TiO₂-decorated porous carbon nanofiber interlayer for Li–S batteries

Abstract

Lithium–sulfur (Li–S) batteries are the most promising energy storage systems owing to their high energy density. However, shuttling of polysulfides detracts the electrochemical performance of Li–S batteries and thus prevents the commercialization of Li–S batteries. Here, TiO₂@porous carbon nanofibers (TiO₂@PCNFs) are fabricated via combining electrospinning and electrospraying techniques and the resultant TiO₂@PCNFs are evaluated for use as an interlayer in Li–S batteries. TiO₂ nanoparticles on PCNFs are observed from SEM and TEM images. A high initial discharge capacity of 1510 mA h g⁻¹ is achieved owing to the novel approach of electrospinning the carbon precursor and electrospraying TiO₂ nanoparticles simultaneously. In this approach TiO₂ nanoparticles capture polysulfides with strong interaction and the PCNFs with high conductivity recycle and re-use the adsorbed polysulfides, thus leading to high reversible capacity and stable cycling performance. A high reversible capacity of 967 mA h g⁻¹ is reached after 200 cycles at 0.2C. The cell with the TiO₂@PCNF interlayer also delivers a reversible capacity of around 1100 mA h g⁻¹ at 1C, while the cell without the interlayer exhibits a lower capacity of 400 mA h g⁻¹. Therefore, this work presents a novel approach for designing interlayer materials with exceptional electrochemical performance for high performance Li–S batteries.

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1. Introduction

Energy storage and conversion devices will play a critical role in the future as the fossil energy sources are diminishing.^1,2 Lithium ion batteries have been widely used in many portable electronics and electric vehicles however safety is still an issue for lithium ion batteries.^3,4 Lithium–sulfur batteries are attracting great interest due to ever-increasing demand for high capacity batteries. Sulfur has a high theoretical capacity of 1675 mA h g⁻¹ with an operating voltage of 2.1 V. Furthermore, it is an environmentally friendly and naturally abundant material. However, the inherent poor conductivity, large volume chance of sulfur and shuttle effect deteriorate the electrochemical performance and prevent commercialization of Li–S batteries.^4–9

Several attempts have been reported including hollow TiO₂-webbed carbon nanotubes,¹⁰ MXene nanosheets decorated with TiO₂ quantum dots,¹¹ hollow porous titanium nitride tubes,¹² CoS @rGO,¹³ CNT/graphene–Li₂S aerogel¹⁴ to improve electrochemical performance of Li–S batteries. However, introducing carbon in sulfur cathode leads to lower energy density and the complicated and high-cost methods are not suitable for scaling up.^5,15 An atomic-scale electrocatalyst was reported to expedite the sulfur redox reactions in Li–S batteries by Li et al.⁸ and significant improvement in electrochemical performance was achieved. Lei et al.¹⁶ coated a thin layer of reduced graphene oxide/sodium lignosulfonate composite on the standard polypropylene separator and a highly robust battery with a capacity retention of 74% over 1000 cycles was reported. In another study, polyacrylonitrile and ammonium polyphosphate were electrospun into a multifunctional separator for stable and safe Li–S batteries.¹⁷ Chen et al.¹⁸ reported atomic interlamellar ion path lithiummontmorillonite for high sulfur content Li–S battery and high rate stable cycling performance was observed. Polyisoprene@sulfur in interlayers of Ti₃C₂T_x was reported as a novel structure to regulate the nucleation and heterogeneous growth of Li₂S to prevent arbitrary Li₂S accumulation on host surface.¹⁹

Inserting porous and conductive carbon interlayer between the cathode and separator has been reported to improve the conductivity of cathode and trap lithium polysulfides. Up to day, carbonized cellulose paper,²⁰ carbon paper,²¹ carbonized filter paper,²² microporous carbon paper,²³ carbon fiber cloth²⁴ were reported, however, the improvement is not satisfactory due to low interaction between carbon and polysulfides. Thicker carbon layer is certainly more effective to physically absorb polysulfides however, it slows down ion movement and lowers volumetric energy density of the cells.^15,25

It has been reported that metal oxides (TiO₂, MnO₂, and MgO, V₂O₅ etc.) and sulfides (MoS₂, TiS₂, VS₂, etc.) have strong adsorption for the polysulfides, because of the intense electrostatic attraction between metal–oxygen bond and polysulfides. TiO₂ also has the low cost advantage.^5,6,15,26 In this work, electrospinning and electrospraying techniques were utilized simultaneously, for the first time, to fabricate interlayer materials for Li–S batteries. PAN/PMMA blend solution was electrospun while TiO₂ dispersion in methanol was electrosprayed at the same time. Then, TiO₂@PCNFs were obtained after the heat treatment in which PAN turned into carbon and PMMA generated pores. The obtained fibrous mat was directly used as an interlayer between the separator and the cathode in Li–S cells. In TiO₂@PCNF interlayer, PCNF layer promotes electron transfer and TiO₂ plays a vital role in entrapment of polysulfides via strong chemical interaction. The high initial capacity of 1510 mA h g⁻¹ and the reversible capacity of 967 mA h g⁻¹ were observed after 200 cycles from the cell with TiO₂@PCNF interlayer, whereas the cell prepared with PCNF interlayer delivered initial capacity of 1326 mA h g⁻¹ and reversible capacity of 500 mA h g⁻¹ after 200 cycles. The cell prepared without interlayer only delivered initial capacity of 967 mA h g⁻¹ and reversible capacity of 206 mA h g⁻¹. The cell resistance and C-rate performance was also improved with conductive nanostructured carbon layer decorated with TiO₂. Therefore, this work demonstrate that employing TiO₂ decorated PCNF interlayer via electrospinning and electrospraying could be very effective approach to fabricate high performance Li–S batteries.

2. Experimental

Polyacrylonitrile (PAN, M_w = 150 000), polymethylmethacrylate (PMMA, M_w = 200 000) and N,N-dimethyl formamide (DMF), sulfur (99%), methanol, 1,3-dioxolane, 1,2 dimethoxyethane, bis(trifluoromethane), sulfonamide lithium, lithium nitrite, N-methyl-2-pyrrolidone were supplied from Sigma Aldrich. Polyvinylidene fluoride (PVDF, Solef 5130) and TiO₂ with D30 nm were supplied from Solvay and SSNanomaterials, respectively.

12 wt% PAN/PMMA (7/3, w/w) blend solution was prepared in DMF and stirred for 24 hours at ambient temperature while TiO₂ dispersion was prepared in methanol. The PAN/PMMA solution was then electrospun into nanofibers and TiO₂ dispersion was electrosprayed simultaneously with the following process parameters: flow rate of 1 ml h⁻¹, voltage of 20 kV, and tip-to-collector distance of 20 cm. To form TiO₂@PCNFs, electrospun PAN/PMMA nanofibers decorated with TiO₂ nanoparticles were stabilized in air at 280 °C for 2.5 h with a heating rate of 5 °C min⁻¹ and were carbonized at 800 °C for 2 h in argon atmosphere with a heating rate of 2 °C min⁻¹, during which PAN nanofibers were converted to carbon nanofibers and PMMA was decomposed to generate pores in the carbon matrix.

The morphology of PCNFs, TiO₂@PCNFs was investigated by JEOL SEM-7100-EDX scanning electron microscope and JEOL TEM-1400-EDX transmission electron microscope. X-ray diffraction (XRD) patterns were examined by using a diffractometer (Japan) with Cu Ka radiation (λ = 0.154 nm) in a 2θ range from 10° to 70°.

The sulfur cathode was prepared by casting a slurry containing 70 wt% pure sulfur, 20 wt% carbon and 10 wt% PVDF binder in NMP on the aluminum foil using doctor-blade technique, followed by drying in an air oven at 60 °C for 12 h. It is punched out and precisely weighed. The electrodes with sulfur loadings between 1 mg were cut into discs of 12 mm in diameter. The electrolyte was kept constant as 40 μl per 1 mg sulfur.

The charge–discharge tests of Li–S cells were conducted by using CR 2032 coin cells. The cells were assembled in an argon filled glovebox using electrolyte consisting of 1.0 M LiTFSI in distilled DME : DOL (1 : 1, v/v), sulfur cathode, separator, Li foil anode with TiO₂@PCNF and PCNF interlayer. For comparison, the Li–S cells without interlayer was also assembled. Furthermore, electrochemical impedance spectroscopy (EIS) was employed to examine the resistance of the prepared cells in the range of 100 kHz to 0.1 Hz with 5 mV amplitude and cycling voltammetry was performed to investigate the electrochemical characteristics.

3. Results and discussion

3.1 Morphology and structure characterization

SEM images of PCNFs and TiO₂@PCNFs are shown in Fig. 1. The average nanofiber diameter was measured to be around 160 nm for PCNFs. SEM image of TiO₂@PCNFs revealed that TiO₂ nanoparticles were decorated on PCNFs. In order to further investigate the morphology of TiO₂@PCNFs, TEM images are reported in Fig. 2. TEM images also proved uniform distribution of TiO₂ nanoparticles on highly conductive carbon nanofiber interlayer which not only could buffer the volume expansion of sulfur cathode during lithiation but also provide pathway for fast transport of electrons during cycling process. Furthermore, TiO₂ nanoparticles chemically adsorb polysulfides owing to strong interaction between TiO₂ and polysulfides and hence leads to enhanced cycling performance with satisfactory reversible capacity.

Fig. 1 SEM images of PCNFs (A), TiO₂@PCNFs (B).

Fig. 2 TEM images of TiO₂@PCNFs.

XRD diffraction patterns of PCNFs and TiO₂@PCNFs are shown in Fig. 3. The peak at about 27⁰ in the two curves demonstrated the existence of nongraphic carbon. The peak at 25⁰ in the curve for TiO₂@PCNFs was the typical and strongest peak of rutile TiO₂ and all the identified peaks in the XRD pattern could be assigned to the rutile TiO₂ phase.²⁵ BET surface area measurement results of PCNFs and TiO₂@PCNFs were also reported in Table 1. The surface areas were 1306 m² g⁻¹ and 1418 m² g⁻¹ respectively, for PCNFs and TiO₂@PCNFs. The surface area of TiO₂@PCNFs was slightly increased compared to PCNFs owing to new surface contributed by the TiO₂ nanoparticle deposition.

Fig. 3 XRD patterns of PCNFs (A), TiO₂@PCNFs (B).

Table 1 BET measurement of PCNFs and TiO₂@PCNFs

Sample	Specific surface area, m^2 g^−1
PCNFs	1306
TiO2@PCNFs	1418

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3.2 Electrochemical characteristics

Cycling voltammograms (CVs) of the Li–S cells with TiO₂@PCNF interlayer, PCNF interlayer, and without interlayer are shown in Fig. 4. For all cells studied, in every cathodic scan, the peaks were observed at 2.3 V and 2 V, associated with the transformation of cyclo S to polysulfides (Li₂S_x, 4 ≤ x ≤ 8) and reduction of polysulfides into Li₂S₂ and Li₂S, respectively.⁵ Two anodic peaks were seen at around 2.3 V and 2.4 V where the insoluble LiS changed into polysulfides and then the element S. The CV of the Li–S cells with TiO₂@PCNF interlayer showed the highest current, indicating higher reversible capacity compared to the cell with PCNF interlayer and without interlayer. Zhuang et al.⁶ prepared MoO₂ containing CNF interlayer and higher current observed from the CV curves of the cell with MoO₂/CNF interlayer compared to that of the cell without interlayer, indicating better sulfur utilization due to strong polysulfide adsorption capacity of MoO₂/CNF interlayer.⁶ Moreover, after the first cycle, no obvious change was observed, indicating superior electrochemical performance.

Fig. 4 Cyclic voltammograms of Li–S cells with TiO₂@PCNF interlayer (A), PCNF interlayer (B) and without interlayer (C).

Fig. 5 shows the first cycle discharge–charge curves for the cells with TiO₂@PCNF interlayer, PCNF interlayer, and without interlayer. The typical two-plateau behavior of Li–S battery was observed at around 2.3 and 2.1 V, which was consisted with the peaks in the CV curves. The cell without interlayer delivered an initial discharge capacity of 884 mA h g⁻¹, meaning 52% utilization while the cells with TiO₂@PCNF interlayer and PCNF interlayer reached the discharge capacities of 1510 and 1326 mA h g⁻¹, corresponding to 90% and 79% utilization, respectively. The enhanced initial discharge capacity for the cells with TiO₂@PCNF interlayer was attributed to conductive nanostructured carbon network decorated with TiO₂ nanoparticles, in which PCNF provides excellent electron pathway while TiO₂ nanoparticles provide strong chemical adsorption for polysulfides.

Fig. 5 Galvanostatic charge/discharge profiles of Li–S cells with TiO₂@PCNF interlayer (A), PCNF interlayer (B) and without interlayer (C).

The rate performance was measured at a stepwise current stage from 0.2C to 0.5C, to 1C, to 2C, and then back to 0.2C. When current changes to 0.2C to 0.5C, to 1C, to 2C, the cell with TiO₂@PCNF interlayer exhibited the discharge capacities of around 1400, 1200, 1150, and 900 mA h g⁻¹ whereas the cell without interlayer delivered the capacities of 515, 375, 272 and 94 mA h g⁻¹.

Superior C rate performance illustrates that TiO₂@PCNF interlayer was effective for preventing shuttle of sulfur species. TiO₂ nanoparticles decorated on PCNFs can capture polysulfides effectively via the strong chemical adsorption resulting from the intense electrostatic attraction between the Ti–O bond and the sulfur species. Xiao et al.⁵ introduced graphene/TiO₂ interlayer and they also reported enhanced C rate performance owing to low resistance, highly efficient entrapment of polysulfides and accommodated volume variations by using graphene/TiO₂ interlayer.⁵ Cycling performance at 1C was also reported in Fig. 6 and discharge capacity of above 700 mA h g⁻¹ was achieved after 200 cycles which was significantly higher than that (around 400 mA h g⁻¹) of Li–S cells without interlayer.¹⁷

Fig. 6 C-rate performance of Li–S cells with TiO₂@PCNF interlayer, PCNF interlayer and without interlayer (A) cycling performance of Li–S cell with TiO₂@PCNF interlayer at 1C (B).

Fig. 7 shows the cycling stability for the cells with TiO₂@PCNF interlayer, PCNF interlayer, and without interlayer. The cells with TiO₂@PCNF interlayer not only delivered the highest reversible capacity but also showed the highest capacity retention. In 200 cycles, the cell with TiO₂@PCNF interlayer delivered the high capacity of 968 mA h g⁻¹ while the cells with PCNF interlayer and without interlayer only had the capacities of 498 and 206 mA h g⁻¹, respectively. The remarkable improvement could be attributed to improved electrical conductivity with nanostructured carbon network and strong adsorption of polysulfides with TiO₂ nanoparticles. Moreover carbon structure in the interlayer provided buffer like structure and reduced fracture in the cathode and provided conductive layer for re-using polysulfides. Jiao et al.⁷ reported TiO/Mxene interlayer and better cycling performance with higher capacity (900 mA h g⁻¹ compared to 600 mA h g⁻¹ in 200 cycles) was attributed to strong polysulfide capturing ability of TiO₂.⁷

Fig. 7 Galvanostatic charge–discharge curves of Li–S cells with TiO₂@PCNF interlayer, PCNF interlayer and without interlayer.

Electrochemical impedance spectroscopy of the cells before and after cycling is shown in Fig. 8. For all the cells studied, a semi-circle in the high to medium frequency region was observed, which is typical for Li–S cells. The semi-circle indicates the charge transfer resistance at the interface between the cathode and the electrolyte.¹⁵ The charge transfer resistance of the cell with TiO₂@PCNF interlayer and PCNF interlayer was 25 and 50 ohm, respectively, whereas the cell without interlayer exhibited the charge transfer resistance of 55 ohm. The lower resistance of the cell with TiO₂@PCNF interlayer was attributed to conductive porous carbon network, which provides excellent electron pathway and thus leads to higher sulfur utilization. After cycling the resistance values were 30, 60 and 80 ohm for the cells with TiO₂@PCNF interlayer, PCNF interlayer, and without interlayer, respectively, indicating the formation of an insulating layer of solid Li₂S₂/Li₂S between the separator and the cathode.²⁵ After 200 cycles, the cell with TiO₂@PCNF interlayer, showed the lowest resistance due to the sufficient infiltration of the electrolyte to the interlayer and the sulfur cathode, indicating that TiO₂@PCNF interlayer restricted polysulfide shuttling, supported lithium ion movement and thus leading to lower resistance with superior cycling and C rate performance. All these results proves that TiO₂@PCNF interlayer was effective in prohibiting the shuttle effect of polysulfides in Li–S batteries.

Fig. 8 Electrochemical impedance spectra of the Li–S cells with TiO₂@PCNF interlayer, PCNF interlayer and without interlayer before (A) and after (B) cycling.

4. Conclusion

A facile and effective way to fabricate TiO₂ decorated PCNF interlayer was developed for Li–S batteries. The enhanced cycling and C rate performance was achieved. The conductive PCNF network provided excellent electron pathway, and thus improved electron transfer and help reutilization of active material while TiO₂ nanoparticles adsorbed polysulfides effectively through strong chemical adsorption. This study reported one step feasible approach to fabricate conductive interlayer with highly adsorbent nanoparticles and tunable properties to improve electrochemical performance of Li–S batteries without electrode modification. This novel porous and conductive structure not only adsorb polysulfides but also help reutilization of the active material on the cathode side. The findings in this work reveal a scalable and effective method to fabricate interlayers that could provide remarkable development for Li–S batteries.

Conflicts of interest

The authors declare that they have no conflict of interest.

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