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.