1、文章编号:摇 1007鄄8827(2014)04鄄0301鄄08用于锂离子电池的 Fe3O4/ C 纳米结构的可控制备邓洪贵1,摇 金双玲2,摇 詹摇 亮1,摇 金鸣林2,摇 凌立成1(1. 华东理工大学 化学工程联合国家重点实验室,上海 200237;2. 上海应用技术学院 材料科学与工程学院,上海 201418 )摘摇 要:摇 采用溶剂热反应并经在氮气中煅烧的方法制备出不同形貌的 Fe3O4/ C 纳米复合物。 无需表面活性剂或模板剂,仅通过调控反应物的浓度,合成出花状、纳米片状、空心球形结构 3 种纳米结构,并对不同形貌的形成机理进行探讨。 此外,三种不同形貌样品的电化学结果表明,花状样
2、品的电化学综合性能显著优于另外两种形貌,在5 C 的充放电电流下,其可逆比容量能达到 227mAh/ g,而空心球形、纳米片状结构样品的容量则分别为 45、10mAh/ g。关键词:摇 Fe3O4;纳米复合材料;负极材料;锂离子电池中图分类号: 摇 TM912. 9文献标识码: 摇 A收稿日期:2014鄄02鄄15;摇 修回日期:2014鄄08鄄12基金项目:国家自然科学基金(20806024, 51002051);中央高校基本科研业务费专项资金(WA1014016).通讯作者:詹摇 亮,副教授. E鄄mail: zhanliang ecust. edu. cn;凌立成,教授. E鄄mail:
3、 lchling ecust. edu. cn作者简介:邓洪贵,博士. E鄄mail: hongguideng gmail. comMorphology鄄controlled synthesis of Fe3O4/ carbonnanostructures for lithium ion batteriesDENG Hong鄄gui1,摇 JIN Shuang鄄ling2,摇 ZHAN Liang1,摇 JIN Ming鄄lin2,摇 LING Li鄄cheng1(1. State Key Laboratory of Chemical Engineering, East China Unive
4、rsity of Science and Technology, Shanghai200237, China;2. School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai201418, China)Abstract: 摇 Morphology鄄controlled Fe3O4/ carbon nanocomposites were synthesized by a solvothermal reaction followed by calcina鄄tion under a n
5、itrogen atmosphere. Flower鄄like structures, dispersed nanoflakes and hollow microspheres could be readily obtained byadjusting the concentrations of the reactants. Based on the time鄄dependent structure evolution, a possible mechanism for the forma鄄tion of the different morphologies under various con
6、ditions was discussed. The lithium storage properties of the different Fe3O4/ car鄄bon composites were compared. The flower鄄like sample shows the best electrochemical performance with the highest specific capaci鄄ty of 227mAh/ g at a current rate of 5 C while hollow microspheres and dispersed nanoflak
7、es have specific capacities of only 45 and10mAh/ g, respectively.Keywords:摇 Fe3O4; Nanocomposite; Anode material; Lithium鄄ion batteryFoundation item: National Natural Science Foundation of China (20806024, 51002051); Fundamental Research Funds for the Cen鄄tral Universities (WA1014016).Corresponding
8、author: ZHAN Liang, Associate Professor. E鄄mail: zhanliang ecust. edu. cn;LING Li鄄cheng, Professor. E鄄mail: lchling ecust. edu. cnAuthor introduction: DENG Hong鄄gui, Ph. D. E鄄mail: hongguideng gmail. comEnglish edition available online ScienceDirect ( http:蛐蛐www. sciencedirect. com蛐science蛐journal蛐1
9、8725805 ).DOI: 10. 1016/ S1872鄄5805(14)60139鄄61摇 IntroductionNano鄄sized transition metal oxides are one of thecandidates for next鄄generation anode materials for lith鄄ium鄄ion battery due to their high theoretical capaci鄄ties. This category of anode materials assumes a dis鄄tinct lithium storage mechan
10、ism based on conversionreactions: MOx+2x Li圮M+x Li2O (M = Fe, Co,Ni, Cu, Mn, etc. )1. Among the above mentionedtransition metal oxides, recently, magnetite (Fe3O4)has been intensely investigated due to its environmen鄄tal benignity, low cost and natural abundance2鄄9.However, the application of Fe3O4i
11、n practical batter鄄ies is hampered by the poor electronic conductivityand fast capacity fading resulting from severe aggrega鄄摇第 29 卷摇 第 4 期2014 年 8 月新摇 型摇 炭摇 材摇 料NEW CARBON MATERIALSVol. 29摇 No. 4Aug. 2014摇tion of active particles and large volume variation thatinherently accompanies the conversion
12、reactions2.In this context, many efforts have been devotedto synthesize Fe3O4/ carbon nanocomposites, whichinclude (i) embedding Fe3O4nanoparticles into a dis鄄ordered carbon matrix3鄄5or wrapping Fe3O4nanopar鄄ticles with graphene nanosheets6,7, and (ii) coatingcarbon onto Fe3O4particles8,9.During cyc
13、ling,however, the nanoparticles in the hybrid structures arefound to be aggregated strongly, which is unstableand detrimental to the cyclability and/ or rate perform鄄ance of Fe3O4. To further enhance the performance ofFe3O4, Fe3O4/ carbon composite with rationally de鄄signed nanostructure is necessar
14、y.Recently we have reported a two鄄step method tosynthesize a flower鄄like Fe3O4/ carbon nanocomposite,in which the carbon is in鄄situ generated during the cal鄄cination from the organic components of the ironalkoxide precursor prepared by solvothermal reac鄄tion10. Such an approach successfully avoids t
15、he needto separately prepare iron oxides nanostructures and ad鄄ditional steps of coating or hybridization with carbon.The carbon framework formed in Fe3O4/ carbon com鄄posite is demonstrated to favorably maintain the goodperformance of anode material, probably because itprevents the detachment of car
16、bon and aggregation ofFe3O4nanoparticles during cycling.In the presentwork, we extended this synthetic method to control themorphology of Fe3O4/ carbon nanostructures, and fur鄄ther investigated their electrochemical properties.2摇 Experimental2. 1摇 Materials preparationFe3O4/ carbon nanostructures we
17、re prepared by asample method. Briefly, 0. 4 g of FeCl36H2O and1. 0g of hexamethylenetetramine (HMT) were addedto 60mL of ethylene glycol under magnetic stirring togive a cloudy solution.The resulting mixture wasplaced into a 90mL Teflon鄄lined autoclave. Then theautoclave was sealed and heated at 16
18、0益 for 6h. Af鄄ter cooling, the product was harvested by centrifuga鄄tion and washed with alcohol for several times beforedrying at 60 益 in an oven overnight. Subsequently,the obtained product was heated to 450益 at a rate of5益 / min and held at this temperature for 3h under thenitrogen flow to obtain
19、the black powder. Three sam鄄ples with different morphologies can be obtained byadjusting the weight of FeCl36H2O and HMT, andthe detail synthesis conditions are listed in Table 1.Table 1摇 Experimental conditions and morphologies of the products.SamplesmFeCl36H2O/ gmHMT/ gSolvothermal temperature/ ti
20、meMorphologyI0.41.0160益 /6hFlower鄄likeII0.40.25160益 /9hDispersed nanoflakesIII0.82.0160益 /6hHollow microspheres2. 2摇 Materials characterizationThe crystal phase of products was characterizedby X鄄ray powder diffraction (XRD) on a Rigaku D/max鄄2500 diffractometer using a Cu K琢 radiation.The morphology
21、 and structure of the products wereobservedunderascanningelectronmicroscope(SEM, FEI Quanta 200F) and a transmission elec鄄tron microscope (TEM, JEOL 2100F). The specificsurface area was measured by the Brunauer鄄Emmett鄄Teller (BET) method using nitrogen adsorption iso鄄therms on a Micrometrics ASAP 20
22、20 system.2. 3摇 Electrochemical measurementThe working electrodes were prepared by disper鄄sing in N鄄methyl鄄2鄄pyrrolidone (NMP) a blend of as鄄obtained active material, acetylene black, and polyvi鄄nylidene difluoride (PVDF) at a weight percent ratioof 75 颐15 颐10. The counter electrode and the referenc
23、eelectrode is lithium foil, the separator is Celgard 2400, and a solution of 1mol/ L LiPF6in ethylene car鄄bonate ( EC) / dimethyl carbonate ( DMC) / diethylcarbonate (DEC) (1 颐1 颐1, mass ratio) is the electro鄄lyte. The discharge鄄charge tests were performed in avoltage range of 0. 01鄄3. 0 V (vs. Li/
24、Li+) at currentrates from 0. 2 to 5 C. Note that both the current den鄄sity and specific capacity were calculated on the baseof mass of the composite rather than that of Fe3O4,unless otherwise stated.3摇 Results and discussion3. 1摇 Morphology of precursorsFig. 1 shows the SEM and TEM images of thealko
25、xide precursors obtained by varying the reactantconcentrations. Fig. 1a depicts the general morphologof the sample I, revealing that it contains a relativelyuniform 3D flower鄄like architecture with a diameter ofabout 2滋m. The entire architecture is made of severaldozens of nanoflakes with smooth sur
26、faces.Thesenanoflakes are about 50nm thick and 1滋m wide, anda connection of each other through the center forms a3D flower鄄like structure. It is observed that the sam鄄ple II consists of irregular nanoflakes in nanoscale tomacroscale sizes, with widths of several hundred nan鄄203摇新摇 型摇 炭摇 材摇 料第 29 卷om
27、eters to a few micrometers (Fig. 1b). Fig. 1cshows that sample III is composed of the microsphereswith diameters of 1鄄2 滋m. The external surfaces ofthese microspheres are densely covered with intercon鄄necting nanoflakes. The hollow interior of the micro鄄sphere is revealed by TEM ( Fig.1d), where ast
28、rong contrast of the dark edges with the pale centercan be clearly observed.Fig. 1摇 (a) SEM image of sample I, (b) TEM image of sample II, (c) SEM and (d) TEM images of sample III before calcination.3. 2摇 Formation mechanismsTo gain insight into the growth mechanisms ofthe nanostructures with differ
29、ent morphologies, time鄄dependent studies of the morphology under differentsynthetic conditions were performed by SEM andTEM.As for the flower鄄like precursor, productsformed at different growth stages were collected(Fig. 2a鄄c). At the initial stage (45 min), the re鄄sultant product exhibits irregular
30、agglomerates accom鄄panied with many fine nanoparticles (Fig. 2a). After1 h of reaction, dispersed flower鄄like microsphereswith unequal sizes ranging from 1 to 2滋m are formed(Fig. 2b). Furthermore, some of small sized flower鄄like particles are hollow in their cores. After 1. 5 h,some rupture parts of
31、 small flowers can be found.With prolonged solvothermal treatment (e. g. , 6 h),the product remains the flower鄄like structure but thesize distribution becomes narrower.The images inFig. 2d鄄f show the interesting morphology evolutionof the dispersed nanoflakes with the reaction timefrom 45min to 4h.
32、As shown in Fig. 2d, at the be鄄ginning of the reaction (45min), the product is com鄄posed of non鄄uniform flower鄄like microspheres withsize in the range of 1鄄3滋m. When the reaction time isprolonged to 2 h, the product consists of uniformflower鄄like aggregates with a mean size of 3滋m,accompanied with a
33、 small amount of flower fragments(Fig. 2e). Compared with Fig. 2d, the density ofthe nanoflakes, as shown in the inset of Fig. 2e,decreases with the increase in their length. When thereaction time is up to 3h, most of the flower鄄like ag鄄gregates collapse, and some disheveled nanoflakesdispersed arou
34、nd them (Fig. 2f). After 9h, onlynanoflakes are obtained and flower鄄like nanostructuresdisappear. Fig. 2g鄄i clearly shows the evolution ofthe hollow microspheres with an increase of the reac鄄303第 4 期DENG Hong鄄gui et al: Morphology鄄controlled synthesis of Fe3O4/ carbon nanostructures 摇tion time.In Fi
35、g.2g, irregular agglomerates areformed at the early stage (45 min), similar to theinitial stage of flower鄄like sample. In the sample of 1h ( Fig. 2h), flower鄄like microspheres with solidcores are formed. After 4h, the hollow structure startsto appear (Fig. 2i). As the reaction increases further(6h),
36、 the interior cavity further expands, eventuallyforming the hollow spheres.Fig. 2摇 SEM and TEM images of precursors at different growth stages: (a鄄c) sample I synthesized for 45 min, 1h and 1.5h;(d鄄f) sample II synthesized for 45 min, 2 h and 4 h; (g鄄i) sample III synthesized for 45min, 1h and 4h, r
37、espectively.摇 摇 On the basis of the above time鄄dependent experi鄄mental evidences, the influence of concentration onthe final morphologies of the composites reveals a hy鄄brid mechanism for crystal growth. The possible for鄄mation processes of the morphologies with varied con鄄ditions are schematically
38、illustrated in Fig. 3. At thebeginning, the reaction is so fast that lots of poorlycrystalline nanoparticles form and these fresh nanopar鄄ticles self鄄aggregate into microspheres of unequal sizesdriven by the minimization of interfacial energy ofsystem. Due to the anisotropic structure of crystal,the
39、 primary nanoparticles in the respective aggregatesfurther grow into 2D nanoflakes through oriented at鄄tachment.Furthermore, withlongersolvothermaltreatment, the smaller flower鄄like aggregates may dis鄄assemble, with their core first dissolve into the solu鄄403摇新摇 型摇 炭摇 材摇 料第 29 卷tion and then re鄄crys
40、tallize on the large flower鄄likemicrospheres. The final stage for the formation of sta鄄ble flower鄄like structure could be due to the Ostwaldripening11,12. The ripening process has been com鄄monly observed in crystal growth for more than a cen鄄tury and involves “the growth of large crystals fromthose
41、of smaller ones which have a higher solubilitythan the larger ones冶13. The direct evidence of thisripening process is the relative uniform size of the fi鄄nal flower鄄like structures. When the concentration ofHMT is low, the nucleation and growth rate are de鄄creased, resulting in longer nanoflakes in
42、the micro鄄structures. Because of the strain or stress caused bythe overlapping of the flakes, the flower cores can notsupport the flakes any more. So the rupture occurs atthis time.With the further aging, the independentnanoflakes form eventually. This similar phenomenonhas also been previously obse
43、rved for the synthesis ofMnO214, SnO215, ZnO16, BiVO417, Bi2S318,etc. At a high concentration limit, after the self鄄as鄄sembly of the initial crystallites (which formed at afast rate due to the initial high supersaturation condi鄄tions) into microspheres, the crystallites on or nearthe surface of the
44、microspheres continue to growslowly into 2D nanoflakes due to the low supersatura鄄tion conditions created after the primary nucleation.With the growth of surface nanoflakes, the smallercrystallites located at central cores because of theirhigher surface energies tend to dissolve and serve asthe sour
45、ces for the growth of nanoflakes, which is atypical outward Ostwald ripening process19,20. Thecentral void space is getting bigger with continuingevacuation, and as a result the hollow microspheresare formed with a complete depletion of the cores.Fig. 3摇 Schematic illustration of the formation and m
46、orphology evolution of the nanostructures.3. 3摇 Structure and morphology of the Fe3O4/ car鄄bon nanocompositesThe crystal phase of three samples obtained afterannealing at 450 益 under nitrogen was analyzed byXRD, as shown in Fig. 4, where all the identifiedpeaks can be well assigned to Fe3O4(Magnetit
47、e, JCP鄄DS No.74鄄1910 ).The mean crystallite size ofFe3O4is calculated to be about 11, 13 and 14 nm forsamples I, II and III, respectively, using the Scher鄄rer爷s formula based on the peak of (311). The car鄄bon content of the sample I was measured to be about12. 2% by inductively coupled plasma atomic
48、 emis鄄sion spectrometer. Because the carbon is derived fromin situ carbonization of the organic components ofalkoxide precursor, the carbon contents of the samplesII and III should be the same as the sample I.As shown in Fig. 5a, c and d, the subsequentcalcination at 450益 for 3h has no noticeable ef
49、fect onthe morphologies of these samples.Fig. 4摇 XRD patterns of sample I, II, and III.摇摇Fig. 5b is a typical TEM image taken from anindividual flower鄄like structure. It can be seen thateach piece of flake of the flower鄄like structure hasbeen transformed from a dense structure with asmoothsurfaceint
50、oahighlyporousstructureconsisting of well 鄄 dispersed nanoparticles . The BET503第 4 期DENG Hong鄄gui et al: Morphology鄄controlled synthesis of Fe3O4/ carbon nanostructures 摇Fig. 5摇 SEM images of (a) sample I, (c) sample II and (d) sample III, and (b) TEM image of sample I.specific surface areas measur
