What are electrolyte anode film-forming additives?

What are electrolyte anode film-forming additives?

It is well known that the solid-state electrolyte interface (SEI film) formed on the carbon anode determines the battery performance, such as reversible capacity, storage life, cycle life, and safety. The formation of the SEI film originates from the decomposition of the electrolyte during charging. At the first cycle potential of about 1.4 V for Li/Li+, the electrolyte (especially ethylene carbonate) begins to decompose, and this process leads to the generation of irreversible capacity Qirr (difference in charge-discharge capacity Q), as shown in Figure 1. From the second cycle, the irreversible capacity decreases and maintains a stable reversible capacity Qrev during charge/discharge. When PC is used as solvent, the electrolyte continues to decompose at about 0.9V to Li/Li+ (as shown in Fig. 2), and the graphitic (crystalline) carbon falls off due to co-intercalation of the solvent and cannot be charged, which is the replacement of EC with EC The main reason for PC, and this substitution has been applied in the first generation of commercial Li-ion batteries with non-graphitic (amorphous) carbon anodes. And some compound additives, such as VC (ethylene carbonate), inhibit the exfoliation of graphite, so that the graphite can be charged normally.

Figure 1 - First charge/discharge curve of natural graphite, Figure 2 - First charge curve of graphite in PC-based electrolyte
Figure 1 – First charge/discharge curve of natural graphite, Figure 2 – First charge curve of graphite in PC-based electrolyte

The reductive decomposition of EC and chain carbonates at room temperature can effectively promote the formation of SEI films, but problems arise at high temperatures. It has been verified that a variety of compounds can be used as additives to improve the properties of the passivation film on the graphite anode.

  1. Unsaturated carbon compounds

Ethylene carbonate (VC), the first electrolyte solvent to be developed, has an extremely high relative permittivity (ℇr = 127) and provides good electrical conductivity. It is found that adding a small amount of VC can suppress gas generation and obtain high cycle efficiency at the first charge, and VC can also inhibit the decomposition of easily reducing solvents such as trimethyl phosphate (TMP), so VC is a typical anode film formation. additive. Adding 1% (mass fraction) VC to lmol/L LiPF6/EC+DMC+DEC (33:33:33, mass ratio) can improve the cycle life of Li-ion polymer batteries, which shows that this passivation layer has excellent stability.

Because VC has a double bond structure, its lowest unoccupied molecular orbital (LUMO) energy is lower, so it is generally believed that VC is more easily reduced than other carbonates such as EC and DMC. The reduction potential of VC can be tested by the gold electrode in tetrahydrofuran (THF) solvent, and its reduction potential is higher than that of other carbonate solvents, indicating that the decomposition of VC precedes other carbonates and can form on the surface of the negative electrode. A good SEI film while suppressing further solvent decomposition and graphite exfoliation due to solvent co-intercalation.

We have detected the composition of the gas escaping at different potentials during the first charge, as shown in Figure 3. When VC was not added to 1mol/L LiPF6 EC+DMC (50:50 volume ratio), the main decomposition products of EC, ethylene and carbon monoxide, gradually accumulated. On the other hand, when 2% (mass fraction) of VC was added, carbon dioxide was the main decomposition product. This shows that carbon dioxide is the reductive decomposition product of VC, and the reductive decomposition of VC can significantly inhibit the decomposition of the solvent. It was observed by scanning electron microscopy that when the voltage to Li/Li+ was 1.0 V, granular products existed on the surface of the graphite negative electrode. With the intercalation of Li+, the gel organic film further covered the graphite surface, as shown in Figure 4.

Figure 3 - Changes in electrolyte decomposition spillover components, Figure 4 - SEM image of graphite surface
Figure 3 – Changes in electrolyte decomposition spillover components, Figure 4 – SEM image of graphite surface

There are various ways to characterize the films formed with the participation of VC. CHz=CHOCHO2Li can be detected by infrared reflection absorption spectroscopy (IRRAS) [30], and it is also speculated that the formation of a polymer with an OCOLi functional group improves the viscosity and toughness of the passivation film. When the potential to Li/Li+ is 1.3 V, VC starts to deposit by reductive deposition, which can be reduced by in situ to a film thickness of about 10 nm at 0.8 V [30,33]. Ex situ AFM revealed an ultrathin film (less than 1 nm thick) on the laminated base of the highly pyrolytic graphite (HOPG) anode [34] o AC impedance analysis found that the film impedance decreased, which was due to LiF on the graphite anode. formation is inhibited.

The presence of polymers on SEI films has been first confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and nuclear magnetic resonance spectroscopy (1H-NMR). . FTIR and XPS (O18) spectra indicated the presence of C=C double bonds and polymers, respectively. TOF-SIMS showed the presence of polyacetylene on the outer surface of the SEI membrane. To identify the structure of this polymer, the SEI film formed in 1 mol/L LiPF6/VC was extracted with dimethyl sulfoxide (DMSO) solvent, and the polycarbonate was successfully identified by two-dimensional 1 H-NMR. Presence of vinylidene esters and other ring-opening compounds.

The relative molecular mass of these polymers formed on the lithium metal anode was between 1000 and 3000 as measured by gel permeation chromatography (GPC), and the addition of VC could enhance the lithium cycling efficiency. The SEI film could be determined by temperature-programmed mass spectrometry (TPAMS). of thermal stability. Adding VC can make the decomposition temperature of the SEI film become higher. The improvement of thermal stability can inhibit the reaction between the anode and the electrolyte when the SEI film is damaged and repaired. According to the observations, although the quality of the SEI film is largely dependent on the charcoal species and charging conditions, the formation of a thinner and uniform SEI film by adding VC is also the reason for the improved battery performance.

The reaction mechanism of VC has not yet been determined. The possibility of EC splitting due to intermolecular electron transfer from VC to EC was proposed by density functional theory (DFT) calculations of supramolecular (EC)nLi+(VC). Although the polymer reaction of the one-electron reduction product is likely to proceed simultaneously with the establishment of the SEI film base, among cyclic carbonates, VC is the most prone to two-electron reduction, which may be the reason for its ability to form a SEI film. One of the properties of additives.

In addition to VC, ethylene ethyl carbonate (VEC) [44~49], styrene carbonate (PhEC) [50], vinylene carbonate (PhVC) [51], catechol carbonate (CC) [51 , 52], amino methyl carbonate (AMC) [53, 54], AEC (amino ethyl carbonate) [55], vinyl acetate (VA), other vinyl compounds [53, 54, 56], acrylonitrile (AAN) [57, 58] and 2-cyanofuran (CN-F) [59], whose chemical structures are shown in Fig. 4.10, have similar effects on graphite exfoliation inhibition in PC solvent systems.

  1. Sulfur-containing organic compounds

Sulfite compounds, including vinyl sulfite (ES), propylene sulfite (PS), dimethyl sulfite (DMS), and diethyl sulfite (DES), have been validated as film formers in PC. Adding 5% (volume fraction) of additives to the electrolyte can make the graphite electrode perform normal charge and discharge. These compounds are consumed when the voltage to Li/Li+ is about 2V, forming a passivation layer, the ability of which hinders the co-intercalation of PC electrolyte in the graphite layer is in the following order: ES>PS>DMS>DES. Besides sulfite compounds, it has been reported that cyclic sulfite compounds such as 1,3-propanesultone (1,3-PS) can also form good SEI films, which can improve the hard carbon/LiMn2 04 cylindrical shape. Cycling and storage performance of batteries.

The researchers speculate that the reduction of ring-opened ES via single-electron transfer is more beneficial than carbonates such as EC and VC. In situ AFM observations indicate that when the voltage to Li/Li+ is IV, the SEI film formed is about 30 nm thick, which is thicker than that of VC, due to the HOPG (highly oriented pyrolytic graphite) expansion due to PC co-intercalation.

The SEI films formed in Imol/L LiPF6/PC+5% (mass fraction) or 10% (mass fraction) ES were also characterized by various analytical means such as SEM, XPS, TPD-GC/MS, TOF-SIMS, etc. At a voltage of 1.8 V for li/Li+, ES is more easily reduced than PC, and the composition of the formed SEI film depends largely on the current density. At high current densities, the inorganic Li2SO3 is the first precipitation on which an organic layer composed of CH3CH(OSO2Li)CH2OCO2Li is formed. In situ electrochemical impedance spectroscopy revealed that the impedance of the SEI film formed by ES was higher than that of the SEI film formed by VC.

  1. Halogen-containing organic compounds

It is easy to know from molecular orbital calculations that the introduction of halogen atoms into ring-opened carbonates can improve their reducibility. Vinyl chloride carbonate (CIEC) is reduced at a potential of about 1.8 V to Li/Li+ to form a passivation film and release CO2 at the same time. It can be speculated that the LiCl generated by reductive cleavage becomes an internal chemical shuttle, which leads to low current efficiency. This deficiency can be compensated by using fluoroethylene carbonate (FEC), which produces less soluble LiF. In situ AFM showed that the thickness of FEC-generated films was intermediate between VC and ES-generated films, but degraded significantly during cycling. Trifluoropropylene carbonate (TFPC) is also reduced at a potential of about 1.8V to Li/Li+ to form a passivation layer with an interfacial impedance greater than CIEC. Halogen-containing GBL (BrGBL or FGBL and N,N-dimethyltrifluoroacetamide (DTA) also have similar effects.

  1. Other organic compounds

Although some other organic compounds such as asymmetric dihydrocarbyl carbonates, dihydrocarbyl pyrocarbonates, and trans-2,3-butene carbonate (t-BC) are considered co-solvents, they have also been shown to be beneficial additive. The alkoxy carbon releases CO2 through natural decomposition, and the CO2 continues to react on the surface of the active material to form Li2CO3. For solvated Li+, t-BC is too large to even intercalate into graphite, thus suppressing graphite exfoliation. The researchers also detected a series of GBL derivatives with different side chains at the 5-position carbon, some of which reduced the number of PC molecules due to the incorporation of Li+, thereby effectively inhibiting the decomposition of PC; also reported that succinic anhydride and succinic anhydride Imide derivatives have the same effect. Tetrakis(ethylene glycol) methyl ether (TEGME), whose molecular composition is similar to that of organic SEI compounds, forms a stable non-porous passivation layer. ) phosphite (TTFP) can also effectively inhibit PC decomposition.

  1. Inorganic compounds

Inorganic compounds include CO2, N2O, SO2, CS2. Gate and Sx2- (the electrochemical reduction product of S8) play an important role in the stability of Li-graphite and Li-metal SEI films. When the above additives are contained, Li2CO3, Li20, Li2S, Li2S2O4 will be formed, forming a good passivation Floor.

  1. Ionic compounds

It is well known that the type of lithium salt also affects the composition and quality of SEI films, however, the superposition effect of lithium salts is not fully understood. It has been confirmed that organoboron complexes, such as lithium bis(salicylate)borate and lithium bisoxalatoborate (LiBOB), can form stable SEI films on graphite anodes because their organic groups can serve as the constituents of SEI films. point. Lil, LiBr and NH4 are commonly used to inhibit the reduction of Mn(II) on the negative electrode of C/LiMn2O4 batteries. It has been reported that the addition of Na+ can reduce the irreversible capacity loss of graphite anodes during the first charge. It has been confirmed by testing that AgPF6 and Cu(CF3S()3)2 can form a metal protective layer.

Read more: Current status of cathode materials

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