The Synthesis Of Three-dimensional Porous Carbon Based On Bacterial Cellulose & The Application In Lithium Sulfur Batteries

In recent years, with the rapid increasing demand of high-performance energy storage system for new energy electric vehicles and portable electronics, the scope of battery energy storage system has been broadened. Lithium-sulfur batteries as a high energy density secondary batteries quickly attracted widespread attention in the research community and industrial circle. The theoretical energy density and specific capacity of lithium-sulfur batteries are up to 2600 Wh·g-1, 1673 mAh·g-1, respectively.It is more than 10 times that of the commercially available cathode materials for lithium-ion batteries. The sulfur element is rich in natural storage, and it is non-toxic,inexpensive and environmentally friendly. So lithium-sulfur battery is considered as a potential competitor in terms of efficient energy storage secondary batteries of the next generation.However, there are currently two main problems to be solved for the lithium-sulfur batteries. On the one hand, the elemental sulfur and its discharge products have poor conductivity, making the battery polarized, resulting in low efficiency of active materials in the positive electrode. On the other hand, the active intermediate products are easily dissolved in the electrolyte, and can spread to the surface of negative electrode, namely"shuttle effect". The shuttle effect can result in loss of the active materials, and reducing the actual specific capacity and cycle performance of the batteries.In view of the above problems, we focus on synthesizing a series of novel composite cathode materials, aiming to improving the conductivity and distribution of the sulfur,suppressing the dissolution and diffusion in the electrolyte, reducing the shuttle effect,enhancing the capacity retention ratio and cycle performance.Firstly, we aim to synthesize a kind of nitrogen-doped porous carbon fiber material using bacterial cellulose (BC) as the base carbon source because of its ultra-fine network structure, high crystallinity and degree of polymerization, good molecular orientation and other characteristics. The nitrogen-doped porous carbon fiber material (NPCM) having a high specific surface area and excellent conductivity was prepared by calcining the mixture of BC and a different proportion of potassium hydroxide activator. Then the new type of porous carbon material was mixed with sulfur, and the sulfur element was introduced into the pores of the porous carbon material matrix by melt injection to form S@ NPCM composite. The NPCM as a matrix can provide sufficient electrode reaction space for the active material of the positive electrode, greatly improve the conductivity and utilization of the active substance, and effectively reduce the dissolution of the intermediate reduction products (lithium polysulfide). The cycle performance of the batteries can be improved to some extent. At the current rate of 0.1C, the initial discharge specific capacity of the batteries was up to 1137 mAh·g-1. It was still maintained at 890 mAh·g-1 after 500 cycles.In addition, the coulombic efficiency of the assembled batteries is around 99%.Secondly, in order to improve the current carrying capacity per unit area and to further optimize the experimental scheme, carbon fiber paper (CP) with three-dimensional continuous conductive network structure was used as the current collector. The prepared S@NPCM composite material was infiltrated into the inside of CP by simple coating method, being continuous distribution within CP. Compared with ordinary aluminum foil,carbon fiber paper can have a higher conductivity, and maintain long-range, continuous,high-speed electron transport channel in the electrode. In addition, carbon fiber paper can provide a larger space for electrode reaction. Good spatial structure can well accommodate and limit the active substance. The low electrochemical impedance of S@NPCM-CP with the electrode structure was verified by EIS, and good electrochemical reversibility was verified by CV. The initial discharge capacity of S@NPCM-CP electrode was about 1300 mAh·g-1 at the current rate of 0.1C, and the reversible specific capacity was about 980 mAh·g-1. The average capacity decay rate of the batteries.