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研究生:楊綠曄
研究生(外文):Lye-Ye Yang
論文名稱:聚矽氧烷改質超分歧寡聚物用於高電壓型鋰離子電池之研究
論文名稱(外文):Study of modified hyperbranch oligomer by phenlsiloxane and additive on high voltage lithium material and its performance evaluation of lithium ion battery
指導教授:陳崇賢陳崇賢引用關係王復民
指導教授(外文):Chorng-Shyan ChernFu-Ming Wang
口試委員:陳崇賢王復民
口試委員(外文):Chorng-Shyan ChernFu-Ming Wang
口試日期:2016-07-19
學位類別:碩士
校院名稱:國立臺灣科技大學
系所名稱:化學工程系
學門:工程學門
學類:化學工程學類
論文種類:學術論文
論文出版年:2016
畢業學年度:104
語文別:中文
論文頁數:137
中文關鍵詞:鋰離子電池安全性添加劑過量鋰鎳錳氧寡聚物
外文關鍵詞:lithium ion batterybattery safetyadditiveLNMOSTOBA
相關次數:
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鋰離子電池泛用於生活中,如高科技產品的儲能裝置,與動力車中。因此如何提升鋰離子電池安全性,一直都是大家關注的議題。由於充放電過程中,最不穩定的材料為正極,所以直接添加於正極的正極添加劑,逐漸受到學界與業界重視。正極添加劑加入於電池中,經過充放電程序後,會於正極材料表面覆蓋一層固液電解質界面(Solid electrolyte interface, SEI),此SEI層能夠隔絕正極材料與電解液接觸,或短路時與負極材料之接觸,進而提升電池的安全性。本研究是合成一種新型高分歧型寡聚物作為鋰離子電池中的安全性添加劑・藉由聚矽氧烷(APTES-PhSLX)的加入,可使整體高分歧型寡聚物的活化能上升,進而增加其熱穩定性。由研究結果指出,此新型高分歧型寡聚物添加入高電壓型過量鋰系層狀正極材料中,可以增加其安全性與電池性能。電化學實驗結果也指出,鋰離子擴散能力也大幅提升。
Lithium ion betteryhave been generally used in our live, i.e digital cameras, laptop and electric vehicles. Research and development has focused on improving the performance of existing battery systems, and safety. When lithium batteries charge/discharge , cathode is unstable .Recently, additives for cathode material in lithium ion battery.After an electrochemical process, these additives form the Solid Electolyte Interface (SEI) layer on the surface of cathode material, which can isolate cathode material to prevent contact of electroly and anode as well asenhance the safety in lithium ion batteries.In the study, bismaleimide (BMI), barbituric acid(BTA) and phenylsiloxane oligomer were used to synthesize BMI/BTA/APTES-PhSLX as an additive by Michael addition reaction and free radical reaction. The BMI/BTA/APTES-PhSLX as additive into Li1.2Ni0.2Mn0.6O2 material to investigate theeffect of performance and thermal stability on battery.According to charge/discharge and cycling tests,half-cell which added 130(phslx) increased the capacity and cycling performance. In the thermal exothermic,the 130(phslx) heat toapproximately 18.1% in room temperature.therefore, the cell would enhance the thermal stability when it operated after 40℃ all day.Finally, in cyclic voltammetry, the 130(phslx)’s cell has better diffusion of lithium ion.
摘要 I
Abstract II
致謝 III
目錄 IV
圖目錄 VIII
表目綠. XIV
第一章 緒論 1
1-1前言 1
1-2研究動機與目的 12
第二章 文獻回顧 14
2-1 正極材料LNMO的介紹 14
2-2鋰電池內部反應機制 16
2-2-1 固液電解質界面(Solid electrolyte interface SEI) 16
2-2-2 鋰電池在高溫下的反應機制 20
2-3鋰電池之安全機制 23
2-3-1電子電路設計(Electronic Design) 23
2-3-2單電池機構設計(Mechanic Design) 24
2-3-3電池內部材料設計(Material Design) 25
2-4添加劑之種類 27
2-4-1電解液添加劑 27
2-4-2 電極添加劑 41
第三章 實驗方法 46
3-1實驗儀器與設備 46
3-2實驗藥品 48
3-3實驗步驟 50
3-3-1 BMI/BTA製備[77] 50
3-3-2 Phenylsiloxane製備[77] 51
3-3-3 BMI/BTA-APTES-PhSLX製備[77] 52
3-3-4 BMI/BTA-APTES-PhSLX一次合成的製備 53
3-3-5陰極(正極) 極片之製備 55
3-3-6電池組裝[77] 56
3-4分析與鑑定 57
3-4-1掃描式電子顯微鏡(SEM) 57
3-4-2熱差式掃瞄卡計儀(DSC) 58
3-4-3 核磁共振儀分析(NMR) 59
3-4-4 X-ray能量散佈光譜分析儀(EDS) 60
3-4-5 旋轉流變儀 60
3-4-6光學視頻接觸角量測儀 61
3-5電化學性能分析 62
3-5-1電池性能測試 62
3-5-2交流阻抗分析 62
3-5-3循環伏安法(Cycle Voltammetry) 63
第四章 結果與討論 66
4-1材料合成鑑定與結構分析 66
4-2流體性質分析 70
4-3極片之表面張力 72
4-4電化學電性分析 73
4-4-1 添加劑對電池充放電與循環測試之影響 74
4-4-2交流阻抗分析 81
4-4-3循環伏安分析 93
4-4-4利用不同掃描速率計算鋰離子擴散係數 105
4-5添加劑對電池之熱穩定性的影響 113
4-6添加劑對正極材料表面之分析 118
4-6-1添加添加劑於正極極片表面之SEM分析 118
4-6-2添加添加劑之正極極片表面之EDS分析 122
第五章 結論 124
第六章 未來展望 127
第七章 參考文獻 128
[1]立法院國會圖書館-外國法案介紹
[2]經濟部能源局-2012年01月能源報導-再生能源
[3]我國二次鋰電池產業狀況概論—可攜帶電子設計.2001.7.16
[4]必翔電能—電池性能之比較
[5]工業材料雜誌 290期二月號探討「動力鋰電池材料」及「綠色窗智慧建材」技術
[6]F.Chenga, J.Chen, H. Zhou, and A.Manthirama, Structural and Electrochemical Characterization of (NH4)2HPO4-Treated Lithium-Rich Layered Li1.2Ni0.2Mn0.6O2 Cathodes for Lithium-Ion Batteries., J. Electrochem. Soc. volume 160, issue 10(2013),A1661.
[7]黃可龍、王兆翔、劉素琴,鋰離子電池原理與技術Lithium Ion Batteries Principles and Key Technologies,五南文化事業機構 (2010).
[8]H.S.Ryang,U.S.Patent 1381396(1983).
[9]H.S.Ryang,U.S.Patent 4404350(1984)
[10]H.S.Ryang,U.S.Patent 14533737(1984)
[11]V.J.Eddy and J.E.Hallgren.U.S.Patent 4542226(1984)
[12]V.J.Eddy and J.E.Hallgren.J.Org.Chem.52.1903(1987)
[13]S.A.Swibt and M.A.Buese.J.Organmetallic Chem.
[14]V.J.Eddy and J.E.Hallgren.R.E.Colborn J.Polym. Sci. Part A.Vol.28,2417(1990)
[15]A. Robert Armstrong, Michael Holzapfel, Petr Nova'k, Christopher S. Johnson,S.H. Kang,M..M. Thackeray, and P G. Bruce, Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery CathodeLi[Ni0.2Li0.2Mn0.6]O2, J. AM. CHEM. SOC. 128(2006), 8694.
[16]K. J. Carroll,D. Qian, C. Fell,S.t Calvin, G. M. Veith, .F. Chi, L. Baggettod , Y.S. Meng, Probing the electrode/electrolyte interface in the lithium excess layered oxide Li1.2Ni0.2Mn0.6O2 ,Phys.Chem. Chem. Phys.,15(2013), 11128
[17]P.Yan,A.Nie,J.Zheng, Y.Zhou,D.Lu,X.Zhang, R.Xu, I.Belharouak, X.Zu,Jie Xiao,K.mine,J.Liu,F.Gao, R.S.assar, J.G. Zhang, C.M.Wang, Evolution of Lattice Structure and Chemical Composition of the Surface Reconstruction Layer in Li1.2Ni0.2Mn0.6O2 Cathode Material for Lithium Ion Batteries.,Nano Lett. 15(2015),514.
[18]E. Peled,The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems—The Solid Electrolyte Interphase Model., J. Electrochem. Soc., 126,12(1979),2047.
[19]D. Aurbach, E.Zinigrad, Y. Cohen, H. Teller,A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions., Solid State Ionics, 148(2002),405.
[20] S. S. Zhang,A review on electrolyte additives for lithium-ion batteries”, Journal of Power Sources, 162(2006),1379-.
[21]趙信豪,「鋰電池三元系正極材料之添加劑製備及電池性能探討」,碩士論文,國立中央大學,桃園市(2012)
[22]S. S.Zhang, K. Xu, T.R. Jow, EIS study on the formation of solid electrolyte interface in Li-ion battery, Electrochimica Acta, 51(2006),1636.
[23]G.Cherkashinin, K. Nikolowski, H. Ehrenberg, S. Jacke, L. Dimesso, W. Jaegermann,The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte–cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation, Phys. Chem. Chem. Phys., 14 (2012),12321.
[24]Q.Wang, P. Ping,X. Zhao, G. Q. Chu, J. Sun, C. Chen,Thermal runaway caused fire and explosion of lithium ion battery, Journal of Power Sources, 208 (2012),210.
[25]E. Markevich,G.alitra,D. Aurbach,Influence of the PVDF binder on the stability of LiCoO2 electrodes, Electrochemistry Communications, 7(2005),1298.
[26]楊長榮,「鋰離子電池安全技術發展」,工業材料雜誌,264 (2008).
[27]朱愛敏、朱華,「鋰離子電池保護電路設計」,常州工學院學報,18 (2005).
[28]呂孟哲,「FEM輔助鋰離子二次電池安全閥之開發研究」,碩士論文,國立台灣大學,台北 (2007).
[29]Marshall Brain, How Lithium-Ion Batteries, How Stuff Works (2006).
[30]X. P. Ai, C. Lin, Q. L. Liao, B. D. Liu, H. X. Yang,A potential-sensitive separator capable of providing a reversible overcharge protection for lithium ion. J. Wuhan Univ., 52,2 (2006),149.
[31]H. Zhong, C. Kong, H. Zhan, C. Zhan, Y. Zhou,S batteries afe positive temperature coefficient composite cathode for lithium ion battery, Journal of Power Sources, 216(2012),273.
[32]H. J.Kweon, J. J.Park, J. W.,Seo,G. B. Kim , B. H. Jung, H. S. Lim, Effects of metal oxide coatings on the thermal stability and electrical performance of LiCoCO2 in a Li-ion cell, Journal of Power Sources, 126(2004), 156.
[33]F. M.Wang, H. M. Cheng ,H. C. Wu, S. Y. Chu , C. S.Cheng , H. R.Yang, Novel SEI formation of maleimide-based additives and its improvement of capability and cyclicability in lithium ion batteries, Electrochimica Acta, 54(2009),3344.
[34]T.Sasaki , T. Abe , Y.Iriyama, M. Inaba, Z.Ogumi, ,Suppression of an alkyl dicarbonate formation in Li-ion cells, J. Electrochem. Soc., 152 (2005),A2046.
[35]Y. S.Hu, W. H. Kong, Z. X.Wang, H. Li , X.Huang, L. Q. Chen, Effect of Morphology and Current Density on the Electrochemical Behavior of Graphite Electrodes in PC-Based Electrolyte Containing VEC Additive, Electrochem. Solid-State Lett., 7,11 (2004), A442.
[36]H.Gan , Takeuchi, hosphonate or phosphite additives for nonaqueous organic electrolytes lithium ion rechargeable batteries, U.S. Patent, 6(2002),495,285.
[37]M. W.Wagner, C.Liebenow, J. O. Besenhard, Effect of polysulfide-containing electrolyte on the film formation of the negative electrode, Journal of Power Sources, 68 (1997),328.
[38]R. J.Chen, F.Wu, L.Li, Y.Guan, X. P.Qiu, S. Chen, Y. J. Li , S. X. Wu, Butylene sulfite as a film-forming additive to propylene carbonate-based electrolytes for lithium ion batteries, Journal of Power Sources, 172(2007),395.
[39]Y. J. Ein-Eli, Electroanal. Chem., 531 (2002),95.
[40]Y.Ein-Eli, S. R.Thomas, V. R.Koch ,The role of  SO2 as an additive to organic Li‐ion battery electrolytes, J. Electrochem. Soc., 144(1997),1159.
[41]C.Wang, H.Nakamura, H.Komatsu, M. Yoshio, H. Yoshitake, Electrochemical behavior of a graphite electrode in propylene carbonate and 1,3-benzodioxol-2-one based electrolyte system, Journal of Power Sources, 74(1998),142.
[42]J.Zhen,S.usking,K.mine,Lithium difluoro(oxalato)borate as a functional additive for lithium-ion batteries, Electrochemistry Communications, 9(2007),475.
[43]S. H.Wu, Effects of tris(pentafluorophenyl) borane (TPFPB) as an electrolyte additive on the cycling performance of LiFePO4 batteries, J. Electrochem. Soc., 160,4 (2013),A684.
[44]L. F.Li, H. S. Lee, H. Li, X. Q.Yang, X. J. Huang, A pentafluorophenylboron oxalate additive in non-aqueous electrolytes for lithium batteries, Electrochemistry Communications, 11(2009),2296.
[45]E.Wang, D. Ofer,W.Bowden, N.Iltchev, R.Moses, K.Brandt, tability of lithium ion spinel cells. III. improved life of charged cells, J. Electrochem. Soc., 147 (2000),4023.
[46]M. Y.Saidi, F.Gao, J. Barker, C.Scordilis-Kelley, U.S. Patent, 5(1998),846,673.
[47]Z.Chen, W. u, Q. J.Liu, K.Amine, LiPF6/LiBOB blend salt electrolyte for high-power lithium-ion batteries, Electrochimica Acta, 51 (2006),3322.
[48]O.Hiroi, K.Hamano, Y. Yoshida,S.oshioka, H. Shiota, J. Aragane, S.Aihara, D. Takemura, T. Nishimura , M.Kise, H. Urushibata, H.Adachi, U.S. Patent, 6(2001),305,540.
[49]B.Sun, Z. T. Zhou, X. D.Xia, Tris(2,2,2-trifluoroethy1)phosphite as a flame-retardant additive in electrolytes for Li-ion batteries, Chinese Journal of Power Sources, 29,9(2005),586.
[50]X.Wang, H.Naito, Y. Sone, G. Segami, S. Kuwajima, New additives to improve the first-cycle charge–discharge performance of a graphite anode for lithium-ion cells, J. Electrochem. Soc., 152(2005),A1996.
[51]W.Li, C. Campion, , B. L. Lucht, B.Ravdel, J. DiCarlo, K. M.Abraham , Additives for stabilizing LiPF6-based electrolytes against thermal decomposition, J. Electrochem. Soc., 152(2005),A1361.
[52]M. Ishikawa, M. Morita, Y. Matsuda, In situ scanning vibrating electrode technique for lithium metal anodes, Journal of Power Sources, 68 (1997),501.
[53]R. D.Rauh, S. B.Brunner,The effect of additives on lithium cycling in propylene carbonate, Electrochim. Acta, 22(1977),75
[54]T.Hirai, I.Yoshimatsu, J. I. Yamaki, Effect of additives on lithium cycling efficiency, J. Electrochem. Soc., 141(1994),2300.
[55]E.Eweka, J. R. Owen, A.Ritchie, Electrolytes and additives for high efficiency lithium cycling, J. Power Sources, 65(1997),247.
[56]M.Ishikawa, H.Kawasaki, N.Yoshimoto, M. Morita, Pretreatment of Li metal anode with electrolyte additive for enhancing Li cycleability, Journal of Power Sources, 146(2005),199.
[57]M.Adachi, K. Tanaka, K.Sekai, Aromatic compounds as redox shuttle additives for 4 V class secondary lithium batteries, J. Electrochem. Soc., 146(1999),1256.
[58]L. M.Moshurchak, C.Buhrmester, J. R. Dahn, Spectroelectrochemical studies of redox shuttle overcharge additive for LiFePO4-based Li-ion batteries, J. Electrochem. Soc., 152(2005),A1279.
[59]J. R. Dahn, J. Jiang, M. D. Fleischauer, C. Buhrmester, L. J.Krause, High-rate overcharge protection of LiFePO4-based Li-ion cells using the redox shuttle additive 2,5-Ditertbutyl-1,4-dimethoxybenzene, J. Electrochem. Soc., 152(2005),1283
[60]A.Omaru, A. Yamahuchi, M. Nagamine, U.S. Patent, 6,146,790 (2000).
[61]Lain, M. J., Neat, R. J., U.S. Patent, 6,387,571 (2002).
[62]Kerr, J. B., Tian, M., U.S. Patent, 6,045,952 (2000).
[63]Z.Chen, K.Amine, Bifunctional electrolyte additive for lithium-ion batteries, Electrochemistry Communications, 9 (2007),703.
[64]K.Abe, Y. Ushigoe, H. Yoshitake, M. Yoshio, Functional electrolytes: Novel type additives for cathode materials, providing high cycleability performance, J. Power Sources, 153(2006),328.
[65]H.Lee, J. H.Lee, S.Ahn, H. J.Kim, J. J. Cho, Co-use of cyclohexyl benzene and biphenyl for overcharge protection of lithium-ion batteries, Electrochemical and Solid State Letters, 9 ,6 (2006)A307.
[66]A.Yoshino, Proceedings of the 3rd Hawaii Battery Conference, ARAD Enterprises, Hilo, HI (2001).
[67]J. N.Reimers, B. M. Way, U.S. Patent, 6, 74(2000), 777.
[68]X. M.Wang, E.Yasukawa, Nonflammable trimethyl phosphate solvent-containing electrolytes for lithium-ion batteries, J. Electrochem. Soc., 148(10) (2001)A1058.
[69]C.Y.Hu, X. H. Li, Non-flammable electrolytes based on trimethyl phosphate solvent for lithium ion batteries, Trans. Nonferrous Met. Soc. China, 15(6) (2005)1380.
[70]K.Xu, M. S. Ding, S. S.Zhang, J. L. Allen,T. R. Jow, An attempt to formulate nonflammable lithium ion electrolytes with alkyl phosphates and phosphazenes., Journal of The Electrochemical Society, 149(2002)A622.
[71]Xu, K., Zhang, S. S., Allen, J. L., Jow, T. R.,“Nonflammable electrolytes for Li-ion batteries based on a fluorinated phosphate”, Journal of The Electrochemical Society, 149:A1079-A1082 (2002).
[72]J.Arai ,Nonflammable methyl nonafluorobutyl ether for electrolyte used in lithium secondary batteries”, Journal of The Electrochemical Society, 150(2003)A219.
[73]K.S.Park, D. Im, A. Benayad, A.Dylla,K. J. Stevenson,J. B.Goodenough, LiFeO2-incorporated Li2MoO3 as a cathode additive for lithium-ion battery safety, Chem. Mater. 24(2012)2673.
[74]R.Guo, P. Shi, X. Q. Cheng, Y. L.Ma, Z.Tan, Effect of Ag additive on the performance of LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion battery, Journal of Power Sources, 189(2009)2.
[75]F. M.Wang, S. C. Lo, C. S.Cheng, J. H.Chen, B. J. Hwang, H. C.Wu, Self-polymerized membrane derivative of branched additive for internal short protection of high safety lithium ion battery, Journal of Membrane Science, 368(2011)165.
[76]L. Xia, D. Wang, H. Yang, Y. Cao, X. Ai, An electrolyte additive for thermal shutdown protection of Li-ion batteries, Electrochemistry Communications 25 (2012) 98
[77]李文涵,「超分歧寡聚物添加劑用於正極材料安全性影響及其在高溫對於對鋰離子電池性能分析之研究」,碩士論文,國立台灣科技大學,台北(2013)
[78]Q.T.Pham, M.P.Chen, W.J. Shao, S.C.Huang, F.E.Yu, C.H.Chiang, J.M.Hsu, C.S. Cherna, Synthesis and characterization of bismaleimide/barbituric acid polymers modified by aminopropyl phenyl siloxane oligomer, EIS poster.
[79]X 光微區分析 EDS) - 臺灣大學化學系:http://www.ch.ntu.edu.tw/~rsliu/solidchem/Report/Chapter3_report2.pdf
[80]流變wiki-
https://zh.wikipedia.org/wiki/%E6%B5%81%E5%8F%98%E5%AD%A6
[81]美商沃特斯產品官網:http://www.waters.com/waters/home.htm?locale=zh_TW
[82]接觸角分析儀:140.136.176.3/joom/data/menu/files/exp/CA.ppt
[83]劉茂煌,「分析化學實驗數位學習平台」,輔仁大學教學成果發表,輔仁大學化工系
[84]M.Takahashi, S.I.Tobishima, K. Takei, Y. Sakurai, Reaction behavior of LiFePO4 as a cathode material for rechargeable lithium batteries, Solid State Ionics, vol.148(2002) 283,.
[85]S.H.Wu, J.J.Shiu, J.Y.Lin, Effects of Fe2P and Li3PO4 additives on the cycling performance of LiFePO4/C composite cathode materials, Journal of Power Sources, vol.196(2011) 6676.
[86]江佳璇,「含苯基矽氧烷與不同墨爾比的雙馬來醯亞胺/巴比妥酸枝反應電力學」碩士論文,國立台灣科技大學,台北(2015)
[87] F.E.Yu, H.Y. Lee, J.M. Hsu,J.P.Pan, T.H.Wang,C.S. Chern , Kinetics of polymerization of N,N’-bismaleimide-4,4’-diphenylmethane with Barbituric acid., J. Taiwan Inst. Chem.Eng., accept (Jun,3 2014).
[88]J.P. Pan ,G.Y. Shiau,S.S. Lin, and K.M. Chen, Effect of Barbituric Acid on the Self-Polymerization Reaction of Bismaleimides., Journal of Applied Polymer Science, Vol. 45, (1992) 103.
[89]F.E. Yu, H.Y. Lee, J.M. Hsu,J.P. Pan, T.H. Wa ng, C.S. Chern, Effects of composition and temperature on the kinetics of polymerizations of N,N’-bismaleimide-4,4’-diphenylmethane with barbituric., Journal of the Taiwan Institute of Chemical Engineers 52 (2015) 181.
[90]Z. Chen ,L. Christensen, J.R. Dahn,Large-volume-change electrodes for Li-ion batteries of amorphous alloy particles held by elastomeric tethers., Electrochemistry Communications 5 (2003) 919
[91]S.Hy,F.Felix,J. Rick, W.N. Su, and B. J. Hwang., Direct In situ Observation of Li2O Evolution on Li-Rich High-Capacity Cathode Material, Li[NixLi(1−2x)/3Mn(2−x)/3]O2 (0 ≤ x ≤0.5), J. Am. Chem. Soc. 136(2014) 999
[92]L.J. Zhang, B. Wu, N. Li, D. Mu, C.Z. Zhang, F. Wu.,Rod-like hierarchical nano/micro Li1.2Ni0.2Mn0.6O2 as high performance cathode materials for lithium-ion batteries, Journal of Power Sources 240 (2013), 644
[93]Q.T. Pham,J.M. Hsu,J.P. Pan, T.H. Wang and C.S.Chern, Synthesis and characterization of phenylsiloxane-modified bismaleimide/barbituric acid-based polymers with 3-aminopropyltriethoxysilane as the coupling agent., Polym Int 62(2013) 1045
[94]袁中直、周震濤、李偉善,電解液組成對鋰離子電池碳負極SEI膜性能的影響,華南理工大學材料科學與工程學院(2002/6)
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