Preparation And Performance Investigation Of Novel Carbon-based Materials For Lithium-air Batteries

Nonaqueous lithium-air batteries have recently attracted much attention owing to their super-high theoretical specific energy. The commonly used commercial carbon materials in the cathode electrode have poor catalytic activity and relatively small pore sizes and volumes, and these lead to a huge polarizations and low discharge capacity. Although functional carbon materials have been used as such porous electrodes which could obtain a huge discharge capacity, their high cost will hamper the commercialization of lithium-air battery technology. In this thesis, we prepared a low-cost and high-performance carbon-based material that could be used in lithium-air batteries.Initially, A series of new carbon-based materials with different microstructure were prepared by carbonize of phytic acid doped polyaniline aerogels. The most suitable new carbon-based material for lithium-air batteries was selected by comparing there electrochemical properties. The research showed that lithium-air batteries based on the best performance of carbon-based materials could display a discharge capacity of 12 607.1 m Ah/g, a discharge voltage of 2.70 V at a current desity of 100 m A/g, a cycleability up to 28 cycles with a fixed capacity of 500 m Ah/g at a current density of 100 m A/g, a super rate capability of 6 930.1 m Ah/g at a current density of 500 m A/g. This series of new type of carbon nanomaterials are doped with nitrogen and phosphorus. Impurity atoms of nitrogen and phosphorus were not introduced in carbon from outside, the nitrogen and phosphorus contained in the precursor had been left inside the carbon material forming the doped carbon material during carbonization. The research showed that carbon nanomaterials doped with nitrogen could greatly improve the oxygen reduction reaction(ORR) catalytic activity of carbon nnomaterials, and the higher the doping content, the stronger the ORR catalytic activity. The improvement of nitrogen is much more than that of phosphorus in the ORR catalytic activity. In the same time, the effect of microstructure of carbon nanomaterials on the discharge behavior was also evaluated. Studies showed that the discharge specific capacity is proportional to the pore volume forming by the pore with size above 10 nm.Carbon nanofiber was selected by the above research to be the best performance of carbon based materials. In this paper, the carbon nanofiber was used as air electrode, and the mechanism of charge and discharge was further studied. It is found that the carbon nanofibers have a saw-tooth-like surface and can construct hierarchical pore structure, so that the structure can provide more catalytic active sites and unobstructed oxygen transfer channels. The ORR catalytic activity of the carbon material is greatly promoted by the pyridinic-like nitrogen and the pyrrolic-like nitrogen. Compared the electrochemical performance with the commercial carbon material Super P, the electrochemical properties of the carbon nanofiber are found to exceed the standard of the commercial carbon material Super P, and the discharge specific capacity reaches the level of graphene. The study also found that the discharge and charge process of lithium-air batteries based on carbon nanofibers is primarily the formation and decomposition of Li 2O2. The toroidal shaped Li2O2 can be seen clearly on the electrode after the discharge, and the Li2O2 particles disappeared while the undecomposed Li2CO3 left over the porous electrode surface after the charge. The main reason for the poor cycle performance of the battery is the hole was plugged by Li2CO3, and the non-conductive Li2CO3 leads to the increase of the impedance after the cycle. In the same time, the effect of lithium salt on the electrochemical performances of carbon nanofibers was also evaluated. The study found that the resistance of the electrolyte and the charge transfer impedance of the battery are both decreased, thus it enhances the oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) catalytic activity of carbon nanofiber, reduces the charge and discharge overpotential, and the cycle performance and rate performance also gets a promotion after replacing of the lithium salt from Li PF6 to Li TFSI.In this paper, the economic analysis of carbon nanofiber had also been gone through. Through the analysis, it was found that the carbon nanofiber has a low price, and it has a good application prospect in the lithium-air batteries.