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High Performance Si/SiO Nanosphere Anode Material by Multipurpose Interfacial Engineering with Black TiO

2-x

Juhye Bae, Dae Sik Kim, Hyundong Yoo, Eunjun Park, YoungGeun Lim, Min-Sik Park, Young-Jun Kim, and Hansu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10707 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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ACS Applied Materials & Interfaces

High Performance Si/SiOx Nanosphere Anode Material by Multipurpose Interfacial Engineering with Black TiO2-x Juhye Baea, Dae Sik Kima, Hyundong Yooa, Eunjun Parka, Young-Geun Limb, Min-Sik Parkb,*, Young-Jun Kimb and Hansu Kima,*

a

Department of Energy Engineering, Hanyang University, 133-791, Seoul, Republic of Korea

b

Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Republic of Korea

KEYWORDS Si/SiOx nanosphere, TiO2-x coating, Si-based anode materials, Li-ion battery, Electrochemistry

ACS Paragon Plus Environment


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Abstract Silicon oxides (SiOx) have attracted recent attention for their great potential as promising anode materials for lithium-ion batteries due to their high energy density and excellent cycle performance. Despite these advantages, the commercial use of these materials is still impeded by low initial Coulombic efficiency and high production cost associated with a complicated synthesis process. Here, we demonstrate that Si/SiOx nanosphere anode materials show much improved performance enabled by electroconductive black TiO2-x coating in terms of reversible capacity, Coulombic efficiency, and thermal reliability. The resulting anode material exhibits a high reversible capacity of 1200 mAh g-1 with an excellent cycle performance up to 100 cycles. The introduction of a TiO2-x layer induces further reduction of the Si species in the SiOx matrix phase, thereby increasing the reversible capacity and initial Coulombic efficiency. Besides the improved electrochemical performance, the TiO2-x coating layer plays a key role in improving the thermal reliability of the Si/SiOx nanosphere anode material at the same time. We believe that this multipurpose interfacial engineering approach provides another route towards high performance Si-based anode materials on a commercial scale.


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Introduction The market for lithium-ion batteries (LIBs) has experienced rapid growth for many applications ranging from mobile IT to electric vehicles (EVs).1, 2 With increasing demand from emerging markets, a further increase in the energy density of current LIBs is regarded as a pending challenge. Since the energy density of LIBs is determined by the reversible capacity and operating voltage of electrode materials, intensive efforts have been devoted to the development of Si-based materials for practical use in commercialized LIBs owing to their high reversible capacity (3580 mAh g-1, Li15Si4) and moderate operating voltage (~0.4 V vs. Li/Li+).2, 3 Compared with commercial carbonaceous anode materials (i.e., graphite, soft carbon, and hard carbon), Si-based materials show approximately three to five times higher reversible capacity and slightly higher operating voltages. As is well recognized, however, a large volume expansion (~300%) of Si induced by alloying/de-alloying with Li+ during cycles is unavoidable.2, 4, 5 Another critical issue is the low electrical conductivity of Si-based materials.6, 7 These technical problems of Si-based anode materials cause undesirable fading of battery performance due to the repeated charge and discharge process. In order to overcome these drawbacks, various Si-based anode materials have been proposed, such as constructing nanoarchitectures and/or composites with conductive materials (e.g., carbon, metal, and conductive polymers).6, 8-12 In particular, Si-based nanostructured materials have been known to facilitate stain/stress relaxation as well as shorten Li+ diffusion length. These research efforts significantly improved the reversible capacity, rate capability, and cycle performance of Si-based anode materials for LIBs.5, 7, 13-15 On the other hand, constructing sufficient electric conduction pathways is also essential for improving the electrochemical performance of Si-based anode materials. One of the most popular ways to improve the electrical conductivity of Si-based materials is by surface



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coating with highly conductive materials. By forming a conductive coating layer, Si-based anode materials showed a noticeable improvement in electrochemical performance.3, 6, 10, 15 Recently, oxygen-deficient black titanium oxide (TiO2-x) has been explored as a promising coating material for Si-based anode materials as well as various lithium and sodium storage materials. Jeong et al. reported that the TiO2-x shell could improve the electrical conductivity of the Si nanoparticles electrode because Ti3+ formed by partial oxygen loss in the structure directly affects the local electronic structure.16-20 As a result, a TiO2-x shell with a relatively smaller band-gap provides higher electric conductivity for Si nanoparticles compared to bare ones. Inspired by this progress, we demonstrate a TiO2-x coated Si/SiOx (denoted as TiO2x@Si/SiOx

hereafter) nanosphere as part of a continuous effort for exploring robust and high-

performance SiOx based anode material.3,

21, 22

Apart from the improved electrical

conductivity of the material, the introduction of TiO2-x on the surface of the Si/SiOx nanosphere not only led to changes in the oxidation state of the Si species in the amorphous SiOx matrix phase towards more reduced states, but also played a key role in enhancing the thermal reliability of Si/SiOx nanosphere anode material. Using various analytical characterization techniques, we also provide direct evidence why black TiO2-x coating can improve the electrochemical performance as well as the reliability of Si/SiOx nanosphere anode materials.


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Experimental Section Material Preparation To prepare the HSQ nanosphere, 10 mL of triethoxysilane ((C2H5O)3SiH, Aldrich, 99.8%) was first slowly dropped into a 0.1 M HCl solution (500 mL) under continuous stirring at 800 rpm. The solution was filtered and washed with deionized water repeatedly. The precipitates were collected and dried in a convection oven at 80 oC to eliminate residual moisture. After fine grinding, the obtained powder was heated at 900 °C under a 4 % H2/Ar atmosphere with a flow rate of 0.5 L min−1. The heating rate and time were fixed at 20 °C min−1 and 1 h, respectively. For a TiO2-x coating, the obtained Si/SiOx nanosphere was dispersed in ethanol (C2H6O, Fluka, ≥99.8%) and titanium(IV) butoxide (Ti(OC4H9)4; Ti(OBu)4, Aldrich, 97%) was added by dropwise to dispersed solution at room temperature. Then, the solution was heated at 70 °C to evaporate ethanol under stirring at 300 rpm. The obtained powder was loaded into a vertical furnace and heated at 500 °C for 5 h under Ar atmosphere with a heating rate of 200 °C h−1. Structural Characterizations The morphologies and microstructures of the Si/SiOx nanosphere and TiO2-x@Si/SiOx nanosphere were examined using a field emission scanning electron microscopy (FESEM; JEOL JSM-7000F) and high-resolution transmission electron microscopy (HRTEM; JEOL 2100F). Powder X-ray diffraction (XRD) patterns of the Si/SiOx nanosphere and TiO2x@Si/SiOx

nanosphere were obtained using an X-ray diffractometer (Empyrean,

PANanalytical) equipped with a 3D pixel semiconductor detector using Cu Kα radiation (λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra of the Si/SiOx nanosphere and TiO2x@Si/SiOx

nanosphere were collected using a FT-IR spectrophotometer (Bruker VERTEX70).



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The surface chemistry of the Si/SiOx nanosphere and TiO2-x@Si/SiOx nanospheres was investigated by using X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific Co.). Electrochemical Measurements The electrodes were prepared by coating slurries containing the TiO2-x@Si/SiOx nanosphere (80 wt%) as active materials, conducting agent (Super-P, 10 wt%), and polyacrylic acid binder (PAA, 10 wt%) dissolved in deionized water on Cu foil. After coating, the electrodes were dried at 120 °C for 10 h and pressed under a pressure of 200 kg cm−2. The loading amount of the electrodes was fixed at 0.5 mg cm−2 and the thickness of electrodes was about 7.0 µm with a density of 0.7 g cc-1. The CR2032 coin type half-cells were carefully assembled in an Ar-filled glove box to evaluate their electrochemical performance. A porous polyethylene (PE) membrane was used as a separator, and the electrolyte was 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (EMC) (1:2, v/v; Panax Etec Co. Ltd.) with 2.0 wt% fluoroethylene carbonate (FEC). The cells were galvanostatically charged (Li+ insertion) and discharged (Li+ extraction) in the voltage range of 0.005−2.0 V vs Li/Li+ at different current densities at room temperature. For storage test, the cell was charged to 5 mV vs. Li/Li+ at a constant current of 200 mAg-1 at room temperature and then stored in the oven at high temperature (60 oC) for 48 h. After storage, the cell was discharged to 2.0 V vs. Li/Li+ at room temperature.23, 24


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Results and Discussion A TiO2-x@Si/SiOx nanosphere is proposed by multipurpose surface modification with TiO2-x on the surface of the Si/SiOx nanosphere derived from hydrogen silsesquioxane (HSQ, HSiO1.5) nanoparticles, in which a very thin TiO2-x shell is successfully introduced on the surface of the Si/SiOx nanosphere through a wet chemical process using a titanium butoxide (Ti(OBu)4) precursor, as described in Figure 1a. The morphologies of the as-prepared hydrogen silsesquioxane (HSQ, HSiO1.5) nanoparticles, HSQ-derived Si/SiOx nanosphere, and TiO2-x@Si/SiOx nanosphere prepared by adding 2 wt% of titanium butoxide (Ti(OBu)4) were investigated using a field-emission scanning electron microscopy (FESEM). Spherical HSQ nanoparticles (~100 nm in size, Figure 1b) were obtained as a result of a sol-gel reaction of triethoxysilane, and each nanoparticle was agglomerated by forming microscale secondary particles. After heating at 900 oC for 1 h, an HSQ-derived Si/SiOx nanosphere was successfully synthesized (Figure 1c). The TiO2-x@Si/SiOx nanosphere also has a similar morphology even after the TiO2-x coating process (Figure 1d). Note that the proposed TiO2-x coating process does not affect the particle size or shape of the Si/SiOx nanosphere. The microstructure of the TiO2-x@Si/SiOx nanosphere was further investigated using a transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS). As shown in Figure 1e and 1f, the TiO2-x layer was uniformly formed on the surface of the Si/SiOx nanosphere, and the thickness of the TiO2-x layer can be controlled by adjusting the amount of Ti(OBu)4 precursor. With an increase in the amount of Ti(OBu)4, the Ti signal in the energy dispersive x-ray spectroscopy (EDS) got stronger without a change in signals for Si and O in the results of the EDS analyses (Figure S1). This result indicates that a greater amount of TiO2-x is coated on the surface of the Si/SiOx nanosphere with an increase in the amount of Ti precursor added. Note that the light-brown Si/SiOx nanosphere (Figure 1g) changed to a dark-brown TiO2-x@Si/SiOx nanosphere after the TiO2-x coating



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process, indicating formation of a black TiO2-x layer. As the amount of Ti(OBu)4 increased, the color of the particle became darker (Figure S2). The powder X-ray diffraction (XRD) patterns of TiO2-x@Si/SiOx nanosphere prepared with different amounts of Ti(OBu)4 were compared with that of the pristine Si/SiOx nanosphere in Figure 2a. A broad Bragg peak was observed at a low angle without any sharp peak in the XRD patterns of the Si/SiOx nanosphere, which is a typical characteristic of an amorphous structure.21, 25, 26 Interestingly, a growth of crystalline peak at 25.3º, corresponding to crystalline TiO2-x,18, 27-30 was evident by increasing the amount of Ti(OBu)4 during the synthesis. According to the Fourier-transform infrared (FT-IR) spectra (Figure 2b), symmetric and asymmetric Si-O-Si stretching vibrations were clearly observed at 1100 and 806 cm-1 26, 31, 32

respectively, together with a strong Si-O-Si bending mode at 470 cm-1 ,31, 32 which was

mainly attributed to the Si/SiOx framework. As expected, formation of Ti-O-Ti bonding was evident as confirmed by a newly appeared peak at 663 cm-1 after the TiO2-x coating process.20, 32

Thus, this implies that TiO2-x phase can be successfully introduced on the surface of Si/SiOx

nanosphere after heating with Ti(OBu)4 during the process. In order to clarify whether TiO2-x coating has the capability to reduce Si species in the amorphous SiOx phase, the surface chemistry of the TiO2-x@Si/SiOx nanosphere was investigated by X-ray photoelectron spectroscopy (XPS). Figure 3a compares the Ti 2p spectra collected from the TiO2-x@Si/SiOx nanospheres prepared by different amounts of Ti(OBu)4 and commercial TiO2 powder as a reference. The collected XPS Ti 2p and Si 2p spectra were carefully de-convoluted based on the excitation of C 1s at 284.6 eV. From a comparison, we found a typical peak corresponding to the binding energy of Ti4+ at 458.2 eV in all the samples,33-35 as well as an additional small peak at the lower binding energy (457.6 eV), which corresponds to Ti3+ in the TiO2-x@Si/SiOx nanospheres.16, 36, 37 This result reveals


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that a partially reduced TiO2-x phase containing Ti3+ was formed on the surface of the Si/SiOx nanosphere during the coating process. As expected, the growth of Ti3+ peak is evident by increasing the amount of Ti(OBu)4. Interestingly, we found that a gradual peak shift in the Si 2p spectra toward a lower binding energy with an increased amount of Ti(OBu)4 (Figure 3b). This shift in the Si 2p spectra supports the evidence that the oxidation state of the Si species was further reduced in the Si/SiOx nanosphere after the TiO2-x coating process, which is probably due to the Ti atoms easily taking away the oxygen atom from the amorphous SiOx matrix (Figure S3). We also investigated the valance band (VB) spectra for a TiO2-x@Si/SiOx nanosphere with different contents of TiO2-x. While commercial TiO2 displayed a VB maximum edge at 2.17 eV,16 the TiO2-x@Si/SiOx nanosphere showed much smaller values as summarized in Figure 3c and S4. It should be noted that the electronic conduction of the TiO2-x phase is facilitated by its smaller band gap compared with commercial TiO2, which can be further supported by previous reports on the density of state (DOS) of TiO2-x (Table S1).18, 20, 38

The electrical conductivities of the TiO2-x coated Si/SiOx electrodes were measured by

four-point probe method after coating on the glass substrate. For comparison, each sample was measured five times and average values are presented in Table S1. As might be expected, the electrical conductivities of TiO2-x@Si/SiOx nanospheres showed the improved value compared with that of bare Si/SiOx nanosphere. The beneficial characteristics of the TiO2-x phase directly affect the electrochemical performance of the Si/SiOx nanosphere as explained below. Figure 4a shows the galvanostatic voltage profiles of the TiO2-x@Si/SiOx nanospheres with different amounts of TiO2-x (2-10 wt%). The initial capacity of the TiO2-x@Si/SiOx nanosphere is dependent on the content of TiO2-x, showing an optimum capacity at 2 wt%TiO2-x@Si/SiOx nanosphere. Further reduction of Si by forming TiO2-x might play a role in increasing the initial Coulombic efficiency and reversible capacity of electrode materials;



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however, the further increase in the relative amount of TiO2-x causes a decrease in reversible capacity of TiO2-x@Si/SiOx nanosphere because of the lower reversible capacity of TiO2-x compared with the Si/SiOx nanosphere. Thus, the TiO2-x@Si/SiOx nanosphere prepared with 2 wt% Ti(OBu)4 exhibited an initial discharge capacity of 1163 mAh g-1 with an initial Coulombic efficiency of 52.2%, which could be improved by increasing the amount of Ti(OBu)4 (inset of Figure 4c). Figure 4b shows the cycle performance of the TiO2-x@Si/SiOx nanospheres at a current density of 200 mA g-1 for 100 cycles. In particular, the optimized TiO2-x@Si/SiOx nanosphere still maintained 98.1% of its initial capacity even after 100 cycles, showing a high Coulombic efficiency of more than 99.5% during cycles (Figure 4c), which is one of the most important characteristics to be satisfied before being deployed on a commercial scale. The rate capability of the optimized TiO2-x@Si/SiOx nanosphere was also examined at various current densities ranging from 50 mA g-1 (0.05 C) to 20 A g-1 (20 C), as shown in Figure 4d. The TiO2-x@Si/SiOx nanosphere exhibited a higher reversible capacity than that of the pristine Si/SiOx nanosphere, even at a high current density of 20 A g-1 (20 C). The thermal reliability of electrode material is one of the most important properties to be adopted in commercial LIBs. We anticipated that TiO2-x coating has the capability to further stabilize the interfacial stability of the Si/SiOx nanosphere anode material. To clearly elucidate the positive effect of TiO2-x coating on the Si/SiOx nanosphere, the thermal stability was also investigated as well. The cell was stored at a high temperature of 60 oC for 48 h after being charged to 5 mV vs. Li/Li+, and then cycled at a constant current of 200 mA g-1 (0.2 C) at room temperature. Figure 5a and 5b shows the voltage profiles of pristine Si/SiOx nanosphere and optimized TiO2-x@Si/SiOx nanosphere before and after the storage test, respectively. After thermal storage at 60 oC, the TiO2-x@Si/SiOx nanosphere exhibited higher capacity retention (82.5 %) than the pristine Si/SiOx nanosphere (79.5 %), supporting the view that the TiO2-x@Si/SiOx nanosphere is more stable against thermal shock. The recovery


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capacity of the TiO2-x@Si/SiOx nanosphere was also higher than that of the pristine Si/SiOx nanosphere (Figure 5c and 5d) because the TiO2-x coating layer could effectively suppress the heat flow. As a result, better thermal stability could be attained in the TiO2-x@Si/SiOx nanosphere. Note that the thermal stability of Si-based materials is crucial for practical use because it is directly related to safety. Conclusion A rationally designed TiO2-x@Si/SiOx nanosphere with a core-shell structure was prepared by a simple surface coating of Si/SiOx nanosphere with Ti(OBu)4 in aqueous solution. The formation of electroconductive TiO2-x on the surface of the Si/SiOx nanosphere allows a further reduction in Si because Ti atoms can easily take oxygen from the Si/SiOx phase by forming the TiO2-x phase. As a result, the initial Coulombic efficiency and reversible capacity of the TiO2-x@Si/SiOx nanosphere increased by introducing the TiO2-x coating layer. More importantly, the TiO2-x phase plays an important role in securing sufficient electric conductivity because the smaller band-gap of TiO2-x containing Ti3+ directly affects the local electronic structures. Moreover, a robust TiO2-x coating layer is also essential for improving the thermal reliability of the Si/SiOx nanosphere for practical use in LIBs. We believe that this multipurpose interfacial engineering approach can be regarded as another path towards high performance Si-based anode materials on a commercial scale.



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Supporting Information Available EDS results, photographs, and XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author * Corresponding authors: [email protected]; [email protected]

Acknowledgements This work was in part supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is funded by the Ministry of Trade, Industry & Energy, and Republic of Korea (No. 20128510010080) and in part supported by the research fund of Hanyang University (HY-2012-T).


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Figure 1. (a) Schematic illustration of the proposed synthetic route for TiO2-x coated Si/SiOx nanosphere. FESEM images of (b) spherical HSQ nanosphere, (c) HSQ-derived Si/SiOx nanosphere and (d) 2wt%-TiO2-x coated Si/SiOx nanosphere. (e) TEM image combined with EDS line-mapping profiles (Red : Ti, Green : Si, Cyan : O) and (f) EDS elemental mapping result of Ti for 5wt%-TiO2-x coated Si/SiOx nanosphere. Digital photographs of (g) pristine Si/SiOx nanosphere and (h) 5wt%-TiO2-x coated Si/SiOx nanosphere.



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Figure 2. (a) Powder XRD patterns and (b) FT-IR spectra of TiO2-x coated Si/SiOx nanospheres synthesized with different amounts of Ti(OBu)4 for TiO2-x coating compared with those of pristine Si/SiOx nanosphere.


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Figure 3. (a) XPS Ti 2p spectra and (b) XPS Si 2p spectra of TiO2-x coated Si/SiOx nanospheres synthesized with different amounts of Ti(OBu)4 together with TiO2-x and pristine Si/SiOx nanosphere references. (c) Comparison of valence band XPS spectra of 2wt%-TiO2-x coated Si/SiOx nanosphere synthesized with different amounts of Ti(OBu)4.



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Figure 4. Electrochemical performance of TiO2-x coated Si/SiOx nanospheres synthesized with different amounts of Ti(OBu)4 compared with pristine Si/SiOx nanosphere: (a) galvanostatic voltage profiles in the voltage range of 0.01-2.0V vs Li/Li+ at a constant current density of 0.05C (50 mAg-1) for the first cycle, (b) cycle performance and (c) Coulombic efficiencies of TiO2-x coated Si/SiOx nanospheres during 100 cycles compared with pristine Si/SiOx nanosphere and (d) rate capabilities of 2wt%-TiO2-x coated Si/SiOx nanosphere and pristine Si/SiOx nanosphere at different current densities (1 C = 1000 mAg-1).


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Figure 5. Galvanostatic voltage profiles of (a) pristine Si/SiOx nanosphere and (b) 2 wt%TiO2-x coated Si/SiOx nanosphere before and after thermal storage at 60 oC for 48 h. Comparisons of charge and discharge capacities of (c) pristine Si/SiOx nanosphere and (d) 2 wt%-TiO2-x coated Si/SiOx nanosphere before and after thermal storage at 60 oC for 48 h.



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