The difference in specific capacity may be ascribed to the increased polarization of the battery resulting from the poor wettability. The cell using STD electrolyte shows a continuous and fast capacity decay. Meanwhile the coulombic efficiency is greatly reduced as the cycle goes on. Continuous electrolyte decomposition contributes to lower coulombic efficiency and eventually leads to failure of the battery. However, with the introduction of the HFE, capacity retention was substantially enhanced.
Apart from cycling performance, improvement of separator wettability also has a significant effect on the rate performance. With the rate raised from 0. When the current density is low, the STD electrolyte with poor separator wettability can still meet the cycling requirements, demonstrating similar discharge capacity and coulombic efficiency to the E2 electrolyte. However, the STD electrolyte is unable to meet the requirements under high current density due to its poor separator wettability.
Greatly increased polarization of the battery results in serious electrolyte decomposition and significantly reduced coulombic efficiency. Meanwhile the E2 electrolyte still performs well with lower polarization and higher discharge capacity. In addition, due to the shortened charging and discharging time at high rates, accompanied side reaction decreases and the coulombic efficiency is improved instead.
Furthermore, cell with the E2 electrolyte possesses a rather smaller ohmic drop particularly at 1 C and higher rate, perhaps on account of lower Li-ion diffusion impedance. When the cell is charged and discharged at a constant current, the current will accumulate in limited transfer channels, resulting in high local current density, serious polarization and intensified oxidative decomposition of the electrolyte. In contrast, with better separator wettability the Li-ion flow is able to pass through the separator evenly, eliminating the local polarization and reducing the internal resistance of the cell.
Additionally, the crowding effect of HFE with higher oxidative stability than carbonate solvents is the other reason for the good high-voltage stability of HFE-containing electrolyte. Therefore, better cycling and rate performance are produced. Schematic diagram of the electrolyte interphase and current distribution of STD electrolyte a, c and E2 electrolyte c, d. Effect of separator wettability toward high-voltage electrolyte is investigated in this paper. As a result, viscosity of the electrolyte was reduced and electrolyte uptake of the PE separator was raised obviously.
Therefore, conductivity of the wet separator was increased by more than ten times although ionic conductivity of the electrolyte decreased to some extent. Further electrochemical tests indicated that high voltage stability of the electrolyte and cycling performance of the battery can be greatly improved and CV tests confirmed HFE cannot form a CEI layer. This can be ascribed to two reasons. The other reason is that improved separator wettability increases the diffusion channel of lithium-ion flow, which will facilitate the uniform current distribution and reduce the local polarization of the electrode.
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User Name Password Sign In. High-Voltage LiNi 0. Previous Section Next Section. Calculation methods Quantum chemical calculations were performed by using the Gaussian 09W package.
Electrodes and electrolytes preparation LiNi 0. Separator wettability characterization The separator used in this work is Celgard polyethylene PE separator. Electrochemical measurements and characterizations All cells were assembled in an argon-filled glove box with the concentration of oxygen and moisture less than 0. Figure 1. View this table: In this window In a new window. Table I. Calculations In order to understand the beneficial effect of HFE besides increasing the wettability of separator toward electrolyte, DFT calculations is further conducted since it can successfully reflect the molecular properties including oxidation potential and ionization energy in a semiquantitative manner.
Table II. Electrochemical performances and characterizations Electrochemical floating test of different electrolytes was performed using the LiNi 0.
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CrossRef Medline Google Scholar. Energy Mater. CrossRef Google Scholar. Cao Z. Abu-Lebdeh Y. Power Sources , , Arai J. Wszystkie nowe produkty. This product is not yet available at Kamami. If you are interested in it please click here to send us an email. We will do our best to add it to Kamami. Fluorinated materials for energy conversion offers advanced information on the application of fluorine chemistry to energy conversion materials for lithium batteries, fuel cells, solar cells and so on. One way to increase energy density is to raise the operating voltage of LIBs.
Conventional carbonate-based electrolytes generally decompose when the voltage is raised above 4. For instance, novel solvents with higher oxidative potentials than carbonate solvents, including nitriles, 4 sulfones 5 or fluorinated carbonates, 6 , 7 have been proved to be effective in some high-voltage applications. However, effect of separator wettability toward nonaqueous electrolyte on high-voltage applications was rarely taken into consideration.
High dielectric constant of the PC molecule means high polarity so that the PC-based electrolyte is not facile to spread on the nonpolar PE separators. Therefore, using PC-based electrolytes for comparison could better demonstrate the effect of separator wettability on high-voltage performance of the cells. The results shown here highlight the key role of separator wettability toward nonaqueous electrolyte for the high-voltage cathode and battery applications.
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Quantum chemical calculations were performed by using the Gaussian 09W package. All calculations were based on the polarizable continuum model PCM. LiNi 0. Positive electrode was prepared by mixing LiNi 0. The capacity of the cathode was 1.
Electrolytes were prepared in an argon-filled glove box MBraun with the oxygen and moisture level less than 0. Compositions of the electrolytes are shown as follows:. The separator used in this work is Celgard polyethylene PE separator. Then the excessive electrolyte was absorbed by the filter paper. All cells were assembled in an argon-filled glove box with the concentration of oxygen and moisture less than 0.
After activated at 0. The Li LiNi 0. Both cells were cycled at 1 C 0. Cyclic voltammetry CV scans were performed on the same texting system at a scan rate of 0. Electrochemical impedance spectrum EIS measurements were conducted on a CHIe electrochemical workstation with frequency ranging from 0. The surface morphology of LiNi 0. With dropping the electrolyte without HFE, the separator is not wetted with a high contact angle of Upon increasing the content of HFE, the electrolyte uptake is increased from At the same time the viscosity is decreased from 3. In addition, the dipole moment of HFE molecule is 4.
The synergy between reduced viscosity and polarity of electrolyte leads to enhanced separator wettability. Optical characterizations a , electrolyte uptake and viscosity of PE separator b. Nyquist plots for the SS SS symmetrical cells c. It is obvious that introducing HFE greatly reduces the impedance of the separator, indicating increased ionic conductivity of the wet separator.
For the electrolyte without HFE, conductivity of the wet separator is merely 0. Upon increasing the content of HFE, the conductivity of the wet separator reaches to a maximum of 0. As the concentration of HFE increases, ionic conductivity of the electrolyte is gradually decreased. The ionic conductivity of the STD electrolyte is 6. Conductivity and MacMullin number of various electrolytes and wet separators. According to previous investigations, MacMullin number is another key criterion associated with the wettability of separator toward electrolyte.
Usually a smaller MacMullin number indicates a better separator wettability. MacMullin number of the E2 and E3 electrolyte are 8. Therefore, the HFE is helpful for improving separator wettability. The above results clearly reveal that the conductivity of the wetted separator is tightly associated with the conductivity of the electrolyte and the wettability of separator.
As the HFE content is increased, on the one hand, conductivity of the electrolyte is decreasing, and on the other hand, reduced MacMullin number indicates the improvement of separator wettability. In order to understand the beneficial effect of HFE besides increasing the wettability of separator toward electrolyte, DFT calculations is further conducted since it can successfully reflect the molecular properties including oxidation potential and ionization energy in a semiquantitative manner. The molecule's highest occupied molecular orbital HOMO energy is related to its oxidation resistance.
It is worth noting that the theoretical oxidation potential of the ether can be greatly increased, from 5. Structure, HOMO energy, ionization energy, theoretical oxidation potential and dipole moment of different molecules. Based on the DFT calculation results shown above, HFE is more difficult to be oxidized than the conventional electrolyte. As a result, electrolytes with HFE as co-solvent are more stable under certain high voltage conditions. Electrochemical floating test of different electrolytes was performed using the LiNi 0.
When the potential is further raised to 5. This finding clearly indicates that the oxidation stability of the electrolyte has been greatly improved by adding HFE. The enhanced electrolyte stability is attributed to two reasons.