SnO2 quantum dots modified N-doped carbon as high-performance anode for lithium ion batteries by enhanced pseudocapacitance
来源期刊:Rare Metals2021年第1期
论文作者:Cui-Ping Wu Kai-Xuan Xie Jia-Peng He Qing-Peng Wang Jian-Min Ma Shun Yang Qing-Hong Wang
文章页码:48 - 56
摘 要:SnO2 is considered to be a promising candidate as anode material for lithium ion batteries,due to its high theoretical specific capacity(1494 mAh·g-1).Nevertheless,SnO2-based anodes suffer from poor electronic conductivity and serious volume variation(300%) during lithiation/delithiation process,leading to fast capacity fading.To solve these problems,SnO2 quantum dots modified N-doped carbon spheres(SnO2 QDs@N-C) are fabricated by facile hydrolysis process of SnCl2,accompanied with the polymerization of polypyrrole(PPy),followed by a calcination method.When used as anodes for lithium ion batteries,SnO2 QDs@N-C exhibits high discharge capacity,superior rate properties as well as good cyclability.The carbon matrix completely encapsulates the SnO2 quantum dots,preventing the aggregation and volume change during cycling.Furthermore,the high N content produces abundant defects in carbon matrix.It is worth noting that SnO2 QDs@N-C shows excellent capacitive contribution properties,which may be due to the ultra-small size of SnO2 and high conductivity of the carbon matrix.
稀有金属(英文版) 2021,40(01),48-56
Cui-Ping Wu Kai-Xuan Xie Jia-Peng He Qing-Peng Wang Jian-Min Ma Shun Yang Qing-Hong Wang
School of Chemistry and Materials Science,Jiangsu Normal University
Institute of Biopharmaceutical Research,Liaocheng University
School of Physics and Electronics,Hunan University
Key Laboratory of Materials Processing and Mold (Zhengzhou University),Ministry of Education,Zhengzhou University
作者简介:Qing-Peng Wang e-mail:wangqh@jsnu.edu.cn;Jian-Min Ma e-mail:nanoelechem@hnu.edu.cn;Qing-Hong Wang e-mail:lywqp@126.com;
收稿日期:5 March 2020
基金:financially supported by the National Natural Science Foundation of China (Nos.51702138 and 21817056);the Natural Science Foundation of Jiangsu Province (Nos .BK20160213 and BK20170239);the Postgraduate Research & Practice Innovation Program of Jiangsu Province (No.KYCX202358);
Cui-Ping Wu Kai-Xuan Xie Jia-Peng He Qing-Peng Wang Jian-Min Ma Shun Yang Qing-Hong Wang
School of Chemistry and Materials Science,Jiangsu Normal University
Institute of Biopharmaceutical Research,Liaocheng University
School of Physics and Electronics,Hunan University
Key Laboratory of Materials Processing and Mold (Zhengzhou University),Ministry of Education,Zhengzhou University
Abstract:
SnO2 is considered to be a promising candidate as anode material for lithium ion batteries,due to its high theoretical specific capacity(1494 mAh·g-1).Nevertheless,SnO2-based anodes suffer from poor electronic conductivity and serious volume variation(300%) during lithiation/delithiation process,leading to fast capacity fading.To solve these problems,SnO2 quantum dots modified N-doped carbon spheres(SnO2 QDs@N-C) are fabricated by facile hydrolysis process of SnCl2,accompanied with the polymerization of polypyrrole(PPy),followed by a calcination method.When used as anodes for lithium ion batteries,SnO2 QDs@N-C exhibits high discharge capacity,superior rate properties as well as good cyclability.The carbon matrix completely encapsulates the SnO2 quantum dots,preventing the aggregation and volume change during cycling.Furthermore,the high N content produces abundant defects in carbon matrix.It is worth noting that SnO2 QDs@N-C shows excellent capacitive contribution properties,which may be due to the ultra-small size of SnO2 and high conductivity of the carbon matrix.
Keyword:
Tin dioxide; Quantum dots; Nitrogen-doped carbon; Lithium ion batteries;
Received: 5 March 2020
1 Introduction
Excessive exhaustion of fossil resource has caused serious energy crisis and environmental damage.Therefore,exploiting/refining high-performance energy storage/conversion systems is of great importance
Tailoring specific nanostructures
Combined with various carbon matrices,such as carbon nanotubes (CNTs)
Herein,SnO2 quantum dots embedded in N-doped carbon spheres (SnO2 QDs@N-C) are obtained by facile hydrolysis-high temperature calcination method using SnCl2 and pyrrole monomer as starting materials.The hydrolysis of SnCl2 and polyreaction of pyrrole monomer proceed at the same process,thus realized the uniform dispersion of Sn specie in the carbon precursor.The asobtained SnO2 QDs@N-C structure possesses multiple advantages.(1) The carbon matrix completely encapsulates the SnO2 quantum dots,preventing their aggregation and volume change during cycling.(2) The superfine nanoparticles SnO2 significantly decreases the diffusion distance for both ions and electrons,thence achieving improved rate capability
2 Experimental
2.1 Materials preparation
The SnO2 QDs@N-C composite was synthesized through a liquid-phase process and a subsequent high temperature calcination method.In a typical process,0.15 g of tin(II)chloride hydrate was dissolved in 120 ml of deionized water under ice bath to form a clear solution.Then,pyrrole monomer (1 ml) was drop wise added into the above solution under magnetic stirring.Afterward,0.6 g ammonium persulfate was added to the above solution.The hydrolysis of SnCl2 and polymerization of pyrrole were implemented for 6 h under stirring.Then,Sn-polypyrrole(PPy) precursor was collected by filtration,washing and drying at 60℃in a vacuum oven.Finally,the SnO2 QDs@N-C composite was acquired by annealing the Sn-PPy precursor in Ar atmosphere for 2 h at 600℃with the ramp rate of 2℃-min-1.
To investigate the effects of annealing temperature on the structures and morphologies of the products,parallel experiments were carried out.The temperatures were set to be 500,600 and 700℃,and the products were denoted to be SnO2 QDs@N-C-500,SnO2 QDs@N-C-600 and SnO2QDs@N-C-700,respectively.
2.2 Materials characterization
The structures of the SnO2 QDs@N-C were characterized by powder X-ray diffraction (XRD,Bruker,D8ADVANCE powder diffractometer,Cu Ka radiation,λ=0.15418 nm).The surface morphologies were studied carefully using field-emission scanning electron microscope (FESEM,Hitachi SU8010).Transmission electron microscope (TEM,FEI,Tecnai G20) and high-resolution transmission electron microscope (HRTEM,FEI,Tecnai G20) equipped with an energy dispersive spectroscopy(EDS) were used to characterize the fine structures of samples.The oxidation states were confirmed by X-ray photoelectron spectroscopy (XPS,a VG Multilab 2000(VG Inc.)) using monochromatic A1 Kαradiation under vacuum of 2×10-6 Pa.The exact contents of Sn,C,N and O in composites were evaluated with inductive coupled plasma atomic emission spectroscopy (ICP-AES,USA Themo Jarrel-Ash Corp.) and elemental analysis (EA,Perkin-Elmer 2400 SeriesⅡCHNS/O).
2.3 Electrochemical measurements
The working electrode was prepared by mixing the active materials,acetylene black and poly (vinyl difluoride),at a weight ratio of 70:20:10 in N-methyl-2-pyrrolidinone(NMP) solvent.The resulting slurry was pasted onto Cu foil and dried at 60℃vacuum.The electrochemical tests were performed using CR 2032 coin half-cells.Lithium metal was served as the counter electrode,1.0 mol·L-1LiPF6 dissolved in a mixture containing ethylene carbonate(EC),dimethyl carbonate (DMC),and propylene carbonate(PC)(4:5:1 by volume) was used as the electrolyte.The cells were assembled in an Ar-filled glove box.The cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI 604E) in a potential range of0.01—3.00 V at the scan rate of 0.1 mV·s-1 and electrochemical impedance spectroscopy (from 0.01 Hz to100 kHz for the frequency range) measurements were conducted on a CHI604E electrochemical workstation.The galvanostatic charge-discharge curves,cycling stabilities and rate performances of the as-prepared SnO2 QDs@N-C electrode were conducted on LAND CT 2001A battery testing system in a charge-discharge voltage window from3.00 V to 0.01 V (vs.Li+/Li).
3 Results and discussion
As illustrated in Scheme 1,SnO2 QDs modified N-doped carbon spheres were prepared via a facile two-step method.The first process involves the hydrolysis of Sn specie and the chemical-polymerization of pyrrolemonomer.Sn (Ⅱ)can be easily oxidized in to Sn (Ⅳ) precursor in the presence of ammonium persulfate and adsorbed on pyrrole ring due to the abundant N-H groups.Therefore,during the in-situ chemical-polymerization of pyrrolemonomer,the Sn (Ⅳ) species can be completely and uniformly encapsuled in the PPy spheres to form Sn-PPy precursor(Fig.Sl).During the subsequent annealing process,PPy was carbonized and Sn (Ⅳ) precursor was converted into SnO2 QDs.
The crystal structures and oxidization states of the SnO2QDs@N-C composites were characterized by XRD and XPS measurements.As shown in Fig.la,all the as-prepared SnO2 QDs@N-C samples display a broad peak at~25°,which can be indexed to the (002) plane of soft carbon.Upon increased temperature,the (002) peak shifts to a higher angle,indicating that the graphene interlayer space becomes smaller.Due to the ultra-small particle size and perfect encapsulation of N-C,no obvious peak of SnO2 is observed.
The graphitization degree of the SnO2 QDs@N-C composite was evaluated by Raman spectra.As shown in Fig.1b,all the samples show two characteristic peaks located at 1367 and 1562 cm-1,which are corresponding to the D-band and G-band of carbon materials,respectively.The spectra reveal that the intensity ratio of the G to D band (IG/ID) increases with calcination temperature,indicating an increased graphitization degree of the carbon matrix.
From the XPS survey spectra of SnO2 QDs@N-C-600,Sn,C,N,and O elements were clearly detected (Fig.1c-f).In the high-resolution Sn 3d spectra,two characteristic signals appear at 486.8 and 495.2 eV,corresponding to the Sn 3d5/2 and Sn 3d3/2 peaks of Sn(Ⅳ),indicating the formation of SnO2 quantum dots.The C 1s spectra present four peaks located at 284.5,285.4,287.5 and 289.2 eV,which can be indexed to the C-C,N-sp2C,N-sp3C and C=C,respectively.Two characteristic peaks of pyridinic N (~398.3 eV) and pyrrolic N (~400.5 eV) were detected in the N 1s spectra,which are highly chemically active.The elemental analysis demonstrates that the content of N is as high as 13.74 wt%in SnO2 QDs@N-C-600,which may produce abundant defects in carbon matrix,and is expected to enhance the reversible capacity and improve the fast kinetics.Combined with ICP measurements,the contents of SnO2 are determined to be 28.03 wt%,31.28wt%and 35.37 wt%in SnO2 QDs@N-C-500,SnO2QDs@N-C-600 and SnO2 QDs@N-C-700,respectively(Table S1).Moreover,it can be seen that N content is high in the carbon matrix,which may endow the carbon shell with high conductivity by creating abundant defects
Scheme 1 Schematic illustration of SnO2 QDs@N-C composite synthesis
Fig.l a XRD patterns and b Raman spectra of Sn02 QDs@N-C composites prepared at different temperatures;c XPS survey spectrum of SnO2QDs@N-C-600;high-resolution d Sn 3d,e N 1s and f C 1s spectra of SnO2 QDs@N-C-600
The morphologies of the as-prepared SnO2 QDs@N-C samples were characterized by SEM,TEM and high-resolution TEM.As shown in Fig.2a,b,the SnO2 QDs@N-C-600 is composed of interconnected nanoparticles with?200 nm in diameter.The surfaces of the nanoparticles are rough,which may be beneficial for the fully infiltrates of electrolyte.High-resolution TEM images shown in Fig.2c,d display that a mass of SnO2 quantum dots uniformly disperse in the carbon matrix without aggregation or expose outside.Figure 2e shows that the diameter of the quantum dots is~2 nm.Selected area electron diffraction(SAED) pattern of SnO2 QDs@N-C-600 (Fig.2f) shows well-defined rings,indicating that the as-prepared sample is poly crystalline.The EDS elemental mapping of SnO2QDs@N-C-600 (Fig.2g) demonstrates the incorporation of SnO2 and its uniform distribution over the N-C spheres.SEM images shown in Fig.S1 demonstrate that other SnO2QDs@N-C samples obtained at different annealing temperatures all display similar sphere-like structures.
CV of the ultrafine SnO2 QDs@N-C-600 electrode were measured at a sweep rate of 0.1 mV·s-1 in the voltage range of 0.01-3.00 V (vs.Li+/Li).As show in Fig.3a,in the initial cathodic sweep,the peak located at?0.352 V could be assigned to the formation of a solid electrolyte interface (SEI) film,which is in accordance with the previous reports for SnO2/carbon composite
Figure 3b shows the galvanostatic discharge/discharge performances of SnO2 QDs@N-C-600 electrodes at 0.1A·g-1 at the 1st,2nd,5th,10th and 100th cycle.During the first discharge process,a quasi-plateau located between 0.5and 1.0 V and a long tail extending to 0.01 V can be seen,which is consistent with the CV results.The initial discharge and charge capacities are 795.7 and 563.6 mAh·g-1for SnO2 QDs@N-C-500,1435.9 and 1020.8 mAh·g-1 for SnO2 QDs@N-C-600,and 940.5 and 638.9 mAh·g-1 for SnO2 QDs@N-C-700,showing low first cycle coulombic efficiency of 70.8%,71.1%,67.9%,respectively.The irreversible capacity loss in the first cycle is caused by the formation of SEI films on the electrode surface and the decomposition of the electrolyte
Fig.2 a SEM image,b TEM image,c-e HRTEM images,f SAED image,g TEM image and corresponding C,N,O and Sn elemental mappings of SnO2 QDs@N-C-600 composite
The SnO2 QDs@N-C composites obtained at different temperatures all deliver excellent specific capacity at 0.1A·g-1 (Fig.3c).It is worth noting that the SnO2 QDs@N-C-600 displays the highest specific capacity of 724.5mAh·g-1 after 100 cycles.SnO2 QDs@N-C-500,SnO2QDs@N-C-700 and N-C-600 deliver lower reversible capacities of 327.0,552.5 and 500.0 mAh·g-1 after 100cycles,respectively.
Rate capability is a significant indicator in assessing the high-power practical applications.Figure 3d elucidates the rate performances of the SnO2 QDs@N-C electrodes under various current densities.When the current densities increase from 100,to 200,500,1000 and 2000 mA·g-1,the specific capacities of SnO2 QDs@N-C-600 are quantified as 855.8,796.3 668.9,574.0 and 456.8 mAh·g-1,respectively.What's more,when the current density goes back to 0.1 A·g-1,the specific capacity can be recovered to763.8 mAh·g-1,illustrating the good rate capability and excellent cycling stability of SnO2 QDs@N-C-600.
To further investigate the cycling performances of the SnO2 QDs@N-C composites,the as-prepared electrodes were measured at a higher current density (Fig.3e).It can be found that SnO2 QDs@N-C-600 delivers remarkably better cycling performance than SnO2 QDs@N-C-500and SnO2 QDs@N-C-700 at a current density of 1 A·g-1.After 400 cycles,the specific capacity of SnO2 QDs@N-C-600 keeps 471.6 mAh·g-1,showing superior cycling stability.
Furthermore,the outstanding electrochemical performance of the SnO2 QDs@N-C-600 attracted attention to explore the electrochemical kinetics process.CV measurements of SnO2 QDs@N-C-600 electrode were carried out at different sweep rates from 0.2 to 1.0 mV·s-1.As shown in Fig.4a,the CV curves demonstrate similar shape at different scan rates,implying a small polarization voltage and fast kinetics.The obtained peaks current (i) and scan rate (v) obey the following equations
Fig.3 a CV curves at a scanned rate of 0.1 mV·s-1 and b discharge-charge voltage profiles measured at a current density of 0.1 A·g-1 of SnO2QDs@N-C-600 electrodes;c cycling performances of SnO2 QDs@N-C and N-C-600 electrodes at 0.1 A-g-1;d rate performances of SnO2QDs@N-C electrodes at different current densities;e long-term cycling performances and coulombic efficiencies of SnO2 QDs@N-C and N-C-600 electrodes at 1 A·g-1
where a and b are constants.According to the previous studies,when the b value equals to 0.5 or 1
Fig.4 a CV curves of SnO2 QDs@N-C-600 electrode for Li-ion storage at different scan rates from 0.2 to 1.0 mV-s-1;b calculation of b values;c capacitive contribution curve of SnO2 QDs@N-C-600 at a scan rate of 1.0 mV-s-1;d contribution ratios of capacitive and diffusion-controlled capacities at increasing sweep rates from 0.2 to 1.0 mV·s-1
With the increase of sweep rate,the capacitive contribution of total charge storage gradually rises.The ratio of the capacitive contribution at a scan rate of 1.0 mVs·-1is~71.33%(Fig.4c,d),indicating that capacitive process is dominant.The calculation results are also consistent with the results of b values.Scilicet the unique structure of the ultrafine SnO2 QDs@N-C composite facilitates the pseudocapacitive process,thus resulting in a high reversible capacity and elevated rate performance.
Finally,the SnO2@N-C-600 demonstrates the best electrochemical properties.The excellent performance may be ascribed to multiple advantages.Firstly,SnO2 quantum dots not only decrease the diffusion length for ion/electrons,but also extremely improve the utilization of active substances due to uniform dispersion.Secondly,the carbon matrix completely encapsulates the SnO2 quantum dots,relieves aggregation and volume change of SnO2 during the lithiation-delithiation process.As shown in Fig.S2,the function of the carbon matrix is obvious.After 400 cycles,the SnO2 quantum dots are still embedded carbon spheres.Moreover,the electrode still maintains the intact structure and exhibits excellent specific capacity.
Thirdly,SnO2 QDs@N-C are equipped with excellent conductivity.As shown in Fig.S3,the Nyquist plot consists of two semicircles and an approximately 45°slope.The high-frequency semicircle is related to the formation of SEI film.According to the fitted results (Table S2),SnO2QDs@N-C-600 delivers the smallest SEI layer resistance(RS),indicating its lowest contact resistance,which is of great benefit to electrode performance.The medium-frequency semicircle can be assigned to charge transfer resistance (Rct) at the interface of the electrolyte/electrode.The Rctvalues of SnO2 QDs@N-C-500,SnO2 QDs@N-C-600 and SnO2 QDs@N-C-700 are 13.33,11.12 and 8.59Ω,respectively.Clearly,SnO2 QDs@N-C all demonstrate small Rct,which indicates superior capacitive performance and enhanced electronic conductivity by carbon materials.In the low frequency region,the sloping line is attribute to the Warburg impedance (W),which is related to Li-ion diffusion in the bulk electrode.The steep slope reveals an ideal capacitive behavior,which is in good agreement with the above results.
4 Conclusion
In summary,Sn02 quantum dots modified N-doped carbon spheres are obtained by facile hydrolysis-high temperature calcination method using SnCl2 and pyrrole monomer as precursors.As anodes for lithium ion batteries,the SnO2QDs@N-C-600 shows a high discharge capacity of 724.5mAh-g-1 at 0.1 A·g-1 after 100 cycles and superior rate capability (456.8 mAh·g-1 at 2 A·g-1),as well as excellent cycling stability (471.6 mAh·g-1 at 1 A·g-1 after 400cycles).Excellent electrochemical performance is attributed to the satisfactory structure.The superfine nanoparticles SnO2 significantly decreases the diffusion distance for ion/electrons,thence rate capability is improved.Then,N-C can create defects in carbons and increases the active sites for ion storage.Furthermore,the encapsulation structure of the carbon sphere is beneficial to enhance the structural stability.The decreased particles size and good conductivity are beneficial for the improvement of capacitive contribution.This work provides an effective way to obtain electrode materials with high specific capacity and good cycling performance for energy storage.
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