So far, there is less information on the positive interface than on the negative interface. However, some recent reports insist that there is also a film at the positive interface, which is also called SEI film like that of the negative electrode because the products of the oxidation reaction on the positive electrode are not fixed like the products of the reduction reaction on the negative electrode, and the number of SEI films on the positive electrode Much less than the negative. Due to the analytical difficulty, few literatures report data on the effect of electrolyte additives on cathodes. It has been reported that adding VC reduces the surface impedance and slightly improves the rate performance. When VC contains a polymerization inhibitor such as BHT, the above effect does not exist. It is speculated that this effect is caused by VC forming a polymer on the positive electrode and inhibiting the deposition of lithium fluoride. This speculation is scientific because VC has a lower oxidation potential than other carbonate solvents.
As shown in Fig. 1, it was confirmed by in situ normalized interfacial Fourier transform infrared spectroscopy (SNIFTIR) that VC in LiCoO2 started to form polymers at a potential of 4.3 V to Li/Li+; below 4.3 V, Upward peaks are formed at 1830 cm-1, 1805 cm-1 and 1750 cm-1, which correspond to the consumption of VC, EC and EMC, respectively. The polymer film formed at 4.3V suppressed the further decomposition of EC and EMC above 4.3V, where downward peaks were formed at 1850~1800cm-1 and 1650~1550cm-1 due to the decomposition caused by the product. The chemical structure of the polymer was determined by XPS to be polyethylene carbonate.
However, the reaction of VC with Lio.5CoO2 at 85°C cannot account for the presence of polymers, and these results show that the passivation film formed by VC on Li0.5CoO2 has poor thermal stability. The amount of SEI film in C/LiCoC2 cells has been measured by ion chromatography (IC), but pure VC has not been observed to significantly inhibit LiF deposition on the cathode. In situ electrochemical impedance spectroscopy also indicated that the addition of VC could increase the impedance of the membrane during discharge.
We also successfully characterized for the first time by X-ray absorption near-band-edge structure spectroscopy (XANES) the formation of the cathode in the electrolyte of Imol/L LiPF6/PC+DMC (50:50, volume ratio) + 5% (mass fraction) ES SE1 membrane [65]. The compounds deposited on the positive electrode are different from those deposited on the negative electrode, and the instrument can detect Li2SO4 and other organic sulfides, as shown in Figure 2. When Li2SO3 is formed on the negative electrode, ES on the positive electrode is oxidized to SO2. In situ electrochemical impedance spectroscopy indicated that the impedance of the SEI film formed by ES was higher than that of the SEI film formed by VC, which was the same as the case on the negative electrode.
The SEI film formed on the positive electrode by GBL solvent alleviates the exothermic reaction between the electrolyte and the positive electrode, which is the basis for the high safety of the battery. These phenomena can illustrate that it is possible to reduce the reaction between the cathode surface and the liquid electrolyte by passivating the active sites [20]. Adding a very small amount (0.1%~0.2%) of aromatic compounds, such as biphenyl (BP), o-terphenyl (o-TP), and meta-terphenyl (m-TP), can improve the cycling performance of the cathode, which is due to The electrolyte polymerized to form a Li+ conductive film covering the surface of the positive electrode.
We also found that adding some sulfur-containing compounds can reduce the reaction of the positive electrode without affecting the performance of the battery. Figure 3 shows the additive effect of cathode protection additives. The addition of 1% (mass fraction) sulfides (1,3-PS and disulfonate X) with higher oxidation potential can significantly reduce the amount of evolved gas and maintain the original after storage at high temperature and low current charging test. some positive capacity. The open circuit voltage (OCV) was higher after the test, indicating that these additives maintained the state of charge of the positive electrode, and the film was formed due to the presence of positive electrode additives, which suppressed the generation of CO2 gas and increased the interfacial impedance.
Read more: What is the charge-discharge mechanism of cathode materials?
