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.