Electrochemical tests showed that LiDFBP effectively reduces the amount of the by-product of PN on the NCM811 cathode, improving the capacity recovery after discharge rate testing and enabling the reversible cycle with capacity retention from 26.6% (LiDFBP-free PN) to 78.5% after 50 cycles at 0.5C rate at 45oC. The synergistic effect of PN cosolvent and LiDFBP additive is contact angle, XPS, SEM and XRD analyses. Schematic illustration explaining the positive effect of the LiDFBP additive on SEI formation on Li-rich cathode. EC, FEC and LiDFBP in neutral charge of the molecule. represented in grey, white, red and cyan spheres respectively). a) Charge-discharge curves and (b) charge differential capacity (dQ/dV) plots on NCM/gra-SiC full cells during pre-cycling below C/10 at 25oC. a) Comparison of the discharge rate capability of NCM/gra-SiC full cells with electrolytes, whether containing nitrile cosolvent and LiDFBP additive or not.
XRD patterns of the gra-SiC anodes in the electrolyte with and without propionitrile and LiDFBP.
Demands for lithium-ion batteries (LIBs)
Problems of high-energy-density electrode materials: NCM and Si
Problems of high-mass-loading electrodes
Discharge voltage curves depending on electrode thickness at different C-speeds. a) Average discharge voltages at different cathode thicknesses; (b) A comparison of the volumetric energy density in a stack cell at different cathode thicknesses at the current rates of C/5, C/2, 1C and 2C.
Discharge rate capability
Functional electrolyte design for discharge rate capability
In the field of electrolyte material, the choice of components is crucial, such as low viscosity solvent and suitable additives. For the low viscosity electrolyte, the choice of the solvent that comprises most of the electrolyte is the most important step. The dielectric constant of the solvent has a tremendous influence on the dissociation and association of Li ions.
However, it has a fatal disadvantage of high volatility due to the low boiling point, which delays the application of pouch cells. Regarding the viscosity of the solvent, ethyl propionate (EP) and butyronitrile (BN) are excluded due to relatively high viscosity. Moreover, the functionalized SEI layer can help the rate capability of the batteries by protecting the electrode surface and forming the ion-permeable SEI layer.
In terms of cycling performance, on the other hand, the VC additive outperforms the FEC additive due to the formation of the highly flexible polymeric protective film that can tolerate the volume changes during cycling. The stable polycarbonates formed by VC reduction contribute to the thermal stability of the battery and suppress additional electrolyte degradation. Therefore, battery performance can be improved by the synergistic effect of using a mixture of VC and FEC as SEI layer-forming additives.
LiDFBP is one of the lithium salt-type additives, which prevents the decomposition of the electrolyte in the cathode from the stable formation of the CEI layer by oxidation as shown in Figure 12. Schematic illustration explaining the positive effects of the LiDFBP additive from the SEI formation in the cathode of rich with Li.
Experimental
Electrochemical measurements
Characterization
The wettability of the electrolytes was examined by contact angle measurements on the NCM cathode and gra-SiC anode using a Phoenix 300 and a 10 μL electrolyte was dropped to take all the pictures after the drop within 2 s. An obtained cathode, rinsed with DMC solvent after the preliminary cycle and additional charging or a clean cathode was stored in 2 g of electrolyte to confirm the content of transition metal dissolution by the interaction of cathode and electrolyte. To examine the electrochemical window of the organic solvent and additives, molecule geometry optimization was obtained using density functional theory with Gaussian 09 at the B3LYP/6-311+G level.
After two forming cycles, the electrochemical impedance spectroscopy (EIS) measurements for whole cells were performed using an IVIUM frequency response analyzer. For electrode analysis after electrochemical testing, whole cells were carefully separated in a glove box. Rinsing with dimethyl carbonate (DMC) was performed to remove the residual electrolyte on the electrodes and they were dried at room temperature.
Ex-situ X-ray photoelectron spectroscopy (XPS, Thermo Fisher) measurements were performed to investigate the surface compositions with Al Kα (hν = 1486.6 eV) radiation in an ultrahigh vacuum environment. The crystal structure of the delithiated anode was investigated after 2C rate discharge cycles on the rate capability test by X-ray diffraction (XRD). After the cycle test at 45oC, the degradation of the NCM cathode was observed through a field emission scanning electron microscope (FE-SEM, JEOL, JSM-6700F) under vacuum condition.
Electrode cross-sections were obtained by ion milling (HITACHI IM4000) with argon ions at beam angles between 0o and 60o from normal incidence.
Result and discussion
Effects of PN cosolvent and LiDFBP additive on the electrochemical performance
The electrochemical float test was performed to confirm the voltage stability of PN cosolvent on the NCM cathode. Figure 16) The nitrile electrolyte showed a higher oxidation current, implying the instability of the nitrile with respect to the NCM cathode, which is the opposite result of the calculated HOMO of PN indicating the stability of oxidation. Density functional theory (DFT) was used in the calculation of the energy levels of typical components in the electrolyte in Figure 19. Figure 20(a) shows the comparison of the voltage profiles of full NCM/gra-SiC cells, precycled with and without LiDFBP, and whether or not PN is included.
The formation of the LiDFBP-derived SEI layer on the gra-SiC anode can be expected by shifting FEC reduction peak to higher voltages. a) Charge-discharge curves and (b) charge differential capacity (dQ/dV) plots on the NCM/gra-SiC full cells during pre-cycling under C/10 at 25 oC. The rate capability performance at various discharge current densities shows the positive effect of the propionitrile cosolvent and the LiDFBP addition confirmed in Figure 21. Figure 21(b) shows the AC impedance spectra of the NCM cathode/gra-SiC anode after full cells precycle at room temp., depending on the electrolytes used, whether or not they contain the propionitrile cosolvent and the LiDFBP addn.
The LiDFBP addition slightly increased the interfacial resistance comprising the SEI resistance and charge transfer resistance because it tends to form an SEI layer on both the surface of the cathode and the anode as expected by HOMO-LUMO energy level calculation. This indicates that the low viscosity of the electrolyte using the propionitrile cosolvent exerts a favorable influence on the lithium ion transport. Moreover, the LiDFBP addition can prevent the cathode from HF attack, which will lead to the growth of LiF compounds in the protective film by promoting hydrolysis of LiPF6 salt.[28] Nevertheless, the cycle instability of the PN cannot be completely overcome by the Nitrile + LiDFBP electrolyte and the discharge capacity retention is relatively lower (78.5% at 50th cycle) than that of the Carbonate electrolyte (86.1% at 50th cycle).
This is estimated from the deterioration of the electrodes and the additives cannot perfectly prevent the effect of PN on them. To confirm whether the Nitrile affects an adverse reaction to the NCM cathode during cycling, the SEM analysis of the NCM cathodes was performed. However, the degree of degradation of the active materials does not seem that different, as the silicon in the SiC particle is stretched to be separated from the graphite.
Thus, the low cycle performance of the Nitrile + LiDFBP electrolyte is believed to be the result of the degradation of the gra-SiC anodes.
Surface Analysis
The nitrile electrolyte peak shows a shape similar to the carbonate electrolyte after the initial cycles. The uncontrolled decomposition of the PN co-solvent appears to cause the formation of an N-based SEI layer, which deteriorates the cycle retention (Figure 23(a)). A relatively high amount of LiF does not seem to affect the degradation of the discharge capacity during the cycle test as carbonate shows similar peak intensity, showing superior cycle performance.
As shown in the F 1s and P 2p spectra, the SEI layer derived by the Carbonate + LiDFBP electrolyte after initial cycling consists of a large amount of LiF, which forms a stiff SEI layer. The LiDFBP-containing electrolyte such as Carbonate + LiDFBP and Nitrile + LiDFBP presents a new peak at 134.5 eV and 133.4 eV on P2p spectra, corresponding to the ionic conductive P-O component in the LiDFBP-derived SEI layer, which is beneficial for speed performance (Figure 21(a)). However, the peak size of the P-O component of the nitrile + LiDFBP is smaller than that of the carbonate + LiDFBP, even though they both contain the same weight percent of the LiDFBP additive.
Moreover, the peak at 687.3 eV indicating LixPOyFx peak at 687.3 eV and LiF peak at 684.9 eV shows no significant difference with the nitrile electrolyte without LiDFBP. It would be the effect of PN coverage of N-based reduction products after the reductive decomposition of LiDFBP on the gra-SiC anodes. The increased peaks of C≡N and C=N on N 1s after cycle test would be the reason for the degradation of discharge capacity retention of Nitrile and Nitrile + LiDFBP electrolytes.
Therefore, the PN cosolvent and LiDFBP additive decompose on both electrodes, forming the SEI layer and affecting the electrochemical performances.
Conclusion
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