Hollow Nanostructured Metal Silicates with Tunable Properties for

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Hollow Nanostructured Metal Silicates with Tunable Properties for Lithium Ion Battery Anodes Seung-Ho Yu, Bo Quan, Aihua Jin, Kug-Seung Lee, SoonHyung Kang, Kisuk Kang, Yuanzhe Piao, and Yung-Eun Sung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07075 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 5, 2015

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ACS Applied Materials & Interfaces

Hollow Nanostructured Metal Silicates with Tunable Properties for Lithium Ion Battery Anodes Seung-Ho Yua,b,‡, Bo Quanc,‡, Aihua Jina,b, Kug-Seung Leed, Soon Hyung Kange, Kisuk Kanga,f, Yuanzhe Piaoc,g,*, Yung-Eun Sunga,b,*

a

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742,

Republic of Korea. b

School of Chemical and Biological Engineering, Seoul National University, Seoul 151-

742, Republic of Korea. c

Graduate School of Convergence Science and Technology, Seoul National University,

Seoul 151-742, Republic of Korea. d

Beamline Department, Pohang Accelerator Laboratory (PAL), Pohang 790-784,

Republic of Korea. e

Department of Chemistry Education and Optoelectronics Convergence Research

Center, Chonnam National University, Gwangju 500-757, Republic of Korea. f

Department of Materials Science and Engineering, Seoul National University, Seoul

151-742, Republic of Korea. g

Advanced Institutes of Convergence Technology, Suwon 443-270, Republic of Korea.

*Corresponding authors. E-mail addresses: [email protected] (Y. Piao), [email protected] (Y. –E. Sung) ‡

These authors contributed equally to this work.

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ABSTRACT: Hollow nanostructured materials have attracted considerable interest as lithium ion battery electrodes because of their good electrochemical properties. In this study, we developed a general procedure for the synthesis of hollow nanostructured metal silicates via a hydrothermal process using silica nanoparticles as templates. The morphology and composition of hollow nanostructured metal silicates could be controlled by changing the metal precursor. The as-prepared hierarchical hollow nanostructures with diameters of about 100~200 nm were composed of various shaped primary particles such as hollow nanospheres, solid nanoparticles, and thin nanosheets. Furthermore, different primary nanoparticles could be combined to form hybrid hierarchical hollow nanostructures. When hollow nanostructured metal silicates were applied as anode materials for lithium ion batteries, all samples exhibited good cyclic stability during 300 cycles, as well as tunable electrochemical properties.

KEYWORDS: hollow nanostructured material; metal silicate; lithium ion battery; conversion reaction; nanoparticle; anode material

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1. INTRODUCTION Over the last decade, nanostructured materials have been intensively studied as promising electrode materials for lithium ion batteries (LIBs) because of their unique properties.1-7 Their nanoscale dimensions facilitate electron and lithium ion transport owing to shortened pathways, which consequently enhances the rate capability compared to that of bulk or microscale materials. Since the early 2000s, it has been reported that nanostructured transition metal oxides, which react with lithium based on conversion reactions, show much higher specific capacities than those of state-of-the-art carbonaceous anodes.8 For example, the theoretical capacities of α-Fe2O3 and MoO3 are 1007 and 1117 mAh g-1, respectively, which are two to three times greater than that of graphite. Similarly, many transition metal fluorides, sulfides, nitrides, and phosphides exhibited high specific capacity through conversion reaction with lithium, showing their potential as next generation anode materials.9-12 Recent reports revealed that metal silicates could also deliver a high specific capacity through conversion reactions.13-15 Metal silicates have many advantages as electrode materials for LIBs such as their abundance, low cost and high thermal stability.13-24 Furthermore, their unique porous structure and tunable composition are very attractive in application to battery electrodes.13-24 However, conversion reaction-based materials suffer from stress induced by huge volume changes during lithium insertion and extraction, which results in poor cyclic stability. Many strategies have been proposed to overcome this issue, such controlling the size, shape, and composition of the active materials.25-28 Among the potential strategies, designing hollow nanostructures is one of the most effective ways to 3



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accommodate the volume changes and enhance the cyclic stability. In addition, hollow nanostructures have higher surface areas, which can further reduce the diffusion length in lithium ion transport and allow for a higher rate capability. So far, various hollow nanomaterials with different components and shapes have been employed as anode materials for LIBs, and the electrochemical properties of hollow nanostructures are significantly affected by their composition and structural features.29-32 For example, our pioneering work showed that spindle-like α-Fe2O3 hollow nanocapsules synthesized by a wrap-bake-peel approach displayed enhanced cyclic stability compared to that of aggregated Fe2O3 particles.33 Hyeon and co-workers reported that hollow MnxFe3-xO4 cubic nanocages prepared by galvanic replacement delivered a high specific capacity of about 1000 mAh g-1 with excellent cyclic stability during 50 cycles.34 Although significant improvements have been achieved in regard to battery performance by using hollow nanostructures, the facile control of the component and morphology of hollow particles is still a challenge. Herein, we present a general approach to prepare hierarchical hollow nanostructures by a hydrothermal method, using various metal silicates that show high electrochemical performance as anode materials. By modifying the experimental conditions such as the precursors, the porosity, composition, and morphology of the nanomaterials could be tailored, while maintaining their overall hollow shape. When assynthesized hollow structured metal silicates are applied as anode materials for LIBs, they showed remarkably high cyclic stabilities during 300 cycles. In addition, two different metal silicates could be integrated, which allowed for facile tuning of the electrochemical properties. We expect that our approach will be highly useful in 4


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designing and tuning high-performance anode materials for LIBs.

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2. EXPERIMENTAL SECTION 2.1. Reagents. Tetraethyl orthosilicate (TEOS, 98.0%), KMnO4 (99.3%), ammonia solution (NH4OH 28.0-30.0%), sodium hydroxide (NaOH), and ethanol (95.0%) were purchased from Samchun (Seoul, South Korea). Potassium ferrate (VI) (K2FeO4, >90%), nickel (II) chloride (NiCl2), urea, and glucose were purchased from Sigma-Aldrich (Milwaukee, WI, USA). Deionized water was filtered using a water purification system (UP90 model, Seoul, South Korea). All chemicals were used as received without further purification. 2.2. Synthesis of silica nanoparticles. Typically, 1.0 mL of ammonia solution was added to a mixture of 80 mL of ethanol and 5 mL of water. After mixing, 6 mL of TEOS was added and stirred at room temperature for 12 h. 2.3. Synthesis of manganese silicate hollow nanostructures (Mn-HNs), iron silicate hollow nanostructures (Fe-HNs), and nickel silicate hollow nanostructures (Ni-HNs). Typically, silica nanoparticles (100 mg) were dispersed in 25 mL of ethanol by sonication, and 0.15 g of KMnO4 was dissolved in 30 mL of water. Next, the two suspensions were mixed for 10 min and placed in a 100 mL Teflon-lined autoclave that was then sealed and heated at 190 °C for 36 h. After the autoclave was cooled down to room temperature, the products were dispersed in a 2M solution of NaOH in a 60 °C oven for 3 h. The final products were washed with ethanol and water several times and were subsequently dried at 90 °C in an oven to yield Mn-HNs. FeHNs and Ni-HNs were prepared via a similar process, in which 1 mmol of K2FeO4 and 1 mmol of NiCl2 were used as the iron and nickel sources, respectively. 2.4. Synthesis of hybrid manganese and iron silicate hollow 6


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nanostructures (Mn/Fe-HNs). Silica nanoparticles (100 mg) were dispersed in 25 mL of ethanol by sonication, and 0.2 g of K2FeO4 and 0.6 g of urea were dissolved in 32 mL of water. Next, the two suspensions were mixed for 10 min and placed in a 100 mL Teflon-lined autoclave that was then sealed and heated at 190 °C for 24 h to yield silicairon silicate core-shell nanostructures. After the autoclave was cooled down to room temperature, 0.15 g of KMnO4 was added into the reaction media. The autoclave was then sealed and heated at 190 °C for another 36 h. After the autoclave was cooled down to room temperature, the products were dispersed into a 2M solution of NaOH in a 60 °C oven for 6 h; the final products were washed with ethanol and water several times, and then dried at 90 °C in an oven. 2.5. Synthesis of hybrid manganese and nickel silicate hollow nanostructures (Mn/Ni-HNs). Silica nanoparticles (100 mg) were dispersed in 25 mL of ethanol by sonication, and then 0.20 g of NiCl2 and 0.15 g of KMnO4 were dissolved in 32 mL of water. The two suspensions were mixed for 10 min and placed in a 100 mL Teflon-lined autoclave that was then sealed and heated at 190 °C for 24 h to yield silica-nickel silicate core-shell nanostructures. After the autoclave was cooled down to room temperature, 0.15 g of KMnO4 was added into the reaction media. The autoclave was then sealed and heated at 190 °C for another 36 h. After the autoclave was cooled down to room temperature, the products were dispersed into a 2M solution of NaOH in a 60 °C oven for 6 h; the final products were washed with ethanol and water several times, and then dried at 90 °C in an oven. 2.6. Synthesis of hybrid iron and nickel silicate hollow nanostructures (Fe/Ni-HNs). Silica nanoparticles (100 mg) were dispersed in 25 mL of ethanol by 7



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sonication, and 0.2 g of K2FeO4 and 0.6 g of urea were dissolved in 32 mL of water. The two suspensions were mixed for 10 min and placed in a 100 mL Teflon-lined autoclave that was then sealed and heated at 190 °C for 24 h to yield silica-iron silicate core-shell nanostructures. After the autoclave was cooled down to room temperature, 0.15 g of NiCl2 was added into the reaction media. The autoclave was then sealed and heated at 190 °C for another 36 h. After the autoclave was cooled down to room temperature, the products were dispersed into a 2M solution of NaOH in a 60 °C oven for 6 h; the final products were washed with ethanol and water several times, and then dried at 90 °C in an oven. 2.7. Characterization. The morphology of the nanoparticles was studied using transmission electron microscopy (JEOL, JEM-2100F). The surface area was measured by nitrogen adsorption-desorption isotherms using the Brunauer-Emmett-Teller (BET) method. The pore diameter and pore size distribution were determined by the BarrettJoyner-Halenda (BJH) model. Powder X-ray diffractometry (XRD) was carried out using a PANalytical X'Pert Powder diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 45 kV and 40 mA. X-ray photoelectron spectroscopy was measured by a Sigma Probe (ThermoFisher Scientific) with Al Kα (1486.8 eV) as the X-ray source. X-ray absorption spectroscopy analysis was carried out at the 8C nano-probe XAFS beamline of PLS-II (South Korea). 2.8. Electrochemical characterization. All samples were coated with carbon for the battery test. The samples (0.15 g) and glucose (0.5 g) were dispersed in a mixture of water (40 mL) and ethanol (20 mL). The mixtures were stirred for 10 min and placed in a 100 mL Teflon-lined autoclave that was then sealed and heated at 8


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190 °C for 15 h. The products were washed with ethanol and water several times, and then dried at 90 °C in an oven. After drying, the samples were heated at 400 °C for 2 h under a flow of N2 gas. The resulting carbon-coated samples were employed as the active materials. To prepare the working electrodes, the active materials were mixed with conductive carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidone (70:15:15 w/w/w). The mixture was coated on copper foil using the doctor blade technique. Then, it was dried at 120 °C in vacuum oven for 8 h, and was subsequently pressed to enhance the contact between particles. The coin-type half cells were assembled in an Ar filled glove box. Lithium foil was used as the counter and reference electrodes. 1.0 M LiPF6 dissolved in ethylene carbonate and diethyl carbonate (1:1 v/v) was used as the organic electrolyte. The electrochemical measurements were carried out using a WBCS3000 cycler (WonAtech) at room temperature.

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3. RESULTS AND DISCUSSION Figure 1 illustrates the procedure for the preparation of various hollow nanostructured metal silicates. Silica template-based approach to preparation of hollow structured materials has been widely used, which enables to control the inner-core size, morphology and produce complex structures easily.35-43 Manganese silicate hollow nanostructures (Mn-HNs), iron silicate hollow nanostructures (Fe-HNs), and nickel silicate hollow nanostructures (Ni-HNs) were developed via a facile, one-pot hydrothermal process using silica nanoparticles as templates. Silica nanoparticles were synthesized using the Stöber method.44 Selected metal precursors (KMnO4 for Mn-HNs, K2FeO4 for Fe-HNs, and NiCl2 for Ni-HNs) and silica nanoparticles were added to a mixture of ethanol/H2O at the required volume ratio. Typically, when the precursor (KMnO4) was dissolved in water, it generated KOH. This reaction was carried out under alkaline conditions (pH=9.3) without using other reducing agents or surfactants such as hydrazine, which is harmful to human health and the environment.45 Residual silica templates were etched out by the washing process after the reaction to yield the hollow nanostructured single metal silicates. To investigate the formation of the Mn-HNs, reactions were carried out using different reaction times of 2, 4, 6, 12 and 36 h. Figure S1a and b show that the solid particles were deposited on the surface of silica nanoparticles without small voids up until 2 h. Since the surface of silica nanoparticles were broken by hydroxide ions and silicate anion were dissolved, which react with manganese cations to produce manganese silicate particles.37-39 After 4 h, small voids began to form on the shell layers (Figure S1c). Previous works have demonstrated that the small hollow structures were produced 10


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by CO2 bubbles as soft templates, which were generated during hydrothermal reactions.46, 47 After 6 h, the larger hollow structures could be clearly determined, and a small number of small voids had been formed (Figure S1d). The BET surface area of the sample (reaction time of 6 h, 145.5 m2 g-1) was seen to be much smaller than the well-defined Mn-HNs (reaction time of 36 h) (Figure S2 and Table S1). The pore size distribution of the sample (at 6 h) showed a broad peak centered at around 103 nm and a weak peak centered on 4 nm (Figure S2b). These are in good agreement with the TEM results. However, with a longer reaction time of 36 h, well-defined Mn-HNs were obtained, which indicates that if Mn-HNs are formed with an adequate reaction time, then the surface area can be increased. Moreover, two different metal silicates could be integrated by combining two processes. In order to obtain the manganese and iron silicate hollow nanostructures (Mn/Fe-HNs), we typically used silica nanoparticles and iron metal salts as starting materials, and produced silica-iron silicates core-shell nanostructures using a hydrothermal process (see Figure S3 and S4 for TEM images of silica nanoparticles; silica-iron and silica-nickel silicates core-shell nanostructures). The cores of the silica nanoparticles are extremely important for the formation of hybrid hollow structures, as manganese silicates will be deposited on the surface of the remaining silica cores. Manganese and nickel silicates hollow nanostructures (Mn/NiHNs) and iron and nickel silicate hollow nanostructures (Fe/Ni-HNs) were also prepared using this process (see Figure S5 for detailed synthesis scheme). Nitrogen adsorption-desorption isotherms and pore size distribution curves of MnHNs were analyzed (Figure S2). The Mn-HNs samples show a BET surface area of 539.0 m2 g-1 and a total pore volume of 0.84 cm3 g-1. The isotherm exhibits a type-IV 11



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profile with a H3-hysteresis loop; in previous researches, hollow particles with a porous wall nanostructure shown H3-hysteresis loops.48 The pore size of Mn-HNs exhibited a bimodal distribution (Figure S2d), with a peak centered at around 4 nm and another broad peak centered at around 139 nm. The smaller pore distributions are attributed to the voids of the ultra-small hollow nanospheres, and the larger pore distributions are attributed to the removal of the silica templates. These are in good agreement with the TEM images of hierarchical and hollow structures. The morphologies of the as-prepared samples were analyzed by transmission electron microscopy (Figure 2). The TEM images clearly showed that hollow structures were composed of primary particles of various shapes and they were formed over a large area. The diameters of all samples were 100-200 nm, which matched that of the silica nanoparticle template. As mentioned above, the silica nanoparticle template plays a very important role in the formation of the hollow structure and acts as a silicon source; hollow structured metal silicates could not be formed without silica nanoparticles. For example, when KMnO4 was used as a precursor without the silica template, needle-like Mn3O4 nanorods were obtained (Figure S6). Mn-HNs, Fe-HNs, and Ni-HNs were composed of hollow nanospheres, solid nanospheres, and ultra-thin nanosheets, respectively (Figure 2b, d, and f). The hollow-structured samples rendered a high surface area (Mn-HNs = 539.0 m2 g-1, Fe-HNs = 90.1 m2 g-1, Ni-HNs = 269.6 m2 g1

. Figure S7 and Table S1, see Supporting Information). From the observation of BET

surface area results, small hollow nanosphere and ultra-thin nanosheets assembled hollow structures showed higher surface area than solid nanospheres assembled hollow structures. Integrated metal silicates had two different shaped primary particles in one 12


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secondary hollow structure. Specifically, Mn/Fe-HNs consisted of hollow nanospheres and solid nanospheres (Figure 2h), whereas Mn/Ni-HNs were made of hollow nanospheres and ultra-thin nanosheets (Figure 2j). Fe/Ni-HNs were composed of solid nanospheres and ultra-thin nanosheets (Figure 2l). These different species and morphologies composed of complex hierarchical hollow nanostructures also showed high surface areas (Mn/Fe-HNs = 122.6 m2 g-1, Mn/Ni-HNs = 294.2 m2 g-1, Fe/Ni-HNs = 309.8 m2 g-1. Figure S7 and Table S1, see Supporting Information). As can be seen in the X-ray diffraction patterns (Figure S8), all hollow structured metal silicates exhibited low crystallinity, and it was difficult to determine the exact composition of the hollow nanostructured metal silicates. Therefore, the elemental composition of the samples was determined by energy dispersive X-ray spectroscopy (EDX). The relative ratios of silicon to metal were between 1 and 2, and the actual value for the hollow structures could be less because small amounts of residual silica were detected by EDX. The distribution of the components was analyzed by EDX mapping (Figure 3). Oxygen, silicon, and targeted metals were homogenously dispersed in the hollow structure. X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical and oxidation states of the hollow structured metal silicates (Figure 4a and b). Using the wide scan spectra (Figure 4a), it was confirmed that all samples contained C, O, Si, and the metal species (Mn, Fe, and Ni), and no other impurity elements were present. The binding energies of the Si 2p peaks were at about 102.1-102.2 eV (Figure 4b), which suggested that all samples were silicates rather than composites of SiO2 and metal oxides because the binding energies of Si 2p in many silicates are 102-103 eV, whereas that in SiO2 is higher (~103.5 eV).49 In order to understand the local structure and 13



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oxidation state of Mn, Fe, and Ni, XANES experiments were carried out (Figure 4c-4h). The normalized Mn K-edge XANES data of Mn-HNs, Mn/Fe-HNs, and Mn/Ni-HNs had similar features, indicating that the local structures of Mn in single and hybrid hollow nanostructures were similar. All Mn K-edges were located between those of MnO and Mn3O4 (Figure 4c), which suggested that the average oxidation states were between +2 and +2.67. The Fe K-edge XANES of Fe-HNs, Mn/Fe-HNs, and Fe/NiHNs also exhibited similar features, and were slightly higher than that of Fe2O3 (Figure 4d). The Ni K-edge XANES of Ni-HNs, Mn/Ni-HNs, and Fe/Ni-HNs were positioned between NiO and NiO2, revealing that the oxidation states of Ni were between +2 and +4 (Figure 4e). The oxidation states of Mn, Fe, and Ni could be confirmed by the first derivatives (Figure 4f-4h). The produced hollow nanostructured metal silicates were coated with glucose, and were subjected to heat treatment at 400 °C for 2 h under an N2 atmosphere to produce the active materials. The overall structure is well retained after carbon coating (see Figure S9 for SEM and TEM images). Figure 5a shows the cycle performances of all samples. The capacity increased during the initial cycles in most samples (Figure 5a), which is similar to the reversible growth of a polymeric/gel-like film on the surface.50 The gradual capacity increase is also seen in other conversion-based materials.51-53 After the capacity stabilized, all samples displayed excellent cycle stability. In addition, the specific capacities were much higher than the theoretical capacity of graphite. From the 150th to the 300th cycle, the cycle retentions of Mn-HNs, Fe-HNs, Ni-HNs, Mn/FeHNs, Mn/Ni-HNs, and Fe/Ni-HNs were 95.2%, 87.9%, 93.8%, 97.7%, 121.9%, and 77.5% with average capacities of 691.0, 719.2, 673.8, 629.2, 471.4, and 519.9 mAh g-1, 14


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respectively. Figure 5b shows the charge-discharge voltage profiles of all samples during the initial 2 cycles. The initial discharge capacities of all samples were between 1000 and 1500 mAh g-1. All samples exhibited distinct plateaus during the first discharge, but the reaction voltages were different. The difference in reaction voltage can be more clearly seen in the differential capacities of the first lithiation curves. Accordingly, the reaction voltage in this system can be tuned by changing the components of nanostructures.34 The reaction voltage of the Mn-HNs was the lowest (~0.3 V), whereas that of the Fe-HNs (~0.7 V) was the highest. Those reaction voltages were similar to those of manganese and iron oxides, respectively.14,

33, 34, 51-55

Furthermore, cyclic voltammograms of hollow nanostructured metal silicates also indicate that the redox potentials of hollow nanostructured metal silicates are related to the components of nanostructures (Figure S10). The shape of cyclic voltammograms during initial 3 cycles and reduction peak positions during initial cathodic process are very similar to those of corresponding transition metal oxides. The peaks related to silicon reaction are rarely observed in the differential capacity plots and cyclic voltammograms in hollow nanostructured metal silicates. Recently, it is reported that metal silicates react with lithium through a conversion reaction forming metallic clusters, the Li2O matrix, and SiO2 during initial discharge, and no silicon alloying reaction is observed.13-15 Figure 5d shows the relative capacity of the metal silicate hollow nanostructures at 300, 500, and 1000 mA g-1. Mn/Fe-HNs and Fe/Ni-HNs exhibited the highest (62.7%) and lowest (46.1%) relative capacities at 1000 mA g-1, respectively (see Figure S11 for rate properties). In order to further analyze the structural changes that occurred during the 15



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electrochemical test, TEM images were obtained before and after lithiation (Figure 6). Many primary hollow nanoparticles assembled to form bigger secondary hollow structures before the electrochemical test. After lithiation, partial expansion and aggregation of the primary particles was observed. However, the overall morphology of the secondary particles was maintained. Notably, the wall of the secondary particles thickened, but the overall size remained almost unchanged. This is a huge advantage for electrode materials, because the change in the overall size of the active material induces contact loss between active materials and conductive agent or current collector. In summary, the overall performances of all samples were excellent. The hollow structures could accommodate volume expansion during cycling. In addition, the high surface area can reduce the lithium ion pathway. Furthermore, each hollow nanostructured metal silicate has unique physical and chemical properties such as morphology of the primary particle, composition, and surface area, which leads to changes in various electrochemical properties including specific capacity, reaction voltage, cycle performance, and rate properties.

4. CONCLUSIONS Hierarchical hollow structured metal silicates were synthesized via a hydrothermal reaction using a silica-template approach. The components, porosity, and morphology of the primary particles in the hierarchical hollow structures could be tailored, which allows for modulation of various electrochemical properties, such as reaction voltage and rate. All developed hollow structured metal silicates showed high cyclic stabilities during 300 cycles, because their hollow interior could accommodate voltage expansion 16


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during lithiation. In particular, Mn-HNs and Mn/Fe-HNs exhibited excellent electrochemical properties including specific capacity, cyclic stability, and rate. They showed almost no cycle fading with an average capacity of 678.3 and 660.4 mAh g-1 during 300 cycles, respectively. Our approach will serve as a promising route for the design of high performance electrode materials.

■ ASSOCIATED CONTENT Supporting Information Available: TEM images of silica nanoparticle, silica-iron silicates core-shell nanostructures and silica-nickel silicates core-shell nanostructures. X-ray diffraction patterns and surface area of metal silicate hollow nanostructures.

■ ACKNOWLEDGEMENTS Y. –E. S. acknowledges financial support from IBS-R006-G1. Y. P. acknowledges financial support from IBS-R006-D1.

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Figures

Figure 1. Schematic illustration of synthetic process used to produce various metal silicate hollow nanostructures.

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Figure 2. TEM images of (a, b) Mn-HNs, (c, d) Fe-HNs, (e, f) Ni-HNs, (g, h) Mn/Fe-HNs, (i, j) Mn/Ni-HNs, and (k, l) Fe/Ni-HNs.

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Figure 3. TEM images and EDX mapping of (a) Mn-HNs, (b) Fe-HNs, (c) Ni-HNs, (d) Mn/FeHNs, (e) Mn/Ni-HNs, and (f) Fe/Ni-HNs.

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Figure 4. XPS spectra: (a) wide-scan and (b) narrow-scan at Si 2p region. XANES spectra of samples at (c) Mn K-edge, (d) Fe K-edge, and (e) Ni K-edge and corresponding first derivatives (f for Mn K-edge, g for Fe Kedge, h for Ni-K edge).

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Figure 5. (a) Cycle performance of all samples at a current density of 200 mA g-1 during 300 cycles. (b) Charge-discharge voltage profile for two initial cycles. (c) Differential capacity of initial discharge curve. (d) Relative capacity compared to that at current density of 100 mA g-1.

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Figure 6. TEM images of MN-HNs (a) before and (b) after lithiation.

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TOC

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Hollow Nanostructured Metal Silicates with Tunable Properties for

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