Preparation And Performance Study Of Carbon Air Cathode For Secondary Lithium Air Battery

Li-air batteries have extremely high theoretical energy densities. However, due tothe many constraints, the actual performance of Li-air batteries is far lower than that ofthe theoretical value. As an important part of Li-air batteries, air electrode is one of themain factors that affect the performance of the batteries. This paper firstly discussed theimpact of microstructure of different carbon materials on the performance of the airelectrode. On the basis of the result, a novel type of air electrode with self-supportingstructure based on carbon nanotube was prepared, and the effects of the preparationprocess on the electrode performance. were also discussed.Firstly, five kind of carbon material with different microscopic structure wereselected as objects for research The pore structure of carbon materials werecharacterized by nitrogen adsorption method, and the discharge performance of aircathodes made of different carbon materials were characterized by constant currentdischarge test. The results show that the pore structure of the porous carbon consists ofmicroporous adsorbent type pores with a pore size less than8nm and mesoporousadsorption type pores with pore size above8nm.In the discharge process, microporousadsorbent type pores are the main location of oxygen adsorption and mesoporousadsorbent type pores form the channel of oxygen diffusion, the performance of theelectrode are determined by the synthetic action of these two types of holes. Therefore,there is sufficient gas diffusion path as well as abundant space for the deposition ofdischarge products in the carbon materials when the content of these two types of poreshas an optimal ratio. Carbons with optimal ration of the two types of pores can achievegood performance. Among the five kinds of carbon materials, Super P that have theratio of the microporous adsorbent type pores to the mesoporous adsorbent type pores is0.3attains the maximum discharge capacity(1349.1mAh/g).Secondly, in order to overcome the disadvantages of conventional air electrodepreparation, we selected multi-wall carbon nanotubes, which have the ratio ofmicroporous adsorbent type pores to mesoporous adsorption type pores is0.29, as rawmaterials for the preparation of self-supporting carbon nanotube electrode. Theself-supporting electrode was obtained by the filtration method and its application inLi-air batteries was also been discussed. The results show that: Electrodes prepared byfiltration method using different surfactant as dispersing agent demonstrate differencedischarge performance. Electrodes using CTAB as dispersant that have the smallestamount of glomerate carbon nanotubes and the minimum porosity attain the highestdischarge capacity (887mAh/g). Meanwhile, The application of the surfactant increasesthe wettability between carbon nanotubes and electrolyte and the increased wettabiltiyleads to the deterioration of the electrode stability in electrolyte. To improve the stability of the electrodes in electrolyte, as-prepared electrodeswere treated by pressure and heat treatment. The influence of post treatment on theperformance of electrodes was also investigated. The results show that, pressuretreatment can make the structure of electrode becomes densely and reduce the porosityin electrode. However, pressure treatment can not fundamentally change the wettabilitybetween electrode and electrolyte, the structural stability of electrode in electrolyte doesnot get obvious improvement. By completely removing the residual surfactant, heattreatment can change the chemical composition on the surface of carbon nanotubes andreduce the wettability between carbon tubes and electrolyte. Because of theimprovement of the electrode stability in the electrolyte, the discharge performance ofelectrode after heat treatment demonstrate significant amelioration.Finally, by optimizing the process parameters of preparation and post-process, weprepared self-supporting air electrodes with carbon nanotubes. The novel type of aircathodes demonstrate better performance than traditional air electrodes, which thedischarge capacity is1507mAh/g. The result shows the broad prospect of theapplications of self-supporting carbon nanotubes electrodes as air cathodes tolithium-air batteries.