The Effect Of Antioxidant TBHQ, BHT And TP As Electrolyte Additives On The Electrochemical Performance Of Graphite Electrodes

In recent years,the problems of the energy and environment become increasingly prominent. The government strongly supports to develop new energy vehicles, and promote the development of green energy resource to protect and improve environment. The core technology of new energy electric vehicles was battery technology. It is kown that the solid state interface(SEI) film formed in eletrode/electrolyte has significant effects on the properties of lithium-ion batteries,such as the faradic effieieney,safety and irreversible capacity. Therefore, choosing favorable electrolyte additives for lithium-ion battery to help form better SEI film is one of the most simple and economic methods to improve the properties of lithium-ion batteries. In this paper, TBHQ, BHT and TP antioxidants were investigated as reaction type film-forming additives for lithium-ion batteries, and the mechanism of improvements was investigated. Meanwhile, the influence of thickness on the electrochemical impedance spectroscopy(EIS) spectrum had been studied, and I try to explain its mechanism theory. Testing methods including constant-charge/discharge test(GC), cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS) and scanning electron microscopy(SEM) were used in this paper.The main contents and the conclusions were listed as follows:(1) Simulation results show, while the electrode slice was relatively thin, the intermediate frequency line related to lithium ion transport in porous electrode pore process was not obvious, however, with the increase of thickness of electrodes, an gradually obvious slash arised in intermediate frequency. The simulation results showed a close agreement with experimental results. From the aboved results,the following conclusion could be obtained the diagonal lines of graphite electrode appear in intermediate frequency area in the range of 3.0 V to 0.3V potential was relate to the thickness of the electrode, in other words, it could be attributed to lithium ion transport processes in the porous electrode.(2) Adding antioxidant TBHQ to 1 mol dm-3 LiPF6-EC:DMC electrolyte could significantly improve the performances of graphite electrode, it could be a new kind of reactive type film-forming additives. CV results showed when the TBHQ content was lower, there was no obvious reduction decomposition process of TBHQ on the interface before the solvent molecules decompose during the whole potential range, which was according with the theory of reactive type film-forming additives. Charge-discharge test showed that the capacity retention rate was 96.4%, the charge capacity was 342.6 mAh · g– 1 after 100 weeks in the electrolyte with 0.1 wt% TBHQ, and it was higher than the 95.4%, 303.4 mAh·g- 1 in blank electrolyte. EIS and SEM results indicated that the SEI film of the graphite electrode in the electrolyte of adding 0.1 wt% TBHQ was thiner, more smooth and dense compare with the blank electrolyte.(3) The effects of 5 wt%?10 wt% and 15 wt% antioxidant BHT as film-forming additive on graphite electrode for lithium-ion battery was negative,the BHT was not fit using as a kind of additives for lithium-ion battery system. CV test showed that adding BHT in the electrolyte could suppress the reduction process of electrolyte in some extent. The results of charge-discharge test showed that the charge and discharge capacity of graphite electrode in adding BHT electrolyte was debased, but with the increasing of additive content, the retention capacity was from 84.4% to 94%. In addition, the EIS and SEM result indicated, although the adding BHT can form stable SEI film, the resistance was bigger which was not beneficial to lithium ion intercalation and deintercalation.(4) Adding saturated antioxidant Tea Polyphenol(TP) to EC-based electrolyte could obviously improved the electrochemical performance of graphite electrode, so it could be used as reactive type film-forming additive in lithium-ion battery. The CV results also were in accord with the principle of reactive type film-forming additives. Charge-discharge test showed, the capacity retention rate was 99.1% and the charge capacity was 331 mAh·g-1 after 50 weeks in the electrolyte with TP, and it was higher than the 315 mAh·g-1 in blank electrolyte. EIS result and SEM indicated that, the SEI resistance and the whole resistance of the graphite electrode are reduced after adding BHT, forming more toughness and thiner SEI film in the electrolyte with TP, this was the reason that TP can significantly improve the electrochemical performance of graphite electrode.