Facile Preparation of Porous Mn - ACS Publications - American

Feb 11, 2016 - Department of Vascular Surgery, the First Affiliated Hospital of Sun Yat-Sen University, Guangzhou, 510080, People,s Republic of. China...
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Facile Preparation of Porous Mn2SnO4/Sn/C Composite Cubes as High Performance Anode Material for Lithium-Ion Batteries Kuang Liang,† Tuck−Yun Cheang,‡ Tao Wen,† Xiao Xie,† Xiao Zhou,† Zhi−Wei Zhao,† Cong−Cong Shen,† Nan Jiang,† and An−Wu Xu*,† †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, 230026, People’s Republic of China ‡ Department of Vascular Surgery, the First Affiliated Hospital of Sun Yat-Sen University, Guangzhou, 510080, People’s Republic of China S Supporting Information *

ABSTRACT: In recent years, Sn-based materials have been explored as potential anodes for high energy lithium-ion batteries. However, their severe volume expansion in lithiation procedure could lead to poor cycling property and inferior rate capability, which limit the applications of Sn-based materials in lithium-ion batteries. In this study, we have designed and prepared uniform Mn2SnO4/Sn/carbon composite cubic particles with a porous structure as anode material through facile hydrothermal method and subsequent annealing process. The results demonstrate that the well-crystallized threedimensional (3D) cubes with rounded corners consist of nanoparticles uniformly arranged in carbon matrix. The as-made Mn2SnO4/Sn/C anode exhibits excellent electrochemical performance and delivers 908 mA h g−1 of discharge capacity up to the 100th cycle at a current density of 500 mA g−1, with a Coulombic efficiency above 97%. The remarkable performance can be attributed to the formation of unique conductive frameworks and the porous structure.

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g−1 in the process of forming alloy LixSn (0 ≤ x ≤ 4.4) (about 2.6 times the theoretical capacity of graphite), along with severe volume expansion (as much as 300%).5−8 (2) Sn-based binary anodes mainly include SnO, SnO2, SnS2, and so forth, and much of the literature illustrated their mechanisms during the processes of lithium intercalation and deintercalation.9−11 Taking SnO2 for example, a two-step reaction is involved in the discharge process: SnO2 + 4Li+ + 4e− ↔ Sn + 2Li2O, Sn + xLi+ + xe− ↔ LixSn (0 ≤ x ≤ 4.4).12 (3) Ternary materials of Sn-basd anodes are usually expressed as the formula of MxSnOy (M = Co, Zn, Ni, Mn, Sr, etc.). Similar to tin oxides, the mechanisms in the Li+ cycling process of ternary oxides are involved with lattice amorphization of the original mixed oxide and the pulverization of the crystal structure.8,13 As a result, metal (M) or metal oxides (MOx) and Sn nanograins are generated and uniformly dispersed in amorphous Li2O, followed by forming the alloy of LixSn.14 To the best of our knowledge, Mn2SnO4 has seldom been investigated as anode material because of quick capacity fading during the cycling test, which is attributed to disruption of the structure caused by large volume change and the aggregation of inorganic nanoparticles with high surface energy.15

INTRODUCTION Rechargeable lithium-ion batteries are more and more indispensable in human life and are produced for their wide applications on portable electronic devices and the next generation of electric vehicles with superiorities of low cost, long life span, high energy density, and environmental friendliness. Graphite has long successfully served as a commercial anode material because of its low cost and ideal voltage platform (vs Li+/Li). Moreover, it is facile for carbon materials to form a stable solid electrolyte interface (SEI) film, which plays a crucial role in decreasing capacity fading and increasing the stability of reversible capacity. However, the limited theoretical capacity of 372 mA h g−1 restricts the electric application of graphite, and it is more and more difficult to meet the increasing market requirements for high-performance batteries.1,2 Therefore, developing new kinds of anode materials with large capacity and energy density will be a big breakthrough in the field of energy storage. In recent decades, Sn-based anode materials have emerged as potential substitutes for graphite on account of their much higher theoretical capacity than that of commercial graphite.3,4 Generally, the currently researched Sn-based anode materials can be mainly categorized into three series on the basis of the different number of elements: (1) Sn element is a typical unitary anode material that has attracted considerable attention because of its relative low charge potential less than 0.66 V (vs Li+/Li) and high theoretical capacity that can reach 990 mA h © XXXX American Chemical Society

Received: December 6, 2015 Revised: January 26, 2016

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DOI: 10.1021/acs.jpcc.5b11935 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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then were dried in a vacuum oven at 60 °C overnight to obtain the precursor MnSn(OH)6/C. The final sample of cubic Mn2SnO4/Sn/carbon composite was obtained by annealing the precursor MnSn(OH)6/C at 550 °C under nitrogen atmosphere for 3 h with a heating rate of 3 °C min−1 in a ceramic tube furnace system. For comparison, Mn2SnO4/C composite and Mn2SnO4 were prepared by the same methold. Mn2SnO4/ C is obtained through adjusting the ratio of Mn2+/Sn4+ to 2:1 before hydrothermal treatment, and Mn2SnO4 was synthesized under the same conditions, excluding the addition of carbon source. Characterization. The X-ray powder diffraction (XRD) patterns of the products were performed on a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å), and the operation voltage and current were maintained at 40 kV and 200 mA, respectively. The field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM6700F. High-resolution transmission electron microscopic (HRTEM) images, scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDX) elemental mapping analysis were taken on a JEOL JEMARF200F atomic resolution analytical microscope with a spherical aberration corrector. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin−Elmer RBD upgraded PHI-5000C ESCA system. The specific surface area was determined by a Brunauer−Emmett−Teller (BET) test using a Micromeritics ASAP-2020 nitrogen adsorption−desorption apparatus. Thermogravimetric analysis (TGA) was carried out on a Shimadzu DTG-60H thermal analyzer under N2 flow at a heating rate of 10 °C min−1. Raman spectrum was collected on a Raman microscope (Renishaw) with 514.5 nm as an excitation source operating on a LabRAM HR Evolution Raman microscope. Electrochemical Measurements. The working electrode was prepared by mixing active material, acetylene black, and polyvinylidene fluoride (PVDF) binder with a mass ratio of 8:1:1 to form a homogeneous slurry with addition of N-methyl2-pyrrolidione (NMP) as solvent. The slurry was finally cast onto copper foil as a current collector and then was dried in a vacuum oven at 120 °C overnight to remove residual solvent before being pressed at 8 MPa by a roll-press machine. The mass loading of active material on copper foil was about 1 mg cm−2. Electrochemical cells were assembled using CR2032 coin cell configuration in an argon-filled universal glovebox under the condition of vapor pressure of oxygen and water less than 0.1 ppm. Li metal and Celgard 2400 microporous membrane were used as the reference/counter electrode and separator, respectively. The electrolyte solution consisted of 1 M LiPF6 in a mixture of ethylene carbonate−diethyl carbonate (EC−DEC) with a volume ratio of 1:1. Galvanostatic charge−discharge cycling properties were measured at various constant currents of 100−2000 mA g−1 between 0.01 and 3.0 V on a multichannel Neware battery testing system. Cyclic voltammetry (CV) and AC electrochemical impedance spectra (EIS) were carried out on a CHI-660E (CH Instruments, Inc.) workstation. CV profiles were obtained at a scan rate of 0.1 mV s−1, and EIS were measured with 5 mV AC amplitude in a frequency scope from 0.01 Hz to 100 kHz.

The biggest problem restricting the practical application of these Sn-based anode materials lies in the poor cycling performance caused by their severe volume expansion and continuous SEI layer formation during the charging and discharging process. To overcome the problem and to promote the electrical properties of electrodes, a series of strategies have been adopted to design satisfactory electrode materials: (1) Electrode materials of nanosize particles are employed because the nanograins consisting of a small number of atoms and the abundant voids between the nanosize particles make them absorb a portion of volume changes.16−18 (2) Doping of elements such as Mn, Co, Ni, Sn, Al, Mg, C, and so forth is an important method to effectively improve the electrical conductivity of anodes and to absorb structural strain by volume changes to a certain extent.19−21 (3) Some unique structures that can provide extra free space, for instance, hollow interior, porous or hierarchical structure, and so on, help to alleviate the large volume expansion and buffer structural destruction.22−24 In addition, materials with porous or hollow structures that possess inherent large surfaces can dramatically shorten the diffusion path and improve kinetics of Li+ migration between the electrode materials and electrolyte, thus contributing to the improvement of rate capacity and cycling performance.25 To date, there have still been limited reports on Mn2SnO4 as the anode for a lithium battery. In this work, we have successfully fabricated porous Mn2SnO4/Sn/C composite cubic particles as anode material for lithium-ion batteries by calcining the precursor MnSn(OH)6/C, which is prepared through a simple hydrothermal treatment. The obtained Mn2SnO4/Sn/C cubes with a porous structure would efficiently restrict the volume expansion and extremely reduce transport distance of electrons and ions during the cycling test. Meanwhile, the formation of carbon network in a different phase of charge− discharge procedure plays an important role as a remarkable electron conductor to increase the conductivity of the anode and benefits the formation of a stable SEI film that inhibits the decomposition of the electrolyte and stabilizes the cycling performance.26 Such a tricomponent electrode material has great potential as the anode for the application in lithium-ion batteries.

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EXPERIMENTAL SECTION Chemicals. All reagents in this experiment were of analytical purity without further purification. The main reagents were manganese(II) chloride tetrahydrate (MnCl2·4H2O), tin(IV) chloride pentahydrate (SnCl4·5H2O), potassium hydroxide (KOH), and D (−)-fructose (C6 H 12 O 6 ) obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Milli-Q water (18.2 M Ω cm−1) was used throughout the experiments. Preparation of Mn2SnO4/Sn/Carbon Composite Cubic Particles. Mn2SnO4/Sn/carbon composite was prepared by a simple hydrothermal process and subsequent calcination. In a typical synthesis, 2 mmol of SnCl4·5H2O and 2 mmol of MnCl2·4H2O were dissolved in Milli-Q water (20 mL), and then 1.5 g of D(−)-fructose was dissolved into the solution with stirring. After stirring for 10 min, 3 g of KOH was slowly added into the solution under vigorous stirring to form a gel-like mixture. The mixture was then transferred into a 30 mL Teflonlined stainless steel autoclave and was kept at 180 °C for 12 h. The resulting precipitates were collected by centrifugation and were washed several times with Milli-Q water and ethanol and

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RESULTS AND DISCUSSION Mn2SnO4/Sn/carbon composite cubes were synthesized via a two-step method. As illustrated in Scheme 1, Mn2+ and Sn4+ were rapidly coprecipitated in the alkaline solution, and B

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(OH)6/C within temperatures ranging from 50 to 550 °C. A weight loss of 24.1% occurred (Figure S3), which can be ascribed to the release of water from the thermal decomposition of the precursor and the loss of oxygen from carbothermal reduction of SnO2 by carbon. In addition, the contents of carbon in Mn2SnO4/Sn/C and Mn2SnO4/C composites are determined to be 13.2 and 18.4 wt % from the results of combustion infrared detection, respectively.27,28 The morphologies of the samples were investigated by SEM measurements. As shown in Figure 2a and c, it can be observed

Scheme 1. Synthetic Route of Mn2SnO4/Sn/C Composite Cubic Particles

MnSn(OH)6/C cubes were fabricated with addition of D(−)fructose as carbon resource after hydrothermal reaction at 180 °C for 12 h. The final Mn2SnO4/Sn/C product was obtained by thermal decomposition of MnSn(OH)6/C precursor in the nitrogen atmosphere. During the annealing process, MnSn(OH)6 was thermally decomposed into Mn2SnO4 and SnO2, and the generated SnO2 grains were subsequently reduced by carbon, and metallic Sn was produced. The process could be inferred on the basis of the X-ray powder diffraction (XRD) results of the annealed products under 450 and 500 °C (Figure S1). The decomposition byproducts of H2O, CO2, and so forth were removed, and many voids were left because of the volume shrinking under high temperature. The crystal structures of the products prepared in different steps were investigated by X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). As seen from Figure 1a, the XRD pattern shows that the crystal structure of the precursor is in good agreement with that of the MnSn(OH)6 standard card (JCPDS no. 72−0007). The final product consists of cubic phase Mn2SnO4 (JCPDS no. 74−2378) and tetragonal phase Sn (JCPDS no. 89−4898), as clearly shown in Figure 1b. No additional diffraction peaks were detected. The results of XRD demonstrate that MnSn(OH)6 precursor was completely transformed into Mn2SnO4 and Sn with the assistance of carbon under annealing temperature of 550 °C. No obvious carbon peaks were found in the XRD pattern, manifesting the amorphous feature of carbon in the sample. The XRD of Mn2SnO4/C and Mn2SnO4 was also carried out at the same testing conditions, shown in Figure S2. Thermogravimetric analysis (TGA) analysis was used to determine the thermal decomposition process of MnSn-

Figure 2. Representative SEM images of (a, b) the precursor MnSn(OH)6/carbon and (c, d) Mn2SnO4/Sn/carbon cubes composite at low magnification and high magnification, respectively.

that the precursor and the calcined sample have a cubic shape with rounded corners and a uniform size (the average side length around 1 μm). The magnified SEM image in Figure 2b exhibits flat faces of the precursor cubes while a large number of nanopores appear on the surface after annealing, as shown in Figure 2d. In addition, each cube consists of nanoparticles, caused by volume shrinking in the annealing process.

Figure 1. XRD patterns of (a) the precursor of MnSn(OH)6/C and (b) the final product of Mn2SnO4/Sn/C composite. C

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expansion of anode materials in the cycling process. The representative HRTEM image (Figure 3b) obtained from the edge of Mn2SnO4/Sn/C single cube in Figure 3a further reveals the presence of different components in the composite. Obvious lattice fringes with interplanar lattice spacing of 0.257 nm are readily assigned to the (222) plane of tetragonal Mn2SnO4, and the lattice spacing of 0.279 nm is ascribed to the (101) plane of tetragonal Sn, in good agreement with the XRD results. This unique structure makes Mn2SnO4 and Sn grains that are embedded in a carbon matrix, connecting the nanoparticles with each other by carbon and enhancing the electrical conductivity of the cubic composite. EDX element mapping analysis was performed to determine the distribution of Mn, Sn, O, and C elements in the Mn2SnO4/Sn/carbon composite (Figure 3c−f). It is clearly seen that Mn, Sn, O, and C elements are uniformly distributed all over each cube, indicating the homogeneous distribution of Mn2SnO4 and Sn nanoparticles in the carbon matrix. Raman spectrum of the obtained Mn2SnO4/Sn/C composite is shown in Figure 4a; it can be clearly seen that there are two characteristic peaks at 1338 and 1595 cm−1, which correspond to the D band and G band of carbon, respectively. Typically, the D band of carbon indicates the defects and disordered portions, while the G band stands for the ordered graphitic crystallites of carbon. The high intensity of the G peak and the ID/IG value of 0.9372 demonstrate the graphitization nature of carbon in the final product. Brunauer−Emmett−Teller (BET) surface area analysis of the sample was conducted by N2 adsorption−desorption measurements. A specific BET surface area of 29 m2 g−1 and a pore volume of 0.11 cm3 g−1 (P/Po = 0.985) were generated from the porous structure (Figure 4b). The corresponding Barrett−Joyner−Halenda (BJH) pore size distribution (inset in Figure 4b) displays the pore sizes of Mn2SnO4/Sn/C composite around 15 nm. The high BET surface area and large total pore volume indicate a nanoporous

Therefore, the annealing process plays a crucial role in the formation of a porous structure. By contrast, the SEM images of Mn2SnO4/C composite and Mn2SnO4 are displayed in Figure S4. As observed, Mn2SnO4 cubes were formed in the hydrothermal process, while Mn2SnO4/C did not keep a cubic shape, probably attributed to the joint influence of the Mn2+/Sn4+ ratio and D(−)-fructose. To gain an insight into the interior structure, scanning transmission electron microscopy (STEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) element mapping analysis were carried out. Figure 3a

Figure 3. (a) STEM image of Mn2SnO4/Sn/C single cube and (b) HRTEM image. EDX elemental mapping of (c) Mn, (d) Sn, (e) O, and (f) C.

shows STEM image of the obtained Mn2SnO4/Sn/C single cube. It is found that each cube has disordered porous structure formed by numerous nanoparticle aggregation in carbon matrix, and enough spaces are provided for relieving the volume

Figure 4. (a) Raman spectrum of Mn2SnO4/Sn/C composite. (b) Nitrogen adsorption−desorption isotherms of Mn2SnO4/Sn/C composite, and the inset shows the pore size distribution plot which was calculated according to the BJH formula. High resolution XPS spectra of (c) Sn 3d and (d) Mn 2p for Mn2SnO4/Sn/C composite. D

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Figure 5. (a) CV curves of Mn2SnO4/Sn/C composite for the initial three cycles. (b) Cyclic performance of Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4 at a current density of 500 mA g−1. (c) Rate capabilities of Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4 tested at different current densities at 100, 200, 500, 1000, and 2000 mA g−1. (d) Nyquist plots for the electrodes based on Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4.

Sn/SnO2 and Mn/MnO, respectively.39 The detailed electrochemical reactions can be expressed as follows:

structure of the obtained Mn2SnO4/Sn/C cubic particles, which would make a significant contribution to the excellent cycling performance of the anode. The elemental composition and the chemical state on the surface of the Mn2SnO4/Sn/C composite were investigated by X-ray photoelectron spectroscopy (XPS) measurements. The XPS survey spectrum reveals the binding-energy values of Mn 2p, Sn 3d, O 1s, and C 1s, as seen from Figure S5. The corresponding high resolution XPS spectra of Sn 3d and Mn 2p are displayed in Figure 4c and d, respectively. The Sn 3d spectrum can be divided into four peaks: two peaks at 495.0 and 486.5 eV are attributed to Sn 3d3/2 and Sn 3d5/2 of Sn4+; the shoulder peaks at about 492.9 and 484.5 eV are ascribed to Sn 3d3/2 and Sn 3d5/2 of Sn0.29,30 These results confirm that partial Sn4+ ions are reduced to Sn0 by carbothermal reduction at high temperature. As shown in Figure 4d, there are two peaks observed at 652.7 and 640.6 eV, corresponding to the spin− orbit peaks of the Mn 2p1/2 and Mn 2p3/2 of Mn2+. In addition, two shoulder satellite peaks positioned at 658.0 and 646.0 eV are considered to be typical behaviors in Mn2+ systems.31,32 To estimate the potential lithium storage properties, the electrochemical properties of Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4 serving as anodes were measured in coin cells. Cyclic voltammetry (CV) measurement was first conducted to evaluate the electrochemical performance of Mn2SnO4/Sn/C. Figure 5a shows the representative CV profiles for the first and second cycles in the 0.01−3.0 V (vs Li+/Li) voltage window at a scan rate of 0.1 mV s−1. In the first cycle, three reduction peaks could be clearly identified, which might be ascribed to the decomposition of Mn2SnO4 into Mn and Sn (0.69 V),33,34 the formation of SEI film that leads to formatting amorphous Li2O (0.48 V),35 and the alloying reaction of Sn with Li (0.16 V).36 Three other oxidation peaks observed at 0.57, 1.28, and 1.55 V usually corresponded to dealloying of LixSn and oxidation of Sn and Mn, respectively.37−39 In the second cycle, three pairs of cathodic/anodic peaks appeared at the potentials of 0.04/0.57 V, 0.64/1.28 V, and 1.05/1.55 V and could be related to the alloying/dealloying reaction of LixSn and the redox reactions of

Mn2SnO4 + 8Li+ + 8e− → 2Mn + Sn + Li 2O

(1)

Sn + x Li+ + x e− ↔ LixSn

(2)

(0 ≤ x ≤ 4.4)

Sn + 2Li 2O ↔ SnO2 + 4Li+ + 4e−

(3)

Mn + Li 2O ↔ MnO + 2Li+ + 2e−

(4)

The discharging−charging curves of the Mn2SnO4/Sn/C electrode at a current density of 500 mA g−1 between 0.01 and 3.0 V are displayed in Figure S6. In the first cycle, mesoporous Mn2SnO4/Sn/C hybrid anode exhibited a high specific discharge capacity of 2650 mA h g−1 and a charging capacity of 1579 mA h g−1, with an initial Coulombic efficiency of 59.6%. This huge loss of initial capacity can be ascribed to the formation of an SEI layer on the surface of the electrode, which was produced by the electrolyte decomposition.40−42 The formation of the SEI could efficiently prevent the further decomposition of the electrolyte and thus improve the cycling performance of the battery. Figure 5b shows the galvanostatic charge−discharge cycling performances and rate capabilities of Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4 in a wide voltage window of 0.01−3.0 V (vs Li + /Li) at a current density of 500 mA g−1. For Mn2SnO4/Sn/C composite, a high specific capacity of approximately 908 mA h g−1 is maintained after 100 charge−discharge cycles, demonstrating that a stabilized reversible cycling performance of the as-prepared anode material results from the reciprocal eqs 2−4. The reversible capacities of the previous cycles were higher than the theoretical value (1000 mA h g−1). The extra capacity above the theoretical capacity can be attributed to the formation of a thicker SEI film or an interfacial lithium storage mechanism.43−45 During the 100 cycles, the Coulombic efficiency of Mn2SnO4/Sn/C composite revealed a value of above 90% after the first cycle and rapidly rose to around 100% after 15 cycles (Figure S7), which manifested a tremendous reversibility of the E

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MnO and Sn grains in the charging−discharging process but also buffers the volume expansion of LixSn during cycling,46 which also contributes to the excellent electrochemical performance of our materials.

cell. As comparisons of the electrochemical performance of Mn2SnO4/Sn/C composite cubes, Mn2SnO4/C composite and Mn2SnO4 were prepared. In contrast, the Mn2SnO4/C delivered a lower discharge capacity of 550 mA h g−1, and Mn2SnO4 retains that of 323 mA h g−1 with continuous capacity fading after 100 cycles at 500 mA g−1. Therefore, Mn2SnO4/Sn/C composite exhibited remarkably higher reversible capacity and cycling stability, which was largely ascribed to the mesoporous structure and the amorphous carbon network. Rate performances of Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4 were tested at a series of current densities from 100 mA g−1 to 2000 mA g−1, further demonstrating the excellent electrical properties of this kind of Mn2SnO4/Sn/C electrode material (Figure 5c). When the charge−discharge rates increased to 200, 500, 1000, and 2000 mA g −1, the Mn2SnO4/Sn/C composite presented relatively stable reversible capacities that were maintained at about 1162.5, 970.8, 775.0, and 550.0 mA h g−1, respectively. The continuous decrease of specific capacities implied the diffusion controlling kinetics process for the anode material in electrode reactions. More importantly, a specific capacity could recover to 941 mA h g−1 while the current rate was reverted to 200 mA g−1 from 2000 mA g−1. However, for the Mn2SnO4/C and Mn2SnO4, smaller discharge capacities of 386 mA h g−1 and 233 mA h g−1 at 2000 mA g−1 were delivered, respectively. These results indicate that the unique mesoporous Mn2SnO4/Sn in carbon matrix possesses an improved rate capacity performance. Electrochemical impedance spectroscopy (EIS) measurements were carried out to characterize the resistance evolution, providing additional insight into the electrochemical performance of anode materials. All the spectra (Figure 5d) exhibit typical Nyquist plots of Mn2SnO4/Sn/C, Mn2SnO4/C, and Mn2SnO4 electrodes. Mn2SnO4/Sn/C presented a smaller semicircle at high-medium frequency, indicating a smaller charge transfer resistance Rct (∼70 Ω), compared with Mn2SnO4/C (∼90 Ω) and Mn2SnO4 (∼204 Ω), which demonstrates improved kinetic transport for the good electrical contact and electrode reactions at Mn2SnO4/Sn/C anode. The sloped line at the low frequency region corresponds to ion diffusion (Warburg impedance). The bigger Rct of Mn2SnO4/C and Mn2SnO4 may be responsible for their lower capacity. The morphology of the cycled Mn2SnO4/Sn/C electrode was investigated to reveal its structure stability after 100 cycles at a current density of 500 mA g−1. As shown in SEM images of the electrode (Figure S8), most of the particles maintained cubic shape without obvious cracks or ruptures. These results further demonstrate that the porous structure can effectively accommodate the volume change of the anode during lithiation/delithiation process and that carbon frameworks can greatly enhance the structure stability. On the basis of these discussions, the high performance of the synthesized material of Mn2SnO4/Sn/carbon cubic particles can be attributed to the following factors: (1) A threedimensional (3D) carbon conductive matrix is created in the synthetic process, which greatly improves the electrical conductivity of the whole electrode, resulting in fast transport of electrons in the electrode. (2) The porous structure of Mn2SnO4/Sn/C hybrid can offer the large surface areas that dramatically shorten the diffusion path and that facilitate the migration of Li+ between the material and electrolyte. (3) The carbon network homogeneously distributed in the anode materials not only effectively prevents the aggregation of

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CONCLUSIONS In conclusion, we have successfully designed and prepared a novel mesoporous Mn2SnO4/Sn/C hybrid as anode material for lithium-ion batteries via a hydrothermal method and subsequent annealing step. Carbon tends to form a conductive network, and Mn2SnO4 and Sn are uniformly located in the carbon matrix, wherein the participation of carbon helps to limit the volume expansion and the formation of an excellent SEI film. Such a composite provides a very stable structure and rich voids that can further offer enough spaces to buffer the large volume changes upon lithium insertion/extraction process. The synthesized mesoporous composite anode shows an excellent cycling performance, and the reversible capacity remains at 908 mA h g−1 after 100 cycles at a current density of 500 mA g−1. From the 50th cycle on, no obvious degradation of capacity is observed. In addition, the anode also exhibits remarkable capacity retention even at increased current densities. Therefore, the outstanding cycling and rate performances of such novel Mn2SnO4/Sn/C hybrid cubes with mesoporous structure make it a very promising anode material for lithium-ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11935. XRD of MnSn(OH)6/C after annealing treatment at different temperatures; XRD and SEM of Mn2SnO4/C composite and Mn2SnO4; TGA of MnSn(OH)6/C; charging−discharging curves and Coulombic efficiency of Mn2SnO4/Sn/C composite anode. SEM images of cycled Mn2SnO4/Sn/C electrode (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 0551 63602346. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The special funding support from the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (51572253, 21271165, 81372821), Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048), and cooperation between NSFC and Netherlands Organisation for Scientific Research (51561135011) is acknowledged.

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REFERENCES

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.5b11935 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (43) Zhang, J.; Liang, J.; Zhu, Y.; Wei, D.; Fan, L.; Qian, Y. Synthesis of Co2SnO4 Hollow Cubes Encapsulated in Graphene as High Capacity Anode Materials for Lithium−Ion Batteries. J. Mater. Chem. A 2014, 2, 2728−2734. (44) Liu, H. C.; Yen, S. K. Characterization of Electrolytic Co3O4 Thin Films as Anodes for Lithium−Ion Batteries. J. Power Sources 2007, 166, 478−484. (45) Maier, J. Nanoionics: Ion Transport and Electrochemical Storage in Confined Systems. Nat. Mater. 2005, 4, 805−815. (46) Wang, Q.; Huang, Y.; Miao, J.; Wang, Y.; Zhao, Y. Hydrothermal Derived Li2SnO3/C Composite as Negative Electrode Materials for Lithium−Ion Batteries. Appl. Surf. Sci. 2012, 258, 6923− 6929.

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DOI: 10.1021/acs.jpcc.5b11935 J. Phys. Chem. C XXXX, XXX, XXX−XXX