Electrochimica Acta 125(2014)22–28Contents lists available at ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taEffect of pre-lithiation degrees of mesocarbon microbeads anode on the electrochemical performance of lithium-ion capacitorsJin Zhang a ,Zhiqiang Shi a ,∗,Chengyang Wang b ,c ,∗aLaboratory of Fiber modification and Functional Fiber,College of Materials Science and Engineering,Tianjin Polytechnic University,Tianjin 300387,PR China bKey Laboratory for Green Chemical Technology of Ministry of Education,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,PR China cSynergetic Innovation Center of Chemical Science and Engineering(Tianjin),Tianjin 300072,PR Chinaa r t i c l ei n f oArticle history:Received 28November 2013Received in revised form 5January 2014Accepted 5January 2014Available online 21January 2014Keywords:Li-ion capacitorsPre-lithiation degrees Phase transitionMesocarbon microbeads electrode Activated carbon electrodea b s t r a c tLithium ion capacitors are assembled with pre-lithiated mesocarbon microbeads (LMCMB)anode and activated carbon (AC)cathode.The effect of pre-lithiation degrees on the crystal structure of MCMB electrode and the electrochemical capacitance behavior of LIC are investigated by X-ray diffraction (XRD)and the charge-discharge test of three-electrode cell.The structure of graphite still maintained when the pre-lithiation capacity is less than 200mAh g −1,phase transition takes place with the increase of pre-lithiation capacity from 250mAh g −1to 350mAh g −1.Pre-lithiation degrees of MCMB anode greatly affect the charge-discharge process and behavior,which impact on the electrochemical performance of LIC.The LIC with pre-lithiation capacity of 300mAh g −1has the optimal electrochemical performance.The energy density of LIC300is up to 92.3Wh kg −1,the power density as high as 5.5kW kg −1and the capacity retention is 97.0%after 1000cycles.The excellent electrochemical performance benefits from the appropriate pre-lithiation capacity of negative electrode.The appropriate pre-lithiation ensures the working voltage of negative electrode in low and relative stable charge-discharge platform corresponding to the mutual phase transition from the second stage graphite intercalation compound (LiC 12)to the first stage graphite intercalation compound (LiC 6).The stable charge-discharge platform of negative electrode is conductive to the sufficient utilization of AC positive electrode.©2014Elsevier Ltd.All rights reserved.1.IntroductionElectrochemical double-layer capacitor (EDLC)is called super-capacitor,which contained two symmetrical activated carbon electrodes with high surface pared with secondary bat-teries,it generally shows higher specific power,longer cycle life and better stability,which is considered as the most effective high power energy density storage device [1–4].However,the energy storage is based on the double-layer forming at the elec-trode/electrolyte interface,so the energy density is limited by the capacitance and withstanding voltage of activated carbon,which restricts its application in the field of electric vehicles,hybrid elec-tric vehicles and large scale energy storage.Therefore,considerable research is focused on how to improve the energy density of EDLC.According to the relation for energy,E =12CU 2,where C is the capacitance and U working voltage,the energy density increases∗Corresponding author.Tel.:+862283955816;fax:+862283955055.E-mail addresses:shizhiqiang@ (Z.Shi),cywang@ (C.Wang).with the capacitance and working voltage.Applying electrode materials with high specific capacitance, e.g.nanoporous car-bon [5],transition metal oxide [6],conductive polymers [7,8],and improving the working voltage by the usage of the voltage resistance materials or ionic liquids electrolytes [9–11],are the two methods to improve the energy density.In recent years,an advanced energy storage device that provides both high energy density and high power density has been proposed,known as lithium ion capacitor (LIC)[12–14].LIC consists of a lithium-doped negative electrode,an activated carbon (AC)positive electrode and an organic electrolyte containing lithium salt,it is the combina-tion of lithium ion battery anode material and electrochemical capacitor cathode material.In LIC,the negative electrode materi-als that can be doped with lithium ions typically include Li 4Ti 5O 12[15,16],hard carbon [17],graphite [18,19].Among these materi-als,graphite is particularly attractive due to its high theoretical capacity (372mAh g −1),low and stable charge-discharge platform,natural abundance and relatively low cost.Currently,a LIC based on AC/graphite-Li system in the voltage range of 2.0-4.0V could deliver the energy and power density of 10Wh kg −1and 10kW kg −1,and maintain good cycle performance at 10C rate which benefits from0013-4686/$–see front matter ©2014Elsevier Ltd.All rights reserved./10.1016/j.electacta.2014.01.040J.Zhang et al./Electrochimica Acta125(2014)22–2823the pre-lithiation technology of graphite negative electrode[20].A few of companies including Subaru and Fuji Heavy Industries Ltd,ISR Micro/JM Energy and Asahi Kasei Corporations have also vigorously researched the pre-doped LICs in recent years.The pre-lithiation can improve the working voltage,energy den-sity and cycle stability,reduce the irreversible capacity loss and increase Li+concentration of the electrolyte[20].Sivakkumar et al. achieved pre-lithiation of graphite electrode by short-circuiting graphite electrode and Li metal in AC/graphite/Li3-electrode cell, the pre-lithiation capacity was estimated to be equivalent to71% of the theoretical capacity of graphite after10h[20].The LIC deliv-ered energy density of55Wh kg−1and100Wh kg−1(based on active material weight of two electrodes)in a voltage window of3.1-4.1V and2.0-4.1V.However,they only gave cycle perfor-mance of capacitor with several hundred times,and not provided the power performance of the LIC.Graphite was successfully pre-lithiated by successive charge/self-discharge pulses in1mol L−1 LiPF6in EC/DMC and2mol L−1lithium bis(trifluoromethane)sul-fonamide(LiTFSI)organic electrolyte,respectively,in the literature [18,21].The two kinds of LIC delivered high energy density.How-ever,the work of pre-lithiation process didn’t introduce auxiliary lithium electrode and the charge-discharge test was operated in the same cell,which could cause the lithium ion concentration decrease and impact on the electrochemical performance.Besides, they just pre-lithiated the negative electrode with a certain degree and didn’t study the effect of different pre-lithiation degrees on the performance of LIC.Despite several papers published on the LIC with pre-lithiated graphite,very few reports have discussed the effects of pre-lithiation degrees of negative electrode on the LIC’s electrochemical performance in details.Hence,in this work,we use electrochemi-cal method to realize pre-lithiation and undertake a more detailed evaluation of the electrochemical performance of LIC with different pre-lithiation degrees MCMB negative electrode.2.Experimental2.1.Positive electrode preparationThe positive electrode was prepared by coating a mixture of commercial activated carbon(ACP-60,S BET=1604m2g−1,PCT Co. Ltd,Korea),conductive carbon black(VXC72,Carbot Co.,USA),and polytetrafluoroethylene(PTFE)with a mass ratio of85:7:8and then pressed with roller press to control the thickness.The disc positive electrodes of13mm in diameter were prepared by punching the coatingfilm and dried at120◦C overnight in dynamic vacuum.The aluminium foil with conducting resin was used as current collector.2.2.Negative electrode preparationThe electrode was prepared by pasting a10m thick copper foil with mesocarbon microbeads(MCMB,BTR New Energy Material Co. Ltd,China),conductive carbon black(VXC72)and polyvinylidene fluoride(PVDF)dispersed in N-methylpyrrolidone(NMP)in89:3:8 mass ratio.After evaporation of the solvent(NMP),the electrodes were roll-pressed to improve the mechanical strength.Then nega-tive electrodes were cut into a disc of13mm diameter and dried at 120◦C for12h in a dynamic vacuum.2.3.Cell fabricationPre-lithiation process of MCMB electrode was carried out by galvanostatic charging in coin cell(CR2430)which assembled with MCMB working electrode and lithium countering electrode. The pre-lithiation process was terminated once the capacity of the cell reached the definite value.The capacity of pre-lithiation we designed as0,50,100,150,200,250,300,350mAh g−1, respectively.Based on the capacity value,the pre-lithiated MCMB (LMCMB)electrode was signed LMCMB0,LMCMB50,LMCMB100, LMCMB150,LMCMB200,LMCMB250,LMCMB300,LMCMB350,the corresponding LICs were denoted LIC0,LIC50,LIC100,LIC150, LIC200,LIC250,LIC300,LIC350.Two-electrode LIC was assembled with AC working electrode (positive electrode)and LMCMB counter electrode(negative elec-trode),it was used to evaluate the energy density,power density and cycle life.The mass ratio of positive to negative electrode was kept at1.A three-electrode cell with AC as positive elec-trode,LMCMB as negative electrode and lithium metal as reference electrode was charged and discharged in the voltage range of2.0-4.0V to study the variation of potential both the positive electrode and negative electrode.The electrolyte was1.2mol L−1LiPF6in 1:1EC/DEC(ethylene carbonate/diethyl carbonate),and a porous polypropylene microporous sheet was used as the separator.All cells were assembled in an Argon-filled glove box and all poten-tials in this paper were presented vs.Li/Li+.The energy and power density were calculated per active materials mass of two electrodes. The electrochemical tests of the electrodes and capacitors were per-formed by cyclic voltammetry,galvanostatic charging-discharging.3.Results and discussion3.1.Electrochemical performance of activated carbonThe electrochemical properties of the AC were investigated using a lithium-based organic electrolyte(1.2mol L−1LiPF6in EC/DEC).Fig.1(a)shows the cyclic voltammograms obtained with different potential ranges in a three-electrode cell where AC is the working electrode and lithium metal is used as counter and ref-erence electrode.In the range of2.0to4.0V,the voltammogram is excellent rectangle without redox and reduction peaks in AC electrode,showing that the capacitance of the cell mainly depend on non-faradic adsorption/desorption of anion.When the potential range is extended to1.5-4.5V,the appearance of a small distortion makes the curves deviate from the shape of double layer capacitors. Even so,the capacitance characteristics still maintain.While at low potential of1.0V,the appearance of a reduction peak indicates that the electrolyte is decomposed and a solid electrolyte interface(SEI) is formed on the surface of activated carbon[18,22].It seems that the product of SEI blocks the pores of the AC electrode,which limit a further access of the electrolyte ions into the active surface.So it is suggested that the operating potential range for activated car-bon should be higher than2.0V and lower than4.0V in organic Li+ electrolyte.The charge-discharge curve of the AC given in Fig.1(b) is excellent linear.The specific capacity and specific capacitance obtained from the discharge curve of the AC positive electrode is about49.95mAh g−1in potential range from2.0V to4.0V.3.2.Electrochemical performance of MCMBThe cyclic voltammograms of MCMB/Li cell at the scan rate of 0.05mV s−1and0.02mV s−1are shown in Fig.2.During thefirst cathodic scan,two peaks at the potential of about1.2V and0.7V before the intercalation of lithium ion into MCMB indicate that a reductive decomposition of electrolyte occurs and the decompo-sition products form the so-called solid electrolyte interface(SEI)film on MCMB surface[23].The SEI forming mainly on thefirst cycle suggests that the SEIfilm blocks solvent molecules and solvated Li+into MCMB electrode,protecting the electrolyte from continu-ous decomposition during subsequent cycling.Thus,we can believe that the vast majority of the irreversible capacity loss(ICL)is asso-ciated with SEI forming.As we know,there were mainly several24J.Zhang et al./Electrochimica Acta125(2014)22–28Fig.1.(a)Cyclic voltammograms of the AC at5mV s−1and(b)charge-discharge curve of the AC using1.2mol L−1LiPF6in EC/DEC as electrolyte in a three-electrode cell.graphite intercalation compounds(GICs)in the process of inter-calation of lithium ion[23].The graphite intercalation compound forms with the intercalation of Li+,and the type of GIC changes with the Li+intercalation degree.As expected,three stages of Li+inter-calation occurring below0.25V with distinct intercalation steps at 0.17V,0.08V and0.05V are observed from Fig.2(b).Fig.3(a)shows thefirst galvanostatic charge-discharge pro-file of MCMB/Li cell.It is obvious that the charge-discharge curve is relativelyflat with the intercalation/de-intercalation processes occurring on at low potentials(<0.25V).Thefirst intercalation curve has a plateau at about0.8V,which corresponds to the forma-tion of SEIfilm.Thefirst intercalation capacity and the irreversible capacity loss(ICL)of MCMB is about370mAh g−1and30mAh g−1,respectively.The magnification of discharge(intercalation of lithium ion)curve of MCMB/Li cell in the inset of Fig.3(a)shows that three discharge plateau corresponding to the transition pro-cess of GICs appear at0.17V,0.11V,0.07V,respectively,which was almost consistent with the result of Fig.2(b).In order to study the effect of pre-lithiation of graphite anode on the system of LIC,the pre-lithiation curves of MCMB electrode at different degrees by controlling capacity are given in Fig.3(b). The relevant parameters are listed in Table1.It can be seen that the intercalation degree increases from0to94.09%with the increase of pre-lithiation,while the potential of electrode gradually decreases from over2.1V to0.001V.Therefore,pre-lithiation operation for graphite anode will affect different stages of GICs formation and transition in LIC assembled by AC cathode and MCMBanode.Fig.2.Cyclic voltammograms of the MCMB/Li cell at two kind of scan rate(a) 0.05mV s−1;(b)0.02mV s−1.Moreover,it will affect the working potential changes of AC cathode and the electrochemical behavior of LIC.For further investigating the variation of MCMB’s structure after pre-lithiation with different capacity,ex situ XRD measurement was performed as soon as the pre-lithiation processfinished,the results were presented in Fig.4.As can be seen,when the pre-lithiation capacity is lower than200mAh g−1,the002diffraction peak of LMCMB is about26.50◦and maintains symmetrical,which is the characteristic diffraction peak of graphite materials.More-over,the002diffraction peak slightly shifts to lower diffraction angle with the increase in pre-lithiation capacity.It indicates that the layer spacing of graphite slightly increases with the increase of intercalation capacity,but the main structure of graphite still maintains according to the standard PDF41-1487.However,when the pre-lithiation capacity increases to250mAh g−1,the002peak Table1The potentials of MCMB in MCMB/Li cell with different pre-lithiation capacity.Sample Pre-lithiationcapacity(mAh g−1)Degree ofpre-lithiation(%)*Potential vsLi/Li+(V) LMCMB000.00>2.10 LMCMB505013.440.160 LMCMB10010026.880.119 LMCMB15015040.320.103 LMCMB20020053.760.080 LMCMB25025067.200.053 LMCMB30030080.650.043 LMCMB35035094.090.001*Values based on the theoretical discharge capacity(372mAh g−1)of MCMB.J.Zhang et al./Electrochimica Acta 125(2014)22–2825Fig.3.(a)First galvanostatic charge-discharge profile of MCMB at 0.05C rate in 1.2mol L −1LiPF 6in EC/DEC.Inset:The magnification of discharge (intercalation of lithium ion)curve of MCMB/Li cell.(b)The plots of pre-lithiation of MCMB with different capacity.Inset:The magnification profiles of pre-lithiation.intensity obviously wakens,while two new peaks emerge at 25.62◦and 25.32◦,respectively,which can be explained by the forma-tion of the third stage GIC (LiC 24)and the second stage GIC (LiC 12)according to the standard PDF 35-1047and 35-1046card.Fur-thermore,when the pre-lithiation capacity continues to increase,the peak at 26.50◦disappears and the intensity of peak at 25.32◦increases,another new peak appears at 24.20◦attributed to the first stage GIC (LiC 6)according to the PDF 34-1320card.The results indicate that the pre-lithiation capacity of MCMB anode not only affects the potential according to the Table 1,but also correspond to the formation and transition of different GICs which are important to the stability of graphite anode and LIC.3.3.The electrochemical performance of LICFig.5showed the charge-discharge curves of the three-electrode LIC with a potential range of 2.0-4.0V at a current density of 25mA g −1.The potential change of the LMCMB neg-ative electrode and AC positive electrode in LICs are shown in Table 2.When the pre-lithiation capacity of negative electrode is lower than 200mAh g −1,the charge-discharge profiles of LIC and AC positive electrode show a great difference,the discharge potential range of the AC positive electrode is extended to a higher potential region,while the charge-discharge profile of LMCMB shows a typical U of graphite electrode.In the whole rangeofFig.4.Ex situ XRD patterns of MCMB electrode after pre-lithiation with different capacity.charge-discharge,the charge-discharge curves of LIC,AC positive and LMCMB negative electrode are not completely linear with time.Especially in the initial charge and discharge end stage,the potential of LMCMB and LIC drastically change.With the increase of pre-lithiation capacity,the charge-discharge profiles of LIC,AC positive and LMCMB negative electrode show almost linear with time.Meanwhile,the potential variation range of LMCMB grad-ually reduces which is beneficial to fully use the capacity for AC positive electrode and cycle stability of LIC.It is apparent that both the positive electrode and negative electrode exhibit the maximal capacity.All the charge-discharge curves are still linear in the range 2.0-4.0V when the pre-lithiation capacity is 350mAh g −1,but the potential of LMCMB is negative,indicating the overcharge of LMCMB electrode.The overcharge generally causes the decrease of capacity and cycle performance and thus should be avoided.The cycle performance of LIC in the range of 2.0-4.0V at 2C rate is shown in Fig.6.The energy density of LIC0decreases from 30.3to 8.1Wh kg −1during the first 100cycles,remaining slightly energy density fading in the rest of cycling test.By contract,the energy density and cycle stability improve after pre-lithiation of negative electrode.The energy density are about 48.6,51.0,54.6,75.8,63.3Wh kg −1,and the loss of energy density after 1000cycles are about 92.6%,12.5%,6.1%,3.0%,95.3%for LIC50,LIC100,LIC200,LIC300and LIC350,respectively.The increasing of energy density is directly correlated with the increased utilization of the AC posi-tive electrode.The decreasing energy density of LIC350is because during the overcharge process,the products of reaction between the electrolyte and the deposited lithium on the surface of MCMB forms thicker SEI film than other LICs increasing the resistance of lithium ion diffusion [24,25].The film thickening process with cycling consumes large amount of lithium ion,which can explain the poor stability.LIC300has the highest energy density and best cycle performance in all samples.The maximum energy density is about 7times higher than the energy density obtained with the conventional EDLCs.The Ragone plots are illustrated in Fig.7,which were obtained by calculating the energy density and power density at different current densities.With the increase of the pre-lithiation capacity from 0to 300mAh g −1,both the energy density and power density improve.When the pre-lithiation capacity increases to 350mAh g −1,the energy density and power density get worse because the thickening of SEI film in the overcharge process impedes the dif-fusion of lithium ion.It is clearly that LIC300has the maximum energy density of 92.3Wh kg −1and the highest power density of about 5.5kW kg −1,showing that LIC300is the best combination of energy density and power density.The results clearly show that the energy density,power den-sity and cycle performance of the LIC with pre-lithiated negative26J.Zhang et al./Electrochimica Acta 125(2014)22–28Fig.5.Charge-discharge curves of the three-electrode LIC of the AC/LMCMB with Li reference electrode at the current density of 25mA g −1,a:LIC0,b:LIC50,c:LIC100,d:LIC200,e:LIC250,f:LIC300,g:LIC350.electrode improve.With the increase of pre-lithiation capacity,the discharge potential range of activated carbon positive electrode is extended to a higher potential region.Increased utilization of activated carbon is responsible for improving the electrochemicalperformance of the LICs.When the pre-lithiation capacity is up to 300mAh g −1,LIC300exhibits the optimal electrochemical perfor-mance,which attributes to the appropriate pre-lithiation capacity.It guarantees the negative electrode at the stage of low and stableJ.Zhang et al./Electrochimica Acta 125(2014)22–2827Table 2The charge-discharge potentials change of positive/negative electrode and LIC in three-electrode cell.SamplePositive electrodeNegative electrodeLICC p (F g −1)C n (mAh g −1)V c (V)V max (V) V d (V)V c (V)V min (V) V d (V)V c (V)V max (V)V d (V)LIC00.84 4.250.95 1.020.25 1.05 1.85 4.00 2.0145.2120.58LIC50 1.12 4.18 1.130.840.180.87 1.97 4.00 2.0054.5030.89LIC100 1.48 4.11 1.550.460.110.45 1.95 4.00 2.0076.7540.21LIC200 1.83 4.07 1.840.160.070.17 2.00 4.00 2.0089.7049.05LIC250 1.88 4.05 1.920.110.050.08 1.98 4.00 2.01101.0652.78LIC300 1.89 4.03 1.900.090.030.10 2.00 4.00 2.00102.8753.91LIC3501.843.991.860.11-0.030.131.954.002.0089.7246.98V c :the potential change of charging,V max :the highest terminal potential of charging, V d :the potential change of discharging,V min :the highest terminal potential of discharging,C p :the specific capacitance of positive electrode,C n :the specific capacity of negativeelectrode.Fig.6.Cycle life of LICs using 1.2mol L −1LiPF 6in EC/DEC at 2C rate.Voltage range from 2.0to 4.0V.charge-discharge platform corresponding to the phase transition from the second stage GIC (LiC 12)to the first stage GIC (LiC 6).The stable charge-discharge platform of negative electrode is helpful use and stability of AC positive electrode performance.Therefore,we proposed that the optimal pre-lithiation capacity should be controlled to make the negative electrode at the initial stage of the phase transition from the second stage GIC to the first stageGIC.Fig.7.The Ragone plots for the LICs using 1.2mol L−1LiPF 6in EC/DEC.4.ConclusionsLICs have been built by combining the pre-lithiated MCMB neg-ative electrode and AC positive electrode,respectively.From the experimental data,it was found that the MCMB electrode could retain the structure of graphite when the pre-lithiation capacity was lower than 200mAh g −1.While the pre-lithiation capacity con-tinued to increase,the formation and transition of different GICs occurred.The pre-lithiation played a crucial role in improving the electrochemical performance such as the energy density,power density and cycle stability.LIC300exhibited the highest energy density of 92.3Wh kg −1,power density of 5.5kW kg −1and the best cycle life performance of about 97.0%retention after 1000cycles,which make it have a wide potential application for hybrid electric vehicles (HEVs)and electric vehicles (EVs).AcknowledgementsThis research was financially supported by the 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