Al掺杂和石墨烯改性的V2O5正极材料

Synergetic Effects of Al 3+Doping and GrapheneModification on the Electrochemical Performance of V 2O 5Cathode MaterialsKai Zhu,[a]Hailong Qiu,[a]Yongquan Zhang,[a]Dong Zhang,[a]Gang Chen,[a,b]and Yingjin Wei*[a]IntroductionThe continuous progress in portable,electrical vehicles and stationary energy storage requires advanced lithium ion batter-ies with a low cost,high energy density,and long cycle life.However,traditional cathode materials such as layered LiCoO 2,spinel LiMn 2O 4,and olivine LiFePO 4only deliver small specific capacities of approximately 100–160mAh g À1.To increase the energy storage of lithium ion batteries,it is desired to develop new cathode materials with larger specific capacities and/or a higher output voltage.A major strategy to achieve large spe-cific capacities is to use cathode materials that can accommo-date more than one lithium ion in the formula unit such as S (S 8to Li 2S x )[1]and V 2O 5(V 2O 5to Li 2V 2O 5).[2]Numerous studies on these materials have increased the specific capacities of cath-ode materials to over 250mAh g À1.The electrochemical properties of V 2O 5were first reported by Whittingham in 1976.[3]The material has a specific capacity of 294mAh g À1based on two lithium insertions per formula unit in the voltage widow of 2.0–4.0V versus Li/Li +.However,the intrinsic low lithium diffusion coefficient (~10À12cm 2s À1)and the moderate electronic conductivity (10À2–10À3S cm À1)of V 2O 5hinder the rate capability and capacity retention of thismaterial.[4]To improve the electronic conductivity of V 2O 5,Yu et al.prepared Cu 0.02V 1.98O 5.The electronic conductivity in-creased by approximately 20times with respect to the pristine material to result in a high reversible capacity of 97mAh g À1at the 20C rate.[5]Another effective approach to enhance the electrochemical performance of the V 2O 5cathode is the prepa-ration of nanostructured materials that can offer a range of unique advantages over their bulk counterparts,which include a shorter lithium diffusion distance and larger contact area be-tween the electrode and electrolyte.[6]In addition,the electro-chemical performance of nanostructured V 2O 5can be further improved by hybridization with carbonaceous materials such as porous carbon [7]or carbon nanotubes.[8]Among carbona-ceous materials,graphene which has a high conductivity,large surface area,and excellent structural stability,has attracted particular attention not only in battery science but also in many other applied research fields such as photocatalysis,elec-trochemical sensors,and solar cells.[9]It has been reported that the graphene in battery electrodes provides a 3D electron transport network in the active materials and it acts as an elas-tic buffer to accommodate the volume change of the materials during repeated lithium insertion and removal.[10]Consequent-ly,the modification of nanostructured V 2O 5[11]and many other electrode materials such as Sn,[12]TiO 2,[13]SnO 2,[14]and MoO 2[15]with graphene has attracted a broad interest in the battery community.Even though nanostructured V 2O 5cathode materials have shown a significantly improved electrochemical performance with respect to their bulk counterparts,the capacity retention of these materials is still a big obstacle for their practical appli-cation.For example,the discharge capacity of V 2O 5hollowmi-[a]K.Zhu,H.Qiu,Y.Zhang,Dr.D.Zhang,Prof.Dr.G.Chen,Prof.Dr.Y.WeiKey Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education)College of Physics,Jilin University Changchun 130012(PR China)(Y.J.Wei)E-mail:yjwei@ [b]Prof.Dr.G.ChenState Key Laboratory of Superhard Materials Jilin UniversityChangchun 130012(PR China)Full PapersDOI:10.1002/cssc.201500027crospheres decreased from 290to 187mAh g À1after 100cycles at a current density of 300mAh g À1,which corresponds to a ca-pacity fading rate of 0.35%per cycle.[16]In another example,the capacity fading rate of V 2O 5nanosheet/reduced graphene oxide (RGO)was approximately 0.3%per cycle at the 2C rate.[17]Similar results can also be seen in numerous related publications.[6b,18]One of the key factors that affects the capaci-ty retention of electrode materials is their structural stability during prolonged charge–discharge cycling.As the [VO 5]layers of V 2O 5are linked only by weak van der Waals forces,the com-pound undergoes a sequence of phase transitions easily,that is,a -L x V 2O 5(x <0.01)!e -L x V 2O 5(0.35<x <0.7)!d -L x V 2O 5(0.7<x <1.0)!g -L x V 2O 5(1.0<x <2.0)during lithium intercalation.[19]The involved lattice distortion and volume expansion will ac-celerate the structural deformation of V 2O 5and thus limit the electrochemical performance of the material.If some metal ions with positive charges (M n +)are doped into the VO 5layers,the interaction between the dopant ions (M n +)and the nega-tive oxygen ions (O 2À)will strengthen the combination of [VO 5]layers,which could increase the structural stability of the V 2O 5material.[20]In the present work,we prepared an Al 3+-doped V 2O 5/RGO nanocomposite by a simple soft chemical method.The synergetic effects of Al 3+doping and graphene sandwich-ing are established,which improve the electrochemical per-formance of V 2O 5significantly.Results and DiscussionThe formation mechanism of the V 2O 5/RGO nanocomposite is illustrated in Scheme 1.In the first step,commercial V 2O 5pow-ders are converted into soluble VOC 2O 4under the reductive ef-fects of oxalic acid according to Equation (1).V 2O 5þ3H 2C 2O 2!VOC 2O 4þ3H 2O þ2CO 2ð1ÞThe color of the solution becomes blue,which indicates that the V 5+ions are reduced to V 4+.If graphene oxide (GO)isadded to the solution,VOC 2O 4will adhere to the GO sheets as a result of electrostatic interactions between the negative charges on GO and the positive charges on VOC 2O 4.During the freeze-drying process,small VOC 2O 4nucleates will form around the functional groups on the GO sheets such as hy-droxyl,carboxyl,and carbonyl groups.These VOC 2O 4nucleates will decompose during calcination and then revert into V 2O 5nanoparticles to form a highly porous RGO-supported V 2O 5nanocomposite.Optical images show that the V 2O 5/RGO nano-composite is sponge-like.SEM images show that numerous V 2O 5nanoparticles are em-bedded in the RGO sheets to form a “sandwich”hybrid struc-ture (Figure 1a),which is believed to prevent the stacking of the RGO sheets.The presence of RGO is favorable for electron transport in the material matrix,which gives rise to a high elec-tronic conductivity.An SEM image of the Al 3+-doped V 2O 5/RGO sample is shown in Figure 1b.The material is also com-posed of small particles that are anchored on the RGO sheets.However,this morphology is somewhat different from that ofV 2O 5/RGO,which indicates that the Al ions may influence the nu-cleation and crystal growth of the material.In addition,we present SEM images of V 2O 5(Fig-ure 1c)and Al 3+-doped V 2O 5(Figure 1d).The materials are composed of ordinary nanoparti-cles with a particle size of 20–70nm.The crystal structure of the materials was determined by XRD (Figure 2a).All of the sam-ples can be indexed based on or-thorhombic V 2O 5with the space group Pmmn (PDF #85-0601).No impurity phases such as Al 2O 3or AlV 2O 4are detected in theXRDScheme 1.Formation mechanism of the V 2O 5/RGOnanocomposite.Figure 1.SEM images of (a)V 2O 5/RGO,(b)Al 0.16V 2O 5/RGO,(c)V 2O 5,and (d)Al 0.14V 2O 5patterns,which indicates that the Al ions are successfully doped in the V 2O 5lattice.Inductively coupled plasma atomic emission spectroscopy analysis shows that the Al/V molar ratio is 0.07:1for Al 3+-doped V 2O 5and 0.08:1for Al 3+-doped V 2O 5/RGO.Hereafter,the chemical formulas of the samples are rep-resented as Al 0.14V 2O 5and Al 0.16V 2O 5/RGO,respectively.In addi-tion,the amount of carbon in the RGO-containing samples was measured to be 2.42wt %for V 2O 5/RGO and 2.37wt %for Al 0.16V 2O 5/RGO.The small hump between 2q =20and 258for V 2O 5/RGO and Al 0.16V 2O 5/RGO is caused by disordered (002)stacking layers of RGO.Enlarged views of some typical XRD peaks are shown in Figure 2b.The XRD peaks of V 2O 5shift to lower angles upon Al 3+doping,which indicates the expansion of the lattice.The lattice parameters of the materials were cal-culated by using the Celref program.The lattice parameters of V 2O 5/RGO are similar to those of pure V 2O 5(Table 1).However,Al 0.14V 2O 5and Al 0.16V 2O 5/RGO show larger lattice parameters that result in a volume expansion of ~1.6%with respect to un-doped V 2O 5.Substitution doping of V 5+by Al 3+can be exclud-ed because the ionic radii of Al 3+(0.50 )is smaller than that of V 5+(0.59 ),which would not cause a lattice expansion of V 2O 5.If we consider that the VO 5layers of V 2O 5are linked by weak van der Waals forces,it is believed that the Al ions reside in the interlayer space of the [VO 5]layers,such as those ofCu 2+-and Zn 2+-doped V 2O 5.[21]These guest cations cause the lattice expansion of V 2O 5.Raman scattering was applied to study the effects of Al 3+doping on the local structure properties of V 2O 5.The V atom in V 2O 5is coordinated by two vanadyl (O 1),three chain (O 2),and one bridging (O 3)oxygen atom (Figure 3a).However,the V atom shows distinct displacements away from the center of the [VO 6]octahedron.As a result,the basic structural unit of V 2O 5is viewed as a [VO 5]square pyramid rather than a regular octahedron.The V 2O 5lattice is built up from these [VO 5]pyra-mids by sharing edges and corners.[22]The Raman patterns of the samples are shown in Figure 3b.For undoped V 2O 5,the Raman bands at n ˜=995,701,and 528cm À1are caused by the stretching vibrations of the vanadyl V ÀO 1,bridging V ÀO 3,and chain V ÀO 2bonds,respectively.The bands at n ˜=404and284cm À1correspond to the bending vibrations of the V ÀO 1bond.The bands at n ˜=482and 303cm À1are assigned to the bending vibrations of the V ÀO 3and V ÀO 2bonds,respectively.There is a Raman band at n ˜=144cm À1that corresponds to the external [VO 5]À[VO 5]vibration.This vibration mode is observed at a low wavenumber because of the weak van der Waals forces between the [VO 5]layers.[23]The Raman pattern of V 2O 5/RGO is quite similar to that of V 2O 5,which indicates that RGO does not influence the local structure of V 2O 5.If Al ions are doped in V 2O 5and V 2O 5/RGO,the V ÀO 1and V ÀO 3stretching vi-brations as well as the external [VO 5]À[VO 5]vibration shift to higher wavenumbers with respect to their undoped counter-parts (Figure 3c).This indicates that Al 3+doping strengthens not only the V ÀO bonds of the [VO 5]unit but also the linkage of the [VO 5]layers.Thus,the structural stability of V 2O 5and V 2O 5/RGO is improved by the Al ions.In addition,V 2O 5/RGO and Al 0.16V 2O 5/RGO show two addi-tional peaks at n ˜=1360and 1601cm À1,which are attributed to the D and G bands of carbonaceous materials,respective-ly.[24]The intensity ratio of the D and G bands (I D /I G )is 0.74,which is lower than that of GO (I D /I G =0.86)used in the synthe-sis process.This indicates that the degree of graphite crystallin-ity is increased as a result of the removal of oxygen-containing groups such as C ÀO,C =O,and COOH after reduction and calci-nation.TEM images of the Al 0.16V 2O 5/RGO nanocomposite are shown in Figure 4a and b.The sheet-like morphology of RGO can be seen clearly from the TEM images.There are numerous nano-sized Al 0.16V 2O 5particles that anchor on the RGO sheets.In the high-resolution transmission electron microscopy (HRTEM)image (Figure 4c),the lattice fringes with an interplanar dis-tance of 0.63nm correspond to the (200)planes of orthorhom-bic V 2O 5.The selected area electron diffraction pattern(SAED)Figure 2.(a)XRD patterns of the materials and (b)enlarged views of the (020),(001),and (110)diffractionpeaks.of Al 0.16V 2O 5/RGO is shown in Figure 4d.The (121),(003),and (242)planes of orthorhombic V 2O 5can be seen clearly.In addi-tion,the SAED pattern is composed of a series of diffraction rings,which indicates the polycrystalline nature of the material.The X-ray energy dispersive spectroscopy analysis of Al 0.16V 2O 5/RGO is shown in Figure 5.The even distribution of V,O,and Al confirms that homogeneous Al doping in V 2O 5is successful.In addition,C is dispersed uniformly in the whole sample,which indicates that the Al 0.16V 2O 5particles are loaded on the RGO sheets.X-ray photoelectron spectroscopy (XPS)patterns of the sam-ples are shown in Figure 6.The Al 2p peak of Al 0.14V 2O 5and Al 0.16V 2O 5/RGO is observed at a binding energy (BE)of 74.2eV (Figure 6e),which is consistent with that of Al 3+ions.[25]The V 2p 3/2spectra of all samples (Figure 6a–d)are dominatedbyFigure 4.a,b)TEM images of the Al 0.16V 2O 5/RGO nanocomposite.c)HRTEM and d)SAED pattern of the Al 0.16V 2O 5/RGOnanocomposite.Figure 5.EDS analysis of the Al 0.16V 2O 5/RGOnanocomposite.Figure 3.a)Basic structural unit of V 2O 5.b)Raman scattering patterns of the samples and graphene oxide.c)Enlarged views of the Raman scattering pat-terns.a main peak at BE =517.6eV,which is attributed to V 5+ions.[26]However,a small shoulder peak is observed at BE =516.3eV,which indicates the incomplete oxidation of the V 4+ions inthe VOC 2O 4precursor.[26]According to Smyrl et al.the electron-ic conductivity of V 2O 5is attributed to the small-polarontheory.The residence of V 4+ions in V 2O 5will produce more small polarons and thus increase the electronic conductivity.[4a]By fitting the XPS curves,we can see that Al 0.14V 2O 5andAl 0.16V 2O 5/RGO possess more V 4+ions than V 2O 5and V 2O 5/RGO.This implies that Al 3+doping may improve the electronic con-ductivity of V 2O 5.However,the measurement of electronic con-ductivity was unsuccessful because of the difficulty to preparedense pellets of these nanopowders.The C 1s XPS spectrum of V 2O 5/RGO and Al 0.16V 2O 5/RGO can be fitted by four components that can be assigned to the C =C/C ÀC bonds of graphene (BE =284.7eV),and different carbon-based functional groups,that is,hy-droxyl or epoxide (BE =286.4eV),carbonyl (BE =287.1eV),and carboxyl (BE =288.6eV;Figure 6f–g).[24]Compared with the C 1s XPS spectrum of GO (Figure h),the relative intensities of these functional groups decrease markedly,which suggests that GO is reduced successfully.Galvanostatic charge–discharge measurements were preformed in the voltage window of 2.0–4.0V.The representative voltage profiles of the samples at a current density of 300mA g À1(which corresponds to the 1C rate)are shown in Figure 7a.The initial dis-charge capacities of V 2O 5,Al 0.14V 2O 5,V 2O 5/RGO,and Al 0.16V 2O 5/RGO are 213,216,268,and 274mAh g À1,re-spectively.Clearly,the incorporation of RGO improves the Li capacities of V 2O 5and Al 0.14V 2O 5significantly.RGO is also known as a Li ion storage material.To identify the capacity contribution from RGO,the charge–discharge performance of RGO was measured (Figure 7b).An initial discharge capacity of 7mAh g À1is obtained in the voltage window of 2.0–4.0V.The discharge capacity remains at 16mAh g À1after 50cycles.Thus,the increased discharge capacity of V 2O 5/RGO and Al 0.16V 2O 5/RGO is not caused by the ca-pacity contribution from RGO but from its high elec-tronic conductivity,which improves the Li ion storage in the material.The discharge capacity of V 2O 5after 50cycles is 150mAh g À1,which corresponds to a ca-pacity retention of 70%.The capacity retention is im-proved to 75%(Al 0.14V 2O 5,162mAh g À1)and 79%(V 2O 5/RGO,213mAh g À1)by Al 3+doping or incorpo-ration with RGO (Figure 7c).Importantly,the capacity retention increases significantly to 90%for the mate-rial that contains both Al 3+dopants and RGO (Al 0.16V 2O 5/RGO).This material shows a high discharge capacity of 247mAh g À1after 50cycles.Rate-depen-dent cycling experiments show that Al 0.16V 2O 5/RGO exhibits the best rate performance of all the samples (Figure 7d)and it displays the highest discharge ca-pacity at each current rate.A high discharge capacity of 122mAh g À1is obtained at the 10C -pared with results reported in the literature,the as-prepared Al 0.16V 2O 5/RGO sample shows a better rate capability than cation-doped V 2O 5[4a,5,27]and better capacity retention than V 2O 5/RGO nanocomposites.[11a,b,17]This indicates that the syner-getic effects of Al 3+doping and graphene modification could improve both the rate capability and cycle stability of the V 2O 5nanomaterials.We have also studied the electrochemical performance of the Al x V 2O 5/RGO materials with different Al contents (x =0,0.16,0.32).The cycling performance (at the 1C rate)and rate capability of the materials are shown in Figure 8.The Al 0.32V 2O 5/RGO sample with the largest Al content showsthe Figure 6.a–d)V 2p XPS of V 2O 5,Al 0.16V 2O 5,V 2O 5/RGO,and Al 0.16V 2O 5;e)Al 2p XPS ofAl 0.14V 2O 5and Al 0.16V 2O 5/RGO;f–h)C 1s XPS of V 2O 5/RGO,Al 0.16V 2O 5/RGO,and GO.best capacity retention but it also shows the smallest dis-charge capacities.Even though the Al 3+ions could increase the structural stability of V 2O 5to improve the capacity reten-tion,too many Al ions in the interlayer space will block the in-sertion of Li ions.Therefore,an appropriate Al 3+content at around x =0.16is required to obtain the optimum electro-chemical performance.The excellent electrochemical performance of Al 0.16V 2O 5/RGO indicates that the combination of Al 3+doping and RGO modi-fication has significant advantages for the electrochemical per-formance of V 2O 5.To identify these advantages,we performed cyclic voltammetry (CV)and electrochemical impedance spec-troscopy (EIS)studies.The CVs of the V 2O 5,Al 0.14V 2O 5,V 2O 5/RGO,and Al 0.16V 2O 5/RGO samples at different voltage scan rates are shown in Figure 9.All samples show three pairs of redox peaks that are ascribed to a series of phase transforma-tions of Li x V 2O 5[19]as shown in Equations (2)–(4).$3:3=3:5V :a -V 2O 5þ0:5Li þþ0:5e À!e -Li 0:5V 2O 5ð2Þ$3:1=3:3V :e -Li 0:5V 2O 5þ0:5Li þþ0:5e À!d -Li 1:0V 2O 5ð3Þ$2:2=2:6V :d -Li 1:0V 2O 5þ1:0Li þþ1:0e À!g -Li 2:0V 2O 5ð4ÞThe voltage gap between the redox peaks (the so-called electrode polarization)is a measure of electrochemical kinetics of a specific electrode reaction.The electrode polarizations of the samples at a voltage scan rate of 80mV s À1are summar-ized in Table 2.The a /e redox couple shows the smallest elec-trode polarization,and the d /g couple shows the largest elec-trode polarization.The continu-ous increase of electrode polari-zation indicates that the electro-chemical barrier becomes in-creasingly higher with Li +inter-calation into the material.In addition,the electrode polariza-tions of Al 0.14V 2O 5are smaller than those of V 2O 5,which indi-cates that the electrochemical ki-netics of V 2O 5is improved by Al 3+doping.However,the elec-trochemical kinetics improved more significantly if RGO is in-corporated in the material.More importantly,the material that contains both Al 3+dopants and RGO (Al 0.16V 2O 5/RGO)exhibits the smallest electrode polarization,which indicates that this material has the best electrochemical ki-netics of all the samples.CV has been used to deter-mine the Li +diffusion coeffi-cients (D Li )of electrode materials according to Equation (5).[28]I p ¼2:69Â105n 3=2AD 1=2v 1=2C ð5Þin which n is the number of electrons per specific reaction,which is 1for Li +;A is the surface area of the electrode,which is 0.64cm 2in this work;C is the concentration of Li ions in the material,for example,C is 4.6 10À3mol cm À3at the peak-a /e in which the chemical composition of the material is Li 0.5V 2O 5;I p is the current intensity,and v is the scan rate.At rel-atively low scan rates,Li ions accumulate in the active material thus I p varies linearly with v 1/2(Figure 10).The lithium diffusion coefficients of the samples during the reduction process are listed in Table 2.The lithium diffusion coefficient of V 2O 5de-creases from peak-a /e to peak-e /d and then to peak-d /g .This indicates that the diffusion of Li +in Li x V 2O 5becomes increas-ingly difficult,which may be because of the puckering of the [VO 5]layers that accompanies the structuraltransformation.Figure 7.a)Charge–discharge profiles of the samples,b)cycling performance of RGO,c)cycling performance of the samples at a current density of 300mA g À1,and d)rate-dependent cycling performance of thesamples.However,Al 0.14V 2O 5shows higher lithium diffusion coefficients than those of undoped V 2O 5.Raman scattering has shown that the Al 3+ions not only strengthen the V ÀO bonds of the [VO 5]unit but also strengthen the linkage of the [VO 5]layers.There-fore,the puckering of [VO 5]layers may be inhibited to some extent by these Al 3+dopants,which thus facilitates the diffu-sion of lithium ions.In addition,the lithium diffusion coeffi-cients increase with the incorporation of RGO.It is known that the RGO sheets can act as an electronic conductive network in the electrode.The higher electronic conductivity of V 2O 5/RGO and Al 0.16V 2O 5/RGO will produce a stronger internal electricalfield.This will facilitate the trans-port of electrons and Li +ions and thus give rise to a higher lithium diffusion coefficient.[29]It is not surprising that Al 0.16V 2O 5/RGO shows the largest lithium diffusion coefficients of all of the samples because of the syner-getic effects of Al 3+doping and RGO modification.The Nyquist plots of the sam-ples at 2.0V in the first dis-charge are shown in Figure 11.All samples display two well-de-fined regions:a depressed semi-circle in the high-frequency region and a sloping line in the low-frequency region.These re-gions correspond to the charge-transfer reactions at the elec-trode–electrolyte interface and the Warburg impedance associ-ated with Li +diffusion in the materials,respectively.Here we use a widely accepted equiva-lent circuit to simulate the impe-dance spectra,in which R s is the internal resistance of the battery cell,R ct is the charge-transfer re-sistance at the electrode–electro-lyte interface,CPE is the double layer capacitance on the elec-trode surface,and W is the War-burg impedance.The charge-transfer resistance of Al 0.16V 2O 5/RGO is 59W ,which is the small-est of all the samples.Corre-spondingly,the charge-transfer resistances of V 2O 5,Al 0.14V 2O 5,and V 2O 5/RGO are 399,218,and 184W ,respectively.This further demonstrates that the material that contains both Al 3+ions and RGO exhibits the best electro-chemical kinetics in all samples.ConclusionsWe prepared a series of V 2O 5-based nanomaterials by a simple soft chemical method.Galvanostatic charge–discharge cycling shows that the Al 3+-doped V 2O 5/reduced graphene oxide (RGO)nanocomposite (Al 0.16V 2O 5/RGO)shows the highest dis-charge capacity,best rate capability,and excellent capacity re-tention in all samples.The superior electrochemical per-formance of Al 0.16V 2O 5/RGO can be attributed to the synergetic effects of Al 3+doping and RGO modification.First,the Al 3+ions reside in the interlayer space of the [VO 5]layers.TheseFigure 8.a)Cycling performance and b)rate capability of the Al x V 2O 5/RGO materials with different Al 3+contents (x =0,0.16,0.32).Figure 9.CV curves of the samples at different voltage scan rates.dopant ions strengthen both the V ÀO bonds of the [VO 5]unit and the linkage of the [VO 5]layers,which enhances the struc-tural stability of V 2O 5.Second,the RGO sheets construct an ef-fective network for electron transport,which increases the elec-tronic conductivity of the material.If the V 2O 5material is modi-fied by Al 3+ions and RGO,the above synergetic effects will im-prove the structural stability and electrochemical kinetics of the material,which leads to excellent electrochemical performance.Experimental SectionMaterial preparationGraphene oxide was synthesized from natural graphite by a modi-fied Hummers method.Typically,graphite powder (2.0g)wasadded to H 2SO 4(98wt %,8mL)mixed with K 2S 2O 8(1.67g)and P 2O 5(1.67g).The mixture was kept at 808C for 5h.Then the mixture was cooled to RT and diluted with deionized (DI)water (0.5L).The material was col-lected by filtration,washed with DI water several times,and dried under ambient condition.This preoxidized graphite was treated by H 2SO 4in iced water.Next,KMnO 4(15.0g)was added gradually with constant stir-ring at 358C for 2h.After dilution with water (0.7L),H 2O 2(30%,20mL)was added to the mixture,which was then washed with HCl and DI water.The graphene oxides were finally obtained after centrifugation,wash-ing with copious DI water,and drying.The powders were dispersed in DI water to form a 1g L À1solution by probe sonication.To prepare the Al 3+-doped V 2O 5/RGO nanocomposite,commercial V 2O 5powders (!99.0%,Sigma–Aldrich)and oxalic acid were dissolved in distilled water with con-stant stirring to form a pellucid blue solution.Al(NO 3)3·9H 2O (AR,Beijing Chemical Works,0.02g)and graphene oxides (44mL)were added dropwise,and the mixture was stirred for 4h.The suspension was then transferred into a freeze-drier to obtain a sponge-like precursor.The precursor was then treated at 3508C for 5h to obtain the final product.For comparison,three samples,V 2O 5nanoparticles,V 2O 5/RGO nanocomposite,and Al 3+-doped V 2O 5nanoparticles were prepared by modifying the above process.Material characterizationThe crystal structure of the materials was studied by XRD by using a Bruker AXS D8diffractometer with CuK a radiation.The morpholo-gy of the material was characterized by using a JSM-6700F scan-ning electron microscope.TEM and high-resolution transmission electron microscopy images were recorded by using an FEI Tacnai G2electron microscope equipped with an X-ray energy dispersive spectrometer (EDS,BRUKER AXS).Raman scattering was performed by using a RenishawinVia plus laser Raman spectrometer with a wavelength of 514nm.XPS was performed by using a VG scien-tific ESCALAB 250spectrometer using monochromic AlK a excita-tion.The relative amounts of metal elements (Al and V)of the sam-ples were analyzed by inductively coupled plasma atomic emission spectroscopy (PE,ICP-AES/1000).The amount of carbon in the samples was measured by using a Vario El III elemental analyzer.The electrochemical experiments were performed by using 2032-type coin cells with lithium foil as the anode electrode.To prepare the cathode electrode,a slurry mixture that contained 75wt %of active material,15wt %of super P conductive additive,and 10wt %of polyvinyl difluoride (PVDF)binder was pasted on an Al foil and dried in a vacuum oven at 1208C.Then the electrode was cut into a size of 8 8mm 2.The electrode loading of active materi-al was ~2.5mg cm À2.The electrolyte was a 1m LiPF 6solution dis-solved in ethylene carbonate (EC),dimethyl carbonate (DMC),and ethylmethyl carbonate (EMC)with EC/DMC/EMC =1:1:8v /v .The cathode and anode electrodes were separated by a Celgard 2320membrane.Galvanostatic charge–discharge cycling was performed by using a Land-2100automatic battery tester.CV and EIS were performed by using a Biologic VSP multichannel potentiostatic-gal-vanostatic system.The CV measurement was performed in the voltage window of 2.0–4.0V.The impedance spectra wererecord-Figure 10.Linear fitting of the I p versus v 1/2relationship of thesamples.Figure 11.Nyquist plots of the samples.Inset:equivalent circuit to fit the Nyquist plots.。