Figure 4 (a) The green synthesis of SCNT-S using SO2 gas
phase treatment. (b) The SO2 concentration released by
CaSO3 (20 g) and the S content of SNCNTs at different
reaction times. (c) The S content of SNCNTs and the
CaSO3 decomposition ratio at different calcination
temperatures. (d) Comparison of the S contents of SNCNTs and SCNT-S at
the same S-doping reaction conditions. (e) The SO2concentration in the final outlet gas after CaO absorption using
different CaO doses.
3.3 Application as conductive additive in LFP cathodes
3.3.1 NMP-based slurry
The as-prepared S-doped CNT materials were applied as conductive
additive in LFP cathodes. First, NMP was used as solvent to prepare a
NMP-based slurry containing LFP (active material), CNTs (conductive
additive) and PVDF (binder). Two SCNT-S samples with different S
contents are labelled as SCNT-S-x, where x represents the S content (wt
%). The S content has been adjusted by using different S doping time.
As shown in Figure 5a, the LFP electrodes using SCNT-S-1.4% and
SCNT-S-0.7% as conductive additive deliver discharge capacities of 134
and 129 mAh g-1 at 5 C, obviously higher than the LFP
electrodes using CNTs (124 mAh g-1). The rate
performance of LFP-SCNT-S-1.4% is slightly better than
LFP-SCNT-S-0.7%, showing that the S doping has contributed to the
enhancement. The typical charge-discharge curves at 2 C are shown in
Figure 5b. The extension of charge/discharge capacity for
LFP-SCNT-S-1.4% and LFP-SCNT-S-0.7% indicates that more LFP active
particles have been involved in the energy storage via the
well-established conductive network. Impedance curves of the LFP
electrodes are shown in Figure 5c. The charge transfer resistance
(R ct,corresponding to the width of the
semicircle at the high frequency zone) of LFP-SCNT-S-1.4% is the lowest
among the three electrodes. Generally speaking, theR ct values for the CNT-added LFP electrodes are
mainly affected by two factors: the intrinsic conductivity and the
dispersion state of CNTs. It has been reported that higher S content is
in favor of enhancing the conductivity of CNTs22,30and porous carbons.30,31 Considering that S-containing
functional groups are usually acidic and N-containing groups are
alkaline, the interaction between SCNTs and NMP might have been
strengthened by the attraction force between the S-containing groups in
SCNTs-S and the N atom in NMP. The strengthened solid-solution
interaction might have led to better dispersion of SCNTs, thus
establishing an evenly-distributed conductive network in LFP cathodes.
Therefore, the superior rate performance of LFP-SCNT-S-1.4% is
attributed to both the higher conductivity and the better dispersion of
SCNTs.