Abstract
Sulfur-doped carbon nanotubes (SCNTs) exhibit better conductivity and hydrophilicity, as compared to the undoped carbon nanotubes (CNTs). Low cost mass production of SCNTs is highly desirable for their real application at an industrial scale. Here, scalable green syntheses of SCNTs and S, N dual doped CNTs (SNCNTs) are demonstrated based on the gaseous S doping using SO2 as dopant. The process not only significantly promotes the hydrophilicity and conductivity of CNTs due to the incorporation of S atoms in carbon framework, but also purifies the final product due to burning off the carbon coatings of metal particles. SO2 is generated by CaSO3 pyrolysis and absorbed by CaO, thus realize the recycling of S species. The LiFePO4 cathodes using the as-prepared SCNTs and SNCNTs as conductive additive exhibit enhanced capacity and rate performance than the electrodes prepared with the pristine CNTs, not matter using N-methyl pyrrolidone or water as solvent.
KEYWORDS: sulfur doping; carbon nanotubes; flue gas; CaSO3 pyrolysis; lithium ion battery
1. Introduction
For lithium ion batteries (LIBs), most of the cathode materials faced the disadvantage of high polarization under high charging rate condition due to their low conductivity and sluggish diffusion of Li+ ion. In order to improve the rate performance, many efforts focus on establishing a uniformly distributed conductive network and reduce the contact resistance inside the electrode.1-6 Carbon nanotubes (CNTs) have been widely used as superior conductive additives to fabricate conductive networks in electrodes for LIBs,7-11 due to their unique one-dimensional structure, high electrical conductivity and good chemical stability.3,12,13 Compared to the conventional conductive additive carbon black (CB), well-dispersed CNTs have significantly promoted the electrode performance of LiFePO4 (LFP) cathodes,11,13,14especially at low temperature environment.15-17However, CNTs are extremely prone to agglomeration. The hydrophobicity of the pipe wall makes it difficult to disperse evenly in water. Fortunately, the surface property of CNTs can be changed by doping hydrophilic functional groups containing heteroatom into the framework of CNTs.17-21 As an example, S-doped CNTs (SCNTs) possess improved conductivity and better hydrophilicity as compared to the pristine CNTs, which permits stable dispersion of SCNTs in water-based slurries. As the result, the LFP cathodes prepared using water as solvent and SCNT aqueous slurry as conductive additive deliver unimpaired electrode performance with reduced cost, as compared to the LFP cathodes prepared with N-methyl pyrrolidone (NMP) as solvent.22 The real application of SCNTs in the battery fabrication is promising to further promote the cell performance and reduce the cost. Low cost mass production of SCNTs is highly desirable to realize the industrial application of SCNTs with an acceptable price.
In our previous work, SCNTs were prepared by a solid phase S doping reaction using MgSO4 as S dopant.22However, the redundant steps in the solid phase S doping process, including impregnation of MgSO4, removal of Mg salts by acid washing, repeated mixing, filtration, drying and grinding, have significantly increased the operation cost, especially for the mass production at an industrial scale. Here, we demonstrate a green production of SCNTs and S, N dual doped CNTs (SNCNTs) by a gaseous S doping process using SO2 as S dopant. CaSO3, the main component of flue gas desulfurization ash, is used to release SO2 via a high temperature calcination, and the SO2-containing tail gas is absorbed by a CaO packed column, thus realizing a low emission green process with recycling of SO2. The LFP cathode prepared with the as-prepared SNCNTs as conductive additive exhibits higher capacity and better rate performance as compared to that with the pristine CNTs, no matter in water or NMP-based slurries.