Optimizing pyrolysis of resin carbon for anode of lithium ion batteries

Vol.13 No. 1J. CENT. SOUTH UNIV. TECHNOL. .Feb. 2006Article ID: 1005 - 9784(2006)01 - 0058 - 05Optimizing pyrolysis of resin carbon for anode of lithium ion batteriesGUO Hua-jun(郭华军)'，LI Xin-hai(李新海)'，ZHANG Xin-ming(张新明)2 ,W ANG Zhi-xing(王志兴)', PENG Wen-jie(彭文杰)'，ZHANG Bao(张宝)'(1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083，China;2. School of Materials Science and Engineering, Central South University, Changsha 410083, China)A bstract: Pyrolytic resin carbon anode for lithoum ion batteries was prepared from thermosetting phenolic resin.Pyrolysis of the primary phenolic resin and the dewatered one was studied by thermal gravimetric analysis. Struc-tures and characteristics of the carbon materials were determined by X- ray diffraction, Brunauer Emmer Teller sur-face area analysis and eletrochemnical measurements. With the increase of pyrolyzing temperature and soaking time,the resin carbon material has larger crystallite sizes of Le and L, lower specific surface area, smaller irreversible ca-pacity and higher initial coulombic efficiency. The pyrolyzing temperature and soaking time are optimized to be 1 050C and 2 h. The resin carbon anode obtained under the optimum conditions shows good electrochemical performanceswith reversible capacity of 387 mA●h/g and initil coulombic efficieney of 69. 1%.Key words: lithium ion battery; carbon; phenolic resin; anodeCLC number: TM912. 9Document code: A1 INTRODUCTIONlink density. A higher cross-link density of phenol-ic resin is beneficial to the charge capacity of theLithium ion batteries have attracted worldwidecarbon. The optimized carbon anode has a large re-attention and been developed rapidly due to theirversible capacity of 400 mA●h/g, but its initial cou-high energy density, good charge discharge per-lombic efficiency is only 62. 5%. The present workformances and long cycle life[1-4]. These outstand-aims to prepare disordered carbon from thermosettinging properties result from the use of carbon materi-phenolic resin and reduce its irreversible capacity.als as anode instead of lithium metal. However ，the specific capacity of carbon materials is far2 EXPERIMENTALsmaller than that of lithium metal (3 670 mA●h/g). Much effort has been focused on improving ca-2. 1 Thermal gravimetric analysis of phenolic resinpacity of carbon anodes during the past fewA Mettler Toledo TGA/SDTA851e thermalgravimetric analyzer was used to measure the rela-Recent work indicated that disordered cartive mass loss of phenolic resin vs. temperaturebons, obtained by pyrolyzing organic precursorsduring pyrolysis. The sample was held in a plati-such as sugar, polyfurfuryl alcohol and cottonnumpanandheatedupto1000Cat10C/mininwool at 600- 1 200 C, can reversibly insert lithiuma nitrogen flow of 30 mL/ min.with capacity greater than the theoretical capacityof graphite (372 mA●h/g)8-1]. But the disorder-2. 2 Preparation of pyrolytic resin carbon materialsed carbons have two major deficiencies: large irre-In an oven，thermosetting phenolic resin wasversible capacity and hysteresis between 'charge andheated up to 120 C in air and soaked at the tem-discharge in the voltage profile[10-13]. Therefore,perature for 12 h. The resin was dewatered and so-reducing the irreversible capacity and the hystere-lidified. Then it was ground into powder and usedsis is very critical for the development of disorderedas a precursor for pyrolysis.carbon materials. The electrochemical perform-A quartz boat containing resin precursor wasances of carbon anodes are related to their struc-put into a SK2 -25-12 horizontal tube furnace. Theture and physical properties, which are much de-resin precursors were heated in an argon atmos-pendant on the conditions adopted in the pyrolysis.phere to the pryolyzing temperature tppr and thenXiang et a1[14] prepared carbon materials tsoaked at tpr for a fixed time, and furnace-cooledpyrolyzing of phenolic resins with different cross-to room temperature.中国煤化工CNMHG①Foundation itm: Project(50302016) supported by the National Natural Scie2005037698) supported byPostdoctoral Science Foundation of ChinaReceived date: 2005 -07 - 20; Acepted date: 2005 -10- 20Correspondence:GUO Hua-jun, PhD; Tel; + 86-731-8836633; E mail; ghj@ mail. csu. edu. cnGUO Hua-jun, et al; Optimizing pyrolysis of resin carbon for anode of lithium ion batteries●59.2.3 Characterization of pyrolytic resin carbonPowder X-ray diffraction ( XRD ) measure-。[aments were made with a Rigaku diffractometer100DTG0equipped with Cu Ka radiation. Brunauer-80 t0.1Emmer- Teller(BET) surface area measurementswere made using a quantacgrome monosorb surface50 t-0.2area analyzer.TGA-0.3阳The pyrolytic carbon material, acetylene blackas an electric conductor and poly( vinylidene difluo-20上-0.4ride) (PVDF) as a binder were mixed together.The carbon electrodes were prepared by spreadingthe above mixture onto a copper foil substrate.200 400600 800 1 000And then the electrodes were dried overnight atTemperature/C105 C in vacuum. A Celgard 2300 porous mem-brane of 20 μm thickness was used as a separator,6and the electrolyte was 1 mol/L LiPFs dissolved inDTAa mixture of ethylene carbonate (EC) and dimethyl30 t-0.1carbonate (DMC) with a volume ratio of 1 : 1.0t).2Charge-discharge tests of carbon electrodes wereperformed in the three -electrode cells. The carbon-0.3“was selected as the working electrode and the lithi-um metal served as both the counter and the refer-20 tence electrode. Current density of 0.1 mA/cm2 wasused for the three electrode cells between 0. 005 V600800 1 000and 2. 500 V.3 rESULTs AND DISCUSSIONFig.1 TGA and DTA curves for (a) primaryphenolic resin and (b) phenolic resindewatered at 120 C for 12 h3.1 Thermogravimetry of phenolic resinThermogravimetric analysis(TGA) and differ-ential thermal analysis (DTA) data for primaryThe irreversible capacity of carbon anode isphenolic resin and the dewatered one are shown independant on its specific surface areal1o. In orderFig.1. The samples were heated in N: atmosphereto obtain resin carbon with small specific surfaceat 10 C/min. There are several mass loss steps forarea, a low rate of gas evolution during decomposi-the primary phenolic resin in the temperature rangetion of the resin is favorable. According to the re-of 25- 1 000 C，which is observed more clearly bysults of TGA, a heat-treatment pattern for pyroly-the mass loss peaks in the derivative profile. Theresis of phenolic resin was adopted as follows: theare two high and narrow mass loss peaks at aboutphenolic resin was dewatered and solidified at 120120 C and 170 C，a low and wide one at aroundC for 12h, then heated at 2 C/min to 600 C and550 C. They are attributed to the volatilization offollowed by 10 C/min to tpw，soaked at the topr foralcohol and dissociated phenol, polymerization anda fixed time, and furnace-cooled to room tempera-condensation of the phenolic resin, and carboniza-ture.tion of the phenolic resin, respectively. As to thedewatered resin, the mass loss occurs mostly at3.2 Structure and properties of pyrolytic resinaround 500 C，which is indicated by the intensecarbonpeak of the derivative curve in Fig. 1 (b). SomeIn carbon materials, fundamental structuralweak mass loss is also observed at about 250，400unit is a layer of carbon hexagon[15. These hexa-and 700 C. The mass loss at about 250 C resultsgonal layers form so-called crystallite by stackingfrom further dewatering of the resin. As suggestedin parallel. The crystallites have different degreeby Xiang et a1[14]， the carbonization reaction atof stacking regularity from graphite -like AB stac-around 500 C generates methane, hydrogen, car-king to random one, which is characterized by dif-bon monoxide, and carbon dioxide. The mass lossfere中国煤化Iers L. and perpendic-at 700 C can be assigned to further carbonizationularYHCN M H Ga of resin carbon py-that is mainly a dehydrogenation reaction. Com-pared with the primary phenolic resin, the dewa-rolyzed at different temperatures for 2 h. There aretered one shows much less mass loss during the py-only two wide Bragg peaks observed at around 23°and 44° in the XRD patterns. It suggests that all ofrolysis.Jourmnal CSUT Vol, 13 No. 12006the powders are disordered carbon. With the in-where k is the shape factor (its value is 0. 9 andcrease of pyrolyzing temperature, the Bragg peaks2.0 for L。and La，respectively)[16]; λ is the X-raybecome sharper, which indicates that the regulari-wavelength; β is the half-peak width; and 0 is thety of the carbon crystallite improves. The' XRDBragg angle. The interlayer spacing (door) was cal-patterns for resin carbons obtained at 600 C andculated using Bragg equation750 C show two separated sharp Bragg peaks ataround 44°，which disappear in XRD patterns fordoo2= 2sinθ(2)carbons prepared at higher temperatures. This in-The results of XRD and BET specific areadicates that some crystalline carbon is formed atmeasurements for different carbon materials arethe low pyrolyzing temperature, but the crystallinelisted in Table 1. The pyrolyzing temperature andcarbon is unstable at higher temperatures andsoaking time show similar influences on the struc-transforms to disordered carbon.ture and properties of pyrolytic resin carbon. WithXRD patterns for resin carbons pyrolyzed atthe increase of pyrolyzing temperature or soaking1050 C for different times are shown in Fig. 2(b).time, the sample has a smaller interlayer spacing,All the XRD patterns show two similar diffractionlarger crystallite sizes of Le and L, and lower spe-peaks to those in Fig. 2(a). As the soaking timecific surface area.increases, the resin carbon shows narrower Braggpeaks with improved intensity， which suggestsTable 1 Results of XRD and BET specific surfacethat the regularity of the carbon crystallite im-area measurements for pyrolytic resinproves.carbon materialsBET specificTime/ doo2/re/Lo/surface area/Sample咒nmm(m2●g-1)R1 6000.3746 0. 85958.221 200CR2 7500.3731 0. 86965.840.3700 0. 953. 6361050C1050;0.3708 1. 014.3408.51900 CR51200.3693 1.117 4. 6482.74750 CR6 1 0500.3700 0.919 4. 23412. 84600 CR7 1 05C0.3708 0.998 4. 4969. 30102030405060708090, 0500.3700 1.018 4. 7768.2720/(° )Note: L. for R1 and R2 was not calculated because there were twodiffraction peaks at around 44° in their XRD patterns[6)3.3 Electrochemical characteristics of pyrolyticresin carbonElectrochemical performances of resin carbonanodes pyrolyzed at various temperatures are~J12hshown in Fig. 3. In Fig. 3，profiles of sample B,C, D, E were shifted by0.2, 0.4, 0.6 and 0.8V，5hrespcetivley. The pyrolyzing temperature has a2hdrastic influence on charge discharge characteris-tics of the resin carbon anodes. The resin carbonOhanode pyrolyzed at 600 C shows larger discharge02030405060708090capacity of 1 068 mA●h/g and charge capacity (re-versible capacity) of 536 mA●h/g, but its irre-versible capacity (532 mA●h/g) is very high andFig.2 XRD patterns for pyrolytic resin carbon(a)the initial coulombic efficiency is only 50. 2%. Aspyrolyzed at different temperatures for 2 hand (b) pyrolyzed at 1 050 C for different timesthe pyrolyzing temperature increases, the dis-charge capacity. and charge capacity of the carbonCrystallite sizes of L。and L. for the carbonanode中国煤化工ge of discharge ca-samples were determined from the (002) and {10}pacityHCNMHG=of charge capacity.Thereof the carbon an-band Bragg peaks using the Scherrer equationodes decreases and the initial coulombic efficiencyLau=。k(1)rises. The discharge capacity and charge capacity= Pcosθof the resin carbon anode pyrolyzed at 1 200 C areGUO Hua-jun, et al; Optimizing pyrolysis of resin carbon for anode of lithium ion batteries488 mA●h/g and 361 mA●h/g, respectively, andthe irreversible capacity reduces to 127 mA●h/g，3.5a)while the initial coulombic increases to 74. 0%.3.0Furthermore, the carbon anode pyrolyzed at highertemperature has lower average charge potentialwith charge capacity increasing in the low potential2.0D- 12hrange and with charge capacity reducing in the highpotential range. The ratio of charge capacity below0.5 V to the total charge capacity for the resin car-号1.0F_Dbon anode pyrolyzed at 600 C is only 26. 5%，0.5while that for the resin carbon anode pyrolyzed at1 200 C reaches 52. 9%.0 100 200 300 400 500 600Capacity/(mA.h*g-)3.5[向)3.0 |600C3.5[6)B一750%900CD一1050CA三11.5, 1.5-1.0E. D。C--BA0.00400600 800 1 000100200300400Capacity/(mA.h.gl)Capacity/(mA.h.g')A- 600 CIPFig.4 Electrochemical characteristics of.750C900 Cresin carbon anodes pyrolyzed at 1 050 C forD- 1050Cdifferent timeE- 1200 CProfiles of sample B, C, D areshifted by 0.2, 0.4 and 0. 6 V, respectivley(a) - Discharge voltage profiles;号(b)- Charge voltage profilesand L，so the reversible capacity in low potentialrange increases. The specific surfaces area of the400 600800 1000resin carbon is much lower for the resin carbon py-Capacity(mA.h*g")rolyzed at higher temperature or soaked at the tprFig. 3 Electrochemical characteristics of resinfor longer time, which results in less irreversiblecarbon anodes pyrolyzed at different temperaturescapacity. With a compromise of reversible capaci-(a) - Discharge voltage profiles; (b)- Charge voltage profilesty, irreversible capacity and coulombic efficiency,the optimum conditions for pyrolysis of phenolicFig. 4 shows voltage profiles for the resin car-resin are determined as follows: pyrolyzing tem-bon anodes pyrolyzed at 1 050 C for 0, 2, 5, andperature, 1 050 C and soaking time, 2 h.12 h, respectively. As the soaking time increases，the carbon anode has smaller reversible capacity4 CONCLUSIONSand irreversible capacity, larger initial coulombicefficiency and lower average charge potential, but1) The carbon materials pyrolyzed from phe-the change is very little when the soaking time isnolic resin are disordered carbon. With the increaseabove 2 h.of the pyrolyzing temperature and soaking time，The difference of electrochemical characteris-the:crystallite improvestics of the resin carbon anodes can be attributed toand t中国烧-ea decreases.changes of structure and physical properties.:DHC N M H Gure or soaking timethe pyrolysis temperature or soaking time increa-increases, botn' reversidle capacity and irreversibleses, the resin carbon shows better regularity ofcapacity of the resin carbon anode decrease, whilecarbon crystallites with larger crystallite sizes of L。the initial coulombic efficiency increases.Journal CSUT Vol.13 No.1 20063) The optimum conditions for pyrolysis ofSi-doped composite carbon as anode of lithium ion bat-phenolic resin are determined as follows: pryolyz-teries[J]. Transactions of Nonferrous Metals Societying temperature, 1 050 C and soaking time, 2 h.of China, 2003， 13(5): 1062 - 1065.The resin carbon pyrolyzed under the optimum[8] Jung Y, Suh M C, Shim s C, et al. 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