based electrolyte solution: Implications for the cycle life of Li-air cells. High-performance cells containing lithium metal anodes, cathodes LiNi0.6Co0.2Mn0.2O2 (NCM 622) and fluoroethylene carbonate-based electrolyte solutions with practical charge. Degradation of LiPF 6 in high-energy lithium-ion batteries studied by online electrochemical mass spectrometry.
A Raman spectroscopic study of organic electrolyte solutions based on binary solvent systems of ethylene carbonate with low viscosity solvents dissolving various lithium salts. CO2 and O2 evolution at high voltage cathode materials of li-ion batteries: A differential electrochemical mass spectrometry study. Effect of standard light illumination on the electrolyte stability of lithium-ion batteries based on ethylene and dimethyl carbonates.
４３ 국문초록
