Co3O4-Carbon Cloth free standing cathode for lithium sulfur battery

Lithium–sulfur (Li–S) battery has been considered to be one of the most promising next-generation electrochemical energy-storage systems due to its high theoretical energy of 2600 Wh kg-1 with low cost. The insulating nature of both sulfur and the dissolution of polysulfides are the two primary challenges for the application of lithium sulfur batteries. Here, we developed a binder-free cathode by chemisorption of Co3O4 to carbon cloth (CC), which was used as a 3D current collector to accommodate a large amount of sulfur, multiwall carbon nanofiber (MWCNF) and carbon black (CB) hybrids within the conductive scaffold, enabling the fabrication of ultrahigh sulfur loaded electrodes. The interconnected carbon fibers established a long-range conductive matrix for an efficient electron transport, the multiple conductive pathways guarantee high sulfur utilization. More importantly, the high electrolyte absorbability of the Co3O4-CC-S current collector facilitates well-localized polysulfides within the Co3O4-CC-S, meanwhile, the polar Co3O4 could also effectively entrapped the intermediated polysulfides preventing their free diffusion to the lithium anode, guaranteeing good cycling stability. Consequently, the Co3O4-CC-S electrodes exhibit excellent electrochemical performance with sulfur loading of 4.3 mg cm-2.


Introduction
We significantly stabilized cycle life of high sulfur loading binder-free cathode by chemisorption of Co 3 O 4 to carbon fiber cloth, which was used as a 3D current collector to accommodate a large amount of sulfur, MWCNF and CB hybrids within the conductive scaffold, enabling the fabrication of ultrahigh sulfur loaded electrodes. 1,2 This special nanoarchitecture combines the advantage of strong chemisorption of lithium polysulfides as well as excellent electrical conductivity, enabling high sulfur utilization and effective trap of lithium polysulfides. When applied as cathode materials for lithium sulfur batteries, the cathodes exhibit a reversible capacity of 1007 mAh g -1 after 300 cycles.

Experiment
Firstly, Co 3 O 4 nanocages were constructed on carbon cloth (CC) by a solvothermal method. In a typical procedure, 0.1 of poly(vinylpyrrolidone) (PVP, Mw = 360000 g mol -1 , 99 %) was well dispersed in 7.5 ml of deionized water and 7.5 ml of ethanol (95 %) by stirring treatment, then 0.5 mmol of Co(NO 3 ) 2 6H 2 O (> 98%) were dissolved into the above dispersion to form a light pink solution by continuous stirring for 30 min. CC which was pre-treated by an acid treatment were performed to functionalize CC with oxygen containing groups. Pristine CC were cut into circular disks with a diameter of 1.4 cm and soaked in HNO 3 solution at 60 °C for 3 h. Acid-treated CC were then washed by water and dried at 60 °C for 24 h. Teflon-Lined stainless steel autoclave and reacted at 180 °C for 12 h. When the reaction finished, the solution was cooled down to room temperature naturally and washed to obtain the black Co 3 O 4 -CC. Then heated at 450 °C for 1 h under N 2 atmosphere. Homogeneous sulfur-containing slurry was fabricated by mixing 90 wt% sulfur, 5 wt% carbon black, and 5 wt% MWCNFs in N-methyl-2pyrrolidone (NMP) followed by high power ultrasonication for 0.5 h. The as-prepared Co 3 O 4 -CC was immersed into the slurry for 10 min, then the Co 3 O 4 -CC was removed from the slurry and placed in a vacuum oven at 60 °C overnight to obtain the Co 3 O 4 -CC-S electrode.

Scheme 1.
Schematic illustration for the formation of Co3O4-CC-S. By using our simple method, an activated carbon cloth was used as a template for the preparation of Co 3 O 4 -CC-S composite (Scheme 1), Co 3 O 4 particles were successfully coated on the surface of CFCs. The as-prepared Co 3 O 4 characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1b, the as-made Co 3 O 4 exhibits regular nanocubic morphology and has a uniform particle size of approximate 400 nm, also the surface of Co 3 O 4 was smooth, which was consistent with the TEM result in Figure 1f, the surface of which shows high crystallinity without any detectable by-products as revealed by the X-ray diffraction pattern ( Figure  2b). The structure of the Co 3 O 4 -CC-S was characterized by scanning electron microscopy (SEM) in Figure 1, it can be seen that sufficient void space, tens of micrometers in size, was generated by the randomly interconnected carbon fibers, which is capable of holding a large amount of active material, maintaining high electrolyte absorbability and effectively accommodating the volume expansion of sulfur during discharge. The Co 3 O 4 -CC-S electrodes were prepared by immersing the Co 3 O 4 -CC in a premixed sulfur slurry containing 70 wt% commercial pure sulfur powder, 10 wt% carbon black(CB), and 20 wt% MWCNFs after high-power ultrasonication, it needs to be pointed out that MWCNFs were used in the sulfur slurry instead of conventional polymetric binders in integrated binder-free electrodes. From the X-ray diffraction pattern of the Co 3 O 4 -CC-S shown in Figure 2b, it can be seen that all the diffraction peaks are well indexed as sulfur structure, indicating that sulfur was successfully encapsulated into the porous structure. This seems to be further supported by the Raman spectroscopy analysis. In Figure 2c, we can detect distinct sulfur peaks at around 200 cm -1 , confirming that sulfur effectively diffuse into the Co 3 O 4 -CC composites. The 3D Co 3 O 4 -CC skelton is capable of holding a high, uniformly distributed concentration of the active material within its interconnected pores, enabling a high sulfur loading. We also characterized the structure of the sulfur-MWCNF-CB clusters loaded on the Co 3 O 4 -CC, considering the distribution of sulfur is closedly associated with the electrochemical performance of the electrode. It was seen that the MWCNFs and CB are not simply surrounded around the sulfur particles, but homogenously dispersed through the sulfur and formed an interconnected and embedded conductive network, the corresponding elemental maps of carbon and sulfur revealing that sulfur is uniformly distributed within the carbon conductive network constructed by MWCNFs and CB. In this work, we demonstrate that an embedded conductive scaffold formed by MWCNFs and CB can be constructed through the insulated sulfur, which is highly desirable for high sulfur utilization during the electrochemical redox process. Electrochemical measurements were conducted to compare the performance of Co 3 O 4 -CC-S and CC/S. Both the sulfur mass loading and the amount of electrolyte added in the coin cells were kept at the same level for Co 3 O 4 -CC-S and CC-S composites. Figure 3a, b illustrate the charge/discharge profiles of Co 3 O 4 -CC-S and CC/S in the first, second and 300th cycles, it is noteworthy that Co 3 O 4 -CC-S exhibits higher first-discharge plateau at around 2.37 V and wider second-discharge plateau at around 2.1 V compared with pure CNF/S electrodes, and even after 300 cycles, the discharge plateau still remain well, indicating more sulfur utilization in the Co 3 O 4 -CC-S. The cycling performance and Coulombic efficiency of the Co 3 O 4 -CC-S composite and pure CC-S electrodes over long-term cycles are shown in Figure 3d. After 300 cycles, the reversible capacity of Co 3 O 4 -CC-S is around 1007 mAh g -1 , the capacity retained 81% of the initial capacity after 300 cycles at 0.5C, while the stability of pure CNF/S electrode is significantly lower than that of Co 3 O 4 -CC-S, the capacity degraded to merely 434 mAh g -1 by 300th cycles. We also cycled the Co 3 O 4 -CC-S electrodes at different current rates. Figure  3c shows the results of Co 3 O 4 -CC-S and CC-S electrode cycled at step-wise current rates. During the discharge-charge process, the electrodes were consecutively cycled at 0.1C, 0.5C, 1C, 2C, 3C and then reversed back to low rates. It should be noted that when the current reversed back to 0.1C, the Co 3 O 4 -CC-S electrode could retain a capacity of 950 mAh g -1 , while the figure for CC-S electrodes was only 370 mAh g -1 .

Discusion
The interaction of different sulfur host materials (CC, Co 3 O 4 -CC) with lithium polysulfides were probed by visual discrimination, taking Li 2 S 6 as the representative polysulfide. An equivalent amount of host materials were first added to Li 2 S 6 solution. The superior intrinsic capability of Co 3 O 4 -CC to absorb Li 2 S 6 was clearly obvious, as shown in Figure 4. The adding of Co 3 O 4 -CC rendered the Li 2 S 6 solution light yellow after resting for 3h, implying strong adsorption while the carbon black solution did not exhibit obvious change, indicative of no interaction. Hence, the results from the visible experiment are highly consistent with the cycling performance of the batteries and further confirm the strong capability of Co 3 O 4 -CC-S in confining the LiPS species.

Conclusion
In summary, we designed a binder-free cathode by chemisorption of Co 3 O 4 to carbon fiber cloth, and applied this as a 3D current collector to accommodate a large amount of sulfur, multiwall carbon nanofiber (MWCNF) and carbon black (CB) hybrids within the conductive scaffold. The electrode with a high sulfur loading exhibits low polarization, stable cycling performance, and excellent rate capability compared with pure CC-S cathodes. The excellent performance could be attributed to synergic effects, including the high surface area and high conductivity of the CC-S matrix, as well as the strong polysulfides binding capability of Co 3 O 4 particles. Moreover, Co 3 O 4 nanocrystals probably participate in multiple polysulfide transformation, affecting the redox reaction environment favourably, leading to the improvement in the reversibility of Li 2 S/polysulfides/S conversion. When applied as cathode material for lithium sulfur batteries, it exhibits superior cycling performance and excellent rate capability. This work opens a new opportunity for the realization of high-energy and commercially viable Li-S batteries.