Synthesis And Electrochemical Properties Of Porous Carbon Fibers/Spheres

The porous carbon materials used as anode materials for lithium ion batterieshave good electrochemical properties due to its effective diffusion pathways for Liions, reasonable electrical conductivity and large numbers of active sites for Li-ionstorage. While traditional methods of producing porous carbon fibers/spheres requirea pore-creation step (so called activation), including complex chemical and physicaltreatment processes, while both electrospinning and electrospraying are the especiallyconvenient and effective methods. We used cheaper and environmentally friendlypolyvinyl pyrrolidone (PVP) instead of polyacrylonitrile (PAN) as a carbon precursor,and polymethyl methacrylate (PMMA) as the pore-forming agent to prepare theporous carbon fibers/spheres. Because severe thermal shrinkage of PVP easily ruinsthe fibrous/spheral structure during the heat treatment process and the existence ofPMMA makes the heat treatment process more complicated, we must explore theproper parameters of heat treatment temperature and time to prepare the porouscarbon strcture.In this paper, porous carbon nanofibers (PCNFs) have been synthesized viaelectrospinning hybrid precursors of PVP and PMMA into polymeric compositenanofibers with molecular weight of1,300,000and90,000respectively, followed bya three-step heat treatment process. After many times heat treatment exploringexperiments with different parameters, we ultimately identified the three-step heattreatment temperature and time combined with the results of thermal gravity analysis(TGA) and scanning electronmicroscopy (SEM) images. The as-spun PVP/PMMAnanofibers should be stabilized at150°C in air for24h, then pre-oxidized at280°Cfor6h in air, and finally carbonized at1000°C under nitrogen atmosphere for4h.The effects of different PVP:PMMA mass ratios on the PCNFs have been studiedsystematically by using x-ray diffraction (XRD), SEM, transmissionelectronmicroscopy (TEM), and specific surface area analyzer. The results indicatethat PCNFs with PVP:PMMA mass ratio of3:2are mesoporous and exhibit thelargest specific surface area of545.4m2·g–1without subsequent activation.Coin cells (CR2025) were assembled in an dry-boxthe using as-prepared PCNFsas anode materials. Then the cyclic voltammogram curves and galvanostaticcharge-discharge properties of PCNFs with different PVP:PMMA mass ratios were examined by using multi-channel battery test system and electrochemical workstation.The electrochemical measurement shows that the cyclic reversible capacity of all thePCNFs is higher than that of carbon nanofibers without PMMA, and PCNFs withPVP:PMMA mass ratio of3:2have the highest capacity of about220mAh·g–1at0.1C after50circles. All samples have relatively good cycling stability.We also attempted to synthesize the porous carbon spheres using PMMA withmolecular weight of30,000and PMMA by electrospraying combined with athree-step heat treatment. Being the morphology different between carbon fibers andspheres, porous carbon spheres can’t be successfully prepared through the same heattreatment process. However, we still got part preliminary heat treatment processparameters by improving the heat treatment process and related experiments.