氟化石墨烯用作一次锂电池正极材料的性能

文章编号:1007_8827(2015)01_0079_07氟化石墨烯用作一次锂电池正极材料的性能许耀,詹亮,王赟,王艳莉,施云海(华东理工大学化学工程联合国家重点实验室特种功能高分子材料及其相关技术教育部重点实验室,上海200237)摘要:以异丙醇/水㊁氟化石墨分别作试剂和原料,采用液相剥离法制备出氟化石墨烯㊂所制氟化石墨烯呈二维㊁无定型的多孔结构,其中F含量高达49.7%㊂氟化石墨烯丰富的C_F键及其超薄的二维纳米结构有利于Li+在充放电过程中的存储和扩散,因此所制氟化石墨烯具有优异的电化学性能,尤其有较大的功率密度㊂关键词:氟化钠米石墨片;二维纳米结构;氟化石墨;一次锂电池中图分类号:TQ127.1+1文献标识码:A基金项目:国家自然科学基金(51472086,51002051);上海市自然科学基金(12ZR1407200).通讯作者:詹亮,副教授.E_mail:zhanliang@;施云海,副教授.E_mail:shi yunhai@作者简介:许耀,硕士研究生.E_mail:xuyaoxy@Fluorinated graphene as a cathodematerial for high performance primary lithium ion batteriesXU Yao,ZHAN Liang,WANG Yun,WANG Yan_li,SHI Yun_hai(State Key Laboratory of Chemical Engineering,Key Laboratory of Specially Functional Polymeric Materials and RelatedTechnology(Ministry of Education),East China University of Science and Technology,Shanghai200237,China) Abstract:A free_standing fluorinated graphene(F_graphene)was fabricated by the simple liquid exfoliation of a fluorinated graphite(F_graphite)in a2_propanol/water mixture,and was used as the cathode material of primary lithium ion batteries.As_pre_ pared F_grapheneh as a very high fluorine content(49.7at%),and an amorphous_like two_dimensional nanostructure.These fea_ tures facilitate the rapid diffusion of lithium ions during discharging.The F_graphene exhibits better electrochemical performance, especially,a higher specific power density than the F_graphite.Keywords: Fluorinated graphene nanosheets;Two_dimensional nanostructure;Fluorinated graphite;Pprimary lithium batteriesReceived date:2014_07_13; Revised date:2015_01_17Foundation item:National Science Foundation of China(51472086,51002051);Natural Science Foundation of Shanghai City (12ZR1407200).Corresponding author:ZHAN Liang,Associate Professor.E_mail:zhanliang@;SHI Yun_hai,Associate Professor.E_mail:shi yunhai@Author introduction:XU Yao,Master Student.E_mail:xuyaoxy@English edition available online ScienceDirect(http:／／／science／journal／18725805).DOI:10.1016/S1872_5805(15)60177_91 IntroductionFluorinated carbon(C x F)n has been extensively applied as active cathode materials for primary lithium battery[1_3].Generally,the(C x F)n materials are de_ rived from coke or graphite by reacting with fluorine gas at high temperature(600ħ)[4,5]or catalytically reacting with HF,F2and a volatile fluoride MF n such as IF5,BF3at room temperature[6_9].Theoretically, the carbon atoms in(C x F)n compounds are sp3hy_ bridized with F atoms forming pure covalent C_F bonding,i.e.the atomic F/C ratio is1(denoted as CF1).However,(C x F)n materials have some elec_trochemically inactive defects such as CF2groups if it is produced at high temperature[4,5].Additionally, the residual catalysts(IF5,IF6_,BF4_,ClHF_, FHF_)will be intercalated into the graphitic layers, significantly affect the electrochemical properties[10]. Importantly,due to kinetic limitations associated with the poor electrical conductivity of the strongly cova_ lent CF1,the battery can only run at medium dis_ charge current densities[11_13].Therefore,to meet the requirements of primary lithium batteries with a high power density for new applications such as for space long term exploratory missions,many efforts have been made to develop novel nanostructured(C x F)n第30卷第1期2015年2月新型炭材料NEW CARBON MATERIALS Vol.30 No.1 Feb.2015materials.For example,carbon nanomaterials,i.g., carbon nanofibres,carbon nanotubes,carbon nano_ horns or carbon nanodiscs,were applied as raw car_ bon materials to parpare fluorinated carbon as cathode materials[14].In this work,we demonstrate a simple approach to fabricate free_standing fluorinated graphene(deno_ ted as F_graphene)nanosheets by liquid exfoliation[15] using fluorinated graphite(F_graphite,50%)as raw material[16].The resulting F_graphene nanosheets ex_ hibit a two_dimensional structure,amorphous_like mi_ crostructure and high fluorine content(49.7%).Such unique features render abundant F active sites for lithi_ um storage and facilitate a rapid diffusion of lithium i_ ons during discharging.As a consequence,the F_gra_ phene nanosheets give an excellent discharge perform_ ance and high power density when used as cathode ma_ terials for primary lithium battery.2 Experimental2.1 Synthesis of F_graphene nanosheetsF_graphene nanosheets were fabricated by liquid exfoliation of commercially available F_graphite pow_ ders(Aldrich,the atom ratio of F：C=1：1)via soni_ cation in common solvents,i.e.N_Methyl_2_pyrroli_ done(NMP),dimethylformamide(DMF)and2_iso_ propanol(IPA).In a typical synthesis,50mg of F_ graphite powder was added into200mL reactor. 100mL of solvent was added into above reactor as dis_ persion agent.The sealed reactor was ultrosonicated at room temperature for6h,and then the dispersion was centrifuged at2000r/min for30min to remove aggre_ gates.2.2 Materials characterizationThe morphology and microstructure of the sam_ ples were investigated by SEM(JEOL6500),HR_ TEM(Field Emission JEOL2100),AFM(Digital Instrument Nanoscope IIIA),XRD(Rigaku D/Max Ultima II Powder X_ray diffractometer)and XPS (PHI Quantera x_ray photoelectron spectrometer). Nitrogen sorption isotherms and Brunauer_Emmett_ Teller(BET)surface area were measured at77K with a Micromeritcs Tristar3000analyzer(USA). The thermogravimetric analysis was carried out in ni_ trogen atmosphere up to800ħ.2.3 Electrochemical measurementsElectrochemical experiments were carried out in 2032coin_type cells.The cathode electrodes were prepared by mixing the active materials(F_graphite or F_graphene nanosheets),carbon black and polyvinyli_ dene fluoride(PVDF)at a weight ratio of80：10：10, and then spread onto a stainless steel current collector disk of10mm in diameter.The corresponding elec_ trodes were denoted as F_graphite or F_graphene nanosheets electrode.After magnetically stirred in1_ Methyl_2_pyrrolidinone(NMP)for8h,the slurry was spread uniformly on pure aluminium foil (99.6%,Goodfellow)with a thickness of50μm. To evaporate the solvent,the film was heated in air at 90ħ,followed by heating under vacuum at120ħfor12h.Electrode slices were cut from the film and about6_7mg active material in every slice was ob_ tained.Pure lithium foil(Aldrich)was used as the counter electrode.The coin cells were assembled in a dried argon_filled glove box,and polypropylene mi_ croporous membrane was used as separator which is soaked by1mol/L solution of LiPF6in ethylene car_ bonate(EC)/dimethyl carbonate(DMC)/diethyl carbonate(DEC)(1：1：1by volume)electrolyte.Relaxation was performed for at least24h until open circuit voltage(OCV)was stabilized.Galvano_ static discharges were carried out at room temperature between the initial OCV and1.5V under various cur_ rent densities of10_960mA㊃g-1.When a plateau was clearly observed on the galvanostatic curve,the dis_ charge potential was measured at half of the total dis_ charge capacity,which was noted as E1/2.Aging was simulated by maintaining the cells at60ħfor5days before the electrochemical measurements.3 Results and discussion3.1Effect of exfoliation solvents on the nano_ structure of F_graphene nanosheetsF_graphene nanosheets were fabricated by a liq_ uid exfoliation procedure.In a typical experiment, 50mg F_graphite(50%,Fig.1a)and100mL IPA were mixed and then sonicated for about6h at ambi_ ent temperature.As shown in Fig.1b,many two_di_ mensional nanosheets with sizes from500nm to sever_ al micrometers can be clearly observed.Moreover,a uniform milk_like dispersion of F_graphene nanoshe_ ets/IPA was achieved with the same color as the orig_ inal F_graphite/IPA dispersion(Fig.2).To investi_ gate the effect of solvent type on the properties of the resulting F_graphene nanosheets,DMF and NMP were used for comparison under the same liquid exfo_ liation conditions.Fig.1c indicates that the flakes were much thick,suggesting most of the F_graphite cannot be exfoliated completely in DMF.Although most of the F_graphite can be exfoliated in NMP (Fig.1d),the color of F_graphene nanosheet/NMP dispersion changed from milk_like to black after exfo_ liation as a result of a fluorine removal from the F_ graphene nanosheets(Fig.2a).㊃08㊃新型炭材料第30卷Fig.1 SEM images of(a)F_graphite and synthesizedF_graphene nanosheets by liquid exfoliation of F_graphite in(b)IPA,(c)DMF and(d)NMP for6h.Fig.2 Photographs of(a)the F_graphene nanosheets dispersion in NMP,DMF and IPA and(b)F_graphite dispersed in IPA. To further improve the exfoliated efficiency of IPA solvent,a certain amount of water was added in_ to the solvent to regulate its surface tension.As shown in Fig.3,the added water has a significant effect on the thickness of the resulting F_graphene nanosheets,and the optimum volume ratio of IPA/ water is60/40(denoted as60%IPA).Hence the following mentioned F_graphene nanosheets were fab_ ricated using60%IPA as the mixture solvent.3.2 Nanostructures of F_graphene nanosheetsThe nanostructures of as_prepared F_graphene nanosheets were further investigated via TEM.The TEM image(Fig.4a)shows a two_dimensional structure of F_graphene nanosheets with crude sur_ face.And the high_resolution TEM(HR_TEM,Fig. 4b)image further discloses a amorphous_like structure on the nanosheets,possibly resulted from a high fluo_ rine content,which can cause a breakage of the hexa_ gonal lattice and parallel basal plane of graphene.The energy dispersive spectroscopy(EDS)analysis re_ veals a high fluorine content of49.7%(Fig.4c), corresponding to a C/F atomic ratio of1：1.It sug_ gests that each fluorine atom attaches one carbon at_ om,forming a covalent C_F bond,and the direction of C_F bond obeys the upward_downward sequence a_ long the C C hexagonal lattice as illustrated in Fig. 4d.AFM further displays the two_dimensional feature of F_graphene nanosheets with a thickness of about 10nm(Fig.5).A BET surface area of125m2㊃g-1 is achieved for F_graphene nanosheets based on their nitrogen adsorption/desorption analysis(Fig.5).Fig.3 SEM images of F_graphene nanosheets exfoliated by different IPA/water mixtures.(a)40%IPA,(b)50%IPA,(c)60%IPA,(d)70%IPA.To elucidate the crystal structure of the F_gra_ phene nanosheets,we carried out XRD characteriza_ tion as shown in Fig.6a.The diffraction peaks at2θaround14ʎand42ʎare indexed as the(001)and (100)reflections of F_graphene,respectively.The (001)reflection is particularly for samples with high fluorine contents.The corresponding reticular distance d100is close to0.213nm,with a resulting C C length of about0.142nm[2].But the(002)reflection at2θ=26.4ʎdisappears owing to the high fluorine content,which leads to a crinkle of the carbon sheets during the conversion of carbon atom hybridization from sp2to sp3[12].To probe the chemical state of carbon and fluo_ rine in the F_graphene nanosheets,XPS measurement was conducted.As shown in Fig.6b_d,F1s and C1s are the prominent peaks,suggesting no other discrete peak,which has the same chemical characteristics as raw material F_graphite(Fig.7a).㊃18㊃第1期XU Yao et al:Fluorinated graphene as a cathode material for high performance primary lithium......Fig.4 (a)TEM image,(b)HR_TEM and(c)EDS of the F_graphene nanosheets by liquid exfoliation ofF_graphite in60%IPA for6h.(d)Simulated microstructure of a F_graphene with a F content of50%.Fig.5 (a)Representative AFM image of F_graphene nanosheets and(b)corresponding thickness analysis taken around the white line in(a).(c)N2adsorption_desorption isotherms and(d)pore size distributions of F_graphite and F_graphene nanosheets,respectively.More importantly,the atomic ratio of F/C still maintains at0.98(50%),well consistent with that of F_graphite and the results from EDS(Fig.4c), suggesting that F species have not been removed dur_ ing the ultrasonication.The high resolution C1s of F_graphene nanosheets can be further fitted to two peaks located at289and290eV,which reflect the different bonding structures of the C C and C_F bonds,re_ spectively.And the typical peak at about285eV relat_ ed to the graphite_like sp2carbon cannot be detected,㊃28㊃新型炭材料第30卷indicating all sp2hybridization of carbon atom is con_ verted into sp3hybridization.The F1s peak at a high binding energy of688.3eV is very symmetric(Fig. 6d),indicating that F atom in the F_graphene has on_ ly one chemical state,i.e.C_F bond.For compari_son,the F1s peak of the F_graphene nanosheets fabri_ cated using NMP as solvent at688.3eV is very weak (Fig.7b),suggesting that most of the F atom was removed during the ultrasonication.Fig.6 (a)XRD pattern and(b)XPS spectrum of F_graphene nanosheets.Curve fitting of(c)C1s and(d)F1s spectra,respectively.Fig.7 XPS spectra of(a)the F_graphite raw material and (b)the resulting product by exfoliation of the F_graphite for24h using NMP as solvent.3.3 ElectrochemistryThe galvanostatic discharge curves of the F_ graphite and the F_graphene nanosheets at the current densities of10_960mA㊃g-1were measured at room temperature,as shown in Fig.8.The voltage initially drops and then increases steadily,reaching a voltage plateau after few minutes.The initial drop is charac_ teristic of Li/(C x F)n cells and is attributed to the re_duction of high_electrically isolated CF2and CF3sur_ face groups.Interestingly,although the fluorine con_ tent of F_graphene nanosheets is almost the same as that of the F_graphite,its electrochemical behavior is quite different,specially the plateau potential at the same current density.The average potential of F_gra_ phene nanosheets evolves from2.24to1.95V with increasing the current density from10to960mA㊃g-1,㊃38㊃第1期XU Yao et al:Fluorinated graphene as a cathode material for high performance primary lithium......which is much higher than that of F_graphite(1.22V at10mA㊃g-1and2.17V at960mA㊃g-1).Addition_ ally,the discharge capacity of the F_graphene nanosheets was952mAh㊃g-1at a low current density of10mA㊃g-1,which is also higher than that of the F_ graphite(899mAh㊃g-1).Although the discharge po_ tential(Fig.8c)and discharge capacity(Fig.8d) evolution with increasing current density are almost linear for both F_graphite and F_graphene nanosheets, the average discharge potential and discharge capacity of the F_graphene nanosheets are higher than those of the F_graphite at the same current density.Consider_ ing the electrical conductivities of the F_graphene nanosheets and the F_graphite have no difference be_ cause of the same kinds of covalent C_F bonds,the obvious difference of electrochemistry of F_graphene nanosheets should be attributed to the thin two_dimen_ sional nanostructure of F_graphene.The thin two_di_ mensional nanostructure is beneficial for a rapid diffu_ sion of lithium ions and also provide a high contact surface area between lithium ions and C_F bonds.Fig.8 Galvanostatic discharge curves for(a)F_graphite and(b)F_graphene nanosheets at room temperature at the current density from10to960mA㊃g-1.(c)Average discharge potential and(d)discharge capacity as a function of current density for the F_graphite and the F_graphene nanosheets.A ragone plot showing the association between specific energy density and specific power density for the F_graphite and the F_graphene nanosheets is pres_ ented in Fig.9a to compare the electrode performance under different current densities.Specific energy den_ sity E s and specific power density P s can be obtained from discharge curves according to the equation:E s= CˑE1/2and Ps=dˑE1/2,in which C and d are the dis_ charge capacity and the current density,respectively.As other literatures reported[17_21],the specific power density decreases with the specific energy density for both the F_graphite and the F_graphene nanosheets, but the specific power densities of the F_graphene nanosheets are higher than those of the F_graphite at each specific energy density.The results also should be attributed to the special thin two_dimensional nano_ structure of the F_graphene nanosheets.Because a thin layer can provide a larger contact area for both lithium ion/(C x F)n and electrolyte/(C x F)n,giving a higher diffusion rate.The aging property is an important aspect for the applications of primary lithium batteries used at high temperatures,so we further conduct the aging test by heating the simulated primary lithium battery at60ħfor5days before discharge curve measurements.As shown in Fig.9b,the average discharge voltage and discharge capacity are similar before and after aging, suggesting that the chemistries of the F_graphene nanosheets and the cell components can be maintained at this temperature.㊃48㊃新型炭材料第30卷Fig.9 (a)Rgone plot comparing the performance of the F_graphite and the F_graphene nanosheets used in primary lithium batteries and(b)galvanostatic discharge curves of the F_graphene nanosheets before and after the simulated aging.4 ConclusionsWe developed a facile approach to synthesize free_standing fully fluorinated graphene nanosheets via a liquid exfoliation method.The F_graphene nanoshe_ ets exhibit a high fluorine content(49.7%)and amorphous_like microstructure with a two_dimensional nanostructure.These features endow the F_graphene nanosheets with a high discharge potential,high ca_ pacity and high power density.Moreover,the F_gra_ phene nanosheets are expected to be a competitive candidate in the fields of lubricant,nanodevice,mag_ netic and anode materials for lithium ion battery. 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