超级电容器及其相关材料的研究

【作者】 李会巧；

【导师】 夏永姚；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 博士

【摘要】 超级电容器具有充放电速度快、效率高、循环寿命长、工作温度范围宽、可靠性好等诸多优点,近年来已经成为电化学储能领域的研究热点。但是,与传统的二次电池如锂离子电池相比,超级电容器的能量密度较低。根据超级电容器的能量密度公式E=1/2 CU²,可以通过两种有效的方法来提高电容器的能量密度:一是增大电极材料的比容量（C）,二是提高电容器的工作电压（U）。基于这种分析,本论文做了如下工作:为了提高碳电极的比电容,我们首先从多孔材料的结构优化出发,分别合成了不同孔长和不同孔径的有序介孔碳材料,并研究了长径比和孔径大小对碳材料电化学性能的影响;其次,从非多孔材料出发,开发出一种低成本、大容量的新型碳电容材料:高比表面石墨,并获得了比活性炭高得多的体积比容量;再次,通过在碳材料上负载一种有机聚合物自由基材料来提高整个电极的比容量。为了提高电容器的工作电压,我们引入具有高电位平台的5 V锂电材料LiNi_xMn_2-xO₄作为正极,与活性炭配对后组成混合电容器,获得了高的能量密度。具体内容介绍如下:1.模板法合成反相介孔碳及其长径比对电化学性能的影响介孔碳材料（OMC）具有高度有序的孔道结构,且在中孔范围内孔径分布单一,因此引起了人们的广泛关注。我们以介孔氧化硅为硬模板,以蔗糖溶液为碳前驱体,通过浸渍、煅烧、祛除模板等过程合成了两种孔径相同（4 nm）而孔长不同的有序介孔碳材料。其中,以传统的SBA15复制的介孔碳（LOMC）其孔长度超过了2μm,而以横向生长的新型介孔硅作为模板复制的介孔碳（SOMC）其孔长度只有200～300 nm。循环伏安测试表明SOMC在6 M KOH中的表面比电容达14μF/cm²,而LOMC的表面比电容为10μF/cm²。交流阻抗研究表明电解液在LOMC中的扩散内阻大于SOMC,且当电解液浓度降低时,LOMC的比容量比SOMC衰减更快。这些结果表明,SOMC能为电解液提供更多的开放性入口,因此其表面浸润程度增加,比表面利用率高;同时短的孔长更有利于电解液离子的快速扩散,因而表现出比长径介孔碳更好的电化学性能。2.自组装法合成介孔碳及其孔径大小对电化学性能的影响通过有机-有机两相自组装和有机-无机-有机三相自组装法分别合成了孔径为3.1 nm（di-OMC）和6.7 nm（tri-OMC）的两种介孔碳,并对两者的电化学性能进行了详细的研究。作为双电层电容材料,tri-OMC在有机体系中的比容量达117F/g,且在200 mV/s的高扫速下仍能保持良好的电容行为;而di-OMC由于小的孔径和高的微孔比例,其表面无法被离子半径较大的有机电解液浸润,因而无双电层容量。在水系电解液中,di-OMC的比表面可以被有效的利用,因而表现出117 F/g的比容量,tri-OMC在水系中的比容量为211 F/g,且倍率性能明显优于di-OMC。电化学研究结果表明,对倍率性能而言,碳材料的孔径越大越有利,而对表面比电容来说,不同的电解液所要求的最佳孔径不同,只有孔径与溶液离子半径相匹配时,材料的表面利用率才最高。此外,对这两种碳作为锂离子电池负极的电性能进行了研究,tri-OMC的可逆容量可达1048 mAh/g,约为di-OMC的三倍,且循环寿命优于一般的硬碳类材料。3.新型高比表面石墨材料的制备及其在电化学电容器中的应用以价格低廉的鳞片状天然石墨为原材料,通过高速球磨法制备了一系列高比表面石墨（HSG）材料。球磨前后材料的比表面可从7 m²/g增至580 m²/g,比电容则可从0 F/g增至200 F/g以上。过长时间的球磨会造成石墨晶体结构的严重破坏,使材料的导电性下降,从而影响其倍率性能。中等球磨时间下得到的HSG材料,无论在碱性、酸性还是有机体系的电解液中都可以作为双电层电容材料使用,其高的比电容不仅由高的比表面积贡献,同时也与高速球磨所造成的晶格缺陷和丰富的表面含氧官能团有关。由于结构中仍保留了大量的石墨微晶,因此,HSG的导电性能良好,显示出高的功率性能。经过5000次循环充放电,HSG的容量无明显衰减,表现出良好的循环寿命。此外,由于HSG低的孔隙率,它的体积比容量比活性炭要高的多。综合来看,HSG原料便宜,制备简单,电化学性能良好,有望实现商业化。4.基于有机自由基-碳复合材料的超级电容器通过酯化、聚合和氧化等多步反应合成了一种具有稳定结构的有机聚合物氮氧自由基材料（聚4-甲基丙烯酸-2,2,6,6-四甲基哌啶-1-氮氧自由基酯,PTMA）,其容量为82 mAh/g,在倍率50C放电时仍能显示出明显的放电平台,表现出非常高的功率潜力。由于PTMA本身不导电,因此我们创新性地通过溶解-再沉积的过程将其负载在高比表面的活性炭上形成电容-电池复合材料,以复合材料制备的电极比容量比活性炭电极高30%。以该复合材料为正极、活性炭为负极组成的混合电容器表现出了好的功率性能和长的循环寿命。通过复合材料的方式,即可以同时利用有机聚合物自由基和活性炭两者的比容量,又可以利用活性炭本身良好的导电性来解决有机聚合物自由基不导电的问题,从而发挥出自由基材料的高功率性能,这为更多类似的氮氧自由基材料应用于电化学电容器提供了一种有效的途径。5.基于高电位锂离子嵌入化合物和活性炭的新型非水体系混合电容器首次提出了以高电位的锂离子化合物为正极、以活性炭为负极的非水体系混合电容器。以硝酸盐为前驱体通过溶胶—凝胶法成功合成了尖晶石型LiNi_0.5Mn_1.5O₄材料,它只在4.7 V左右显示一个放电平台,比容量达123 mAh/g,且循环寿命良好。以该材料为正极,商业活性炭为负极,正、负极质量比为1:3,组装成混合电容器,电解液为1 M LiPF6—EC/DMC溶液。与EDLC相比,该混合体系的工作电压从1.4 V提高到了2.1 V,比容量则从17 mAh/g提高到了26 mAh/g。工作电压和比容量的同时提高使得该混合体系的比能量达到了55Wh/kg,超过了EDLC的两倍。以10C的倍率放电,该混合电容器的容量能维持在初始容量的80%以上;经过1000次循环,其容量损失也小于20%。此外,该体系的充放电过程只涉及Li⁺在两极之间的转移,避免了以往电容器如EDLC,Li₄Ti₅O₁₂/AC体系在充电时由于阴、阳离子分离所造成的电解液消耗问题。更多还原

【Abstract】 In recent years,supercapacitors have attracted global attentions due to its fast charge/discharge,high efficiency,long cycle life,wide temperature range and high reliability.However,its energy density is much lower than that of secondary batteries, e.g.lithium-ion batteries.According to the energy equation of supercapacitor E=1/2 CU²,two effective approaches can be used to improve the energy density of supercapacitors:One is to increase the specific capacitance of the electrode material （C）;the other is to promote the output voltage（U）.The present work focuses on improving the capacitance of carbon by the following method:1).Optimize the pore structure of highly ordered mesoporous carbons（OMCs） in both the pore length and pore size,2).Develop a novel non-porous electrode material:high surface-area graphite（HSG）,it showed a high volumetric specific capacitance as well as low cost compared to the conventional activated carbon（AC）;3).Load an organic polyradical material onto AC to obtain a composite electrode with higher capacitance.Finally,we also developed a novel nonaqueous hybrid capacitor of high work voltage by introducing a 5 V lithium intercalation compound LiNi_xMn_2-xO₄ as the positive electrode and AC as the negative electrode.1.Synthesis of OMCs with different pore lengths by hard-template method and its capacitance propertiesOMC has attracted much attention due to its highly ordered pore structure and narrow pore size distribution in mesopore range.We synthesized two OMCs with same pore size（4 nm） but different pore length by using highly ordered mesoporous silica as the hard temple and sucrose as the carbon precursor through a successive process of solution impregnation,pyrolysis,and template elimination.The pore length of LOMC derived from the conventional SBA15 template was over 2μm,while SOMC derived from a novel short-axis-oriented-growth SBA template showed a much shorter pore length of 200～300 nm.Shown by the cyclic voltammetry,a maximum specific capacitance of 14μF/cm² was obtained for SOMC in 6 M KOH solution compared with a capacitance of 10μF/cm² for LOMC.The electrochemical impedance analysis indicated that LOMC accounted for a larger ion diffusion resistance than SOMC.Moreover,the capacitance of LOMC also decayed faster than that of SOMC in the case of low electrolyte concentration.The electrochemical results suggested that SOMC can provide more facility for electrolyte accessibility and rapid ion diffusion thus it exhibited higher surface efficiency and better rate capability than LOMC.2.Synthesis of OMCs with different pore sizes by self-assembly and its electrochemical propertiesDi-OMC with a pore diameter of 3.1 nm and tri-OMC with a pore diameter of 6.7 nm were synthesized by organic-organic di-constituent self-assembly and organic-inorganic-organic tri-constituent co-assembly,respectively.As the electrode material for supercapacitor,tri-OMC showed a large specific capacitance of 117 F/g in organic electrolyte as well as a good retention of capacitive characteristic even at a high scan rate of 200 mV/s;while di-OMC showed nearly no capacitance in nonaqueous electrolyte since its surface area was difficult for the accessibility of organic electrolyte due to its small pore size and high proportion of micro-porosity. In aqueous electrolyte whose ion size was quite smaller than that of organic electrolyte,the surface area of di-OMC can be easily accessed by electrolyte thus it showed a high surface efficiency,e.g.a capacitance of 117 F/g was obtained.As for tri-OMC,it showed a specific capacitance as large as 210 F/g in aqueous electrolyte, and its rate capability was much better than that of di-OMC due to its large pore size. The results indicated that larger pore size was helpful to get better rate capability; while for specific capacitance,different electrolyte desired different pore size,a highest surface efficiency can be reached when the pore size was most appropriated for the electrolyte ion.In addition,we also investigated the possibility of these two carbons as the anode material for lithium-ion battery.Tri-OMC exhibited a reversible capacity of 1048 rnAh/g,nearly three times of that of di-OMC,and the cycle performance of tri-OMC was also superior to that of conventional hard carbon materials.3.Organic polyradical-carbon composite as the electrode material of supercapacitorA quite stable organic nitroxide polyradical material（PTMA, Poly-2,2,6,6-tetramethylpiperidinyloxy methacrylate） was synthesized by a successive process of esterification,polymerization and oxidation。It showed a specific capacity of 82 mAh/g in 1 M LiClO₄/PC electrolyte.When discharged at a high current rate of 50C,it can still exhibit a flat voltage plateau,implying its much higher power potential.However,PTMA was an insulator,which limited its application in supercapacitor.We creatively loaded PTMA onto the high surface activated carbon to prepare a capacitor-battery composite（PTMA-AC） through a dispersing-depositing procedure.The capacity of this composite electrode was 30%larger than that of pure AC electrode.A hybrid capacitor fabricated by a PTMA-AC composite as the positive electrode and AC as the negative electrode exhibited good rate capability as well as long cycle life.AC in the composite served as both active material and electronic conducting support during charge/discharge,by this means,the advantages of AC and PTMA can be well combined.This present work provided an effective approach for the application of other nitroxide polyradical materials in supercapacitor.4.HSG obtained by high-energy ball milling and its application in EDLCA new kind of electrode material called high surface-area graphite（HSG） was developed for EDLC through a high energy ball milling method using natural graphite as the starting material.The BET surface of HSG can be increased from 7 m²/g to 580 m²/g by high energy ball milling.Long time milling led to a much high specific capacitance over 200 F/g but at the same time with deteriorated rate capability due to the decreased conductivity caused by the strong destroy of graphite crystal.HSG sample obtained at a medium milling time can be employed as a good electrode material for EDLC in both aqueous and nonaqueous electrolytes.The large specific capacitance of HSG was demonstrated to derive of not only the high specific surface area but also the high density of lattice defects and surface functional groups produced during high energy ball milling.Attribute to the large quantity of graphite microcrystallines,HSG possessed of a better conductivity than AC thus it can exhibit good power ability.The cycle performance showed that HSG can well maintain its capacitance during 5000 cycles charge/discharge processes.Moreover,the low porosity of HSG promised a high electrode density thus led to a much higher volumetric energy density than AC.The results demonstrated that HSG is hopeful to be commercialized especially take its low cost,simple preparation and good electrochemical properties into consideration. 5.A novel nonaqueous hybrid capacitor using high voltage lithium intercalation compound as the positive electrode and activated carbon as the negative electrodeA high voltage lithium intercalation compound LiNi_0.5Mn_1.5O₄ was first introduced as a positive electrode for a nonaqueous hybrid capacitor combined with an AC negative electrode in 1.0 M LiPF₆ EC/DMC electrolyte.A Ni completely doped compound LiNi_0.5Mn_1.5O₄ was synthesized by a sol-gel process,it showed a single flat discharge plateau at 4.7 vs.Li⁺/Li with a large capacity of 123 mAh/g.The novel hybrid capacitor fabricated by LiNi_0.5Mn_1.5O₄ positive and AC negative with a weight ratio of positive:negative=1:3 showed a capacity of 26 mAh/g in comparison with 17 mAh/g of an EDLC;and the average work voltage of the hybrid capacitor was also promoted to 2.1 V compared with 1.4 V of an EDLC.Both the increase of average voltage and specific capacity resulted in a much high total energy density of 55 Wh/kg for the hybrid capacitor,approximately twice that of an EDLC. The rate performance showed that the hybrid capacitor can maintain 80%of its initial capacity at a discharge current of 10C.After 1000 cycles charge/discharge,the capacity loss was also lower than 20%,showing a desirable cycling ability. Moreover,the charge/discharge process of this novel hybrid capacitor was associated with the transfer of Li-ion between the two electrode,which overcame the drawback of electrolyte depletion during charge process in conventional EDLC and other hybrid systems such as Li₄Ti₅O₁₂/AC.更多还原