简介概要

Preparation and electrochemical properties of Sn/C composites

来源期刊：Rare Metals2019年第10期

论文作者：Bei-Ping Wang Rui Lv Dong-Sheng Lan

文章页码：996 - 1002

摘要：The metal tin（Sn）,as one potential anode material for lithium-ion batteries,rapidly degrades its cyclic performance due to huge volume expansion/contraction during lithium intercalation/de-intercalation process.Amorphous carbon was adopted as conductive and buffer matrix to form Sn/C composites.The products were prepared by hydrothermal reaction and carbothermal reduction using tin tetrachloride and glucose as raw materials.The composites were characterized by X-ray diffraction（XRD）,Raman spectroscopy,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,cyclic voltammetry（CV） and galvanostatic charge/discharge measurements.The results show that relative smaller metallic tin particles in 1:8 Sn/C composite are formed and distributed more uniformly in the carbon matrix.The lithium intercalation capacity of Sn/C composites reaches 820.4 mAh·g^-1,and the capacity retention over 60 cycles remains 54.1%.1:8 Sn/C composite exhibits enhanced rate performance and cyclic stability compared to1:5 and 1:10 samples.

稀有金属(英文版) 2019,38(10),996-1002

Preparation and electrochemical properties of Sn/C composites

Bei-Ping Wang Rui Lv Dong-Sheng Lan

School of Materials Science and Engineering,North Minzu University

School of Materials Science and Engineering,Sun Yat-Sen University

作者简介：*Bei-Ping Wang e-mail:minjer@yeah.net;

收稿日期：2 August 2018

基金：financially supported by the Ningxia Natural Science Fund (No.NZ17096);the Ningxia Science Research Project for Colleges (No.NGY2016148);the Project from Ningxia Key Laboratory of Powder Materials and Special Ceramics (No.1603);

Preparation and electrochemical properties of Sn/C composites

Bei-Ping Wang Rui Lv Dong-Sheng Lan

School of Materials Science and Engineering,North Minzu University

School of Materials Science and Engineering,Sun Yat-Sen University

Abstract：

The metal tin(Sn),as one potential anode material for lithium-ion batteries,rapidly degrades its cyclic performance due to huge volume expansion/contraction during lithium intercalation/de-intercalation process.Amorphous carbon was adopted as conductive and buffer matrix to form Sn/C composites.The products were prepared by hydrothermal reaction and carbothermal reduction using tin tetrachloride and glucose as raw materials.The composites were characterized by X-ray diffraction(XRD),Raman spectroscopy,scanning electron microscopy(SEM),transmission electron microscopy(TEM),cyclic voltammetry(CV) and galvanostatic charge/discharge measurements.The results show that relative smaller metallic tin particles in 1:8 Sn/C composite are formed and distributed more uniformly in the carbon matrix.The lithium intercalation capacity of Sn/C composites reaches 820.4 mAh·g^-1,and the capacity retention over 60 cycles remains 54.1%.1:8 Sn/C composite exhibits enhanced rate performance and cyclic stability compared to1:5 and 1:10 samples.

Keyword：

Lithium-ion battery; Tin/carbon composite; Hydrothermal method; Electrochemical performance;

Received： 2 August 2018

1 Introduction

With the development of mobile equipment and instruments,there is a growing demand for higher energy,higher power,longer life and more friendly environmental secondary batteries [ 1, 2, 3, 4] .Lithium-ion batteries are currently dominant in secondary battery applications;nevertheless,the capacity of graphite widely used as anode materials is close to its theoretical specific capacity (372 mAh·g^-1),which impedes further development for lithium-ion batteries [ 5, 6] .Therefore,the development of new anode materials has been attracting more attention.In recent years,the metal tin (Sn),due to high theoretical capacity of994 mAh·g-1,has become a rising concern.However,the volume expansion of the tin in the process of alloying/dealloying with lithium is up to 300%,which results in the deformation and crack of the electrode material,and finally its cyclic performance degrades [ 7] .

To solve this problem,the researchers mainly adopted the following strategies.(1) Reduce the particle size to nanometer level.Many types of nanostructures such as tin and tin oxide nanowires,nanorods and nanotubes were used to improve the lithium storage properties.(2) Form tin-based alloy SnM (M=Sb,Co,Cu,Fe) with other metal elements [ 8, 9, 10, 11] .Guan et al. [ 12] prepared pure tin and tinnickel alloy nanoparticles by liquid-phase reduction and found that the introduction of nickel element can not only refine the particle size,but also change the alloying/dealloying behavior of lithium and improve cyclic performance of the alloy.But the single control of particle size still cannot solve poor cyclic performance.Lee et al. [ 13] prepared a hollow Sn-SnO₂ nanosphere using Kirkendall effect and found that the cyclic performance of hollow nanospheres has been significantly improved compared with solid spherical metal tin.Other literatures [ 14, 15] also show that it is not feasible to solve the cyclic performance of metallic tin and tin alloys as anode materials only through the control of morphology and size.

In order to further improve the cyclic performance of the metal tin,carbon materials were used as the conductive and buffer matrix to form Sn/C composites with different morphologies and sizes [ 16, 17, 18] .Carbon nanotubes have high electrical conductivity,mechanical flexibility,large surface area and cavity space and can be used as good coatings of tin particles.Wang et al. [ 19] prepared carbon nanotube-coated nano-tin composites Sn@CNT and Sn@C@CNT and found that the metal tin particles were pulverized to about 10 nm in size over eighty cycles,and the specific capacity decreased obviously.At present,the combination of metallic tin and carbon nanotubes is mainly pided into three cases:tin particles in the carbon nanotubes,or tin particles outside the carbon nanotubes or both.When the metal tin is pulverized outside the carbon nanotube,it could exfoliate from the carbon matrix and cause the capacity to decay.The space in carbon nanotubes can buffer metal tin volume expansion while maintaining good electrical contact.But the tin particles in carbon nanotubes in the radial and axial directions showed uneven distribution and agglomeration,which affected the electrochemical properties of composite materials.These problems put forward higher requirements on the synthesis process of materials.

In addition to carbon nanotubes as the matrix,graphene has become a popular research in recent years.He et al. [ 20] prepared three-dimensional (3D) porous graphene/-nano-tin composites by NaCl self-assembly template method.The overall electrochemical properties of the core-shell structure Sn@G were obviously improved.Huang et al. [ 21] obtained nano-tin embedded expanded graphite composites by sintering reduction.Wang et al. [ 12] prepared the graphene-tin@CNTs composites by chemical vapor deposition (CVD) reaction,and the specific capacity at 0.1C is still 982 mAh·g-1 after 100 cycles.Although the graphene-based tin-carbon composites have been extensively studied,the bottlenecks faced by metallic tin are still not well addressed.Here,we reported metal tin particles distributed in the carbon matrix by hydrothermal and carbothermal reduction methods.Sn/C synergistic effect results in well electrochemical performance.

2 Experimental

2.1 Materials fabrication

A certain amount of SnCl_4·5H₂O and glucose was dissolved in 60 ml deionized water with magnetic stirring for30 min;then,6 mol·L^-1 NaOH solution was dropped into the above mixing solution with stirring for 1 h,while adjusting the solution pH at 11.0-11.5,followed by reacting in 100 ml reaction still at 180℃for 24 h.The resulting intermediates were washed,filtered and dried at80℃,then treated in a tubular furnace under argon atmosphere at 700℃for 15 min to obtain Sn/C composites.In this paper,Sn/C composite samples with SnCl_4·5H₂O to glucose molar ratios of 1:5,1:8 and 1:10were prepared,and the corresponding samples are noted as1:5,1:8 and 1:10,respectively.According to the above method,the pure carbon material adopting glucose as a precursor was prepared.Additionally,metallic tin powder was synthesized by the chemical reduction reaction using SnCl_4·5H₂O and NaBH₄ as raw materials.All reagents used are the analytical grade.

2.2 Characterization

X-ray diffraction (XRD) was performed by a Rigaku D-Max X-ray diffractometer using Cu Kαradiation(λ=0.15418 nm).The morphology of the samples was examined by scanning electron microscope (SEM,Zeiss Supra55) and transmission electron microscope (TEM,TECNAI F30).Raman spectroscopy was performed with an inVia Raman microscope (Renishaw,England).Spectra were recorded at room temperature using the 532-nm wavelength with a 24.3-mW argon laser.According to the mass ratio of 85:10:5,the Sn/C composites,super P and polyvinylidene fluoride (PVDF) pre-solved in N-methylpyrrolidone (NMP) were mixed and stirred into a uniform slurry,which was coated on copper foil with a drawknife and dried at 80℃under vacuum.After that,the copper foil coated with the composites was cut into the rounds (i.e.,the working electrode),together with Celgard2400 membrane and lithium disks (i.e.,the counter electrode),which were assembled into CR2025 coin cells in an argon-filled glovebox.The electrolyte contains 1 mol·L^-1LiPF₆ in ethylene carbonate/dimethyl carbonate (1:1 in volume).Electrochemical properties of the Sn/C composites were carried out with the coin cells.The galvanostatical charge/discharge was conducted in the voltage range of0.05-2.00 V (vs.Li/Li⁺) at different current densities using a battery testing system (Wuhan LAND,China) at ambient temperature.The cyclic voltammetry (CV) curves were measured at a scanning rate of 1 mV·s^-1 within the potential range of 0.05-2.20 V (vs.Li/Li⁺) using a CHI660D electrochemistry working station (Shanghai Chenhua,China).The electrochemical impedance spectroscopy (EIS) measurements were performed at excitation voltage of 5 mV in the frequency range of 100 kHz-0.01 Hz using the aforementioned working station.

3 Results and discussion

3.1 Structure and morphology

Figure 1 shows XRD patterns of the Sn/C composites,metallic tin powder and the amorphous carbon.It can be seen that the diffraction patterns of the 1:8,1:10 samples and metallic tin powder are consistent with the JCPDS No.65-0296 standard card of metallic Sn,while in 1:5 sample,a few SnO₂ impurities exist besides the metallic Sn phase.In particular,no diffraction peaks of carbon are found in the three samples,which indicate that graphitization process cannot occur in the intermediates under 700℃heat treatment.In Fig.1,the pattern of the pure carbon originated from glucose has no clear sharp peaks and is highly noisy.The very broad (002) peak is located around 21°;accordingly,the interlayer distance (d₀₀₂) is 0.422 nm by using Brag’s equation.The pure carbon exhibits high background intensity,indicating that the material contains a proportion of highly disordered materials in the form of amorphous carbon.The intensity of the amorphous swelling is far lower than the peaks of the metallic tin;hence,the carbon peaks in the Sn/C composites are submerged by that of the metallic tin.Additionally,SnO₂ phase exists in1:5 sample due to the insufficient carbothermal reduction reaction.

To further characterize the disorder degree of the carbon materials in the Sn/C composites,Raman spectroscopy was performed,as shown in Fig.2.In the first-order Raman spectra of all the samples,there are two broad and overlapping bands,a D peak around 1368 cm^-1 and a G peak around 1593 cm^-1.The integrated area ratio (I_D/I_G) varies in the range of 1.04-1.64 (the integrated areas of D and G peaks are marked asI_D andIG,respectively),based on Refs. [ 22, 23, 24] ,which indicates that the carbon materials have intermediate structures between nanocrystalline graphite and amorphous state called turbostratic structure.So,the graphitization degree of the carbon matrix in the samples stays at a low level under 700℃treatment.

Fig.1 XRD patterns of Sn/C composites,metallic Sn powder and pure carbon

Fig.2 Raman spectra of pure carbon and Sn/C composites

Figure 3a shows that the carbon particles are vermicular with the mean diameter of 400-600 nm on the radial direction,and locally aggregated together to form 3D cavities.Figure 4 presents TEM images of the 1:8 Sn/C composites.Combining with Fig.4,some metallic tin particles are spherical with the size of 1-15μm,and the others in size of dozens of nanometers distribute in the carbon matrix.The formation of micron-tin particles may be due to the excessive growth during the carbothermal reduction process.Compared to 1:5 and 1:10 samples in Fig.3b,d,1:8 sample in Fig.3c exhibits that metallic tin particles are relatively smaller and more uniformly distributed in the carbon matrix.Further,in 1:10 sample,relatively serious aggregation of the carbon makes metallic tin particles difficult to uniformly distribute in carbon matrix;thus,more tin particles are isolated from the carbon matrix.

3.2 Electrochemical properties

Figure 5 shows the first charge/discharge curve of Sn/C composites,pure carbon and metallic Sn powder.It can be seen that 1.2-V discharge plateau exists for 1:5 sample,which corresponds to the substitution reaction of lithium with heterogeneous SnO₂ in the active material:4Li⁺+4e^-+SnO₂→2Li₂O+Sn [ 25] .There is no SnO₂ in the other two samples;correspondingly,no 1.2-V discharge plateau occurs.Meanwhile,the approximate 0.6-,0.4-and0.2-V discharge plateaus exist in the 1:5 and 1:10 Sn/C composites,corresponding to the alloying process of the metallic Sn and Li:xLi⁺+xe^-+Sn→Li_xSn (0≤x≤4.4) [ 26, 27] .During the charge process,the Sn/C composites have charge plateaus at about 0.6,0.7 and0.8 V,corresponding to the dealloying reaction of the alloy Li_4.4＿xSn [ 20, 27, 28, 29] .Among the Sn/C composites,the lithiation capacity of the 1:5 sample reaches820.4 mAh·g^-1.In addition,there is no 1.8-V charge plateau during the charging process for the 1:5 sample,which is different from the literature [ 25] ,indicating that the reaction 4Li⁺+4e^-+SnO₂→2Li₂O+Sn is irreversible.The first discharge capacities of metallic Sn and pure carbon exhibit 275.9 and 462.7 mAh·g^-1,respectively,which are obviously lower than that of Sn/C composites.This indicates that the carbon matrix can facilitate the electrochemical activity of metallic Sn.

Fig.3 SEM images of a pure carbon and b 1:5,c 1:8 and d 1:10 Sn/C composites

Fig.4 TEM images of 1:8 Sn/C composite:a micron-tin particles and b nano-tin particles

The cyclic performance curves for the Sn/C composites,pure carbon and metallic tin powder were measured at charge/discharge current density of 800 mA·g^-1 except for the first four cycles of metallic tin powder tested at400 mA·g^-1.As can be seen from Fig.6,the capacity retention of 1:8 sample over 60 cycles is up to 54.1%at discharge current density of 800 mA·g^-1,and better than that of 1:5 (45.7%) and 1:10 (41.3%) samples.This may be due to more uniform distribution and relative smaller size of the tin particles in carbon matrix for 1:8 sample,which effectively relieves volume expansion and reduces the pulverization of metallic tin particles.Additionally,the cyclic performance of metallic Sn powder is markedly inferior to that of Sn/C composites,which manifests that the combination with carbon can boost the electrochemical cyclic performance of metallic Sn.In Fig.6,the coulomb efficiency of 1:8 Sn/C composite keeps up to 98%except for the first cycle at 85.6%.

Fig.5 First charge/discharge curves of Sn/C composites,metallic Sn powder and pure carbon at current density of 80 mA·g^-1

Fig.6 Cyclic performance of Sn/C composites,metallic Sn powder and pure carbon and coulombic efficiency of 1:8 sample

Fig.7 High-rate performance curves of Sn/C composites

Figure 7 shows the high-rate performance for the Sn/C composites.The 1:8 sample displays much more high-rate discharge performance compared to the other two samples,and the specific capacity remains at 255 mAh-g^-1 at the 2C current density (1C=800 mA·g^-1).After 2C charge/discharge cycles,the discharge capacity of the sample recovers back to 630 mAh·g^-1.The 1:8 sample exhibits preferably electrochemical stability at higher current densities due to more uniform distribution and better electrical contact with the carbon matrix,which suppresses the agglomeration and pulverization of tin particles and alleviates the volume change of tin particles during the cycle.Additionally,the relative smaller size of the tin particles in1:8 sample also contributes to the high-rate performance.

In order to understand the reasons for the higher-rate performance of the 1:8 Sn/C composites than that of the 1:5and 1:10 Sn/C composite,the EIS at fresh coin cells was performed.The results are presented in Fig.8.(The horizontal axis Z’represents real impedance,and the vertical axis Z’’represents imaginary impedance.) Generally,the semicircle at the high frequency region represents the solid electrolyte interface (SEI) film resistance (R_SEI) and contact resistance (R_f),and at the middle frequency region the charge transfer resistance (R_ct) on the electrode/electrolyte interface.The inclined line at lower frequency represents the Warburg impedance,which represents the lithium-ion diffusion process in the bulk electrodes [ 17, 27, 29] .The sum of R_SEI,Rf and R_ct reduces considerably from 47.84 for1:10 sample and 42.61 for 1:5 sample to 24.89Ωfor 1:8,which indicates enhanced electrical conductivity and higher Li⁺transfer rates arising from 1:8 Sn/C composite.Accordingly,1:8 Sn/C composite exhibits enhanced rate performance and cyclic stability compared to 1:5 and 1:10samples at high current rates.Additionally,the sum of R_SEI,R_f and R_ct metallic Sn up to 109.97Ωis greater than that of the Sn/C composites,which results in depressed electrochemical kinetics of metallic Sn.

Figure 9 shows the CV curves of 1:8 sample.It can be seen that the reduction peaks appear near 0.90-1.10,0.60,0.25 and 0.10 V in the first cycle,of which 0.90-1.10 V corresponds to the formation of SEI film,0.60,0.25 V peaks correspond to Li-Sn alloying reaction,and the reduction peak near 0.1 V corresponds to the formation of the Li_xC₆ compound [ 20, 30] .The reduction peak near0.9 V in the second cycle disappears,indicating that the more stable SEI film is formed at this time.In the anodic process,the oxidation peaks near 0.75 and 1.00 V correspond to the dealloying reaction of Li_4.4-xSn alloy [ 20] .

Fig.8 EIS spectra of Sn/C composites and metallic Sn powder

Fig.9 Cyclic voltammetry curves of 1:8 Sn/C composite

To further investigate the electrochemical properties of the Sn/C composites,the 1:8 Sn/C composite together with LiNi_0.5Co_0.2Mn_0.3O₂ powders (a commercial product presented by Ningxia Kejie Co.) was assembled into coin-type full cells.Therein,the loading mass of anode electrode is1.3 mg,including the active material,conductive carbon black and polyvinylidene difluoride,and the calculated capacity of the LiNi_0.5Co_0.2Mn_0.3O₂ powders is larger than that of 1:8 sample.The other details about full cell assembling are similar to the depicted above.Figure 10shows the charge and discharge properties of the full cell.The first charge and discharge capacities reach 716.5 and529.9 mAh·g^-1,respectively.Over 100 electrochemical cycles,they markedly decline to 174.2 and173.3 mAh·g^-1,also the discharge plateau performance becomes more declining.The results indicate that the performances of the Sn/C composites need to be further promoted.

Fig.10 Charge and discharge curves of 1:8 Sn/C composites assembled in a full cell (cathode:LiNi_0.5Co_0.2Mn_0.3O₂,anode:1:8Sn/C composite and current density:1C=600 mA·g^-1.)

4 Conclusion

The Sn/C composites were synthesized via hydrothermal reaction and carbothermal reduction,adopting tin tetrachloride and glucose as main starting materials.The results show that relative smaller metallic tin particles in 1:8 Sn/C composite are formed and distributed more uniformly in the carbon matrix.The graphitization degree of the carbon matrix in the samples stays at a low level under 700℃treatment.The maximum lithiation capacity of Sn/C composites reaches 820.4 mAh·g^-1,and the capacity retention rate of the 1:8 sample over 60 cycles is 54.1%.1:8 Sn/C composite exhibits enhanced rate performance and cyclic stability compared to 1:5 and 1:10 samples.Finally,the rational proportion and uniform dispersion of tin in the carbon matrix are beneficial for the promotion of the high-rate performance and electrochemical stability.The combination with carbon can markedly boost the comprehensive electrochemical performance of metallic tin.