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Nanoengineering S-Doped TiO2 Embedded Carbon Nanosheets for Pseudocapacitance-Enhanced Li-Ion Capacitors Libin Wang, Huiling Yang, Ting Shu, Yue Xin, Xue Chen, Yuyu Li, Heng Li, and Xianluo Hu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00191 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Energy Materials

Nanoengineering S-Doped TiO2 Embedded Carbon Nanosheets for Pseudocapacitance-Enhanced Li-Ion Capacitors

Libin Wang, Huiling Yang, Ting Shu, Yue Xin, Xue Chen, Yuyu Li, Heng Li and Xianluo Hu*

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Correspondence to: [email protected]

ABSTRACT: Li-ion capacitors, comprising a battery anode and a supercapacitor cathode, have been expected to bridge the gap between batteries and supercapacitors. However, the kinetics mismatch between the anodic sluggish insertion and the cathodic capacitive process has impeded the energy-storage potential of devices. Developing pseudocapacitive

anode

materials is urgently needed in that

pseudocapacitance can deliver energy in the same timescale as electrostatic adsorption and offer a comparable level of energy storage to that of battery-type materials. Here we demonstrate an inside and outside synergistic nanoengineering strategy to synthesize nanoporous carbon-modified S-TiO2 hybrid nanosheets, through which both the carbon layer and sulfur doping can be simultaneously generated in situ. Benefitting from the in-situ S doping, the electronic and ionic conductivity of anatase TiO2 nanoparticles is enhanced. The carbon-modified S-TiO2 with dominant 1

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pseudocapacitance realizes an unprecedentedly high capacity of 550 mAh g−1 at 0.3 C and excellent rate capability, outperforming that of the ever reported TiO2-based materials. Furthermore, a hybrid Li-ion capacitor based on the as-obtained carbon-modified S-TiO2 electrode has been assembled, delivering a high energy density of 92.7 Wh kg–1 and power density of 26 kW kg–1 with a stable cycling life (85.8% after 10 000 cycles). Our work offers a new avenue for achieving electrode materials with extrinsic pseudocapacitance that is kinetically comparable to capacitive materials.

KEYWORDS: extrinsic pseudocapacitance, titanium oxide, Li-ion capacitors, nanoengineering, rate capability

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The ever-growing demands of electronic and transportation applications promote the development of high-energy and high-power storage devices. By integrating a battery-type anode and a capacitor-type cathode, the lithium-ion capacitors (LICs), also as known as lithium-ion hybrid supercapacitors (LIHSs), hold great potential for bridging the gap between batteries and supercapacitors.1-2 However, the kinetics mismatch, originated from the dynamic differences between the sluggish insertion of anode and the rapid capacitive process of cathode, has restrained the energy-storage capability of devices, especially at high currents.3-4 Therefore, one of the key sciences for constructing high-performance LICs lies in exploiting ideal anode materials with rapid kinetics that could possibly keep pace with the non-faradic capacitive cathode.5 Capacitive energy storage, featuring excellent long-term endurability to deliver energy rapidly in a pronouncedly shorter time than batteries, endows the materials or devices with desirable advantages for widespread applications.6 Generally, electrical double

layer

capacitors

(EDLCs)

are

associated

with

the

physical

adsorption/desorption of electrolyte ions on the electrode/electrolyte interface, which stores less charges by at least one order of magnitude than that of ion (de)intercalation in battery-type faradic processes. Therefore, a tradeoff between power and energy densities has significantly limited the application potential of capacitive energy storage.7 In this regard, pseudocapacitance has attracted considerable interest in achieving high-energy materials at no expense of power density, through underpotential deposition, surface-confined redox reactions at the surface of electrodes (redox pseudocapacitance) or even rapid ion (de)intercalation into the bulk 3



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(intercalation pseudocapacitance).8 During the past decade, remarkable progresses have been made in achieving extrinsic pseudocapacitance via engineering the structure,9 surface area,10 porosity,11-12 and particle size of electrode materials.13 Driven by the nanotechnology-enabled revolution (e.g., nanostructuring and compositional

control

at

the

nanoscale),

anode

materials

with

extrinsic

pseudocapacitance that behave a pseudocapacitive charge-storage mechanism on the nanoscale are promising candidates for LICs. To this end, much effort has been devoted to develop capacitive anode materials for LICs by means of reducing the particle size,14-16 hybriding heterostructures17-19 and cooperating with carbonaceous materials.4,

20-23

Despite these advances, the investigation of capacitive anode

materials with robust lithium storage and excellent rate capability is impeded by the intrinsic limited storage capacity of (de)intercalation-type reactions, and still remains a big challenge. Anatase TiO2 has been considered to be an attractive anode material for Li-ion batteries (LIBs) because of its outstanding stability for Li-ion (de)intercalation. Nevertheless, it suffers from the sluggish kinetics during ion (de)intercalation and low theoretical capacity of 167 mAh g–1. Sulfur doping is an effective method to enhance the intrinsic conductivity and realize robust ion-storage capacity. Ni et al. has achieved encouraging progress in improving the ion-storage capacity in TiO2 by treating with S powder.24 Yet, the S-doped TiO2 is formed on the surface layer instead of bulk and the consequent structural disorder may hinder the ion transport so that the potential of the storage capacity at high current densities cannot be released 4


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completely. Herein, we propose an inside and outside synergistic nanoengineering strategy to prepare mesoporous carbon-modified S-TiO2 (STO/C) hybrid nanosheets through a topological conversion reaction, whereby TiS2 nanosheets with organics adsorbed on the surface were first prepared and applied as the precursors. Benefitting from the oxidation of TiS2 and carbonization of adsorbed organics, the S dopant is in situ generated from the crystal lattice and a carbon layer is formed simultaneously. Results demonstrate that both the electronic and ionic conductivity of anatase TiO2 are enhanced by in situ sulfur doping, ensuring the facile (de)intercalation kinetics. Further modified by carbon, the reaction kinetics is prominently boosted, contributing to the dominant extrinsic pseudocapacitance. Importantly, the electrodes of as-prepared STO/C nanosheets have exhibited an unprecedentedly high capacity of 550 mAh g–1 at 0.3 C and a high rate capability of 102 mAh g–1 at 50 C (1 C = 335 mA g–1), which is much superior to that of the previously reported TiO2-based materials, to the best of our knowledge. In virtue of the synergistic effects of the homogenous in-situ S doping in crystal lattices and surface carbon modification, it is found that the STO/C material is pseudocapacitance-dominated and able to deliver high Li-ion capacity at high rates, which is intrinsically important to the use in the high-energy and high-power devices. As expected, the as-prepared STO/C electrode has been integrated into a device of a hybrid Li-ion capacitor, delivering 92.7 Wh kg–1 at the power density of 260 W kg–1 and 33.2 Wh kg–1 at 26 kW kg–1.

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RESULTS AND DISCUSSION

Scheme 1. The Inside and Outside Synergistic Nanoengineering Strategy for Preparation of Nanoporous STO/C Nanosheets.

The inside and outside synergistic nanoengineering strategy for the preparation of mesoporous STO/C nanosheets is displayed in Scheme 1. The STO/C nanosheets can be obtained by thermally converting TiS2 nanosheets with organics adsorbed on the surface into TiO2, through which both the carbon layer and S dopant can be simultaneously generated in situ. A one-pot solution method was applied to prepare the precursor of TiS2 nanosheets with an average thickness of ~10 nm and diameter of ~200 nm, as demonstrated by X-ray diffraction (XRD) (Figure 1a) and scanning electron microscopy (SEM) (Figure 1b). Typically, as can be seen from the high-resolution transmission electron microscopy (HRTEM) images (Figure 1c,d), a layer of organic residues is formed automatically on the surface of the well-crystalized TiS2 nanosheets during the preparation (Figure S1). Fourier-transform infrared (FT-IR) spectroscopy further conforms the existence and composition of the organic 6


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residues (Figure S2), which could be transformed into a layer of carbon according to the thermogravimetric and differential scanning calorimetry (TG-DSC) results (Figure S3). Through thermally treating in air atmosphere, the as-prepared TiS2 nanosheets will undergo four procedures, from the oxidation of TiS2 (240–320 oC) to the stabilization of the carbon layer (320–400 oC), then the decomposition of the carbon layer (400–530 o

C), and finally the removal of the doped sulfur atoms (610–800 oC). Hopefully, the

sulfur doping and carbon coating can be realized in the STO/C product simultaneously when the prepared TiS2 nanosheets are annealed at a relatively low temperature of 400 o

C. After the low-temperature thermal treatment, the STO/C nanosheets were

fabricated through the topological conversion reaction. Inheriting from the TiS2 nanosheets, the lamellate morphology of STO/C has been conformally preserved except for the enhanced roughness on the surface (Figure S4). The XRD pattern was employed to confirm the crystal structure of the STO/C nanosheets. All the peaks in the XRD pattern can be well indexed to anatase TiO2 without any other impurities (Figure 2a). The XRD result agrees well with the Raman spectrum in Figure 2b as well, in which typical scattering modes of anatase TiO2, including the Eg (151 cm–1) arising from external vibration, B1g (407 cm–1), A1g (515 cm–1) and Eg (631 cm–1), can be distinguished easily.25 Additionally, there are two broad peaks located at 1577 cm–1 (G band arising from the symmetric E2g mode) and 1367 cm–1 (D band arising from the

A1g mode), which are related to the sp2 microdomains in amorphous carbons, suggesting the formation of an amorphous carbon layer in the STO/C product.26-27 7



Figure 1. (a) XRD pattern, (b) SEM, (c) HRTEM and (d) side-view HRTEM images of the as-prepared TiS2 nanosheets.

Important information about the surface electronic structure of the products, together with the chemical valence of elements, has been explored by X-ray photoelectron spectroscopy (XPS). It is found that only signals for the Ti, O, S, C and N elements exist in STO/C without any other elements (Figure 2c). More details can be revealed by high-resolution XPS spectra. In the representative S 2p spectrum (Figure 2d), the peaks of 161.7 and 163.6 eV are assigned to the S2– species, indicating the partial substitution of sulfur for oxygen in the lattice and bonding interaction with surrounding atoms. Besides, the presence of S4+ ions (168.0 eV) confirms that some of Ti4+ ions have been substituted by S4+. These results verify the successful doping of sulfur in the crystal lattice and only substitutional sites without interstitial sites are observed.24, 28-29 The peak for C 1s in Figure 2e has been deconvoluted into three peaks, which can be attributed to C–C, C–N, and O–C=O/C=N bonds, respectively.30-31 All the 8


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present peaks are ascribed to the formed carbon layer without any bonding reaction between the C and Ti. Similar results can be obtained from the N 1s spectrum (Figure 2f), where the peaks are assigned to pyridinic (N-6), pyrrolic (N-5) and quaternary (N-Q) nitrogen and the signal from the N-Ti bond is hardly detectable.32 It is therefore concluded that only the sulfur atoms have been doped into the lattice while the C and N atoms remained on the outside carbon layer, due to the intrinsic inferiority of external doping.

Figure 2. Structure and chemical composition of the as-prepared STO/C: (a) XRD pattern, (b) Raman spectrum, (c) survey XPS spectrum, (d–f) high-resolution XPS spectra for S 2p, C 1s and N 1s, (g) HRTEM image, (h) bright-field TEM image, and (i) corresponding EELS mapping for the C, Ti, O and S elements.

9



The well-defined structure of STO/C nanosheets has been validated by HRTEM images, in which the lamellar morphology inherited from the TiS2 nanosheets can be well distinguished (Figure 2g). Notably, the S-doped TiO2 nanoparticles of around 5– 10 nm in size are highly crystalline and the lattice space of (101) planes is ~0.36 nm, indicating negligible expansion and disorder induced by sulfur doping. The geometric distribution of S-TiO2 and carbon matrix is depicted in the side-view TEM images in Figure S5. It is reasonable that, after the topological conversion reaction, the S-TiO2 nanoparticles were embedded into the amorphous carbon matrix and formed into the STO/C nanosheets with an average thickness of 5 nm and diameter of ~150 nm, which is expected to possess a high electronic and ionic conductivity. According to the electron energy loss spectroscopy (EELS) mapping results (Figure 2h,i), the distribution of Ti, O, S and C in the STO/C nanosheets is quite uniform and conformal, further demonstrating the composition of carbon and S-TiO2. The nanoporous structure is confirmed by the N2 adsorption-desorption isotherms (Figure S6). It is found that the STO/C is mesoporous. The majority of pore size is 3.8 nm and the specific surface area is 70.5 m2 g–1, which will facilitate the Li-ion diffusion. TG results in Figure S7 suggest that the quantitative C content is 16.6 wt% and the S content (based on S/Ti) of 8.6 at% is collected, which is almost the same to that of energy dispersive X-ray (EDX) results (Table S1). The half-cell electrochemical performance of the STO/C nanosheets is displayed in Figure 3. To investigate the influence of sulfur doping and carbon modification, the control samples of S-doped TiO2 without carbon (STO) (the related characterization is 10


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presented in Figure S8) and commercial TiO2 were also investigated. All the electrochemical measurements were carried out in the potential range between 0.01 and 3 V. Figure 3a shows the charge/discharge curves for the initial ten cycles for the STO/C electrode at a current density of 0.3 C (1 C = 335 mA g–1). It is noted that the characteristic charge and discharge plateaus for anatase TiO2 are located at 1.9 and 1.7 V, respectively. After the first cycle, an exceptionally high reversible capacity of 550 mAh g–1 can be maintained at 0.3 C and the overlapping of charge/discharge profiles indicates the excellent reversibility of the STO/C electrode, which is well consistent with the CV curves at various sweep rates (Figure S9). Different from the external sulfur doping in TiO2, such an ultrahigh capacity has no relationship with the contribution of Ti2S species involved in the Li2S conversion, as implied by the absence of the Ti0 peak in the Ti 2p spectrum (Figure S10).24 In order to gain an insight into the outstanding electrochemical performance, kinetic analysis is further performed. The reaction kinetics can be explored by utilizing the following equation:

i = avb

(1)

in which, a and b are adjustable values. In general, the b-value plays an important role in determining the charge storage behavior, that is, a b-value around 0.5 indicates the diffusion-controlled current response while a b-value around 1 reflects the surface-defined behavior. The b-value of 0.86 and 0.89 (Figure 3b) for the cathodic and anodic currents can be achieved in the STO/C electrode by calculating the slope of log (i) versus log (v), showing the rapid reaction kinetics for lithium-ion storage. Quantitative analysis results are exhibited in Figure 3c by calculating the capacitive 11



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contribution in total charge storage, according to Dunn’s method11:

i(V) = k1v + k2v1/2

(2)

Here, k1, k2 are constants and can be applied as quantitative indicators of the capacitive and diffusive contributions. As can be seen, the percentage of the capacitive contribution of for the STO/C product is as high as 97% at a sweep rate of 1 mV s–1, and more than 90% of the total capacity is derived from the capacitive contribution at different sweep rates (Figure 3d), demonstrating the dominant pseudocapacitance in the STO/C electrode. It is well accepted that the majority of pseudocapacitance in total capacity will lead to a desirable rate capability, as displayed in Figure 3e. Compared to the commercial TiO2, the superior rate performance of the STO electrode indicates that the sulfur doping plays an essential role in enhancing the capacity and rate capability. Further coated by carbon, an ultrahigh rate capability (550, 505, 473, 365, 278 and 102 mAh g–1 at 0.3, 0.6, 1, 5, 10 and 50 C, respectively) is achieved naturally in STO/C, due to the outstanding capacitive behavior. To the best of our knowledge, such a high capacity at high rates has never been reported previously for TiO2-based electrode materials (Figure S11), which offers the possiblities of using TiO2 in high-power devices. The dependence of sulfur doping and carbon modification has been reflected by investigating the lithium-ion diffusion coefficient and electronic conductivity. The lithium-ion diffusion coefficient was measured by a cyclic voltammetry (CV) method according to the following equation:33

Ip = 0.4463zFA(zF/RT)1/2∆C0DLi1/2v1/2 12


(1)

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where Ip is the peak current, A is the surface area of the electrode material, DLi is the diffusion coefficient, ∆C0 is the surface concentration of the electrode material while

R, T, v represent the gas constant, absolute temperature and scan rate, respectively. By establishing the linear relationship of the peak current (Ip) versus v1/2, the obtained slope can be applied as a qualitative index for comparison of DLi. Obviously, the electrodes of STO/C and STO have higher DLi than that of commercial TiO2 (Figure 3f). The calculated DLi for STO (1.25 × 10–9 cm2 S–1) is much higher than that of commercial TiO2 (1.63 × 10-10 cm2 S–1), suggesting that the DLi can be improved dramatically by the introduction of the sulfur dopant. Owing to the synergistic effect of sulfur doping and carbon modification, a highest DLi (3.19 × 10–9 cm2 S–1) has been achieved for STO/C. Besides, a 4000-fold enhancement in the conductivity for STO/C and 210-fold enhancement for STO is realized, respectively, compared to that of commercial TiO2 (Figure S12). This trend is also unveiled by the decrease of charge transfer resistance in electrochemical impedance spectroscopy (EIS) spectra (Figure 3g and Table S2). Conclusively, the in situ sulfur dopant in the lattice of TiO2 induced the elevated electrochemical performance by enhancing both the electronic and ionic conductivity, and thus the rapid reaction kinetics is achieved. The increased capacity may be attributed to the enhanced electronic and ionic conductivity induced by in situ sulfur doping, which allows for the accommodation of more Li+. Moreover, the merit of synergistic effects between S doping and carbon modification endows the STO/C with dominant pseudocapacitance, leading to the exceptional high capacity and ultrahigh rate capability as well as excellent cyclability. As shown in Figure 3h, the 13



capacities of 450 mAh g–1 at 0.6 C after 500 cycles and 110 mAh g–1 at 20 C after 1000 cycles are retained, respectively.

Figure 3. Electrochemical performance of the STO/C nanosheets for lithium storage. (a) Charge/discharge curves of STO/C at a current density of 0.3 C. (b) b-value determination of the peak currents for STO/C. (c) Capacitive contribution of total charge storage at a sweep rate of 1 mV s–1. (d) Capacitive contribution to the total charge storage at different sweep rates. (e) Rate capability. (f) Li-ion diffusion coefficients. (g) EIS spectra (inset: corresponding the equivalent Randles circuit). (h) Long-term cycling performance at 20 C (1 C = 335 mA g–1) of the STO/C electrode.

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Prompted by the unprecedented high rate capability and high capacity, we expect that the as-formed STO/C via our effective synthesis strategy based on homogenous in-situ doping and surface modification, would be highly desirable for fabricating anodes in hybrid LICs. A hybrid device made of STO/C as the anode and nitrogen-doped porous carbon (N-PC, detailed characterizations by SEM, TEM, Raman, XPS and BET in Figure S13) as the cathode is shown in Figure 4a. The excellent electrochemical performance of the N-PC cathode is presented in Figure S14. Figure 4b shows the discharge/charge curves of the as-assembled LIC measured at different current densities of 0.1–1.0 A g–1 over the potential range between 1.0 and 4.2 V. Figure 4c shows the CV curves of the hybrid LIC device between 1.0 and 4.2 V at scan rates ranging from 10 to 100 mV s–1. No obvious distortion could be observed with the sweep rates increased, indicating that the fast intercalation reaction of the anode matches the rapid physical adsorption/desorption of the cathode. A near-linear correlation of discharge/charge curves and a quasi-rectangular shape of CV curves indicate the ideal capacitive behavior of the as-constructed devices. At a low current density of 0.1 A g−1, the LIC can deliver a reversible capacitance of 40 F g−1 at 0.1 A g−1 and 25 F g−1 at 1 A g−1, respectively. Furthermore, the hybrid LIC device shows excellent cyclability over 10 000 cycles with the capacitance retention of 85.8% at a current density of 5 A g−1 (Figure 4d). The Ragone plot (energy density

vs. power density) of the hybrid STO/C//N-PC device is shown in Figure 4e. As expected, our LIC can achieve a high energy density of 92.7 Wh kg−1 at a power density of 260 W kg−1, much superior than that of the previous reports on TiO2-based 15



LICs.20, 34-38 Even at an extremely high power density of 26 kW kg−1, the hybrid device can still deliver 33.2 Wh kg−1. Here the power density of 26 kW kg−1 suggests that a full discharge/discharge cycle takes a short time of ~10 s. In general, supercapacitors usually deliver typical power densities ranging from 1 to 20 kW kg−1, but their energy densities are limited below 20 Wh kg−1.39-40 Evidently, our electrode made of carbon-modified anatase TiO2 nanoparticles with homogenous in-situ S doping combines the merits of supercapacitors and rechargeable batteries. In this regard, it holds great potential as high performance electrodes for next-generation high-power and high-energy devices.

Figure 4. Electrochemical performance of the hybrid LIC based on STO/C and N-PC. (a) Construction illustration for the device of the LIC. (b) Charge–discharge plots. (c) CV curves. (d) Long-term cycling performance at a current density of 5 A g–1. (e) Ragone plots of STO/C//N-PC devices.

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CONCLUSION

In summary, we have successfully fabricated nanoporous STO/C nanosheets via an inside and outside synergistic strategy, in which the S atoms were doped inside the lattice and a carbon layer was formed outside in situ simultaneously. In the virtue of homogenously in-situ sulfur doping, both the electronic and ionic conductivity of anatase TiO2 were boosted pronouncedly, facilitating the ascent of lithium-storage capacity and rate capability. Further modified by carbon derived from in situ carbonization of adsorbed carbonaceous compounds on the surface, the synergistic effect between S doping and carbon modification ensures the dominant pseudocapacitance of STO/C nanosheets. Reasonably, the electrode of nanoporous STO/C realizes the highest capacity ever reported for Ti-based anodes in LIBs and excellent rate capability. As a demonstration, our assembled device of a hybrid LIC comprising STO/C as an anode and N-PC as a cathode exhibits outstanding electrochemical

performances.

The

present

inside

and

outside

synergistic

nanoengineering strategy for modulating both the bulk phase and the surface layer could potentially be extended to fabricate intriguing pseudocapacitive electrode materials, thus promoting the development of kinetics-comparable anodes for high-energy and high-rate energy storage devices such as Li-ion capacitors, Na-ion capacitors, emergent new-concept devices, etc.

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EXPERIMENTAL METHODS

Fabrication of TiS2 nanosheets. TiS2 nanosheets were synthesized via a hot-injection method with a slight modification.41 Briefly, titanium (IV) chloride (1.1 mL) and oleylamine (10 mL) were added to a 25-mL three-neck round-bottom flask under argon atmosphere. To remove residual air and impurities with a low boiling point, the mixtures were heated at 150 oC for 30 min under an argon flow. Subsequently, the temperature was increased to 300 oC at a heating rate of 5 oC min–1. Once the temperature reached 300 oC, 2.0 mL of CS2 was injected into the mixture and kept for 15 min. Finally, the TiS2 product was precipitated by adding excess n-butyl alcohol and washed twice by hexane, methyl alcohol and ethanol, respectively. Fabrication of STO/C and STO. The black STO/C nanocomposite was fabricated by annealing the obtain TiS2 product in air at 400 oC for 2 h at a heating rate of 10 oC min–1. For comparison, the control sample of STO was obtained by annealing the TiS2 nanosheets in air from room temperature to 600 oC (once the temperature was up to 600 oC, the process was stopped) at a heating rate of 10 oC min–1. After that the yellow powder of STO with a sulfur content of 7.9 at% collected from EDX results was obtained. Besides, commercial anatase TiO2 (Aladdin, 99.8% metals basis, anatase, 5–10 nm) was also investigated for comparison. Preparation of N-PC. The porous carbon was prepared according to the previous report.42 In order to obtain the N-doped porous carbon (N-PC), the resulting porous 18


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carbon was treated at 700 oC for 1 h under a pure NH3 atmosphere at a heating rate of 5 oC min–1. The nitrogen content in N-PC is calculated to be 2.7%. Materials Characterization. The crystal structure of all prepared samples was determined by X-ray diffraction (XRD, PANalytical B.V.). Raman spectra were obtained from LabRAM HR800 with a laser wavelength of 532 nm. The valence states were investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi system using a monochromatic Al Ka1 source of 1,486.6 eV). The morphologies and structures of samples were characterized by field-emission scanning electron microscopy (FESEM, Sirion 200) and field-emission transmission electron microscopy (TEM, Tecnai G2 F30), respectively. The Fourier-transform infrared (FT-IR) spectra of TiS2 nanosheets were recorded by VERTEX 70. The surface area and pore size distribution were identified by Autosorb iQ Station 1. Thermogravimetry (TG) tests were performed under air atmosphere at a heating rate of 10 ºC min–1 in NETZSCH STA 449F3. The content of nitrogen for N-PC is measured by Element Analysis (Vario Micro cube). Electrochemical Measurements. The half-cell electrochemical tests of the electrodes were conducted in coin-type cells (CR 2032) with lithium metal as both the counter and reference electrodes. As for the preparation of the anode, in order to avoid unnecessary impacts, the as-prepared STO/C, STO and commercial TiO2 were mixed with conductive carbon black (Super P) and poly(vinylidenefluoride) (PVDF) in a ratio of 7:2:1 by Micro-Vibration Mill (MSK-SFM-12M) for a constant time. The homogeneous slurry was pasted on the copper foil and the mass loading of active 19



materials was controlled to be ~0.6 mg cm–2. The cathode was prepared by mixing the as-prepared N-PC (75 wt%), conductive carbon (5 wt%) and PVDF (20 wt%) and spreading the mixture on a carbon-coated aluminum foil. For above-mentioned tests, a separator (Celgard 2400) and an electrolyte of LiPF6 (1M) in a 1:1 mixture of ethylene carbonate and dimethyl carbonate were applied for assembling the cells. The potential windows are 0.01–3.0 V and 3.0–4.5 V for the anode and the cathode, respectively. For the assembly of full-cells, the STO/C, which has been pre-lithiated by contacting with Li metal with the electrolyte between them for 1.5 h, was employed as the anode. The N-PC was employed as the cathode, and an optimized ratio of STO/C to N-PC was 1:2. Note that the separator and the electrolyte were the same as the above mentioned ones. The cyclic voltammetry (CV) and galvanostatic charge–discharge curves were collected by electrochemical workstation (CHI660e) in the potential range between 1.0 and 4.2 V. Electrochemical impedance spectra (EIS) were obtained over a frequency range of 106 Hz to 0.01 Hz and the obtained spectra were further handled by the Zview software. The electronic conductivities of the STO/C were tested using a two-electrode setup in a potential range between –0.1 and 0.1 V. The specific capacitance (C, F g–1), energy density (E, Wh kg–1) and power density (P, W kg–1) were calculated according to the following equations:

C = I × t/(m × ∆U) P = 1000×∆V × i/m E = P × t/3600 ∆V = (Vmax + Vmin)/2 20


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in which, i (A), m (g), ∆U (V) and t (s) represent the discharge current, mass of active materials based on both anode and cathode, potential window and discharge time, respectively. Vmax (V) and Vmin (V) are the maximum and minimum working voltages during the charge-discharge process, respectively.

ASSOCIATED CONTENT

Supporting Information available: The Supporting Information is available free of charge on the ACS Publications website. Additional characterization of TiS2, STO/C and STO; cyclic voltammetry curves, comparison of electrochemical performance with reported works, current-potential curves of samples; and related characterization and electrochemical performance of N-PC. (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ORCID Xianluo Hu: 0000-0002-5769-167X Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENTS

This work is supported by National Natural Science Foundation of China (Nos. 51772116, 51522205, and 51472098), and the fund for Academic Frontier Youth Team of HUST. The authors thank Analytical and Testing Center of HUST for XRD, SEM, TEM measurements, etc.

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Nanoengineering S-Doped TiO2 Embedded ... - ACS Publications

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