Synthesis Of Hard Carbon Materials And Electrochemical Performance As Anode Materials Of Sodium Ion Batteries

The current society is facing the problems of energy crisis and environmental pollution. C lean energy sources such as wind and solar energy have the characteristics of instability and relying on natural conditions. Electrochemical energy storage technology has played an important role in the storage and conversion of clean energy. Sodium ion batteries(SIBs) are one of the most promising electrochemical energy storage technologies in the large-scale energy storage field. The electrode materials are the key technologies of SIBs and the research on anode material of SIBs is mainly focused on hard carbon material. In this thesis, we used pomelo peels and interconnected polypyrrole(PPy) as the precursors of hard carbon materials to study the effects of phosphoric acid treatment, specific surface area and binders on the sodium storage performance. The main contents and results are listed below.Firstly, hard carbon materials were synthesized by activation and pyrolysis of pomelo peels. Phosphoric acid treatment can not only obtain a well-developed porous structure but also introduce functionalized groups on the surface of carbon. The results indicate that the best pyrolysis temperature of pomelo peels is 700 oC. By adding phosphoric acid during the pyrolysis process, we successfully obtain the surface functionalized hard carbon material with high specific capacity. The O-containing functional groups can contribute to the sodium storage capacity. After 220 cycles, the specific capacity retains 181 mAh g-1 at the current density of 200 mA g-1, and the efficiency is more than 99%. The capacity retention of 84.6% shows a good cycling stability. At a high current density of 5 A g-1, the specific capacity is still as high as 71 mAh g-1, indicating that phosphoric acid activated hard carbon possesses a good rate capacity. Thus, through the pyrolysis of pomelo peels activated by phosphoric acid, we successfully develop a low-cost, large-scale method to produce high performance anode material of SIBs.In order to investigate the relationship between the specific surface area and the sodium storage performance, we designed a series of hard carbon material with different specific surface area. The precursor was interconnected polypyrrole activated by KOH. The results indicate that the initial efficiency increase at first and then decrease as the specific surface area increase. In our experiments, when the activation mass ratio of KOH : PPy comes to 0.5, the initial efficiency reaches the maximum value of about 46.3%, and the initial charge capacity is as high as 299.3 mAh g-1.Different binders also affect the sodium storage performance of hard carbon material. Here we systematically studied four different binders named CMC, SA, PAA-Na and PVDF, respectively. It is found that the hard carbon material using PAA-Na binder shows the best electrochemical performance, exhibiting that the initial efficiency was 65.7%, the initial charge capacity reached 343.6 mAh g-1, and the reversible capacity was 155 mAh g-1(at the current density of 3 A g-1). When using SA as the binder, the electrode exhibits the similar electrochemical performance. The initial efficiency can reach 51.6%, and retain 200 mAh g-1 after 125 cycles at current density of 200 mA g-1. Furthermore, the SA binder derived from seaweed possesses the advantage of abundance, low cost and industrial mass production, thus can reduce the cost of SIBs.The advantages of hard carbon material such as high specific capacity, low voltage platform, high safety and so on have made it as one of the most potential anode material for SIBs. However, the storage mechanism need to be further investigated and there still exist some problems in practical application. It is believed that hard carbon material will finially be in practical use one day as continual investigation on hard carbon material.