Hierarchical TiO2 spheres assisted with graphene for a high performance lithium–sulfur battery†

Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University’s research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners. Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any format or medium without formal permission of the copyright holder. The full policy is available online: http://nrl.northumbria.ac.uk/policies.html


Introduction
There has been a strong demand for sustainable, renewable energy technologies due to air pollution and global warming. Among the available energy storage methods, rechargeable Li-ion battery represents the state-of-the-art technology in the market. Lithium-sulfur (Li-S) battery is of great interest as a next-generation energy storage solution, particularly for electric vehicles. The ultra-high theoretical energy density (2600 W hk g À1 ) of the Li-S battery is due to the very high specic capacity of sulfur (1675 mA h g À1 ) based on a two-electron reaction (S + 2Li + +2 e À 4 Li 2 S). 1-3 Currently, Li-S batteries suffer from a problem of short cycle-life, which could be caused mainly by a coupled 'chemical' and 'mechanical' degradation of polysuldes during the charge-discharge processes. [4][5][6][7][8] When elemental sulfur reacts with lithium ions to form Li 2 S, the intermediate polysulde species (Li 2 S n ,4# n # 8) are formed, which are soluble in most electrolyte solutions. The polysulde intermediates are gradually dissolved in organic electrolytes and shuttle between the anode and cathode, leading to the deposition of insoluble and insulated Li 2 S and Li 2 S 2 on both the electrodes. This will prevent further lithiation/delithiation processes, eventually causing severe capacity loss. 9,10 In addition, the low conductivity (5 Â 10 À30 Scm À1 ) of sulfur at room temperature and $76% volume expansion/contraction during operation of the conversion reaction (S 8 4 Li 2 S) result in low utilization efficiency and poor rate performance in the chargedischarge processes. [11][12][13] To solve the abovementioned issues, a holistic research approach is needed to radically extend the cycle life and performance of Li-S batteries. For example, the discharge products Li 2 S 2 /Li 2 S with intrinsic insulating properties limit the devices' high-rate operation. Carbon frameworks with good electronic conductivity and copious pores have been regarded as one of the preferred carriers to support sulfur, including double-shell and hollow carbon spheres, 14 brous hybrids of graphene, [15][16][17][18][19] conductive polymer such as polyaniline coated carbon nanosphere, 20 nano-graphene sheets. 21 A substantial amount of sulfur can be effectively constrained in these conductive frameworks due to large surface area and pore volume. Interestingly, three-dimensional porous graphitic carbon composites were suggested to have the capability of increasing the sulfur content up to 90 wt%. 22 Because of high sulfur content, good distribution of sulfur nanoparticles, and covalent bonding between sulfur and porous graphitic carbon, the developed cathodes exhibit excellent performance with a high sulfur utilization, high specic capacity, and excellent rate capability at a high charge/discharge current. A large specic capacity in the range of 1000-1200 mA h g À1 at a low current rate with a superior cycling stability could be achieved for these electrodes. This is due mainly to a rational design, which efficiently minimizes the polysulde diffusion into electrolytes. Nevertheless, there is weak physical absorption between the non-polar carbon materials and polar polysulde intermediates, which causes inefficient prevention of the detachment of suldes and serious degradation problems over long-term cycling. 23,24 To date, metallic oxides, such as TiO 2 and MnO 2 , have been proven to be effective in inhibiting polysulde dissolution processes, which is favorably comparable with the conventional carbon materials. 25 In particular, TiO 2 could be one of the strong candidates for impregnation of sulfur due to its low-cost and facile fabrication process as well as nontoxicity. 8,26 For example, sulfur-TiO 2 yolk-shell nano-architectures were used as the cathode for an Li-S battery, exhibiting a minor capacity decay of 0.033% per cycle undergoing 1000 cycles at 0.5C. 26 The hierarchical TiO 2 spheres, being in possession of polar surface, could strongly bind polysuldes, delivering a high reversible capacity of 928.1 mA h g À1 aer 50 charge-discharge cycles at a current density of 200 mA g À1 . 27 Fundamentally, such improvements present a materials science and manufacturing challenge: normally, metal oxides have relatively low electronic conductivity. 28 Accordingly, various strategies have been proposed by combining carbon materials with TiO 2 to efficiently promote both the TiO 2 electronic conductivity and the binding between polysuldes and cathode. [29][30][31] For instance, a graphene-TiO 2 composite was designed to conne sulfur. 29 Electrochemical characterization revealed that the graphene-TiO 2 -S sandwich electrode could deliver an enhanced cycling stability with a capacity of 737 mA h g À1 (along with a capacity retention of 75%) aer 100 cycles at 1C, due to the highly conductive graphene layers, which facilitate the transportation of electrons. 29 Another design was proposed by Hwang et al. to conne sulfur within hollow-mesoporous and spherical TiO 2 particles that are interconnected via multi-walled carbon nanotubes. 32 The hollow nanostructure and large pore volume of the spherical TiO 2 particles provide sufficient accommodation for the volume expansion of sulfur. Consequently, the electronic conductivity of TiO 2 and Li + ion diffusion can be improved effectively by multiple pathways in a web of carbon nanotubes. An ultra-high capacity of 931 mA h g À1 at 5C was achieved with this electrode.
Based on previously reported results, in this study, we presented a new protocol for high performance Li-S battery using an interconnected architecture based on graphene-modied TiO 2 spheres frameworks. Rather than the hollow TiO 2 and TiO 2 nanoparticles, we for the rst time investigated hierarchical TiO 2 spheres combined with graphene utilized as the sulfur host for the Li-S battery. With this strategy, the sulfur nanoparticles can be distributed uniformly inside the hierarchical TiO 2 spheres, which efficiently mitigate sulfur aggregation and volume expansion, and thus improve sulfur utilization. [33][34][35][36] More importantly, the modication of graphene layers can not only increase the electrode's electronic conductivity, but also effectively form a shield to prevent the polysuldes from detaching and being released into the electrolytes. The assembled Li-S cells show stable cycling performance with only a capacity loss of 0.04% per cycle in the prolonged chargedischarge processes (400 cycles at 1C). This newly proposed structural design allows a signicant improvement in the cycle life performance wherein the battery can be charged/discharged without signicantly losing its capacity. Therefore, this provides a new solution for one of the major issues of rapid degradation of cathode performance for Li-S batteries.

Material synthesis
The hierarchical TiO 2 spheres were prepared as follows. 37 1mL tetrabutyl titanate (TBT) was added into 50 mL acetic acid (HAC) under mild magnetic stirring, followed by a hydrothermal reaction inside a 90 mL polytetrauoroethylene vessel at 150 C for 8 hours. Aer being centrifuged using water, the products were dried at 80 C in an oven and calcined at 400 C for 1 hour to obtain the well-crystallized anatase TiO 2 . Sulfur powder (from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was thoroughly mixed with the synthesized TiO 2 at a weight ratio of 2.5 : 1. The compounds were placed into a polytetrauoroethylene vessel (10 mL) and heated to 200 C for 12 hours, and sulfur was melted and diffused into hierarchical TiO 2 . The obtained products are denoted as TS (short term for TiO 2 -sulfur sample). Graphene oxide (GO) was fabricated according to reported procedure. 38 GO with a concentration of 0.5 g mL À1 was sonicated for 1 hour to obtain a uniform solution with a dark grey color. Then, 30 mg TS was added to the solution to homogeneously blend with GO via continuous sonication for 30 min. 18 mL hydrazine hydrate and 126 mLN H 3 $H 2 O were added into the solution in a sealed glass bottle to reduce GO at 90 C for 1 hour. 39 Through this reduction process, the color of the solution gradually changed into black, demonstrating the successful reduction of GO to graphene. Aer centrifugation for three times, the solution was dried at 60 C for 12 hours and the nal product was obtained, which is denoted as GTS. On the other hand, sulfur and carbon hybrid sample (CS) were prepared superlatively. Carbon black and sulfur powders with a mass ratio of 2.5 : 1 were ground together in a mortar, and then heated at 200 C for 12 hours and then dried at 60 C for 48 hours. Fig. 1 presents the fabrication process of the graphene-modied TiO 2 -S composite (i.e. the GTS). First, the hierarchical TiO 2 nanostructures were fabricated based on the hydrothermal reaction using tetrabutyl titanate (TBT) as the precursor together with acetic acid. The sulfur powders were then mixed thoroughly with TiO 2 and held in a sealed container at 200 C for 12 hours. In this process, melted sulfur was fully inltrated into the inner nanostructures of TiO 2 . Furthermore, the TS composite was successfully encapsulated with graphene via the in situ reduction reaction (as illustrated in the synthesis process of the GTS in Fig. 1).

Characterization
Field-emission scanning electron microscopy (FE-SEM, Nova Nano 450, Netherlands) and energy dispersive X-ray spectroscopy (EDS) were used to observe the morphology of the obtained materials. The porous structures and lattice fringes were further determined utilizing a transmission electron microscope (TEM, Tecnai G220). Crystalline structures of the materials were analyzed using an X-ray diffractometer (XRD, X'Pert PRO, PANalytical B.V., Netherlands) with Cu Ka1 irradiation (l ¼ 1.5406Å). X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD-600W) was used to examine the surface chemical nature of the GTS.
Thermogravimetry (TG) combined with thermogravimetry and differential scanning calorimetry measurements (TGA-DSC) (PerkinElmer Diamond) in N 2 were conducted to characterize the actual mass ratio of sulfur in the TS and GTS powders. Raman spectroscopy (LabRAM HR800, Horiba) was carried out to reveal the chemical property of the encapsulated graphene. The mean pore size distributions and the specic Brunauer-Emmett-Teller (BET) surface areas of TiO 2 were obtained with accelerated surface area and porosimetry based on N 2 isotherm adsorption/desorption measurement (Micromeritics ASAP 2020, US).

Electrochemical test
In a standard electrochemical measurement, the CS, TS and GTS samples were mixed thoroughly with polyvinylidene uoride (PVDF) and carbon black (with the weight ratio of 8 : 1 : 1). The powders were then dissolved into N-methylpyrrolidone (NMP) solvent and stirred for 12 hours at ambient temperature. The solution was then casting coated onto an Al foil and dried at 80 C for 12 hours in a vacuum chamber. The mass loading on an Al foil was controlled in the range of 1.5-2m gc m À2 measured using a micro balance (METTLER TOLEDO, XS3DU, Switzerland). The thickness of the lm coated on the Al foil was around 42.4 mm according to SEM (data not shown). Coin-type cells were assembled to evaluate the electrochemical performance of the as-derived materials and the capacity was calculated based on the mass of sulfur.
The tested electrodes were assembled to 2016 half cells using an Li foil as a counter electrode, and a porous polypropylene membrane as a separator. The device fabrication was carried out in an Ar-lled glove-box, in which the moisture and oxygen contents were less than 1 ppm. The electrolytes were composed of lithium bis-(triuoromethanesulfonyl)imide (LiTFSI, 1 M) in a mixed solvent of 1,3-dioxolane and 1,2-dimethoxyethane (volume ratio 1 : 1) combined with lithium nitrite (1 wt%). The electrochemical performance of the prepared electrodes was evaluated using a Landbattery system (CT 2001A Wuhan, China) with the voltage range of 1.5-2.8 V (vs. Li + /Li) at various current densities. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using a CHI 660D electrochemical workstation. The hierarchical TiO 2 spheres consist of overlapped subunit of nano-akes, which lead to a large surface area and a uniform distribution of pore sizes. Aer the adequate impregnation of sulfur, the ultra-thin nanoakes of TiO 2 disappeared due to an even inltration of sulfur (see Fig. 2b). The intimate decoration of graphene on the TS composite can be veried from the layered architecture with spherical morphology, as illustrated in Fig. 2c. EDS analysis (see Fig. 2d) demonstrated the existence sulfur by a strong peak at energy of $2.3 keV in the GTS sample. In addition, a homogeneous distribution of sulfur nanoparticles within the TS (Fig. S1 †) and GTS (Fig. 2e) samples can be further conrmed by EDS mapping analysis, which also veries that introduction of graphene in the GTS has a minor inuence on the uniform distribution of sulfur inside the TiO 2 spheres. Fig. 2f compares the XRD patterns of the TiO 2 , TS and GTS powders. Apart from the peaks at 25 ,3 8 and 48 for the (101), (004) and (200) phases of anatase TiO 2 from the wellcrystallized TS and GTS, the remaining peaks can be assigned to orthorhombic sulfur (JCPDS no. 08-0247). The sulfur content in various samples was measured by TGA, as shown in Fig. 2g. The TGA plot shows 70.5 wt% weight loss between 100 C and 320 C and 55 wt% weight loss (Fig. S2 †) between 100 C and 295 C for the TS and GTS samples, respectively. This corresponds to the release of sulfur that was presented in the TS and GTS samples. The 55 wt% sulfur in the GTS sample corresponds to a sulfur loading of $1.0 mg cm À2 in the cathode. Notably, there are two step mass losses for the GTS sample in the TGA test. The weight loss between 295 C and 500 C could be attributed mainly to the pyrolysis of oxygen-containing functional groups of graphene in GTS. 40,41 The presence of graphene in the nal GTS sample can be identied easily by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 2h. Two prominent peaks are observed at 1351.6 cm À1 and 1591.8 cm À1 , corresponding to the characteristic D and G bands of the graphene in the GTS sample. 17,42 The intensity ratio of the D band to G band (I D /I G )is commonly used to evaluate the degree of defects in carbon materials. A higher ratio indicates an increased amount of defects. For the GTS sample, the intensity ratio (I D /I G ) was calculated to be 1.24, demonstrating the existence of large amounts of defects. This results in an enhanced electronic conductivity related to the sufficient reduction of GO. 17,18 TEM characterization shown in Fig. 3a and b reveals the hierarchical TiO 2 sphere architecture. Fig. 3a shows the hierarchical TiO 2 spheres of $3 mm in diameter with a layered nanostructure, which agrees well with the aforementioned SEM images. The thickness of TiO 2 nanoakes was evaluated to be $4 nm according to the high magnied TEM images (Fig. 3b). It is noted that there are a plethora of mesopores in the individual TiO 2 nanoakes (Fig. S3 †), which is benecial for the efficient implantation of molten sulfur and also can effectively reduce the size of sulfur nanoparticles. 40,43 Fig. 3c displays a TEM image of the TS sample. The layered nanostructure clearly disappears aer penetration of sulfur. In addition, the higher magnication TEM image in Fig. 3c and the inset reveal numerous dark dots with an average size of 20 nm. This result is different from the inset of Fig. 3b. These can be ascribed to the uniformly distributed sulfur nanoparticles. 44 The corresponding EDS mapping of S, O and Ti within the selected rectangular area in Fig. 3d veries the uniform distribution of sulfur inside the TS matrix. The TEM observation of the GTS sample in Fig. 3e reveals the successful decoration of the graphene on the TS composites (marked with white circles). This result also indicates good contact between TS and graphene, and a homogenous distribution of the sulfur nanoparticles in both the TS and GTS samples. The layered structure corresponding to graphene can be clearly observed, as shown in Fig. 3f.

Results and discussion
The BET-surface area of the TiO 2 sphere was determined to characterize the hierarchical feature of TiO 2 spheres. The adsorption-desorption isotherm curves (see Fig. 4a) of the TiO 2 spheres with typical hysteresis loops indicate the abundant existence of mesopores with two type of pore size distribution centered at 5 nm and 20 nm, respectively (Fig. 4b). 28,45,46 A large BET surface area of 116.6 m 2 g À1 and a specic pore volume of 0.55 cm 3 g À1 can be determined for the hierarchical TiO 2 spheres, which is benecial to uptaking sulfur into the nanostructures. Fig. 4c presents the XPS survey spectrum of the GTS powders, revealing the existence of C, Ti, O and S. 35,47,48 The high-resolution C1s XPS spectrum of GTS sample in Fig. 4d can be tted with four different components of carbon-containing functional groups at binding energies of $284.9 eV (C-C), $285.7 eV (C-S), $287.3 eV (C]O), and $289.9 eV (O-C]O). 17,19 Among them, the carbonyl (C]O) and carboxyl (O-C]O) groups with weaker peak intensity can be assigned to the remaining GO in GTS without sufficient reduction. 19 The C-Speakcanbe attributed to the chemical bond between graphene and sulfur formed during the in situ reduction process of graphene oxide. This bond is benecial for hindering polysulde dissolution, thus improving the cycling performance as discussed below. Fig. 5a and b present the galvanostatic charge-discharge curves for the TS and GTS electrodes in a potential range of 1.5-2.8 V at different current rates. It was found that the polarization of GTS electrode (Fig. 5a) is much smaller than that of TS electrode (Fig. 5b), even at a higher current density. The GTS electrode deliveries higher specic discharge capacities at high rates, i.e., $816 mA h g À1 at 1C, $760 mA h g À1 at 2C, $725 mA hg À1 3C, and 660 mA h g À1 at 5C. Fig. 5c shows the cycling performance and the derived data of the coulombic efficiency for the CS, TS and GTS electrodes at a current rate of 1C (1C ¼ 1675 mA g À1 ) for 100 cycles. A CS electrode (blue symbol) was provided as the reference data to prove the effective polysulde entrapment of the designed framework. It can be seen from Fig. 5c that the GTS electrode possesses the most stable cycling performance with remarkable 94.4% capacity retention of the original capacity. In contrast, the CS electrode shows a rapid capacity loss with a low specic capacity of 572.9 mA h g À1 aer 100 cycles, occupying only 69.9% capacity retention of the original capacity ($821 mA h g À1 ). The drastic capacity decay for the CS electrode is associated with the signicant polysulde dissolution as a result of the weak interactions between the nonpolar carbon and polar suldes. 43 The coulombic efficiency of the GTS electrode was estimated to be $98%, which is much higher than those of the TS and CS electrodes. This can be explained from a minimized shuttle effect and excellent electrochemical reversibility based on the use of the GTS electrode. The synergistic combination of graphene and hierarchical TiO 2 guarantees the excellent cycling performance and high coulombic efficiency of the Li-S battery. Owing to the suitable design of utilization TiO 2 as the sulfur scaffold and graphene as the polysulde barrier, the two-electron reaction process on the cathode (S + 2Li + +2e À 4 Li 2 S) can be efficiently reversible with much less polysulde dissolution, leading to stable cycling performance and high coulombic efficiency. The GTS and TS electrodes were tested continuously for their rate performance aer 100 charge-discharge cycles at a constant current rate of 1C. Fig. 5d displays the rate performance for the GTS and the TS electrodes at various current rates. The GTS electrode exhibited a much higher capacity than that of the TS electrode in the entire charge-discharge process. A specic capacity of 660 mA h g À1 was obtained for the GTS electrode at a rate as high as 5C. When the current rate was changed back to 1C, a capacity of 800 mA h g À1 is remained for the GTS electrode, nearly 100% capacity retention of the original capacity. These results demonstrate an excellent reversibility using the GTS electrode. In addition, the size of hierarchical TiO 2 plays signicant role on the electrochemical performance of Li-S batteries. Thus, it is still urgent to optimize the size of TiO 2 host to further enhance their performance. Long-term cycling tests were further carried out for the GTS electrode to verify its electrochemical stability. Aer 400 cycles at 1C, the electrode still maintained a capacity of 732 mA h g À1 (Fig. 5e), being $83% capacity retention of the primary capacity with a capacity loss of 0.04% per cycle. Fig. 6a presents an SEM image of the GTS electrode aer charge-discharge process for 400 cycles. A spherical morphology can be observed, which was attributed to the TS microstructure, implying the sturdy structure of the hybrids, which can accommodate the sulfur volume change. 44 The retention of sulfur and its distribution aer 400 cycles could be  detected by the EDS characterization of the cycled GTS electrode, as shown in the inset of Fig. 6a. The weak peaks for uorine can be linked with PVDF. 36 To further verify if there are strong interactions between the GTS cathode and polysuldes, the coin cells made using the CS, TS and GTS electrodes were disassembled to investigate the surfaces of the cathode and counter electrodes (Fig. S4 †). The original orange color poly-suldes were found to dissolve into electrolytes on the surface of the CS electrode, whereas such phenomena were not observed on the surfaces of both the TS and GTS electrodes. These results reveal that the polysulde dissolution has been suppressed signicantly via formation of interconnected architectures of the graphene-assisted TiO 2 -S framework, and a synergistic protection function from both TiO 2 and graphene. Fig. 6b presents the typical Nyquist plots for these electrodes at an open-circuit voltage obtained from impedance measurements. The circuit diagram in the inset is used to simulate the lithium ion transportation process, in which the semicircle and oblique lines are related to the medium and low frequency processes, respectively. 49 The charge transfer resistance (R ct ) mostly contributed to the overall resistance for the electrode. 50,51 The value of R ct for the cycled TS and GTS electrodes was calculated to be 103.2 and 47.9 U, respectively. The lower value of the resistance for the GTS electrode is attributed mainly to the modication of conductive graphene on the TS surfaces.
The large surface area and abundant mesopores of the TiO 2 sphere can enhance the impregnation of sulfur and can reduce the dimension of the sulfur particles into the nanoscale during the melt inltration. These can signicantly prevent the volume expansion and improve the sulfur utilization rate. 52 In addition, TiO 2 may also take part in the electrochemical reaction, which could make a contribution to the improvement of the capacity for GTS and TS electrodes. [53][54][55][56] Furthermore, the modication of graphene for the TS microstructures plays an important role in improving the cycling performance and rate capability. Therefore, based on the schematic shown in Fig. 7, we can conclude that the interconnected frameworks provide (a) a good modication of conductive graphene on the hierarchical TiO 2 , (b) the multi-pathways for Li + ions diffusion, and (c) the conductive networks for electron transfer. Furthermore, the chemical bond between graphene and sulfur, together with the adsorption of graphene to sulfur and polysuldes due to the existence of many contact sites, can prevent the loss of suldes during charge/discharge processes. In brief, the synergistic effects from both TiO 2 and graphene stabilize sulfur and polysuldes and promote the efficient transportation of electrons and lithium ions, thus providing stable cycling performance and a high coulombic efficiency using the GTS cathode.

Conclusions
Graphene-assisted TiO 2 -S interconnected frameworks were successfully designed, fabricated and used as the cathode materials for Li-S batteries. The hierarchical architectures of TiO 2 sphere with overlapped subunits of nanoakes can effectively accommodate sulfur nanoparticles. Therefore, the volume expansion and nanoparticle aggregation can be effectively mitigated, thereby facilitating the sulfur utilization rate.   Further modication of graphene for the TS composite can not only improve the electronic conductivity, but also signicantly provide the effective pathways for the Li + ions and electrons and suppress the diffusion of polysuldes to electrolytes, thus providing excellent cycling stability and high coulombic efficiency. In addition, the chemical bond detected in the GTS products between graphene and sulfur can efficiently stabilize polysuldes on the cathode. The GTS electrodes exhibit appreciated performance, even though the sulfur content in this type electrode is relative low. Therefore, it is highly desirable to optimize the electrode composition to further enhance their performance.