3.1 Morphology of (Co, Ni)9S8@CNFs
(Co, Ni)9S8@CNFs were synthesized by a feasible electrospinning technique (Fig. 1a, Experimental Section). The scanning electron microscope (SEM) of (Co, Ni)9S8@CNFs reveals that it has 1 D interconnected scaffolds morphology (Figure S1). The CNFs fiber has a diameter of about 100 nm and its surface contains abundant (Co, Ni)9S8 nanoparticles as well as pores due to the release of CO2 gas from high-temperature calcination (Fig. 1b). The uniform distribution of the abundantly active (Co, Ni)9S8 nanoparticles on CNFs fiber is verified by the energy dispersive spectrometer (EDS) mapping images (Figure S2). Moreover, elemental analysis illustrates the (Co, Ni)9S8@CNFs contains about 16.59 wt% carbon species. The transmission electron microscopy (TEM) of (Co, Ni)9S8@CNFs further reveal that (Co, Ni)9S8 nanoparticles are uniformly decorated on CNFs skeleton and the diameter of sulfides particles are about 3 nm (Fig. 1d, 1e). The high-resolution TEM image (Fig. 1e (Inset)) displays the clear lattice fringes with 0.281 nm space, matching well with the (311) plane of cubic (Co, M)9S8. To confirm the advantages of the fiber as sulfur host, (Co, Ni)9S8@CNFs/S was prepared by S infiltration treatment. The (Co, Ni)9S8@CNFs/S display smooth surfaces as the pores were filled with sulfur (Fig. 1c). Additionally, The STEM image and EDS line-scan analysis of (Co, Ni)9S8@CNFs/S are shown in Fig. 1f. It demonstrates that sulfur has penetrated into the interspace and pores of (Co, Ni)9S8@CNFs fibers and uniformly distributed on surface.
3.2 Physical properties of (Co, Ni)9S8@CNFs
The crystallographic structure of Co9S8@CNFs and (Co, Ni)9S8@CNFs before and after sulfur permeation is investigated by X-ray diffraction (XRD) (Fig. 2a, S3b). Clearly, all the strong diffraction peaks of Co9S8@CNFs agree well with the cubic (Co, M)9S8 exhibiting a space group of Fm-3m (PDF #30–0444), which are consistent with those reported works.[35, 42] For (Co, Ni)9S8@CNFs, no diffraction line corresponds to the Ni-based sulfides has been observed, implying the successful incorporation of the Ni in the tetrahedral and octahedral sites of the pentlandite structure.[43–44] This should be attributed to the similar size of Co2+ (74 pm) and Ni2+ (69 pm). The contents of Co, Ni and S in the (Co, Ni)9S8 detected by an inductively coupled plasma-optical emission spectrometer (ICP-OES) measurements are about 45.05 wt%, 22.59 wt%, and 32.47 wt%, respectively (Figure S3a). The corresponding molar ratio is approximately 6:3:8, which consists with the molar ratio of raw materials. While the CNFs/S, Co9S8@CNFs/S and (Co, Ni)9S8@CNFs/S inherit prominent peaks of sublimed sulfur and (Co, M)9S8 reconfirming the existence of surface sulfur and catalytic host materials (Figure S3b), in line with the TEM results. The sulfur content of the three host materials is measured to be 80 wt% by thermogravimetric analysis (TGA) tests (Fig. 2b). Figure 2c illustrates the Raman spectrum of CNFs, Co9S8@CNFs and (Co, Ni)9S8@CNFs hosts. The ratios of the intensities of the G-band at 1590 cm− 1 and the D-band at 1350 cm− 1 (IG/ID) for the three hosts are calculated to be 0.823, 0.900, and 0.965, respectively. The higher ratio of Co9S8-based CNFs compound indicates higher graphitization degree than that of CNFs,[45] which may be due to the rapid electron-transfer effect between Co9S8 and carbon.[46] The outstanding graphitized carbon is favorable to boost the electronic conductivity of the S host and further reinforce its utilization during cycling processes.[46] The surface resistances of host materials were tested by a four-point probes resistivity measurement system (RTS-9). As displayed in Figure S3c, the resistivities of the three hosts are about 148, 76, and 39 Ω cm, respectively. The high electrical conductivity of (Co,Ni)9S8@CNFs contributes to rapid electron transfer to remedy the insulativity of sulfur and its discharge products.[9, 18] The specific surface and pore structures of CNFs, Co9S8@CNFs, and (Co, Ni)9S8@CNFs were investigated by Brunauer EmmettTeller (BET) method (Fig. 2d). The results show that (Co, Ni)9S8@CNFs have a higher specific surface area of 89.48 m2 g− 1 and a total pore volume of 0.36 cm3 g− 1 than those of CNFs (51.38 m2 g− 1, 0.17 cm3 g− 1) and Co9S8@CNFs (87.48 m2 g− 1, 0.33 cm3 g− 1). The high surface area and large pore volume of (Co, Ni)9S8@CNFs can provide a high sulfur loading and facilitate electrolyte infiltration.[47–48]
3.3 LiPSs adsorption and kinetics characterization
The strong interaction between (Co, Ni)9S8@CNFs and polysulfides (LiPSs) was examined systematically by visual adsorption experiments and UV-vis tests. The same amount of three host composites was separately added into LiPSs solution, which were then placed in glove box for 12 h (Fig. 3a). The LiPSs solution before and after adsorbing was measured by UV-vis tests (Fig. 3b). The peak located at 260 nm is attributed to the S82−/S62− species, the sharp peak at 350 nm is assigned to S62−/S42− species, and the wide peak at about 490 nm is ascribed to S42− species.[49–50] Obviously, (Co, Ni)9S8@CNFs completely decolors the LiPSs solution after 12 h and significantly removes the characteristic peak of Li2S4 (Fig. 3a), confirming that the (Co, Ni)9S8@CNFs has strong chemisorption towards LiPSs. The adsorption mechanism was further probed by XPS analysis. The peak intensity of S element in the XPS spectrums of CNFs/Li2Sx, Co9S8@CNFs/Li2Sx, and (Co, Ni)9S8@CNFs/Li2Sx is stronger than that before soaking Li2Sx (Figure S4), which implies that large amounts of LiPSs were adsorbed. The peaks of S2p located at 162.18 and 164.08 eV should belong to the terminal sulfur (ST−1) and bridging sulfur (SB−1), respectively (Figure S5a).[51] The other peaks at 166.98 and 169.28 eV can be assigned to the thiosulfate and polythionate. The presence of the sulfates has a positive mediator effect on the conversion from long-chain to short-chain LiPSs.[46, 52] Additionally, S-Co and S-Ni bonds at about 163.62 and 161.37 eV corroborates the synergetic adsorption effect of Co and Ni sites.[45, 53] Compared with the original Co 2p3/2 core level, electron binding energies of Co3+ and Co2+ in (Co, Ni)9S8@CNFs/Li2Sx slightly shift to lower energies (Fig. 3c), implying the electron transfer from Li2Sx to Co. Meanwhile, the proportion of Co3+ from 34.2% decreases to 27.0% after adsorbing Li2Sx solution, which originates form the partial reduction of Co3+ induced by the charge transfer from Sx2−.[53] Similarly, the binding energy of Ni 2p peaks is reduced more significantly in Fig. 3d, and the content of Ni3+ from 51.3% decreases sharply to 37.5% after adsorbing Li2Sx, indicating that Li2Sx interact more strongly with Ni than Co sites of (Co, Ni)9S8.[54] Co 2p core level of the Co9S8@CNFs and Co9S8@CNFs/Li2Sx were also investigated (Figure S5b, S5c), the proportion of Co3+ decreases significantly and the content of Co2+ increases obviously. Meanwhile, the binding energy of Co 2p peaks is reduced significantly. These XPS spectra analysis together with the adsorption experiments support the fact that (Co, Ni)9S8@CNFs exhibits a strong chemical affinity towards LiPSs, which is essential for a stable cycle of battery.
To further rationalize the electrocatalytic activity of (Co, Ni)9S8@CNFs at the atomic level, the density functional theory (DFT) calculation was carried out. The total density of states (TDOS) of Co9S8 and (Co, Ni)9S8 are shown in Fig. 3e. The introducing of Ni atoms contributes to the energy levels of Co9S8 shifting to the Fermi level, which suggests that the (Co, Ni)9S8 has better conductivity than Co9S8.[40, 55] According to calculation in Figure S6 (a, b), the band gap of (Co, Ni)9S8 (0.006 eV) is much lower than that of the Co9S8 (0.024 eV), further indicating the enhancement of metallicity. The TDOS analysis of polysulfides adsorbed on the (311) surfaces of Co9S8 and (Co, Ni)9S8 in Figure S6 (c, d) shows that the conductivity of Co9S8 weakens after adsorption of polysulfides while (Co, Ni)9S8 is almost unchanged.[56] Once again proving there is excellent electronic conductivity of (Co, Ni)9S8@CNFs. Figure 3f and Figure S7 display the Li2S4, Li2S6, and Li2S8 adsorption configuration on Co9S8 and (Co, Ni)9S8 (311) surfaces. The binding energy in Fig. 3g verifies that polysulfide molecules are adsorbed more strongly on the surface of (Co, Ni)9S8 than Co9S8, implying that the introduction of Ni is helpful to suppress polysulfides shuttle effect.
To investigate the transformation kinetics of polysulfides, various electrochemical measurements were performed. The electrochemical impedance spectra (EIS) tests of LSBs after first discharge and 400th discharge at 0.1 C were conducted (Fig. 4a, b), and the fitted data were showed in Table S1. After first cycle, both the charge-transfer resistance (Rct) and the resistance of the deposited Li2S2/Li2S on the electrode surface (Rl)[18] of Co9S8@CNFs and (Co, Ni)9S8@CNFs are slight larger than CNFs (Fig. 4a). After 400 cycles, the semicircle in high frequency region gradually become smaller, accompanied with relatively large semicircle in medium-high frequency region (Fig. 4b). It means that the deposition of the insoluble Li2S2/Li2S layer of three composites during the cycling processes is induced.[31] After cycling, as shown in Table S1, the values of Rct and Rl after 400th cycle are decreased sharply to 12.9 Ω and 12.0 Ω, 23.1 Ω and 7.4 Ω for Co9S8@CNFs/S and (Co, Ni)9S8@CNFs/S cathodes, respectively. It is verified (Co, Ni)9S8@CNFs possesses outstanding electronic conductivity, which is consistent with the above TDOS results. Additionally, CNFs/S electrode exhibits higher Rct resistance after 400th cycling, which possibly caused by the accumulation of Li2S2 and Li2S. The discharge products (Li2S2 and Li2S) with low conductivity are physically covered on the surface of CNFs/S electrode, which gives rise to slow electron transfer, enlarged impedance and cracked electrode because of the shuttle effect. On the other hand, for (Co, Ni)9S8@CNFs electrodes, the low resistance can be ascribed to the rapid reversible transformation between Li2S2/Li2S passivation layer and sulfur as well as fast charge transfer during cycling process, owing to the strong chemical adsorption.[57] Fig. 4c presents the typical CV curves of the three kinds of electrodes in window of 1.4 ~ 2.8 V. Two cathodic peaks at. 2.36 V (Peak a) and. 2.04 V (Peak b) correspond to the stepwise reduction from Li2S8/Li2S6 to Li2S4, and then to Li2S2/Li2S. Correspondingly, a sharp anodic peak at 2.42 V (Peak c) is associated with the reverse reaction.[51]
CV tests were also conducted under different scan rates to demonstrate the effect of (Co, Ni)9S8@CNFs on the redox reaction kinetics of intermediate LiPSs (Figure S8). As shown in Fig. 4d-f, it can be found that a linear relationship between redox peak currents (Ip) and the square root of scan rate (v0.5), implying the rate-determining step dependents on the diffusion rate of LiPSs.[9] The Li+ ion diffusion process can accord with the Randles-Sevcik equation:[30, 49]
$${I}_{p}=\left(2.69\times {10}^{5}\right).{n}^{1.5}\cdot A\cdot {D}_{Li}^{0.5}{v}^{0.5}{C}_{\text{L}\text{i}}$$
1
The Li+ ion diffusion rate (DLi+) is positively correlated with the slope of (Ip/v0.5) due to the n, A, and CLi can be seen as constants in LSBs. Obviously, the slopes of the. reduction/oxidation peaks (peak a, b, c) of Co9S8@CNFs/S and (Co, Ni)9S8@CNFs/S electrodes are higher than that of CNFs/S, especially (Co, Ni)9S8@CNFs/S, verifying the rapid diffusion process of Li+ ions. The high DLi+ of (Co, Ni)9S8@CNFs should be attributed to the modified electronic structure induced by the introduction of Ni element, leading to outstanding catalytic performance. To further evaluate the catalytic kinetics of Co9S8@CNFs/S and (Co, Ni)9S8@CNFs/S electrodes, the activation energy is calculated based on the transformation from Li2S4 to Li2S through temperature-dependent CV tests (Figure S9a-c).[52] According to the Arrhenius equation, the peak current (j) is positively correlated with the reaction rate.[54] By fitting the slops (Fig. 4g), the barrier potential of (Co, Ni)9S8@CNFs (15.86 kJ mol− 1) is lower than that of Co9S8@CNFs (16.34 kJ mol− 1) and CNFs (19.43 kJ mol− 1), implying easier to catalytic conversion of polysulfides. The polarization voltage gap (peak b and c) reveals the excellent catalytic property of Co9S8@CNFs species, especially (Co, Ni)9S8@CNFs (Figure S9d).
Potential polysulfides conversion kinetics enhancements were also investigated by the CNFs, Co9S8@CNFs, and (Co, Ni)9S8@CNFs symmetrical cells. Compared with CNFs, Co9S8@CNFs and (Co, Ni)9S8@CNFs symmetric cells present smaller charge-transfer resistances (Fig. 4h), hinting the improved LiPSs conversion kinetics. As shown by the CV profiles in Fig. 4i, all symmetric cells present four main peaks at -0.46, -0.06, 0.07, and 0.46 V. The (Co, Ni)9S8@CNFs symmetric cell exhibits the largest current response, further suggesting its rapid polysulfides conversion kinetics.[51] These results collectively validate that the uniform distributions of (Co, Ni)9S8 nanoparticles on carbon nanofibers will greatly facilitate the conversion rate of sulfur and decrease polarization during cycling process. Meanwhile, they also demonstrate that the reversible conversion of LiPSs can significantly accelerate by the introduction of Ni in Co9S8@CNFs.
3.4 Electrochemical properties of (Co, Ni)9S8@CNFs/S electrode
Further electrochemical evaluations were carried out for different electrodes by CR2032 coin cells. Figure 5a presents the rate capacity of CNFs/S, Co9S8@CNFs/S, and (Co, Ni)9S8@CNFs/S cathodes at 0.1 to 5.0 C and returning to 0.1 C (1 C = 1675 mA g− 1, E/S = 20 µl mg− 1). Evidently, the (Co, Ni)9S8@CNFs/S cathode shows superior rate performance than the other two electrodes under different rates. Based on (Co, Ni)9S8@CNFs/S, at 0.1 C, 0.2 C, 0.5 C, 1.0 C, and 2.0 C, the discharge specific capacity is as high as 976, 850, 704, 584, and 485, respectively. Even current density is up to 5.0 C, a reversible capacity of 342 mAh g− 1 is presented. The excellent rate of (Co, Ni)9S8@CNFs/S electrode benefits from the strong adsorption-catalysis interaction between (Co, Ni)9S8@CNFs and LiPSs. The Co9S8@CNFs/S shows a similar rate performance but lower capacity. and utilization of S. In sharp contrast, the CNFs/S exhibits low rate. capacity. of only 160 mAh g− 1 at the 5.0 C. This kinetic. difference. can also be reflected from the voltage profiles (Fig. 5b and Figure S10a, b). For (Co, Ni)9S8@CNFs electrode, a remarkably steady discharge plateau at ~ 1.9 V representing the conversion from long-chain LiPSs to insoluble Li2S2/Li2S can be shown even under 5.0 C. In contrast, CNFs/S has no obvious discharge plateau at 2.0 C, and it also presents a larger polarization at each current density. When the current density returns back to 0.1 C after diverse rates, the discharge behavior can be self-healed very well for catalytic electrodes, especially (Co, Ni)9S8@CNFs/S, which recurred 95% of its discharge capacity of second cycle suggesting excellent catalytic activity and reversibility. By comparison, CNFs/S recovered to 700 mAh g− 1, only 77% of its discharge capacity of second cycle. So again, the results demonstrate the higher catalytic ability and the faster redox kinetics of the (Co, Ni)9S8@CNFs than CNFs/S.
It is essential to satisfy the practical application of LSBs with low electrolyte/sulfur (E/S) ratio.[17] The cycling performances of (Co, Ni)9S8@CNFs under S loading 2.8 mg cm− 2 with different E/S ratios have been investigated at 0.5 C. As shown in Fig. 5c, it still obtains decent cycling performance at 15 and 20 µl mg− 1. Significantly, when the E/S = 20 µl mg− 1, the (Co, Ni)9S8@CNFs electrode delivers the high initial capacity of 770 mAh g− 1 and retains a desirable capacity of 440 mAh g− 1 (capacity retention of 57% and capacity decay of 0.142% per cycle) after 300 cycles. This is attributable to the effective wetting and penetration of electrolyte across the electrode/electrolyte interface.[51] However, a further increase in the E/S ratio to 25 leads to a highest initial capacity of 826 mAh g− 1 and unsatisfied capacity retention of 45% after 300 cycles, which arises from the LiPSs dissolution and diffusion in the electrolyte resulting in serious shuttle effect and poor utilization of sulfur. At an even low E/S ratio of 5 µl mg− 1, the (Co, Ni)9S8@CNFs/S cell shows a high initial capacity of 582 mAh g− 1 and a reversible capacity of 315 mAh g− 1 after 300 cycles at 0.5 C. The corresponding initial charge-discharge curves are shown in Fig. 5d at 0.5 C. The remarkably steady discharge plateaus at ~ 1.9 V can be sighted even under the low E/S ratios of 5 µl mg− 1, implying the relatively fast conversion kinetics and very weak polarization of (Co, Ni)9S8@CNFs/S.
To further enhance the energy density of LSBs, the electrochemical performance of (Co, Ni)9S8@CNFs/83.3S with 83.3 wt% sulfur content, 5.0 mg cm− 2 areal sulfur loading and 5 µl mg− 1 E/S ratio was also investigated. The sulfur content was determined by TGA (Figure S11). Figure 5e presents the rate and cycling capabilities of (Co, Ni)9S8@CNFs/80S and (Co, Ni)9S8@CNFs/83.3S. Obviously, (Co, Ni)9S8@CNFs/83.3S shows high discharge capacity at various current densities, but also displays a more stable cycling performance, suggesting high sulfur utilization. Significantly, a high discharge capacity (376 mAh g− 1) is presented at 2.0 C, indicating excellent reversibility of high sulfur content and lean electrolyte cell. After being subjected to cycling at different rates, the current density was restored to 0.1 C, (Co, Ni)9S8@CNFs/83.3S electrode still maintain relatively stable cycling performance, which shows a high discharge capacity (590 mAh g− 1) is maintained after 180 cycles at 0.2 C with high CE (> 94%). The corresponding charge-discharge profiles of (Co, Ni)9S8@CNFs/83.3S and (Co, Ni)9S8@CNFs/80S cathodes at different current density are displayed in Fig. 5f and Figure S10c, which further prove the superior rate performance. The results imply that the (Co, Ni)9S8@CNFs/83.3S cathode has a fast redox reaction. Compared with similar works based on catalyst-anchored carbon cathodes for Li-S batteries, (Co, Ni)9S8@CNF/S displays excellent cycling stability at high sulfur content and lean electrolyte (Table S2). These results suggest that the (Co, Ni)9S8@CNFs host delivers potential as efficient sulfur electrocatalysts for LSBs.