Electrocatalytic activity of lithium polysulfides adsorbed into porous TiO2 coated MWCNTs hybrid structure for lithium-sulfur batteries

Lithium-sulfur batteries have attracted great attention because of their high energy density, environmental friendliness, natural abundance and intrinsically low cost of sulfur. However, their commercial applications are greatly hindered by rapid capacity decay due to poor conductivity of electrode, fast dissolution of the intermediate polysulfides into the electrolyte, and the volume expansion of sulfur. Herein, we report a novel composite MWCNTs@TiO2-S nanostructure by grafting TiO2 onto the surface of MWCNTs, followed by incorporating sulfur into the composite. The inner MWCNTs improved the mechanical strength and conductivity of the electrode and the outer TiO2 provided the adsorption sites to immobilize polysulfides due to bonding interaction between TiO2 and polysulfides. The MWCNTs@TiO2-S composite with a mass ratio of 50% (MWCNTs in MWCNTs@TiO2) exhibited the highest electrochemistry performance among all compositing ratios of MWCNTs/TiO2. The performance improvement might be attributed to the downward shift of the apparent Fermi level to a more positive potential and electron rich space region at the interface of MWCNTs-TiO2 that facilitates the reduction of lithium polysulfide at a higher potential. Such a novel hybrid structure can be applicable for electrode design in other energy storage applications.

Scientific RepoRts | 7:40679 | DOI: 10.1038/srep40679 To address these problems, several strategies have been proposed to enhance the performance of lithium-sulfur batteries. Some metal oxides with polar surfaces, including Mg 0.6 Ni 0.4 O, Al 2 O 3 , MnO 2 and TiO 2 [16][17][18][19] have been used as additives or absorbents to confine sulfur and polysulfides. Porous TiO 2 , for example, can serve as on-site adsorption host for encapsulating polysulfides through bonding interaction to minimize the dissolution of sulfur into the electrolyte 20 . Yi Cui and co-workers also reported that conductive Ti 4 O 7 was a highly effective matrix to bind with sulfur species due to the strong adsorption of polysulfides on the low-coordinated Ti sites of Ti 4 O 7 21 . However the relatively low conductivity of metal oxides makes it insufficient to satisfy the high rate performance for lithium-sulfur batteries. In fact, various carbon matrix, including mesoporous carbon or microporous carbon [22][23][24][25] , graphene oxide [26][27][28] , carbon nanotube or carbon nanofiber [29][30][31] , and hollow carbon sphere 32,33 have also been used as conductive matrix in lithium-sulfur batteries due to the conductivity of carbon and the physical confinement property of the nanostructured carbon. Therefore the combination of the conductive network, such as MWCNTs 34 , with metal oxides would be an ideal route to overcome the low conductivity issue associated with metal oxides. Other strategies such as optimization of organic electrolyte 35 , sulfur loading conducting polymer 36 , designing new Li-S batteries device configuration 37 were also employed and stable cycle performance was achieved.
Herein, we report a novel MWCNTs@TiO 2 -S hybrid nanostructure by grafting TiO 2 onto the exterior surface of MWCNTs, followed by incorporating sulfur through the melt-diffusion method. TiO 2 provided adsorption sites to bind with polysulfides, while MWCNTs acted as the efficient current collector. The hybrid structure of TiO 2 coated MWCNT demonstrated the synergistic effect towards electron transfer kinetics for the redox response of polysulfides and an interpretation based on bending of energy bands at the MWCNT-TiO 2 interface was also proposed.

Results and Discussion
The structural characterization. Scanning electron microscopy (SEM) was used to characterize the morphologies of the pristine MWCNTs, TiO 2 , 50%-MWCNTs@TiO 2 and 50%-MWCNTs@TiO 2 -S composites (50% refers to the mass proportion of MWCNTs in the MWCNTs@TiO 2 composite). MWCNTs exhibit smooth surface and one dimensional linear structure, with an outer diameter of about 20 nm and a length of about several microns (Fig. 1a). TiO 2 reveals grain-like morphology with the average length of about 300 nm (Fig. 1b), the fine flower-like nanostructure indicates high specific surface area 38 . The SEM image of 50%-MWCNTs@TiO 2 shows TiO 2 nanoparticles coating on the surface of MWCNTs with a rougher surface (Fig. 1c) comparing with pristine MWCNTs. Interestingly, in contrast to the aggregation of the flower-like morphology of TiO 2 particles in the absence of MWCNTs, TiO 2 is distributed uniformly on the exterior of MWCNTs, probably due to the enhanced nucleation of nanoparticles on hydrophilic surface resulting from carboxylation of MWCNTs 39 . After encapsulating sulfur into the 50%-MWCNTs@TiO 2 , the surface of 50%-MWCNTs@TiO 2 -S becomes smoother without agglomeration (Fig. 1d), indicating the successful incorporation of sulfur into the composites.
To further confirm the nanostructure of the composites, X-ray diffraction (XRD) patterns were acquired for MWCNTs, TiO 2 , 50%-MWCNTs@TiO 2 and 50%-MWCNTs@TiO 2 -S respectively (Fig. 2). It reveals an apparent high-intensity graphitic peak at 2θ value of around 26°, which can be ascribed to the (002) reflection of MWCNTs 34 40 (Fig. 2a). After coating MWCNTs with TiO 2 , MWCNTs peaks become much weaker, while the peaks of anatase TiO 2 remain strong. From the XRD patterns of pure sulfur, 50%-MWCNTs@TiO 2 and 50%-MWCNTs@TiO 2 -S (Fig. 2b), intense and sharp diffraction peaks of sulfur at 2θ values of 23.4° and 28.0° were observed, which match well with the (222) and (040) reflections of the Fddd orthorhombic phase (JCPSD no. 08-0247) 41 , indicating a well-defined crystal structure. After sulfur being encapsulated into the 50%-MWCNTs@TiO 2 , almost all typical sulfur diffraction peaks and the anatase TiO 2 peaks became weaker, indicating that sulfur was dispersed homogeneously into the composite and no obvious aggregates of sulfur or TiO 2 on the surface of MWCNTs, which is consistent with the results from SEM.
Thermal gravimetric analysis (TGA) was utilized to evaluate the content of sulfur incorporated into the composites ( Figure S1). All materials show a plateau of weight loss from 200 °C to 350 °C, reflecting the evaporation of sulfur loaded in the composites. The sulfur contents were determined to be 65.95%, 58.78%, 65.62%, 61.66% and 62.99% respectively, which are lower than the proportion of sulfur and MWCNTs@TiO 2 (75%) utilized for incorporating sulfur into the MWCNTs@TiO 2 , indicating that a small amount of excessive sulfur was removed through the heat treatment process and no excess sulfur remained on the surface.

Electrochemical performance of the MWCNTs@TiO 2 -S composites. The electrochemical features
of different composites can be seen from the cyclic voltammetry curves (CV) recorded at a scan rate of 0.1 mV s −1 in the voltage range of 3.0-1.5 V. As the first cycle showed the activation behavior, the second and the tenth CV curves were selected to evaluate the redox behaviors of the cathodes (Fig. 3a,c,e). During the discharging process, all electrodes show two apparent reduction peaks at the same potential for the second and tenth cycle, indicating excellent cycling stability. The peak at around 2.3 V represents the reduction of element sulfur (S 8 ) to high-order lithium polysulfide (Li 2 S n , 4 ≤ n ≤ 8), while the peak at around 2.0 V corresponds to the reduction of high-order lithium polysulfide to low-order lithium polysulfide (Li 2 S 2 and Li 2 S). In the subsequent anodic process, an oxidation peak develops at around 2.5 V of 0%-MWCNTs@TiO 2 -S and 100%-MWCNTs@TiO 2 -S (Fig. 3a,e), which shows similar oxidation potential for typical microporous and mesoporous carbon structures 24,42 . Interestingly, 50%-MWCNTs@TiO 2 -S shows an additional oxidation peak at about 2.7 V (Fig. 3c), which have only been observed in a few sulfur-conductive matrix systems with high conductivity and low polarization 26,32 , and this oxidation peak is associated with the electrocatalytic oxidation towards the Li-S redox reaction due to the synergistic effect of the best combination of TiO 2 and MWCNTs (with the mass ratio of 50%), which further promoted the electron transfer rate and minimized the polarization of the battery 21,43,44 . The charge -discharge curves of different electrodes from the 2 nd , 20 th and 50 th cycles demonstrate two distinct discharge plateaus for all tested cathodes (Fig. 3b,d,f), including a shorter and higher potential plateau at 2.3 V and a prolonged lower potential plateau at around 2.0 V, in agreement with their redox peaks of CV curves respectively. Other composites (20%-MWCNTs@ TiO 2 -S, 70%-MWCNTs@TiO 2 -S) were also studied ( Figure S2), both of which have the same two reduction peaks at around 2.3 V and 2.0 V, one oxidation peak at 2.5 V, and the optimal performance was observed with the mass ratio of 50% for MWCNTs.
50%-MWCNTs@TiO 2 -S composite shows a reduction peak at higher reduction potential (2.03 V) comparing to electrodes with other TiO 2 loading ratios (Fig. 4), which indicates improved electrochemical kinetics and better utilization of active mass 21,45 . The improvement can be attributed to the immobilization of polysulfides on the TiO 2 surface, as well as the nature of energy band bending on the interface of MWCNTs and TiO 2 . It have been reported that TiO 2 are terminated with hydrophilic Ti-O groups and surface hydroxyl groups, which are known to bind favorably with polysulfides anions 46 . In fact, Li 2 S x shows strong adsorption on TiO 2 (101) from DFT calculations, especially for Li 2 S 2 and Li 2 S 4 , and the chemical bonding between Ti-S bonds and S-O bonds were detected from XPS analysis, confirming that sulfur atom in polysulfides is bonded strongly with two bridging oxygen atoms on the TiO 2 surface 21 . Hence the soluble high order lithium polysulfides (Li 2 S n , 4 ≤ n ≤ 8) can be anchored at the adsorption site, significantly alleviating the undesired shuttle effect 47,48 . In addition, Li 2 S x shows molecular interaction with oxygen vacancies in TiO 2 49,50 . To characterize the density of oxygen vacancies, electron paramagnetic resonance (EPR) spectroscopy was carried out to evaluate the unpaired electrons associated with oxygen vacancies in the TiO 2 crystalline structure ( Figure S3). The strong peak at g = 2.001 was attributed  to oxygen vacancies in the TiO 2 crystalline structure and the density of oxygen vacancy was estimated to be 1.09 × 10 11 cm −3 , which provides additional adsorption sites for polysulfides 49 .
The composite 50%-MWCNTs@TiO 2 -S shows two reduction peaks at around 2.03 V and 1.89 V respectively, indicating the co-existence of two types of electron transfer kinetics. Anatase TiO 2 is known to have a band gap of 3.2 V, with a Fermi level of 1.3 V (work function of 6.0 V vs. vacuum level of − 4.7 V vs. NHE) 51 , which is more positive than the Fermi level of the metallic MWCNTs (0.2 V vs. NHE) 52 . When TiO 2 is in contact with MWCNTs, electrons flow from MWCNTs to TiO 2 until an equilibrium of Fermi energy is reached and the energy bands bend downwards at the interface, leaving an electron rich accumulation layer on the TiO 2 side, and a hole rich surface at the MWCNTs side of the TiO 2 -MWCNTs interface (Fig. 5), which is consistent with the literature 53 . The downward shift of the apparent Fermi level to a more positive potential facilitates the reduction of lithium polysulfide at 2.03 V. To balance the negative charge at the TiO 2 side, holes are accumulated on the MWCNTs surface at the interface (Fig. 5), shifting the reduction potential of lithium polysulfide to a more negative one at 1.89 V. The typical width of the space charge region is around tens of nanometers 54,55 . When it is in the same order with the thickness of the TiO 2 coating, as the case of 50%-MWCNTs@TiO 2 -S, the electron rich space charge region may even expand from the interface to the exterior surface of the TiO 2 layer, leading to a pronounced reduction peak at a relative positive potential (2.03 V). Further increasing of the TiO 2 thickness may reduce the electron density on the exterior surface of the TiO 2 coating (20%-MWCNTs@TiO 2 -S), and deteriorates the reduction kinetics. In summary, the results demonstrate that TiO 2 and MWCNTs composite with a mass ratio of 50% (MWCNTs) may produce a synergistic effect that have a positive catalytic activity towards the redox property of polysulfides.
Electrochemical performances of cathodes with various MWCNTs loading ratio (0%, 20%, 50%, 70%, and 100%) have been investigated by galvanostatic charge/discharge measurements. In this study, all specific capacity values were calculated on the basis of the mass of sulfur. After the first cycle of activation and stabilization, the discharge capacities achieve 677, 1133, and 941 mAh g −1 for 0%-MWCNTs@TiO 2 -S, 50%-MWCNTs@TiO 2 -S, 100%-MWCNTs@TiO 2 -S respectively (Fig. 6a). After 50 cycles, the capacities decrease to 446, 679, 408 mAh g −1 , indicating that a higher performance retention for 50%-MWCNTs@TiO 2 -S after cycling. It is believed that the dissolution of polysulfides is the main challenge for capacity degradation. Capacity retention decreases with the decline of mass fraction of TiO 2 , which indicated that higher TiO 2 mass ratio may lead to higher absorption ability toward sulfur and polysulfides 21,56 . However, higher mass fraction of TiO 2 may also deteriorate the electrical conductibility of the cathode 57 . The results show that 50%-MWCNTs@TiO 2 -S might be the relatively optimal mass proportion, leading to the highest reversible capacity and excellent capacity retention. The coulombic efficiency (CE) of 50%-MWCNTs@TiO 2 -S remained above 95% over 50 cycles (Fig. 6b), indicating minimum shuttle effect of polysulfides and stable cycle performance. Other composites (20%-MWCNTs@TiO 2 -S, 70%-MWCNTs@ TiO 2 -S) were also studied ( Figure S4), and the results demonstrate that the mass ratio of 50% is still optimal.
It is clear that the rate capability of 50%-MWCNTs@TiO 2 -S was much better than the 0%-MWCNTs@TiO 2 -S and 100%-MWCNTs@TiO 2 -S (Fig. 7). It demonstrates a discharge capacity of about 900 mAh g −1 at 0.1 C rate after 10 cycles. Subsequently, cyclings at 0.2 C, 0.5 C, 1 C and 2 C show high reversible capacities of 700 mAh g −1 , 500 mAh g −1 , 360 mAh g −1 , 300 mAh g −1 respectively. And when the current rate switches from 2 C to 0.1 C again, the 50%-MWCNTs@TiO 2 -S reaches a capacity of 640 mAh g −1 , revealing better stability of the cathode material. The higher electrochemical performance might be attributed to the synergetic effect of TiO 2 and MWCNTs, which is consistent with the CV results. In order to shed light on the role of TiO 2 and MWCNTs in the electrochemical reaction, electrochemical impedance spectroscopy (EIS) spectra were acquired (Fig. 8) and analyzed by fitting with equivalent circuits ( Figure S5). It can be seen that all EIS spectra were composed of a depressed semicircle in the high-frequency region before cycling, corresponding to the charge-transfer process generated at the electrode/electrolyte interface, and a sloping straight line in the low-frequency region corresponding to the Warburg diffusion process (Fig. 8a). The charge-transfer resistance (R ct ) decreases with the increase of the mass ratio of MWCNTs, indicating that higher proportion of MWCNTs contributes to higher conductivity of the electrode. Moreover, after 20 cycles, the obtained impedance spectrum of 50%-MWCNTs@TiO 2 -S contains two semicircles (Fig. 8b), the one in the high-frequency reflects the charge-transfer resistance (R ct ), while the other in the middle-frequency may be attributed to the formation of insoluble polysulfides species such as Li 2 S 2 /Li 2 S 58 . Previous report showed that the impedance of interfacial charge-transfer dominated the reduction reaction during the upper voltage plateau, whereas the mass transport dominated the lower voltage plateau 58,59 . After 20 cycles, the semicircle resistance in the high-frequency decreases (from 69 Ω to 46 Ω), indicating faster interfacial charge transfer owing to the synergistic effect of MWCNTs and TiO 2 .   the exterior surface of MWCNTs can provide the adsorption sites to immobilize polysulfides. The improvement of the electrochemical performance might be attributed to the downward bending of energy band on the interface of MWCNTs and TiO 2 , and a charge region of rich electron developed at the TiO 2 side of the interface that could even extend to the exterior TiO 2 surface. The strategy employed in this work provides an efficient way of recapturing polysulfides from the electrolyte while maintaining the high conductivity and redox activity, which can also be applicable for designing electrode materials for other energy stordage applications.

Chemicals and Materials.
Multi-walled carbon nanotubes (MWCNTs purity > 95%) were purchased from Nanotech Port Co., Ltd. (Shenzhen, China). Nitric acid (HNO 3 65%) and dehydrated alcohol were supplied by Aldrich Chemical Company (America). Tetrabutyl titanate (TBT) was purchased from the Aladdin Industrial Corporation (America). All chemical reagents were analytical grade and were used as received.
Materials synthesis. Prior to TiO 2 coating, the pristine MWCNTs were pretreated in 6 M concentrated nitric acid and refluxed at 120 °C for 12 h to remove impurities and introduce carboxyl groups on the surface, subsequently lead to improved dispersion of MWCNTs. Then the acid-treated MWCNTs (with varied mass ratio) were dispersed in 50 mL ethanol under sonication for 30 min, after that, 1 mL tetrabutyl titanate (TBT) and 10 mL glycerol were added into the solution, and stirred for 30 min. Then the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave at 180 °C for 15 h. Subsequently, the precipitation was washed with ethanol and deionized water, dried at 60 °C overnight and calcined at 450 °C in ambient atmosphere for 2 h.
The MWCNTs@TiO 2 -S composites were prepared by a simple melt-diffusion method. Firstly, sulfur and MWCNTs@TiO 2 were mixed by physical grinding at the weight ratio of 3:1, then the mixture was sealed in a Teflon-lined autoclave in an oven and heated at 155 °C for 12 h; followed by calcination at 250 °C in nitrogen atmosphere for 2 h to remove exterior excess sulfur. MWCNTs@TiO 2 -S were obtained after cooling down to room temperature.
Preparation of the cathodes. All the cathodes were prepared by mixing the MWCNTs@TiO 2 -S, Super-P and PVDF (polyvinylidene fluoride) binder (80:10:10 by weight) in N-methyl-2-pyrrolidone solvent to form a slurry. The slurry was uniformly spread onto the Al foil and dried at 50 °C for 12 h in a vacuum oven to act as the working electrode. The mass loading of the active S was 2 mg cm −2 . The 2032-type coin cells were assembled in an argon-filled glove box using Li metal foil as the counter electrode and the reference electrode. The electrolyte was 1.0 M lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M LiNO 3 in 1,3-dioxolane(1,3-DOL) and 1,2-dimethoxyethane(1,2-DME) (volume ratio, 1:1).
Electrochemical measurements. Cyclic voltammetry (CV) measurements were performed with an electrochemical workstation (Biologic VSP 300) using a voltage range of 3.0− 1.5 V vs. Li/Li + at a scan rate of 0.1 mV s −1 . Galvanostatic charge-discharge of the Li-S battery was conducted using Land battery system (CT2001A) operated at different rates between cut-off potentials of 3.0− 1.5 V. Electrochemical impedance spectroscopy (EIS) was measured with the frequency range from 100 kHz to 10 mHz, and the voltage amplitude applied to the coin cells was 5 mV. Specific capacity values were calculated based on the mass of sulfur in the samples.
Structural characterization. The morphology and microstructure were characterized using SEM operating at 10.00 kV (Sirion 200); the crystallographic structure was characterized using XRD (PANalytical B.V. X'pert PRO) operating at 40 kV and 40 mA using Cu Kα radiation (λ = 0.15406 nm); and the mass fraction of sulfur were estimated using TGA (Waters TGA Q 600), performed under an Ar atmosphere with a heating rate of 10 °C min −1 from room temperature to 600 °C.