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Granadilla-inspired structure design for conversion/alloyreaction electrode with integrated lithium storage behaviors Chaoji Chen, Linfeng Peng, Yiju Li, Lei Zhang, Jingwei Xiang, Pei Hu, Shijie Cheng, Yunhui Huang, and Jia Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 22, 2017

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ACS Applied Materials & Interfaces

Granadilla-inspired structure design for conversion/alloy-reaction electrode with integrated lithium storage behaviors Chaoji Chen,1 Linfeng Peng,1 Yiju Li,3 Lei Zhang,4 Jingwei Xiang,2 Pei Hu,1 Shijie Cheng,1 Yunhui Huang2,* and Jia Xie1,* 1

State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of

Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 2

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 3

Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education,

College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China 4

Institute for Superconducting and Electronic Materials, University of Wollongong,

Wollongong, NSW, Australia *

Correspondence to [email protected] and [email protected]

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Abstract Conversion/alloy-reaction electrode materials promise much higher energy density than the commonly used ones based on intercalation chemistries. However, the low electronic conductivity, and specially the large volume expansion upon lithiation hinder their practical applications. Here for the first time, a unique granadilla-inspired structure was designed to prepare the conversion/alloy-reaction anode of carbon coated tin/calcium tin oxide (C@void@Sn/CaSnO3) ternary composite. The granadilla-inspired structure ensures the intimate contact between the Sn/CaSnO3 nanoparticles and the carbon matrix, providing not only conductive networks for electron transport and short distance for Li+ diffusion but also effective space for the electrode volume-expansion towards conversion/alloy reaction. Moreover, the unique structure possesses abundant solid-solid interfaces between the three components as well as solid-liquid interfaces between nanoparticles and electrolyte, contributing to a large percent (58%) of interfacial charge (thus capacity). The integration of alloy-reaction, conversion-reaction and interfacial lithium storage endows the hybrid electrode a high capacity and long cycling life, holding great promise for next-generation high-capacity lithium-ion batteries. Keywords: lithium-ion batteries, anodes, templating method, granadilla-inspired structure, integrated lithium storage.

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Introduction The state-of-the-art lithium-ion batteries (LIBs) based on intercalation chemistries have approached their up limits of energy density due to the limited Li ion (typically less than one) they can store.1,2 Recently lithium storage mechanisms beyond intercalation chemistries such as alloy-reaction, conversion-reaction and interfacial charge storage have gained increasing attention due to their great potential in surpassing the energy density limits of intercalation chemistries by storing more than one Li ion.3-9 Nevertheless, each of these chemistries faces serious

challenges

for

practical

availability.

For

instance,

the

alloy-reaction

and

conversion-reaction chemistries generally cause a huge volume expansion that would lead to the crash of the electrode materials and severe capacity decay. In addition, the low intrinsic conductivity of the electrode materials is also an obstacle in realizing their theoretical energy densities. So far many efforts have been dedicated to addressing these two challenges. Nanoscale structure design including nanotube10, nanofiber11, nanowire12, hollow sphere13 etc. is the most commonly used strategy to reduce the transport distances of Li+ ions and electrons and to relieve the strains of volume expansion. However, nanostructure could also bring some problems such as severe side reactions, low volumetric energy density and weak adhesion between the electrode materials and Al/Cu foils, leading to unsatisfied cyclability.14-16 Moreover, the reaction kinetics should not be high enough since the conductivity of the nanosized material is relatively low. Doping17,18, carbon or metal oxide coating19,20 and hybridizing with conductive matrix21 are effective strategies to improve the conductivity, thus promoting the kinetics of the alloy- and 3



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conversion-reactions. Despite the relatively successful development of the alloy- and conversion-reaction anode materials including Si3, Sn5,9, Al4, SnO26,8,11,21, Fe2O37 et al., the interfacial lithium storage mechanism need further exploration and deeper understanding, especially in the alloy-reaction and/or conversion-reaction anode materials.9 It is reported that solid-solid and solid-liquid interfaces in the batteries can provide active sites for lithium or sodium storage thus contributing to an additional interfacial lithium/sodium storage.22-28 In this context, structure design involving nanoscale interface engineering is preferred for additional interfacial lithium storage. Here for the first time we demonstrate a unique granadilla-inspired structure design of spherical double-shelled carbon coated Sn/CaSnO3 nanoparticles (C@void@Sn/CaSnO3) with void inside by a “green” and facile CaCO3-templated process along with acetylene chemical vapor deposition (CVD) carbon coating and template removing. The unique structure ensures the effective alloy reaction of Sn, conversion reaction of CaSnO3 and interfacial lithium storage in the abundant interfaces between the three components and/or electrolyte, contributing to an integrated lithium storage behavior. Meanwhile, the conductive and connected carbon networks are beneficial for the fast transport of electrons whereas the void between the coated carbon layer and the Sn/CaSnO3 nanoparticles can release the stress generated from the volume expansion

of

the

electrode

materials.

The

above

structural

merits

C@void@Sn/CaSnO3 hybrid electorde an excellent electrochemical performance.

4


endow

the

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Materials and methods Materials synthesis: SnO2 nanoparticles were synthesized via hydrothermal route according to the literature.29 CaCO3@SnO2 microspheres were attained via a co-precipitation process. Firstly, 0.4 g of SnO2 (~50 nm grain size) was added to the Na2CO3 aqueous solution under vigorous stirring for 10 min to form a well-dispersed suspension. Meanwhile, another CaCl2 aqueous solution was prepared with the same volume as the former solution. Then, the latter solution was slowly added into the former solution under stirring to get the CaCO3@SnO2 suspension, which was then collected by filtration with DI water. For carbon coating by CVD treatment, the obtained CaCO3@SnO2 was placed in a tube furnace with Ar protection, and heat treated at 600 °C. Then acetylene gas was introduced into the furnace to replace the original Ar gas at 600 °C, and held for half an hour. Subsequently, acetylene gas was replaced by Ar again during the naturally cooling process, and the sample of C@CaCO3@SnO2 was collected. To obtain the final C@void@Sn/CaSnO3 hybrid, the C@CaCO3@SnO2 powders were added into a diluted HCl aqueous solution for half an hour under stirring to remove the CaCO3 sacrificial layer, followed by filtration and DI washing, and drying at 80 °C for 24 h. Characterizations: The morphology and structure of the samples were investigated by SEM (FEI SIRION-200), TEM (Tecnai G2 F30, FEI Holland), XRD (PANalytical B.V., Holland with Cu Ka radiation of λ = 0.15406 nm), XPS (VG MultiLab 2000 system with a monochromatic A1 Kα X-ray source, Thermo VG Scientific), and thermogravimetry (TG) analysis from 50−800 °C in air atmophere. Raman spectra were measured on a Renishaw Invia spectrometer with an Ar+ laser of 514.5 nm at room temperature. The specific Brunauer−Emmett−Teller (BET) 5



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surface area and pore size distribution were analyzed from the nitrogen adsorption/desorption isotherms collected on a Micromeritics ASPA 2020 instrument. Electrochemical Measurements: Electrochemical lithium storage performance of the electorde materials were studied in 2032 coin type cells. Active material, super P and polyvinylidene fluoride in a mass ratio of 8:1:1 was mixed to make a slurry, which then was coated on a Cu foil to get the working electrode with an active material areal mass loading of 1.0 to 1.5 mg per square cm. Lithium metal was used as counter electrode, Celgard 2300 as separator, and LiPF6 (1 mol L–1) in a mixed solvent of ethylene carbonate and dimethyl carbonate (1:1 by volume) as electrolyte. Galvanostatic charge/discharge measurements were performed on a battery tester (Land, China) at various current rates within a voltage window of 0.01−3 V. Cyclic voltammetry (CV) was tested on a PARSTAT 2273 potentiostat electrochemical station.

Results and discussion Figure 1 graphically illustrates the design concept and fabrication process of the granadilla-structured

C@void@Sn/CaSnO3

hybrid.

Briefly,

the

as-synthesized

SnO2

nanoparticles were added into the Na2CO3 aqueous solution under violent stirring, then the CaCl2 aqueous solution was slowly poured into the former solution. The individual SnO2 nanoparticle then was encapsulated by the CaCO3 framework in 5 min. Surprisingly, the CaCO3 encapsulated SnO2 nanoparticles tend to assemble into microsphere, giving rise to a granadilla-like CaCO3@SnO2 structure. After CVD treatment using acetylene as the carbon source, uniformed carbon layers were formed not only on the surface of each CaCO3 6


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encapsulated SnO2 nanoparticle but the surface of the microsphere due to the excellent penetration of acetylene. The CaCO3 can be easily etched by diluted HCl, resulting in the double core-shelled structure of C@void@Sn/CaSnO3, with Sn/CaSnO3 as the core and carbon layers on nanoparticle and microsphere as the double shells. Note that in our approach, CaCO3 as template with electrode materials embedded inside can be easily processed. The CVD carbon coating using acetylene as carbon source and template removing using dilute HCl as etching agent are also facile routes without involving any toxic chemicals. The above advantages demonstrate that our developed synthesis method is facile and “green” as compared to the commonly reported SiO2-template method that generally involves multiple steps including template synthesis, electrode material coating on the template, carbon coating and template removing by HF.30 It should be also noted that SnO2 could be reduced into Sn accompanied with the formation of CaSnO3 (SnO2 + CaCO3 = CaSnO3 + CO2), which can be validated by XRD patterns wherein all the diffraction peaks can be asigned to Sn (no. 65-0296) and CaSnO3 (no. 31-0312) (Figure S1a). The XPS can further validate the existence of Sn and CaSnO3 in the resultant hybrid. Peaks at 495.5 for Sn 3d3/2 and 487.5 eV for Sn 3d5/2 can be assigned to the Sn-O bond in CaSnO3 whereas another peak at 486 eV belongs to metallic Sn0 (Figure S1b). In the Ca 2p high-resolution XPS spectrum, a couple of peaks at 347.5 and 351 eV indicate the existence of Ca2+ in the form of Ca-O bond (Figure S1c). The BET surface area and pore-size distribution of the C@void@Sn/CaSnO3 composite were determined by nitrogen adsorption/desorption isotherms. Figure S1d shows that the adsorption/desorption isotherms have a typical Type-IV 7



feature with a clear loop between 0.5−1.0 P/P0, suggesting a mesoporous structure of the composite. The Barrett-Joyner-Halenda (BJH) pore-size distribution shows a sharp peak centered at 4−5 nm, confirming that numerous mesopores exist in the C@void@Sn/CaSnO3 hybrid. The carbn content in the C@void@Sn/CaSnO3 hybrid was determined to be around 34 wt.% by TG measurement (Figure S2). Raman spectrum demonstrates obvious D and G peaks, indicating the amorphous nature of the coated carbon layer (Figure S3).31,32

Figure 1. Schematic illustration of the synthesis process of the C@void@Sn/CaSnO3 and C@void products. SP stands for microsphere. The morphology and microstructure of the C@void@Sn/CaSnO3 hybrid and its precursors 8


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are determined by electron microscopies. SEM images show that after encapsulating with CaCO3, the CaCO3@SnO2 composite has a spherical morphology with a diameter of a few micrometers (Figure S4a-b). The magnified SEM image shows that a number of nanoscale particles in size of 20~60 nm decorate on the surface of the microsphere (Figure S4c), revealing that the microsphere is constructed by plenty CaCO3 coated SnO2 nanoparticles. After CVD treatment, the spherical granadilla-like morphology retains well (Figure S4d-f), demonstrating the good stability of the granadilla-like structure. Surprisingly, removing the CaCO3 template from the hybrid by HCl etching doesn’t cause the collapse of the granadilla-like structure (Figure 2a-b) but creates voids in the hybrid (Figure 2c-e), giving rise to the double core-shelled granadilla-like structure of the C@void@Sn/CaSnO3 hybrid. Figure 2a demonstrates the homogeneous microspheres with diameters of 3−5 micrometers. The surface of each microsphere is much smoother than the un-etched precursor due to the coated layer of CVD carbon on the surface (Figure 2b). TEM image in Figure 2c reveals that the hybrid microsphere is highly porous. Double core-shelled structure can be clearly observed by a magnified TEM image (Figure 2d), showing that each nanoparticle has a carbon shell and void between the nanoparticle and shell while all the carbon-coated nanoparticles are coated with an outer carbon shell. Figure 2e shows that void of a few nanometer exists between the carbon shell and the nanoparticle core, in good consistence with the pore-size distribution result. Lattice spacing with distances of 0.29 and 0.39 nm can be clear observed, assigned to the (200) face of Sn and (020) face of CaCO3 respectively (Figure 2f). Selected area electron diffraction (SAED) pattern also confirms the existence of both Sn and CaCO3 (Figure 2i), agreeing well with the 9



former results. Scanning transmission electron microscopy (STEM) image in Figure 2g further validates the porous granadilla-like structure whereas element mapping images in Figure 2h demonstrate the existence and even distributions of C, O, Sn and Ca elements in the composite.

Figure 2. (a,b) SEM images, (c,d) TEM images, (e,f) HR-TEM images, (g) STEM image, (h) element mapping images and (i) SAED pattern of the C@void@Sn/CaSnO3 composite. The lithium storage performance of the C@void@Sn/CaSnO3 composite is evaluated in Li-half cell. Figure 3a shows the rate performance of C@void@Sn/CaSnO3 cycling at different current densities of 0.2 to 30 A g−1. High reversible capacities of ~880, 845, 755, 660, 525, 400, 300 at 0.2, 0.5, 1, 2, 5, 10 and 20 A g−1, respectively, can be obtained. More impressively, a high 10


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capacity of 245 mAh g−1 can be maitained even at an extremely high current rate of 30 A g−1, indicative of an excellent rate capability of the C@void@Sn/CaSnO3 composite electrode. As far as we know, this value represents one of the best rate capability among all CaSnO3 anodes ever reported, and among the most excellent Sn-based anodes as well (Table S1). The discharge-charge profiles at various current densities show a sloped feature (Figure 3b), which should be ascribed to the ultrafine Sn/CaSnO3 nanoparticles with most materials located in the surface and near-surface regions and the abundant interfaces between the Sn/CaSnO3 nanoparticles and the coated carbon layer. Both could benefit the interfacial lithium storage that is characteristics of a capacitive feature. The C@void@Sn/CaSnO3 composite electrode demonstrates an excellent cycling performance as well, with a slowly increasing capacity from ~350 to ~700 mAh g−1 after 500 cycles and stable capacity for the following 500 cycles (Figure 3c). As comparison, the controlled samples of CaSnO3 (termed as CSO) without carbon coating and C@void without Sn/CaSnO3 show much inferior rate performances. As presented in Figure S5, the C@void electrode shows a capacity of ~600 mAh g−1 at 0.05 A g−1 but the capacity quickly drops to below 200 mAh g−1 when the current density increases to 10 A g−1. For the CSO electrode, the rate performance is even worse; the capacity is ~200 mAh g−1 at 0.05 A g−1 but decreases down to nearly 10 mAh g−1 at 1 A g−1. Figure 3d compares the rate-performance of the three samples, further confirming the superior rate performance of the C@void@Sn/CaSnO3 composite electrode. The enhanced rate capability of the C@void@Sn/CaSnO3 composite electrode is mainly due to the unique double-shelled carbon coated Sn/CaSnO3 nanostructure. On one hand, 11



the double-shelled coated carbon can not only provide a continuous conductive network for the fast transportation of electrons, but also improve the structure stability of the electrode by releasing the stress from the volume expansion of the electrode upon lithiation. On the other hand, the nanosized Sn particles have a high theoretical capacity (994 mAh g−1), which can also enable much shorter distance for the transportation of ions and electrons and abundant interfaces for interfacial lithium storage, contributing significantly to the enhanced rate capability of the C@void@Sn/CaSnO3 composite electrode. Note that the capacity of the C@void@Sn/CaSnO3 keeps increasing in the initial 500 cycles. This is due to the reconstruction of the electrode material during the repeated charging/discharging processes, similar to some conversion-reaction or alloy-reaction electrodes reported previously.33 The reconstruction results in two phenomena: one is the decrease in particle size from 20~60 nm to 3~5 nm due to the repeated conversion and alloy/dealloy reactions, as evidenced by the TEM observation of the electrode before (Figure 2) and after 1000 cycles (Figure S6); the other is the increase in (interfacial) capacitive capacity contribution due to the reduction of particle size, as confirmed by the evolution of potential profiles from different cycles (Figure S7). Both phenomena contribute to the continuous increase of capacity, which finally reaches a stable state when the reconstruction of the electrode completes. In our case, the capacity becomes relatively stable (~700 mAh g−1) after 500 cycles.

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Figure 3. Lithium storage performance of the C@void@Sn/CaSnO3 electrode: (a) rate performance, (b) representative charge-discharge curves, (c) cycling performance at 1 A g−1 and (d) capacity-current density curves compared with the C@void and CSO electrodes. In order to get further insight of the superior electrochemical performance and lithium storage behavior of the C@void@Sn/CaSnO3 composite, we carry out CV measurements to investigate the electrochemical kinetics of the electrodes. Figure 4a shows the CV curves of the C@void@Sn/CaSnO3 electrode at 0.1 mV s−1 for the initial 4 cycles. In the lithiation process, a peak at ~1.5 V is assigned to the conversion reaction between Li and CaSnO3, which disappears in the sequential cycles.34,35 The peak generated at ~0.6−0.8 V belongs to the alloy reaction between lithium and tin. The other two peaks below 0.5 V should be assigned to the solid electrolyte interface (SEI) formation, which disappear in the following cycling process.34 The delithiation shows three peaks between 0.5−0.75 V and one peak at ~1.2 V, corresponding to the dealloying process of Sn4.4Li.36 Figure 4b shows the CV curves collected from different scan 13



rates of 0.2 to 20 mV s−1. It can be observed that the oxidation/reduction peaks become broader when the scan rates increase whereas the shapes are similar. Another feature is that the oxidation/reduction peak separations get bigger as the scan rates increase, indicating a higher overpotential required for the lithiation/delithiation proceeding within a shorter time. Based on the equation of i = avb that reflects the relationship between current and scan rate,37-40 the b-value can be quantified by the slope of the log(i)-log(v) curves. Particularly, a b-value of “0.5” indicates that the electrochemical process is dominated by solid-state diffusion, whereas “1” signifies a totally capacitive-controlled process. In our case, a b-value of “0.82” can be obtained both for the anodic and cathodic processes, which indicates a combination of diffusion- and capacitive-controlled charges (thus capacities) (Figure 4c). By using a more detailed method37, the capacitive charge contribution at a fixed scan rate can be quantified. Accordingly, the capacitive charge contribution is quantified to be 58% at 0.5 mV s−1 (Figure 4d), further confirming that integrated lithium storage behaviors of both diffusion and capacitive types occurring in the C@void@Sn/CaSnO3 electrode, consistent with the above analysis. Based on the above kinetic analysis, it can be concluded that the dominated charge contribution comes from the interface-based lithium storage (58% at scan rate of 0.5 mV s−1). This is mainly due to the unique electrode structure with small particles coated by carbon that enables abundant interfaces for interfacial lithium storage.

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Figure 4. Kinetic analysis of the C@void@Sn/CaSnO3 electrode: (a) the initial 4 cycles CV curves at 0.1 mV s−1, (b) CV curves at scan rates of 0.2−20 mV s−1, (c) log (peak current)-log (scan rate) curves (d) capacitive charge contribution at 0.5 mV s−1. The TEM measurements are further performed to investigate the phase and microstructure transformation of the C@void@Sn/CaSnO3 electrode towards repeated lithiation/delithiation processes. As shown in Figure 5a, nanoparticles of several nanometers in size decorated in the amorphous carbon matrix can be clearly observed, suggesting that the primary Sn/CaSnO3 particles with a diameter of ~50 nm break into smaller (3~5 nm) nanoparticles due to the volume expansion of the electrode material caused by repeated lithiation/delithiation. Fortunately, the existence of the outer carbon shell confined the smaller nanoparticles tightly. 15



Clear lattices with a spacing of 0.29 nm belonging to the (200) face of Sn metal can be observed in the high-resolution TEM image in Figure 5b, confirming the existence of Sn after numerous cycles of charging/discharging.

Figure 5. (a) TEM image, (b) HR-TEM image of the C@void@Sn/CaSnO3 hybrid electrode after 1000 cycles. (c) Graphic illustration of the integrated conversion-, alloy- and interfacial-based lithium storage behaviors occurring in the C@void@Sn/CaSnO3 hybrid electrode. Based on the above analyses, the lithium storage behaviors and phase/microstructure evolutions of the C@void@Sn/CaSnO3 composite can be graphically illustrated in Figure 5c. During the initial discharging process, conversion reaction between Li+ and CaSnO3 takes place, 16


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resulting in the formation of nanosized Sn, CaO and Li2O (CaSnO3 + 4Li+ + 4e− = Sn + CaO + 2Li2O)27. Due to the protection by the outer carbon shell, the newly formed Sn, CaO and Li2O nanoparticles will be confined within the carbon shell rather than departing from the current collector. In the further discharging process, alloying reaction between Sn and Li+ occurs, forming Li4.4Sn alloy in the end of the discharging process (Sn + Li+ = Li4.4Sn), accompanying with the volume expansion of the electrode. In this case, the existence of void between the carbon shell and the electrode particles provides enough space for the volume expansion, stabilizing the hybrid structure of the electrode. In the subsequent charging-discharging processes, reversible dealloying/alloying reactions between Sn and Li+ will take place, along with the reversible volume shrinking/expanding cycles. It is worth noting that in addition to the conversion- and alloying-type lithium storage mechanisms, the interfacial lithium storage could occur due to the existence of abundant Sn/CaSnO3/C interfaces and the high proportion of active materials on the surface and near-surface regions. The integration of conversion-, alloy- and interfacial-based lithium storage contributes to an excellent electrochemical performance with high capacity, excellent rate capability and feasible cyclability.

Conclusion In brief, we have successfully developed a “green” and facile route to prepare a granadilla-structured double-shelled carbon coated Sn/CaSnO3 hybrid microsphere. The hybrid as a negtive electrode for lithium-ion batteries presents a high capacity of ~900 mAh g−1 at 200 mA g−1, a superior rate capability with ~245 mAh g−1 at an extremely high current rate of 30 A 17



g−1 and a super-long cycle life of 1000 cycles without obvious capacity decay. Further kinetic and TEM analyses reveal that an integrated convertion-, alloy- and interfacial-based lithium storage takes place simultaneously in the hybrid due to the unique granadilla-inspired structure which combines the conversion-reaction electrode of CaSnO3, the alloy-reaction electrode of Sn and the double-shell coating carbon. As a consequence, abundant interfaces for interfacial lithium storage, conductive networks for electron transport, and stable matrix for strain releasing can be realized. The present “green”, facile and scalable strategy for synthesis granadilla-structured anode materials can be extended to fabricate a variety of other electrode materials for applications in high-capacity lithium-ion batteries and other batteries as well.

ASSOCIATED CONTENT Supporting Information. XRD patterns, XPS spectra, nitrogen adsorption/desorption isotherms, TG curve, SEM images, TEM images and charge/discharge profiles are included. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Emails: [email protected] (Jia Xie) and [email protected] (Yunhui Huang). Notes The authors declare no competing financial interest.

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Acknowledgements The authors acknowledge the support from the National Basic Research Program of China (973 Program, 2015CB258400), the PCSIRT (Program for Changjiang Scholars and Innovative Research Team in University, IRT14R18), the Project Funded by China Postdoctoral Science Foundation (2016M590690, 2015M580642), and the National Natural Science Foundation of China (Nos. 51361130151 and 51632001). We would like to thank the Analytical and Testing Center of HUST for XRD, SEM, TEM, TG, and Raman measurements.

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