CVI法によるリチウムイオン電池負極用低結晶性炭素繊維の表面修飾

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愛総研・研究報告第13号 2011年

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，司法によるリチウムイオン電池負極用低結晶性炭素繊維の表面修飾

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Abstract To reduce the high irreversible capacity ofthe low crystalline carbon fiber for the anode material of lithium-ion battery， pyrolytic carbon (pyrocarbon) was coated at 9500C from C 3Hs(30%)-H2 gas system using pressure-pulsed chemical vapor infiltration. Carbon fiber was coated with the dense pyrocarbon film having the laminar texture and the low surface area ofl.9 m2 _g_-_l_I_t_{was r}_e_v_e_a_l_e_d_企_{omXRD a}_n_d_{Raman s}_p_巴_c_t_r_o_s_c_o_p_yt_h_a_t the crystallinity of pyrocarbon is higher than that of，the core carbon. Electrochemical properties were measured

in ethylene carbonate但C)and propylene carbonateσC) base electrolytes. Irreversible capacity was reduced in EC-based electrolyt巴bycoating with 8 mass% pyrocarbonヲwhichwould be attributed to the high crystallinity，

laminar structure and low surface area of pyrocarbon. Irreversible capacity was also decreased in PC-based electrolyte. The crystallinity ofpyrocarbon was not so high as PC-based electrolyte was decomposed in the case of the high crystalline graphite 1. Introduction Low crystalline carbon such as non-graphitizing carbon (hard carbon) has received the a社ention for its high reversible capacity， which exceeds the theoretical capacity of graphite (372 mA h g-l) for the anode of lithium ion battery. Low crystalline carbons prepared企omnatural materials such as paper and wood also have the large reversible capacity， however， irreversible capacity is0食印刷ghfor these disordered structure [1] Pyrocarbon-coating by CVD was recently applied to graphite司based anodes of lithium-ion secondary ba社ery to improve the anode performance， especially in propylene carbonate σC) containing solvent [2-4，] which was continuously d巴composedby the high crystalline graphite Among the CVD technique， pressure-pulsed chemical vapor deposition(PCVD) / infiltration(PCVI) allows homogeneous coating with the relatively high crystalline pyrocarbon through the thickness of the porous substrate under the suitable conditions [5-8，]resulting in high first coulombic efficiency of 十愛知工業大学工学部応用化学科(豊田市) pyrocarbon-based anode [9]. How巴ver，the increase of the surface crystallinity may cause the PC solvent to decompose. In present study， the relation between surface structure and electrochemical properties was inv巴stigated for the low crystalline carbon fiber and the samples coated with pyrocarbon using PCVI methods in both巴thylenecarbonate (EC) and PC base electrolytes 2. Experim思ntal The carbon fiber substrate was prepared by the carbonization ofthe commercial filter paper (ADV ANTECH MFS) at 10000C inAr for4 h Coating with pyrocarbon was perforrned using the typical PCVI appara旬S5) The source gas mi対ureofC3Hs (30%) -H2

was allowed to flow into a reservoir.It was instantaneously introduced (within 0.1 s) into the reaction vessel up to 0.1 MPa， and the pressure was held under the same condition to allow matrix deposition for 1.0 s (holding time). Then， the reacted gas was evacuated to below 0.7 kPa within 1.5 s. This cycl巴ofthe 29

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30 愛知工業大学総合技術研究所研究報告，第13号， 2011年

sequential steps was defined as one pulse， and repeated to the desired number of times. The temperature for PCVI treatment was kept at 950 'C

The morphology of the samples was observed using scanning electron microscope (SEl¥久 JEOL，JSM820). Stmcture of pyrocarbon was examined by X-ray diffraction (XRD) measurement (Shimazu， XD-610 with Cu Ka radiation) and Raman spectroscopy (Jasco， NRSIOOO with Nd:YV04 laser of

532 nm). The Bmnauer-Emmett嗣Teller(BET) surface area and

mesopore volume distribution were measured using nitrogen gas (MicromeriticsフGemini2375)

Charg巴/dischargecycling was made at 25 'C， using a three

electrode cell with metallic lithium as counter and reference electrodes， in 1 mol L-1 _LiCI0₄_{ECIDEC (}₁_:₁₎_o_r_{PC s}_o_l_u_t_i_o_n

Discharging and charging were performed under the condition of constant current of 30 mA g-l followed at

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nstant potential of3m V vs. Li/L

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for 48 h (CCCV method) and constant current of 30 mA g-l (CC method)ョrespectively.The samples were dried at 150'C for 15 h under vacuum， before using as working electrodes 3. Results and discussioIl Fig. 1 shows the typical SEM images of the original carbon fiber (a) and the pyroc訂bon-coatedsample obtained a丘町20000 pulses in PCVI treatment. 1t can be observed that the relatively rough surface of th巴originalcarbon fiber became smooth in submicron scale. It appears that pyrocarbon film adher巴stightly to the carbonized fiber. It can be also observed that pyrocarbon has the laminar te双ureoriented parallel to the surface of the carbonized fiber. This laminar st孔lctureis effective in reducing the irreversible reaction such as the decomposition of the electrひl戸esbecaus巴thebasal planes of carbon crystallites with low reactivity are mainly exposed to the electrolyte solution. Stmctural properties are shown in Table 1 for the original and pyrocarbon-coated carbon fibers. The do02 of the pyrocarbon-coated samples are below 0.358 nmフ which紅巳 lower than that of the original carbonフhowever，紅巳much higher than that of graphite (0.3354 nm). R value (IDIlG) calculated 企omRaman spectmm of original carbon fiber is slightly decreased by coating with pyrocarbon. From these results by XRD and Raman spectroscopy， it is considered that the ι:rystallinity of pyroc紅bonfilm is higher than that of core carbon. However， the crystallinity of pyroc紅bonis low in comparison with that of high crystalline graph此e.The BET surface area is decreased from 223 m2_g_-1 _o_f_t_h_e_o_r_i_g_i_n_a_l_c_a_r_b_o_n fib巴rto 1.89 m2g司1after coating with 8.3 mass% pyrocarbon Fig. 1.SEM images of original carbon fiber (a) and pyrocarbon-coated sample(b) obtained by PCVI tr巴atmentwith 20000 pulses Surface area is furth巴rreduced with the increase of mass fraction and film thickness of pyrocarbon. From pore volume distribution analysis， i was ft ound that the pores with the diam巴terof 1.5-5 nm were extremely decreased by pyrocarbon coatmg Fig. 2 shows the first charge-discharge curves of original {(a) and (c)} and pyrocarbon-coated carbon fibers {(b) and (の}in 1 mol L-1 _LiCI0₄_EC_厄_EC(₁_:₁₎_e_l_e_c_t_r_o_l_戸_e{₍_a₎_a_n_d₍_b₎_}_a_n_d₁

mol L.1 _LiCI0₄_{PC e}_l_e_c_t_r_o_l_y_t_e_{₍_c₎_a_n_d ₍_の_}_{. The}

charge-discharge profiles of the sample coated with 5-20 wt% pyrocarbon were similar to that observed in 匂pical non-graphitizing carbon having the disordered stmcture. Charge capacity (Li de-intercalation) of the coating sample is 450δ00 mA h g-l in each electrolyte， which is nearly close to that ofthe original carbon fiber. High irreversible capacities of 180 and 132 mA h g-l are observed in EC and PC司based巴lectrolytefor the original carbon fiber， respectively， reflecting the disordered stmcture and high surface訂eaof original carbon.Irreversible capacity is reduced企om180 to 132 mA h g-l in EC-based electrol戸巴bycoating with 8 mass% pyrocarbon. As mentioned above， the pyrocarbon film has higher crystallinity and lower surface area than those of the core carbon. Inaddition， pyrocarbon has the laminar te双ure，in which the basal planes of the crystallites are oriented p訂allelto the surface of the core carbon fiber.These structural features of pyrocarbon would

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一お B m 一 Z CVI法によるリチウムイオン電池負極用低結晶性炭素繊維の表面修飾 Raman R value l.8 'fable 1 Structural prop巴抗iesof original carbon fiber substrate and pyrolytic carbon (pyro圃C)coated samples Number of 恥1ass企actionof Average thickness of pulses pyro回C/ % pyro-C /μm Original 0 0 500 ラ改D d002 / nm 0.386 l.89 1.4 0.358 8.3 0.11 l.46 0.81 1.4 1.3 0.355 0.353 0.52 19.8 59.7 円 u n u n u 唱EA 5000 would be lower than that of the core carbon fiber regarded as non-graphitizable carbonヲresultingin the decrease of capacity with mass fraction of pyrocarbon [10]. In fig. 3 (b)， it is found that irreversible capacity is rapidly reduced by coating with 8-10 mass% pyrocarbonラ reflectingthe change of the nano-scaled surface struc加reas mentioned above. Irreversible capacity can be slightly decreased with increasing the mass企action of pyrocaτbon in both electrolytes， which would be attributed to the small reduction of the BET surfac巴areawith the thickness ofpyrocarbon film as shown in Table1.From the results offig 3 (a) and (b)， it is considered that thin pyrocarbon film with uniform thiclmess is d巴siredin order to achieve high coulombic efficiency at first cycle without reducing the reversible capacity. Irreversible capacity in PC-based electrolyte is slightly lower cause the decrease of irreversible reactions such as decomposing the electrolytes and trapping lithium ions. Irreversible capacity can be also decreased in PC-based electrol戸巴 Thecrystallini勺r of pyrocarbon is low compared with high crystalline graphiteフ on which PC-based electrolyte is decomposed. Fig. 3 shows the dependence of ch訂ge capacity (a) and irreversible capacity (b) of pyrocarbon-coated samples on mass fraction of pyrocarbon. Charge capacity (Li de-intercalation) of the pyrocarbon-coated sample is nearly close to that of the original carbon up to around 20 mass% of pyrocarbon in each electrolyte. Above 20 mass%， howeverヲcapacityis decreased

with mass fraction of pyrocarbon. Pyrocarbon coated in present study is considered to be the graphitizable carbon (i.e. sof王 carbon). Therefore， the reversible capacity ofthe pyrocarbon 一「一一「一

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Fig. 2. First charge-discharge curves of original {(a) and (c)} and pyrocarbon-coated carbon fibers {(b) and (d)} in 1 mol L-1

LiCI04 EC/DEC (1:1)electrol戸e{(a) and (b)}and 1 mol L-' LiCI04 PC electrolyte {(c) and (d)}. Mass企actionof pyrocarbon; (b)

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pyrocarbon-infiltrated sample. Reversible capacity of the sample coated with pyrocarbon up to 20 mass% was 450-500 mA h g-l in each electrolyte， which was nearly close to that of th巴originalcarboniz巴dpape.r 1五ghirreversible capacity was observed in the original carbonized paper， reflecting the disordered structure and high surface area. 1町ev巴rsible capacity was reduced in EC-based electrolyte by coating with 8 mass% pyrocarbon， which would be attributed to the high crystallinityコ laminar structure and low surface area of

pyrocarbon. Irreversible capacity

PC-based electrolyte. The crystallinity of pyrocarbon was low compared with high crystalline graphite， on which PC-based electrolyte was decomposed. Irreversible capacity can be slightly decreased with increasing the mass fraction of pyrocarbon in both electrol戸es，however， reversible capacity is also decreased with mass fraction of pyrocarbon 愛知工業大学総合技術研究所研究報告，第13号， 2011年 than that in EC-based electrolyte in the region below 30 mass% of pyrocarbon as shown in fig. 3 (b). Because PC molecule is bulkier than EC molecule， the surface紅eaofthe poreinwhich PC molecule is accessible would be small in comparison with the area for EC molecule.Itis supposed that the small effi巴ctive surfac巴紅eafor PC electrol戸eresults in the decrease of the electrolyte decomposition to reduce the irreversible capacity However， the details紅eopen to further investigation 32 decreased in also was (a) 550 も)500 2450 h E400 悶 u 邑 350 C百 ..c

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Acknowledgement This work is partially supported by l¥I[E文

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Private University Project Grant under contract # 1001033. The authors gratefully thank Dr. A. Y oshida for企uitfuldiscussion. 80 ECIDEC

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H. Wada， J.Power Sourcesヲ26(1989) 545. References 80 Fig. 3. Dependence of charg巴capacity (a) and irrev巴:rsible capacity (b) of pyrocarbon-coated sample on mass企actionof pyroc訂bon 70 60 40 50 20 30 10 70 Mass fraction of pyrocarbon / % 50 60 20 30 40 10 300 250 0 200 180護 (b) 160 140

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て国立︿戸﹄ ¥hH 一一U 悶且悶 U ω 一且一回﹄ ω ﹀ ω ﹄﹄ 3. Condnsion In present study， the relation between surface structure and electrochemical properties was investigated in ethylene carbonate(EC) and PC bas巴巴lectrolytesfor the low crystalline carbon fiber coated with pyrocarbon using PCVI methods Carbon fibers were coated with the dense pyrocarbon films having the laminar texture oriented parallel to the surface of the carbon fiber.Itwas revealed from XRD and Raman spectroscopy that the crystallinity of pyrocarbon is higher than that ofthe carbon fibe BET s.r urface area was decreased企om 223 m2 _g_-_l_o_f_o_r_i_g_i_n_a_l_s_u_b_s_t_r_a_t_e_t_o₁_{.9 m}2 _g_-_l_o_f_t_h_e