Figure 6 SEM images of the LFP electrode fabricated with the water-based
slurries of the pristine CNTs (a), SCNT-M (b), SCNT-S (c) and SNCNTs
(d).
The electrode performances of the LFP electrodes prepared using
water-based slurries were tested via assembly coin-type half cells. As
shown in Figure 7a, the LFP electrode with SNCNTs conductive additive
shows the highest discharge capacities among all the samples. It affords
the average reversible capacity of 163.4 mAh g-1 at
0.2 C, obviously higher than the capacities of the LFP electrode with
CNTs (155.4 mAh g-1). The reversible capacity of
LFP-SNCNT remains 134.4 mAh g-1 at 5 C, 5.5% higher
than that of LFP-CNT (127.4mAh g-1), showing excellent
rate capability. The cycling performances of the as-prepared electrodes
were shown in Fig 7b. The capacity retention of the LFP-SNCNT electrode
is 99.5% after 100 cycles at 1C, obviously higher than that of LFP-CNT
(96.3%). In the charge-discharge curves at 0.5 C (Figure 7d), extended
charge
and discharge platforms at ~3.4 V are observed for
LFP-SCNT-S and
LFP-SNCNT.
It indicates that more active material particles have been involved in
the charge-discharge processes, due to the connection by
well-established conductive networks. Figure 7f shows the impedance
curves of the as-fabricated LFP electrodes. The charge transfer
resistance (R ct) of LFP-SNCNT is the smallest
among all the simples. Although the conductivity of SNCNTs (806 S/m) is
smaller than that of SCNT-S (1230 S/m), our results indicate that the
better dispersion of SNCNTs in the water-based slurry and the
as-resulted electrode due to their better hydrophilicity has played a
dominant role in reducing the charge transfer resistance of the
electrode.
Although the final discharge capacity at 2.0 V for LFP-SCNT-M is close
to that for LFP-SCNT-S (Figure 7c), the discharge platform of LFP-SCNT-M
drops earlier. The drop in the discharge platform for LFP-SCNT-M is
likely due to not only the higher bulk charge transfer resistance
aroused by the worse dispersion of CNTs, as compared to LFP-SCNT-S, but
also the higher contact resistance between CNTs and LFP particles
aroused by the uneven distribution of S heteroatoms in SCNT-M. The
conductivity and hydrophilicity of SCNT-S are superior to SCNT-M, which
has led to better electrode performance. On the other hand, in the
synthesis of SCNT-M that uses MgSO4 as dopant, the
S-doping reaction between carbon species and MgSO4 was
significantly affected by the solid phase contact between the two
reactants. In order to realize a good contact between CNTs and
MgSO4, the amount of MgSO4 was kept at a
high level. It means that the operation cost for adding and removal of
Mg salts is high, and that the usage efficiency of MgSO4is limited. Therefore, we conclude that the gaseous S doping possesses
the advantage of both lower cost and better product quality, as compared
to the solid phase S doping.