Lin
Gao
a,
Minglei
Cao
a,
Yong Qing
Fu
b,
Zhicheng
Zhong
c,
Yan
Shen
a and
Mingkui
Wang
*a
aWuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China. E-mail: mingkui.wang@mail.hust.edu.cn
bDepartment of Physics and Electrical Engineering, Faculty of Engineering and Environment, Northumbria University, Newcastle Upon Tyne, NE1 8ST, UK
cHubei Key Laboratory of Low Dimensional Optoelectronic Material and Devices, Hubei University of Art and Science, Longzhong Road 296, Xiangyang, 441053, China
First published on 16th September 2016
In this study, we report hierarchical TiO2 sphere–sulfur frameworks assisted with graphene as a cathode material for high performance lithium–sulfur batteries. With this strategy, the volume expansion and aggregation of sulfur nanoparticles can be effectively mitigated, thus enabling high sulfur utilization and improving the specific capacity and cycling stability of the electrode. Modification of the TiO2–S nanocomposites with graphene can trap the polysulfides via chemisorption and increase the electronic connection among various components. The graphene-assisted TiO2–S composite electrodes exhibit high specific capacity of 660 mA h g−1 at 5C with a capacity loss of only 0.04% per cycle in the prolonged charge–discharge processes at 1C.
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 Li2S2/Li2S 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 fibrous hybrids of graphene,15–19 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 specific capacity, and excellent rate capability at a high charge/discharge current. A large specific capacity in the range of 1000–1200 mA h g−1 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 polysulfide diffusion into electrolytes. Nevertheless, there is weak physical absorption between the non-polar carbon materials and polar polysulfide intermediates, which causes inefficient prevention of the detachment of sulfides and serious degradation problems over long-term cycling.23,24
To date, metallic oxides, such as TiO2 and MnO2, have been proven to be effective in inhibiting polysulfide dissolution processes, which is favorably comparable with the conventional carbon materials.25 In particular, TiO2 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–TiO2 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 TiO2 spheres, being in possession of polar surface, could strongly bind polysulfides, delivering a high reversible capacity of 928.1 mA h g−1 after 50 charge–discharge cycles at a current density of 200 mA g−1.27 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 TiO2 to efficiently promote both the TiO2 electronic conductivity and the binding between polysulfides and cathode.29–31 For instance, a graphene–TiO2 composite was designed to confine sulfur.29 Electrochemical characterization revealed that the graphene–TiO2–S sandwich electrode could deliver an enhanced cycling stability with a capacity of 737 mA h g−1 (along with a capacity retention of 75%) after 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 confine sulfur within hollow-mesoporous and spherical TiO2 particles that are interconnected via multi-walled carbon nanotubes.32 The hollow nanostructure and large pore volume of the spherical TiO2 particles provide sufficient accommodation for the volume expansion of sulfur. Consequently, the electronic conductivity of TiO2 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−1 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-modified TiO2 spheres frameworks. Rather than the hollow TiO2 and TiO2 nanoparticles, we for the first time investigated hierarchical TiO2 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 TiO2 spheres, which efficiently mitigate sulfur aggregation and volume expansion, and thus improve sulfur utilization.33–36 More importantly, the modification of graphene layers can not only increase the electrode's electronic conductivity, but also effectively form a shield to prevent the polysulfides 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 charge–discharge processes (400 cycles at 1C). This newly proposed structural design allows a significant improvement in the cycle life performance wherein the battery can be charged/discharged without significantly 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.
Fig. 1 presents the fabrication process of the graphene-modified TiO2–S composite (i.e. the GTS). First, the hierarchical TiO2 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 TiO2 and held in a sealed container at 200 °C for 12 hours. In this process, melted sulfur was fully infiltrated into the inner nanostructures of TiO2. 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).
Thermogravimetry (TG) combined with thermogravimetry and differential scanning calorimetry measurements (TGA-DSC) (PerkinElmer Diamond) in N2 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 specific Brunauer–Emmett–Teller (BET) surface areas of TiO2 were obtained with accelerated surface area and porosimetry based on N2 isotherm adsorption/desorption measurement (Micromeritics ASAP 2020, US).
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-filled glove-box, in which the moisture and oxygen contents were less than 1 ppm. The electrolytes were composed of lithium bis-(trifluoromethanesulfonyl)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.
TEM characterization shown in Fig. 3a and b reveals the hierarchical TiO2 sphere architecture. Fig. 3a shows the hierarchical TiO2 spheres of ∼3 μm in diameter with a layered nanostructure, which agrees well with the aforementioned SEM images. The thickness of TiO2 nanoflakes was evaluated to be ∼4 nm according to the high magnified TEM images (Fig. 3b). It is noted that there are a plethora of mesopores in the individual TiO2 nanoflakes (Fig. S3†), which is beneficial for the efficient implantation of molten sulfur and also can effectively reduce the size of sulfur nanoparticles.40,43Fig. 3c displays a TEM image of the TS sample. The layered nanostructure clearly disappears after penetration of sulfur. In addition, the higher magnification 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 verifies 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.
The BET-surface area of the TiO2 sphere was determined to characterize the hierarchical feature of TiO2 spheres. The adsorption–desorption isotherm curves (see Fig. 4a) of the TiO2 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 m2 g−1 and a specific pore volume of 0.55 cm3 g−1 can be determined for the hierarchical TiO2 spheres, which is beneficial 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 fitted 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 (CO), and ∼289.9 eV (O–CO).17,19 Among them, the carbonyl (CO) and carboxyl (O–CO) groups with weaker peak intensity can be assigned to the remaining GO in GTS without sufficient reduction.19 The C–S peak can be attributed to the chemical bond between graphene and sulfur formed during the in situ reduction process of graphene oxide. This bond is beneficial for hindering polysulfide 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 specific discharge capacities at high rates, i.e., ∼816 mA h g−1 at 1C, ∼760 mA h g−1 at 2C, ∼725 mA h g−1 3C, and 660 mA h g−1 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 = 1675 mA g−1) for 100 cycles. A CS electrode (blue symbol) was provided as the reference data to prove the effective polysulfide 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 specific capacity of 572.9 mA h g−1 after 100 cycles, occupying only 69.9% capacity retention of the original capacity (∼821 mA h g−1). The drastic capacity decay for the CS electrode is associated with the significant polysulfide dissolution as a result of the weak interactions between the nonpolar carbon and polar sulfides.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 TiO2 guarantees the excellent cycling performance and high coulombic efficiency of the Li–S battery. Owing to the suitable design of utilization TiO2 as the sulfur scaffold and graphene as the polysulfide barrier, the two-electron reaction process on the cathode (S + 2Li+ + 2e− ↔ Li2S) can be efficiently reversible with much less polysulfide dissolution, leading to stable cycling performance and high coulombic efficiency. The GTS and TS electrodes were tested continuously for their rate performance after 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 specific capacity of 660 mA h g−1 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−1 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 TiO2 plays significant role on the electrochemical performance of Li–S batteries. Thus, it is still urgent to optimize the size of TiO2 host to further enhance their performance. Long-term cycling tests were further carried out for the GTS electrode to verify its electrochemical stability. After 400 cycles at 1C, the electrode still maintained a capacity of 732 mA h g−1 (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 after 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 after 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 fluorine can be linked with PVDF.36 To further verify if there are strong interactions between the GTS cathode and polysulfides, 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 polysulfides 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 polysulfide dissolution has been suppressed significantly via formation of interconnected architectures of the graphene-assisted TiO2–S framework, and a synergistic protection function from both TiO2 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 (Rct) mostly contributed to the overall resistance for the electrode.50,51 The value of Rct for the cycled TS and GTS electrodes was calculated to be 103.2 and 47.9 Ω, respectively. The lower value of the resistance for the GTS electrode is attributed mainly to the modification of conductive graphene on the TS surfaces.
The large surface area and abundant mesopores of the TiO2 sphere can enhance the impregnation of sulfur and can reduce the dimension of the sulfur particles into the nanoscale during the melt infiltration. These can significantly prevent the volume expansion and improve the sulfur utilization rate.52 In addition, TiO2 may also take part in the electrochemical reaction, which could make a contribution to the improvement of the capacity for GTS and TS electrodes.53–56 Furthermore, the modification 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 modification of conductive graphene on the hierarchical TiO2, (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 polysulfides due to the existence of many contact sites, can prevent the loss of sulfides during charge/discharge processes. In brief, the synergistic effects from both TiO2 and graphene stabilize sulfur and polysulfides and promote the efficient transportation of electrons and lithium ions, thus providing stable cycling performance and a high coulombic efficiency using the GTS cathode.
Footnote |
† Electronic supplementary information (ESI) available: SEM image and corresponding EDS mapping of TiO2–S electrode; TEM image of an individual TiO2 nanoflake; photographs of the disassembled cells corresponding to the CS, TS, and GTS electrodes. See DOI: 10.1039/c6ta06285g |
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