Low cost bio-derived carbon-sprinkled manganese dioxide as an efficient sulfur host for lithium–sulfur batteries

Realization of the lithium-sulfur battery system is of major concern because a theoretical cell capacity of 1675 mA h g−1 can be obtained at an average voltage of 2.1 V. The primary issues that hinder the practical applications of this system include its poor utilization of sulfur, limited cycle life and retarded rate performance. In the present study, hemp-derived carbon (C-hemp) is made into a composite with room temperature-synthesized MnO2, which acts as a host for sulfur in the lithium-sulfur battery system. The composite material is characterized physico-chemically and electrochemically using various techniques. This composite exhibits better electrochemical performance as a sulfur carrier compared to pristine carbon. An initial specific capacity of 926 mA h g−1 is obtained at 0.1 C for C-hemp/MnO2-sulfur, which surpasses that of the C-hemp-sulfur sample. C-hemp provides a conductive matrix as well as porous sites for the accommodation of sulfur, while MnO2 exhibits the ability to absorb polysulfide chemically. Thus, this composite is established as a potential cathode for lithium-sulfur batteries.


Introduction
In the 21st century, the critical need for high performance and long-life energy storage systems is highly dependent upon the generation of safe, cost effective and high energy density rechargeable batteries, which are required for the development of electric vehicles, renewable energy plants and smart grids. 1,2 However, traditional lithium ion batteries fail to meet the requirements because they generate low energy density, utilize raw materials and pose an environmental hazard. This has caused the emergence of other types of batteries such as lithiumair, lithium-sulfur, zinc-air, sodium-ion, and aluminium ion batteries. The conducive characteristics of sulfur such as low cost, non-toxicity, resource abundance make it a promising candidate for next generation energy storage systems. 3,4 Also, it possess a high theoretical specic capacity of 1675 mA h g À1 . 5,6 Furthermore, it delivers a high energy density (2500 W h kg À1 ) when coupled with lithium (Li-S cell), which is much higher than that of other available lithium-ion batteries. [7][8][9] However, the commercialization of these batteries is hindered by critical problems. Primarily, the intrinsic insulating nature of sulfur and lithium suldes lead to a low volumetric energy density, and the large volume change of sulfur and aggregation of the charge discharge products upon cycling lead to a decrease in the electrode stability and internal redox shuttle causing capacity fading, low specic capacity and low coulombic efficiency. [10][11][12] Thus, to address the abovementioned hurdles, research is ongoing in the development and modication of sulfur cathodes, lithium anodes, 13 electrolytes 14 and separators. 15,16 The poor conductivity of sulfur can be overcome by impregnating sulfur into conductive matrices of carbonaceous materials such as porous carbon, 17-21 carbon spheres, 22,23 carbon bers, [24][25][26] carbon nanotubes, [27][28][29][30] and graphene. [31][32][33][34][35][36] Additionally, its electronic conductivity can be effectively enhanced by combination with carbon. The high conductivity of carbon reduces the polarization of the cathode and its high surface area traps the polysulde intermediates by physical interaction. 37 Nevertheless, the non-polar nature of carbon has limited ability in suppressing lithium polysulde dissolution. Accordingly, high performance can be achieved by not only improving the conductivity and reducing polysulde shuttling, but also the rectication of electrode pulverization. 38 Another advantageous strategy for superior performance is exploiting the chemical interaction of polar materials with lithium polysuldes. 39 62 Additionally, combining carbon materials with manganese oxide not only improves their conductivity but also produces absorption agents for polysuldes. Herein, we report a cathode structure based on d-MnO 2 -decorated C-hemp as the host material for Li-S rechargeable cells, which shows an appreciable cycling performance and rate capability. The porous nature of C-hemp provides a conductive matrix and ensures the connement of sulfur in the pores. Additionally, the polar nature of the metal oxide enables the absorption of polysulde intermediates. The composite matrix also stabilizes the capacity to a small degree.

Synthesis
Hemp-derived carbon. This was obtained from the thermal carbonization of hemp bers (Cannabis sativa) followed by activation with alkali. The bers were washed with water, cleaned and dried at 60 C for 12 h. Then, the dried bers were soaked in 10% KOH solution overnight followed by carbonization at 220 C for 3 h. The obtained product was ground well in an agate mortar and heated in a tubular furnace at 750 C for 30 min under a nitrogen atmosphere. The product was washed with 0.2 M HCl followed by several washes with distilled water until it reached neutral pH and then dried. The puried carbon material is henceforth referred to as C-hemp.
MnO 2 nanoparticles. MnO 2 nanoparticles was prepared using a method reported elsewhere. 63 In the present study, an aqueous solution of a 1 : 1 molar ratio of KMnO 4 and amino acid was sonicated for 30 min and the brown precipitate formed was allowed to settle. This precipitate was centrifuged and washed with excess water followed by ethanol. Then, it was dried at 80 C overnight.
C-hemp/MnO 2 -S composite. C-hemp and MnO 2 in a 1 : 2 ratio were dispersed in distilled water by ultrasonication for 1 h and continuously stirred overnight. The nanocomposite was collected by centrifugation, washed repeatedly with water and dried at 70 C in an oven. Aerwards, a given amount of Chemp/MnO 2 and sulfur were mixed well by grinding and heated at 150 C under an N 2 atmosphere for 6 h to ensure the uniform impregnation and distribution of sulfur. A schematic representation of the C-hemp/MnO 2 -S composite employed as a cathode material for Li-S batteries is presented in Scheme 1.

Physico-chemical characterization
The crystal structure and phase purity of the synthesized materials were assessed via powder X-ray diffraction (XRD) on a Philips XRD 'X' PERT PRO diffractometer with a Cu Ka (l ¼ 1.5418Å) source. Thermal analysis was carried out in the temperature range from ambient to 700 C under a nitrogen atmosphere at a heating rate of 10 Cmin À1 using a TA Instruments Model SDT Q600 thermogravimetric analyzer. The chemical state of the surface and the elemental composition of the sample materials were determined via X -ray photoelectron spectroscopy (XPS) using a Thermo Scientic MULTILAB 2000 with a Twin Anode Mg/Al (300/400W) X-Ray source and 110 mm radius hemispherical analyser with 7 channeltrons as the detector. The morphological characteristics of the samples were analysed via FE-SEM (Carl Zeiss, SUPRA 55VPFEI, Germany). A Denver CE10 (0111) microbalance with 10 mg sensitivity was used for weighing materials and electrodes. Deionized water was used for all experiments.

Electrochemical characterization
The C-hemp/MnO 2 -S composite was mixed with super P conductive carbon and PVDF in a ratio 7 : 2 : 1 using NMP as the solvent. The slurry was coated on aluminium foil and dried at 50 C. The electrodes were cut into circular discs with a diameter 15.4 mm and had an average sulfur loading of 3 mg cm À2 . 2032 coin cells were fabricated in an argon-lled glove box (MBraun, Germany) with the C-hemp/MnO 2 -S composite as the cathode and Li metal as the anode. The electrolyte was composed of 1 M LiTFSI and 50 mM anhydrous LiNO 3 in a 1 : 1 mixture of DME and DOL. All electrochemical experiments were conducted at room temperature. Cyclic voltammograms were recorded on a Solartron, USA at a scan rate of 0.1 mV s À1 between 3 and 1.5 V. Galvanostatic charge-discharge proles at different current densities were obtained using a computercontrolled battery test system (Arbin, USA) between the voltage range of 1.5 and 3 V. Electrochemical impedance spectroscopy measurements were performed before and aer cycling using a Biologic Instrument, France and the resistive parameters were calculated using the Zt soware.

Results and discussion
The structural characteristics of the composite formed were investigated using the XRD technique. The XRD pattern of the sulfur impregnated C-hemp is shown in Fig. 1. The broad peak at around the 2q value of 26 can be assigned to the (002) crystallographic plane of graphitic carbon. The sulfurized sample exhibits well-dened sulfur peaks corresponding to orthorhombic sulfur (JCPDS 00-008-0247), which indicates the presence of sulfur on the surface of C-hemp.
The surface area of C-hemp aer activation with alkali was calculated to be 746 m 2 g À1 , while that without activation was only 34 m 2 g À1 . This enhancement in surface area inuences the electrochemical behaviour of the material. Fig. 2a shows the nitrogen adsorption-desorption isotherm, which suggests the type IV nature of the carbon material with mesoporous structures. The mean pore radius of C-hemp is approximately 16Å, as evident from Fig. 2b.
To conrm the sulfur content, the sample materials were subjected to thermogravimetric analysis and the results are shown in Fig. 3. Considering that MnO 2 is not likely to feature large weight changes below 700 C; here, the sulfur loading in Chemp-/MnO 2 -S is estimated to be $65% by TGA. Thus, the porous nature of C-hemp enables the accommodation of the maximum amount of sulfur.
To study the chemical composition and surface properties of the materials, XPS measurements were carried out and the results are displayed in Fig. 4. The survey spectrum given in Fig. 4a evidences the existence of manganese Mn2p and sulfur S2p in addition to carbon C1s and oxygen O1s, which indicate the successful incorporation of manganese and sulfur in the carbon matrix. The peaks centered at approximately 286.0 eV and 533.0 eV in all the survey spectra correspond to C1s and O1s, respectively. For the composite material, C1s is deconvoluted into three peaks at 284.8, 286.4 and 289.5 eV corresponding to C]C, C-C and C-S, respectively, as shown in Fig. 4b. [64][65][66] The O1s spectrum shown in Fig. 4c is deconvoluted into three peaks at 531.9 (C]O), 534.3 (C-O) and 536.3 (chemisorbed oxygen or water). 67 Fig. 4d shows that the deconvoluted XPS S2p spectrum is tted into three peaks positioned at 163.9 (C-S-C), 165.1 (C]S) and 169.1 eV (C-SO x ). 64,68 In the Mn 2p region shown in Fig. 4e, the 2p3/2 and 2p1/2 doublet is observed at 642.7 and 654.2 eV is consistent with previous reports. 69,70 The morphology of C-hemp and C-hemp/MnO 2 -S is shown in the FE-SEM images in Fig. 5a-d. C-hemp exhibits a bundlelike morphology with a porous nature, which is visible in Fig. 5a and b. The composite of C-hemp/MnO 2 -S has a similar structure with sulfur coated and impregnated onto the porous C-hemp structure. Granular particles of MnO 2 nanoparticles are also found all over the carbon substrate. Fig. 5e and f show the energy dispersive spectroscopy mapping proles of C-hemp/ MnO 2 -S. These images show that S and MnO 2 are uniformly distributed over the C-hemp matrix. This uniform distribution of MnO 2 is benecial for the effective trapping of polysuldes.
The electrochemical performance of the material was assessed using cyclic voltammetry. The electrochemical evaluation of C-hemp-S was evaluated with the same sulfur content for comparison. Fig. 6a shows the comparative cyclic voltammograms of the C-hemp-S and C-hemp/MnO 2 -S cells. The Chemp-S cathode exhibits typical cathode behaviour with well-dened cathodic and anodic peaks. During the cathodic    This journal is © The Royal Society of Chemistry 2018 sweep, two reduction peaks were obtained at 2.3 V and 2.0 V, which suggest the multi-step reduction of sulfur. The rst reduction peak at 2.3 V corresponds to the reduction of cycloocta sulfur (S 8 ) to higher order polysuldes (Li 2 S n , 4 # n < 8) and that at 2.0 V indicates the decomposition of the long chain polysuldes to shorter polysuldes (Li 2 S 2 or Li 2 S). During the anodic scan, a single intense oxidation peak was observed at 2.41 V due to the slow kinetics of the oxidation of lithium sulde to high order lithium polysuldes. In the case of the cathode with MnO 2 additive, two cathodic peaks and one anodic peak appeared, which were shied slightly to lower and higher potentials, respectively. This implies that MnO 2 does not electrochemically participate in the charge-discharge process of the cell. Aer three consecutive cycles, the anodic peak was stabilized, which indicates the good durability of the material as a cathode (Fig. 6b). Fig. 7a describes the initial charge-discharge curves of C-hemp-S and C-hemp/MnO 2 -S at a rate of 0.1 C. They both have characteristic plateaus at 2.3 V and 2.0 V, which is in accordance with the earlier reports. The upper discharge plateau at 2.3 V corresponds to the reduction of S 8 to long-chain polysuldes (Li 2 S n , 4 # n < 8), while that at 2.0 V corresponds to the subsequent reduction of long-chain polysuldes to Li 2 S 2 /Li 2 S. C-hemp-S and Chemp/MnO 2 -S deliver an initial discharge capacity of 874 and 927 mA h g À1 , respectively, at a rate of 0.1 C. The increased capacity of C-hemp/MnO 2 -S indicates the polysulde binding ability of the   MnO 2 nanoparticles scattered on the surface of the C-hemp matrix. Fig. 7b represents the rate performance of the C-hemp-S and C-hemp/MnO 2 -S electrodes at different current densities. It is found that the discharge capacity of C-hemp-S is 1079, 874, 651, 455 and 326 mA h g À1 at the rates of 0.05 C, 0.1 C, 0.2 C, 0.5 C and 1 C, whereas the C-hemp/MnO 2 -S electrode delivers a capacity of 1131, 926, 711, 502, and 426 mA h g À1 at the same rates, respectively. When switched back to a rate of 0.1 C, Chemp-S and C-hemp/MnO 2 -S deliver a capacity of 611 and 700 mA h g À1 , respectively. However, the obtained value in the present study is lower compared with the previously reported values. This may be due to various factors including the structure of MnO 2 and the properties of the carbon materials. Hence, further attention is needed to improve the cell characteristics, such as increase the specic capacity and cycle stability. A comparison of the present data with the literature is provided in Table S1 (ESI Table 1 †). The cycling performance of C-hemp/ MnO 2 -S and C-hemp-S at 0.1 C is shown in Fig. 7c. A capacity retention of 74% (675 mA h g À1 ) and coulombic efficiency of 91% are obtained over 100 cycles for C-hemp/MnO 2 -S, whereas for C-hemp-S, a capacity retention of only 68% with a coulombic efficiency of 86% is obtained.
To retrieve further information regarding the electrochemical processes, electrochemical impedance spectroscopy was carried out. The Nyquist plots of both cells before the rst discharge and aer the 100th charge are given in Fig. 8(a and b). As seen in this gure, the impedance plots of the cells before cycling consist of a semi-circle in the high frequency region, which corresponds to charge-transfer resistance and interfacial impedance, while the inclined line at the low frequency zone corresponds to Warburg impedance (W). The real axis intercept in the high frequency region corresponds to the solution resistance. The Nyquist plots of the cells aer cycling consist of an additional depressed semi-circle in the high frequency region, which corresponds to the resistance offered by the SEI layer. Aer cycling, the charge transfer resistance is found to decrease for both cells owing to the uniform distribution of sulfur as well as the availability of pores, which minimize the volume change. The reduction in charge-transfer resistance is greater for C-hemp/MnO 2 -S, which indicates more charge transfer between sulfur and the C-hemp/MnO 2 material. The tted impedance values are given in Table 1.
To evaluate the interaction of MnO 2 with polysuldes, around 60 mg MnO 2 was added to 10 mL of 2 Â 10 À2 M Li 2 S 6 in dimethoxyethane. The Li 2 S 6 solution was initially yellow in colour (Fig. 9a(i)). Then, immediately upon contact with MnO 2 , it changed to light yellow and was completely colourless aer 15 min, which indicates the strong adsorption of polysuldes ( Fig. 9a(ii)). Also, the above solution became colourless upon the addition of C-hemp/MnO 2 (Fig. 9a(iii)). Accordingly, the UVvisible absorption spectra of the above solutions were measured, which are presented in Fig. 9b. The broad peak at around 410 nm suggests the presence of higher order poly-sulde anions. This distinct peak is absent for the solution aer the addition of MnO 2 , which suggests the polysulde adsorption ability of MnO 2 nanoparticles.

Conclusions
In the present study, a novel bio-source (hemp) was used for the synthesis of carbon, which was successfully employed as a sulphur host in the Li-S system. Further, amorphous MnO 2 was synthesised via a simple solution process, which also showed the ability to accommodate sulphur in its structure. The combined effect of carbon and MnO 2 leads to better polysulde absorbing ability in the cathode matrix, which is the most promising nding in this study. This new C-hemp/MnO 2 -S composite signicantly enhanced the specic capacity of the cell (926 mA h g À1 at 0.1 C) compared to the C-hemp/S composite (874 mA h g À1 at 0.1 C). The cell fabricated using   C-hemp/MnO 2 -S showed better columbic efficiency (91%) and specic capacitance retention (74%) than the C-hemp-S composite even aer 100 cycles at a rate of 0.1 C. Thus, the studied new C-hemp/MnO 2 -S composite is a potential cathode for the Li-S system. However, this study needs to be extended further for improving its cycle life by optimising the composite content.

Conflicts of interest
There are no conicts to declare.