Morphology Evolution of Tin-Based Oxide Hierarchical Structures

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Morphology evolution of tin-based oxide hierarchical structures synthesized by molten salt approach and their applications as anode for lithium ion battery Xueying Li, and Yongquan Qu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00670 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015

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Crystal Growth & Design

Morphology evolution of tin-based oxide hierarchical structures synthesized by molten salt approach and their applications as anode for lithium ion battery Xueying Li†, Yongquan Qu†‡* †

Center for Applied Chemical Research, Frontier Institute of Science and Technology, and State

Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an Jiaotong University, Xi’an, China, 710049 ‡

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter,

Xi’an Jiaotong University, Xi’an, China 710049

ABSTRACT: A molten salt strategy with SnO2 nanoparticles as precursor in a mixed molten salt system (NaCl+Na2CO3) was used to prepare tin-based oxide microstructures with various morphologies, including porous hollow, bimodal mesoporous and solid core-porous shell structures in a large scale. The morphology of the as-synthesized products exhibits a strong correlation with the weight ratios of NaCl/Na2CO3 in the mixed solvent. With NaCl: Na2CO3 =1:1, the liquefaction of eutectic salt and the reaction between SnO2 nanoparticles and Na2CO3 would be happened simultaneously, which allow formation of the porous hollow particles by Ostwald ripening and decomposition of Na2CO3. As the weight ratios of NaCl to Na2CO3 are increased to 2:1 and 4:1, the morphology of oxides is changed from porous hollow structure into bimodal mesoporous structure and solid core-porous shell structure, respectively.

The

morphologies of as-synthesized nanostructures are determined by the release rate of CO2 from

ACS Paragon Plus Environment

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solvent decomposition and the aggregation rate of small nano crystals at high temperature. Porous hollow oxides exhibit the best performance as anode for lithium ion battery.

The

enhanced performance can be originated from the structural features, which alleviate the volume changes and mechanical stress during charging/discharging cycling. KEYWORDS:

tin oxide; molten salts synthesis; hollow hierarchical structure; lithium-ion

battery


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1. INTRODUCTION Tin oxide (SnOx) as a n-type semiconductor oxide has been widely employed in broad fields such as solar cells,1-3 gas sensors4, 5 and lithium ion batteries.6-8 For example, tin oxide can give rise to Li storage and cycling behavior by the virtue of the alloying-dealloying reactions at V ≤ 1.0V vs. Li metal and hence can be considered as prospective anodes of lithium ion battery (LIB).9 Tin oxide delivers a high theoretical specific capacity of ~790 mA h g-1, when it is cycled between 0.005-1.0V.9 If the voltage is high to 3V, the theoretical capacity of SnO2 reaches ~ 1400 mAh g-1 exceeding commercially available graphite anode (372 mAh g-1).9 However, SnOx-based anodes suffer from an enormous volume change of 200% during the lithium alloying/dealloying process, which results in the material disintegration and eventually leads to a quick capacity fading with the cycling.10 Different strategies have been proposed to resolve the volume expansion of the SnOx as the anodes of LIBs. Many unique structures including nanotubes11, nanowires12, nanosheets13, hollow14, 15 and porous16, 17 structures show the ability to afford the huge volume swing during the charging/discharging cycles. Especially, the porous nano/micro tin oxides with hierarchical structures provide large surface areas for surface electrochemical reactions, improve electrode conduction by reducing amount of grain boundary and accommodate the volume expansion, which can significantly enhance the performance of tin oxides as the anode of LIBs.6 However, the synthesis of such hierarchical structures usually suffers from the disadvantages related to the small-scale production, low yield and tedious synthetic procedures. The molten salt synthesis, providing strong polarizing force to reduce the stabilization of metallic, ionic or covalent bonds at high temperature by utilizing a mixture of molten salts as


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highly reactive medium, is a powerful synthetic strategy for crystal design and growth.18 Molten salt method has been widely explored on the synthesis of metal oxides. The common molten salt systems involve the mixed solvents of LiCl/KCl, NaCl/KCl, AlCl3/NaCl, KCl/ZnCl2, LiF/NaF/KF, LI/KI, NaOH/KOH, LiNO3/KNO3, Li2SO4/K2SO4 and Li2CO3/K2CO3.18 Different molten salt systems have different eutectic points and solubility, which can affect the size, morphology and phase of the final products significantly.

Series of investigations on the

synthesis of metal oxides and their composites (Table S1) including Co3O4,19, 20 CuCo2O4,21 CuO ﹒ Co3O4,21 (V1/2Sb1/2Sn)O4,22 (Fe1/2Sb1/2Sn)O4,22 (In1/2Sb1/2Sn)O4,22 xZnx)Fe2O4,

25

Fe2O3,

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ZnO ﹒ Fe3O4,26 TiO2,27,

28

NiFe2O4,23 CuO,24 (Ni1-

Y2Sn2O7,29 Li(Co1-xNix)O2,30 LiCoO2,31

Li(Ni1/3Co1/3Mn1/3)O2,32 Li(Ni2/3Mn1/3)O2,33 MgCo2O4,34 MnCo2O4,34 Li(Mg1-xCox)O235 and SnO2 nanoparticles36, 37 have been explored. Fine SnO2 nanoparticles (5-10nm) with the SnCl2﹒ 4H2O as precursors were prepared by molten salt method in a mixed solvent of LiNO3+LiCl at 280 °C.36 Afterwards, they further developed the synthesis of SnO2 nanoparticle aggregates (~40-60nm) with the SnCl2﹒4H2O as precursors in LiNO3+LiOH at low temperature (180°C).37 However, seldom investigations have been performed on the morphology evolution of tin oxides in the mixed molten salts of Na2CO3 and NaCl with different ratios. Herein, we report a facile and scalable synthesis of the nano/micro hierarchical tin-based oxides with controllable morphology by molten salts method, which can simultaneously satisfy the need of a high yield and large scale production and continue to require great ingenuity. Tin oxides, having a high eutectic temperature, are less soluble than the two molten salt solvents (NaCl and Na2CO3). Compositions of the molten salts can significantly affect the ions transport in the salt mixture and


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the reaction rate at high temperature, which can be used to tailor the morphology of the products. In this work, tin-based oxides with the controllable morphologies have been prepared by the molten salt method, in which commercial SnO2 nanoparticles were chosen as the reactive phase and the salt mixture of NaCl and Na2CO3 with different weight ratios was used as the reactive medium. The porous hollow structure (NaCl : Na2CO3 = 1 : 1), bimodal mesoporous structure (NaCl : Na2CO3 = 2 : 1) and solid core-porous shell structure (NaCl : Na2CO3 = 4 : 1) were successfully achieved. Among various morphologies, the porous hollow oxides deliver the best electrochemical performance as the anode materials for LIBs. 2. EXPERIMENT 2.1.

Synthesis of tin-based oxides with controllable structures. The SnO2 nanoparticle

(Alfa-Aesar, 22-43 nm), NaCl and Na2CO3 were ground with various weight ratios (1:1:1, 1:2:1 and 1:4:1) in the mortar for the completed mixing. The mixture was transferred into the crucible and heated at 950°C for 8h at a ramping rate of 5°C min-1. After cooling down naturally, the products were thoroughly washed by copies amount of hot water to remove the solvents. For a typical synthesis, 0.2 g of SnO2 nanoparticles was used for each synthesis. The weights of NaCl and Na2CO3 were determined by their weight ratios. 2.2.

Characterization. The morphologies of oxides with different nano/micro structures

were characterized by transmission electron microscopy (TEM, HT7700, Hitachi).

X-ray

powder diffractometry (XRD) patterns were recorded using Cu-Kα radiation (Smart Lab, Rigaku).

The surface characteristics of each sample were tested by X-ray photoelectron

spectroscopy (XPS, AXIS ULTRA DLD, Kratos). The elemental analysis of each sample was derived from electron probe X-ray microanalysis (EPMA, EPMA1600, Shimadzu). The element mapping was characterized by energy dispersive X-ray spectra (EDX) performed on field


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emission scanning electron microscope (FESEM, S-4800, Hitachi). Thermal behaviors of samples were examined by thermogravimetric analysis (TGA, TGA/DSC-1, Mettler Toledo) at a heating rate of 10 °C min-1 from room temperature to 1000°C. The X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Kratos) was used to investigate the surface property of samples. The surface areas measured by nitrogen physisorption (Micromeritics, ASAP 2020 HD88) based on the Brunauer-Emmet-Teller (BET) method in the liquid nitrogen. The pore size distributions of samples were calculated based on the Barrett-Joyner-Halenda (BJH) method applied to the adsorption branch of the N2 adsorption isotherm. The samples were pre-treated at 80°C for 1h and at 200°C for 6h under vacuum. The ramping rate was 10 °C/min and the pressure was controlled at 10 mmHg. 2.3.

Electrochemical storage behavior. For electrochemical studies, the electrodes were

fabricated with the active material (nano/micro tin-based oxides), super P carbon black and polyvinylidene fluoride binder (PVDF) in the weight ratio of 70:15:15 using Nmethylpyrrolidone (NMP) as a solvent. The slurry was coated on a copper foil as current collector and dried at 120°C for 12h in a vacuum oven and then transferred to an Ar-filled glove box which maintains

Morphology Evolution of Tin-Based Oxide Hierarchical Structures

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