POSS-聚合物在锂离子电池中应用的研究进展
来源期刊:中国有色金属学报2013年第10期
论文作者:梁 波 刘燕平 唐思绮 赖延清 刘业翔
文章页码:2851 - 2863
关键词:POSS-聚合物;锂离子电池;聚电解质
Key words:POSS-polymers; lithium-ion batteries; polyelectrolyte
摘 要:低聚笼形倍半硅氧烷(POSS)-聚合物结合了POSS和聚合物各自的结构和性能的优势,显示出独特的电化学性能和力学性能,在锂离子电池领域得到越来越多研究者的关注。聚合物电解质的高离子电导率、适宜的力学性能及稳定的电化学性能对提高电池的性能有很重要的影响。POSS的优势在于通过较少的添加量,提高聚合物电解质的热稳定性和力学性能,进一步提高聚合物电解质的室温离子电导率,获得更好的电池充放电性能。分析POSS-聚合物结构和电池性能之间的关系,综述具有代表性的POSS-聚合物在锂离子电池电解质中应用的研究进展,并对该种聚合物在锂电池中的应用前景进行展望。
Abstract: Polyhedral oligomeric silsesquioxane (POSS) is a silicone compound with unique structure. POSS-based polymer (POSS-polymer) attracts much attention due to its unique electrochemical and mechanical properties. Polymer electrolyte materials with high ionic conductivity, suitable mechanical properties, and stable chemical and electrochemical properties play an important role in developing batteries with high qualities. POSS-polymer can improve the thermal stability and mechanical properties, further enhance the ionic conductivity of polymer electrolyte, and accordingly it is considered as one of the most promising candidates for the future electrolyte materials of lithium ion batteries. Additionally, with a little addition of POSS in the polymer electrolytes, better battery charge/discharge performance can be achieved. This review was intended to cover the more recent advances in structure-property relationships of POSS-polymers. An application prospect of POSS-polymers in lithium ion batteries was also proposed.
11文章编号:1004-0609(2013)10-2851-12
梁 波1, 2,刘燕平1,唐思绮1,赖延清2,刘业翔2
(1. 长沙理工大学 汽车与机械工程学院,长沙 410114;
2. 中南大学 冶金与环境学院,长沙 410083)
摘 要:低聚笼形倍半硅氧烷(POSS)-聚合物结合了POSS和聚合物各自的结构和性能的优势,显示出独特的电化学性能和力学性能,在锂离子电池领域得到越来越多研究者的关注。聚合物电解质的高离子电导率、适宜的力学性能及稳定的电化学性能对提高电池的性能有很重要的影响。POSS的优势在于通过较少的添加量,提高聚合物电解质的热稳定性和力学性能,进一步提高聚合物电解质的室温离子电导率,获得更好的电池充放电性能。分析POSS-聚合物结构和电池性能之间的关系,综述具有代表性的POSS-聚合物在锂离子电池电解质中应用的研究进展,并对该种聚合物在锂电池中的应用前景进行展望。
关键词:POSS-聚合物;锂离子电池;聚电解质
中图分类号:TB383;O634.4 文献标志码:A
LIANG Bo1, 2, LIU Yan-ping1, TANG Si-qi1, LAI Yan-qing2, LIU Ye-xiang2
(1. School of Automobile and Mechanical Engineering, Changsha University of Science and Technology, Changsha 410114, China;
2. School of Metallurgy and Environment, Central South University, Changsha 410083, China)
Abstract: Polyhedral oligomeric silsesquioxane (POSS) is a silicone compound with unique structure. POSS-based polymer (POSS-polymer) attracts much attention due to its unique electrochemical and mechanical properties. Polymer electrolyte materials with high ionic conductivity, suitable mechanical properties, and stable chemical and electrochemical properties play an important role in developing batteries with high qualities. POSS-polymer can improve the thermal stability and mechanical properties, further enhance the ionic conductivity of polymer electrolyte, and accordingly it is considered as one of the most promising candidates for the future electrolyte materials of lithium ion batteries. Additionally, with a little addition of POSS in the polymer electrolytes, better battery charge/discharge performance can be achieved. This review was intended to cover the more recent advances in structure-property relationships of POSS-polymers. An application prospect of POSS-polymers in lithium ion batteries was also proposed.
Key words: POSS-polymers; lithium-ion batteries; polyelectrolyte
随着微电子机械系统(MEMS)、超级智能卡及射频识别(RFID)等技术的发展,具有较高的能量密度、优良的安全性和循环性的锂离子电池受到越来越多的关注[1-7]。近年来,通过静电纺丝[8]、喷涂打印[9]等工艺制备聚合物薄膜电池简化了制作工艺并降低了成本,使得大规模生产薄膜电池成为研究热点。
聚合物易于制成各种形状,其独特的弯曲性能和安全性能满足MEMS和智能卡的需求,被广泛应用于聚合物薄膜锂离子电池中[10]。聚苯胺(PANI)[11-14]、聚吡咯(PPy)[15-18]、聚噻吩(PTP)[19-22]等聚合物大多作为电极材料应用于薄膜锂电池中。优化聚合物电解质途径有[23-27]:采用适度的交联;共聚或共混;添加无机纳米材料,扰乱或抑制结晶,从而降低玻璃态转变温度(tg),提高离子电导率[28];线性结构转化为超支化结构[29]。
低聚物笼形倍半硅氧烷(Polyhedral oligosilsesquioxane,POSS)-聚合物电解质表现出较高的离子电导率的同时保持了较高的机械强度。Si—O—Si键构成的无机笼状结构的核心,具有良好的热稳定性,外部由活性或非活性R基官能团环绕,有机外壳使它具有很好的聚合物相容性、生物相容性和其他一些表面相容性。研究最多的POSS衍生物结构通式为(RSiO1.5)8,其结构示意图如图1所示。
图1 POSS的结构示意图
Fig. 1 Schematic diagram of structures of POSS
POSS添加在聚合物基体中能够提高基体的力学性能和热性能,使其多功能化并应用在生物医学领域和光电器件的半导体材料中,相关合成和应用的综述已见报道[30-32],但POSS应用于锂离子电池的综述还未见报道。本文作者综述了近年来有关POSS聚合物在锂离子电池中的研究进展,并结合相关的研究体系,总结和评述了POSS在锂离子电池聚合物研究中的应用。
1 POSS-聚合物结构与性能关系
在POSS-聚合物中POSS作为纳米填料表现出特殊的性能,例如单一分散性、低密度、高热稳定性和可调的表面性能。POSS主要通过3种方式与聚合物结合:(a) POSS作为核和大分子引发剂引发POSS表面活性基团与聚合物基体聚合制得星形大分子;(b) 多功能团POSS作为纳米填料或单分子与有机单分子或聚合物聚合制得络合纳米复合材料;(c) 单功能团POSS接枝到聚合物主链上形成悬挂共聚物或封端共聚物,如图2[33]。图3所示为采用图2中(c)方法制备的POSS-聚合物。图3(a)[34]所示物质主要用于研究POSS-聚合物结构与性能的关系,3(b)[35]和3(c)[36]所示物质目前主要应用于光电领域研究,利用POSS封端达到提高其电化学性能的目的。
图2 制备POSS聚合物的3种主要方法[33]
Fig. 2 Three main types of POSS-based nanocompoiste[33]
研究POSS-聚电解质结构和性能之间关系的方法主要有:建立模型和模拟,微观采用光谱学和宏观采用性能测试。MATHER等[34]采用光谱学和性能测试的方法研究POSS-纳米复合材料的结构和性能之间的关系。通过改变嵌段和无规POSS-降冰片烯共聚物(图2(a))的相对分子质量分布和选择不同的POSS种类研究POSS对聚合物的影响,并研究了不同取代基对聚合物基体的影响,如环戊基-POSS(Cp-POSS)直径为6 nm,长度为36 nm,环己基-POSS(Cy-POSS)直径达到12 nm,长度为62nm;POSS接枝在Cy上的数目是接枝在Cp上的7倍。结果表明,R基团影响圆柱体的尺寸,对力学性能产生较大影响。WANG等[37]用钯-二亚胺催化剂,合成了共价键连接的POSS聚合物。因为POSS纳米粒子高度化的笼形结构,相对于同等质量的纯聚乙烯(PE),POSS-PE共聚物的固有黏度极大幅度地减小。热力学研究表明: POSS的引入较大的提高了聚合物的热稳定性,并且随着POSS含量的增加tg增大。
通过共价键将POSS连接在聚合物主链上或者用POSS封端聚合物链,同样可提高聚合物的热性能和力学性能。POSS在杂化聚合物中以分子级别的形式分散,能提高聚合物的热稳定性和机械强度。POSS分子较大的体积和空间位阻,极大地降低了取代反应的聚合度。
基于POSS-聚丁二烯和聚乙烯共聚体系,ZHENG等[38]提出了POSS聚合物的一种结构模型,如图4所示。模型图与透射电子显微镜(TEM)测试结果一致,表现出POSS和聚合物基体的“筏状”和结晶结构。尽管大多数人认为POSS-聚合物是三维结构,但是受到聚合物链的约束,POSS聚合物最大可能是二维网状结构。热重分析(TGA)表明,POSS对提高聚合物基体的热稳定性有较大作用。12%(质量分数)POSS-苯乙烯的分解温度提高了70 ℃。通过比较含5%(质量分数)辛基和十八烷基POSS的聚苯乙烯(PS)-POSS聚合物纳米复合材料的td20(损失20%质量时的分解温度)值,分别为24.5 ℃和28.6 ℃;含大量不饱和键苯基POSS的PMMA-POSS,其td20值提高了64 ℃。
XU等[39]详细研究了POSS-聚合物中POSS对tg的影响机制。tg值的大小主要由3个因素决定:1) POSS的稀释效应降低聚合物分子偶极矩之间的相互作用;2) POSS硅烷和聚合物有机分子的极性羰基之间的偶极矩作用;3) POSS-POSS分子内和分子外部之间的相互作用。当POSS含量较低时,因素1起主要作用,能降低tg值;POSS含量较高时,因素2和3起主要作用,能提高tg值。
图3 POSS接枝共聚物[34-36]
Fig. 3 POSS tethered with polymer materials[34-36]
图4 POSS共聚物结构的Coughlin模型[38]
Fig. 4 Coughlin model for structure of POSS-copolymer[38]
聚甲基丙烯酸甲酯(PMMA)和聚乙烯基吡硌烷酮(PVP)是目前研究较多的聚合物电解质体系之一。YEN等[40]比较了LiClO4/OP-POSS(8(二甲基(4-羟基苯))硅烷基-POSS)/PMMA-PVP和LiClO4/PMMA-PVP的离子电导率。共聚物(PMMA-co-PVP)通过与POSS共混提高PMMA-co-PVP基体的热性能。因为聚合物链之间的相互作用受OP-POSS的影响,载流子的流动性与聚合物基体的流动性直接联系。所以,OP-POSS可促进聚合物电解质分子链的流动性,从而进一步提高聚合物电解质在室温下的离子电导率,表明了OP-POSS在LiClO4/OP-POSS/PMMA-co-PVP固体聚合物电解质中提高离子电导率方面起到了重要作用。
可见,含POSS-聚合物的结构对其在锂电池中的应用有较大的影响。因此,探索POSS-聚合物结构与性能的关系有利于设计出性能优良的POSS-聚合物材料。
2 POSS在锂离子电池中的应用
2.1 电解质材料
2.1.1 固态聚合物电解质
聚环氧乙烷(PEO,也叫聚乙二醇(PEG)),通过醚氧上的孤对电子与锂离子络合,从而溶解锂离子作为一种重要的聚合物电解质应用于锂离子电池中[41-43]。POSS-PEO(n)聚合物电解质的低结晶性以及相对较低的tg值,使其在锂离子电池低温溶剂中具有很好的应用潜力。通过PEO短链的接枝形成梳状聚合物或树状聚合物,或通过作为无机物支架或网络聚醚的臂来控制结晶,提高室温下PEO基聚合物电解质的电导率[44]。
图5 PEO(n)-POSS合成路线[47]
Fig. 5 Reaction scheme to prepare PEO(n)-POSS[47]
美国Temple大学的Wunder课题组制备了低聚物PEO链接枝在Q8M8H上的多功能PEO倍半硅氧烷POSS,研究了结构对聚合物的离子电导率以及结晶度等方面的影响。2002年[45]他们报道了POSS接枝的PEO在电池和药物方面的应用。PEO与POSS接枝后能有效抑制PEO结晶,例如当n=4时,PEO呈完全结晶态,而POSS-PEO仍是液体,且其tg降至-85 ℃。2004年,MAITRA等[46]将上述PEO(n)-POSS与锂盐混合得到黏性电解质溶液,研究其离子电导率与结晶度和tg的关系。POSS-PEO(n=4)8的tg在-60~-70 ℃之间。图5所示为制备POSS-PEO(n)的合成路线[47]。tg随着n的增大而提高,且随着n的增大,聚合物链结晶的可能性也增加。低温(接近tg)时,POSS-PEO(n)/LiClO4的离子电导率随着PEO组分的降低而提高;高温时,离子电导率则随着PEO组分含量的增加而提高。当温度为90 ℃时,POSS-PEO (n=4)8/LiClO4(O/Li=32:1)的离子电导率为2×10-4 S/cm。50 ℃时,POSS-PEO(n=x)/LiClO4的离子电导率大于1×10-3 S/cm,室温离子电导率大于1×10-4 S/cm。2006年,ZHANG等[47]比较了在不同条件下化合物POSS-PEO(n=4)8/LiX的离子导电率,X分别取ClO4-、(CF3CF2SO2)2N-、(CF3SO2)2N-、CF3SO3-、PF6-、AsF6-和BF4-,结果表明:tg随着锂盐浓度和PEO链长度的增加而提高,离子电导率随tg的升高而降低。高温(t≥tg,熔融温度tm),低黏度时,结合性弱的锂盐(LiClO4,LiN(CF3CF2SO2)2,LiN(CF3SO2)2,LiPF6,LiAsF6)比结合性强的锂盐(LiBF4和LiCF3SO3)对电荷迁移率的贡献更大,其离子电导率也更高。在低温、高黏度时,锂盐的tg越低,则离子电导率越高。可通过大的有机阴离子((CF3CF2SO2)2N-)增塑作用降低锂盐的tg值,或与PEO络合来影响tg值。结合力弱的锂盐更易与PEO络合,提高tg值。例如在30 ℃下,POSS-PEO(n=4)8/LiN(CF3CF2SO2)2(n(O)/n(Li)=16:1)的室温离子电导率达到1.1×10-4 S/cm,10%甲基纤维素和90% PEO-POSS/LiClO4组成了力学性能极好的自组装薄膜,薄膜的室温电导率为10-5 S/cm数量级[47-48]。
2007年,ZHANG等[48]通过调节POSS-PEO(n=4)8、大相对分子质量PEO(Mw=600 000)以及LiClO4的不同比例研究黏性液态电解质(固态聚合物电解质的基础)的离子电导率与相态的关系。当n(O)/n(Li)=8:1和12:1时,混合物具有由微相分离成一种高的tg非晶体相 (由PEO决定)和一种低的tg非晶体相(由POSS-PEO(n=4)8决定)的趋势。当n(O)/n(Li)=16:1,混合物具有非结晶相和完全包含PEO的结晶相。离子电导率由连续相的性质决定:较低tg非结晶连续相的离子电导率较高;结晶态连续相电导率较低。室温下这类混合物的最高离子电导率为8×10-6 S/cm,而在n(O)/n(Li)=12:1条件下PEO的离子电导率仅为2×10-6 S/cm。
图6 POSS-PEG8、POSS-benzyl7Li3和POSS-benzyl7(BF3Li)3结构式[49]
Fig. 6 Structures of POSS-PEG8, POSS-benzyl7Li3 and POSS-benzyl7 (BF3Li)3[49]
图7 POSS-benzyl7(BF3Li)3/ POSS-PEG8的两相结构示意图[49]
Fig. 7 Schematic diagram of two-phase structure of POSS-benzyl7(BF3Li)3/POSS-PEG8[49]
将两种POSS纳米材料混合可得到纳米复合电解质。2011年,CHINNAM等[49]结合两方面优势,构建POSS-PEG8 /POSS-benzyl7(BF3Li)3复合电解质材料,充分发挥了POSS-PEG8良好导电性能和POSS- benzyl7(BF3Li)3空间骨架结构的特性,提高了该复合电解质材料的电化学行为。POSS-PEG8无挥发性,具有较高的热稳定性(400 ℃),添加少量锂盐后,室温电导率大约1×10-4 S/cm。通过POSS-benzyl7Li3和BF3(OC2H5)2反应制备得到多离子锂盐POSS- benzyl7(BF3Li)3,结构式如图6[49]所示。POSS-benzyl7 (BF3Li)3具有类Janus性能,既有主要的疏水基封端,又有—Si—O—BF3Li离子基团封端。路易斯酸BF3促进在锂电极上形成钝化层,生成保护层使电阻减小,阻止锂和溶剂之间的反应[50],改善PEG/聚锂羧化物的电化学稳定性[51]。由于POSS-PEG8促进了POSS- benzyl7(BF3Li)3的疏水苯基的聚集和结晶,复合电解质形成两相形态,即苯环成为结构相,POSS-PEG8成为导电相(图7[49])。POSS-benzyl7(BF3Li)3的—Si—O—BF3—Li+基团发生取向并分离,所以Li+溶解在POSS-PEG8中。纳米复合电解质是一种黏性材料,在自身重力下不会发生流动。10 ℃时该纳米复合电解质的离子电导率为1.5×10-5 S/cm;30 ℃时,POSS-PEG8/POSS-benzyl7(BF3-Li)3(n(O)/n(Li)=16:1)的离子电导率约为2.5×10-4 S/cm,是POSS-PEG8/ POSS-LiBF4相同条件下离子电导率的17倍;90 ℃时的离子电导率为1.6×10-3 S/cm。研究还表明,该复合电解质中较大的阴离子抑制了分子链的流动性,其锂离子迁移数为0.50.01。在80 ℃存放3 d,表现出相对稳定的界面阻抗,表明POSS在不降低离子电导率的前提下可改善聚合物电解质的界面稳定性。
LEE等[52]通过甲基丙烯酰-POSS作为交联剂,聚乙二醇二甲醚(PEGDME)作为增塑剂,采用自由基聚合法制备固态聚合物电解质(SPE)甲基丙烯酰-POSS,结构如图8所示。5% POSS,95% PEGDME(质量分数)时体系的最大离子电导率为5.3×10-4 S/cm,可满足实际应用的要求。室温下,Li/SPE/LiCoO2锂聚合物电池在0.1C倍率下放电,经过20次循环后仍保持原放电容量的80%。
2.1.2 离子液体及凝胶电解质
离子液体(Ionic liquids, ILs)具有较高的室温离子电导率、不挥发性和不燃烧性,成为改善和提高电池安全性能的新选择[53-55]。ILs可以通过调节阴阳离子制备特殊性能离子液体,而且其对环境友好的特性也促进离子液体成为当今新材料研究的前沿与热 点。纳米结构聚合物材料的独特结构和形态特征,相对于普通小分子对提高离子液体性能方面更有发展潜力[56-61]。POSS的星形结构,即在每个顶点连接有取代基团,有助于提高离子传输能力,表明含POSS的离子液体在电解质应用方面能够表现出优异的性能[62-63]。
图8 甲基丙烯酰-POSS化学结构[52]
Fig. 8 Chemical structure of methacryl-POSS[52]
日本京都大学的TANAKA等[64-65]研究了POSS-离子液体在锂离子电池中的应用。他们首次成功合成了POSS基室温离子液体[64]:即八羧基-POSS- ([POSS-(COO-)8])为阴离子,碱性离子液体咪唑鎓盐([Bmim+])为阳离子的纯净透明的离子液体,结构如图9(a)所示。通过溶解性气体(DGA)检测和示差扫描量热法(DSC)分析认为POSS的刚性立方结构对降低熔融温度(25 ℃),提高热稳定性具有重要作用。
随后,他们制备了一系列改性的咪唑鎓盐-POSS羧基类的离子液体,研究了结构和热性能之间的关系(图9(b))[65]。POSS立方结构形成的星形结构提高了聚合物的热稳定性并降低了其熔融温度。POSS-Im6和POSS-Im8的热分解温度(Td)高于POSS-(COOH)8和Arm-COOH盐的热分解温度,是因为刚性硅阻碍了分子的移动,从而抑制了材料的热分解。在DSC曲线上,POSS-Im6和POSS-Im8的吸热峰分别为19和23 ℃。相反地,POSS-Im2和POSS-Im4相比于Arm-COOH盐和POSS-(COOH)8表现出较低的td值。通过热动态和构象分析,他们认为离子对的星形结构影响了连接POSS离子对的物理性能。
图9 离子对的化学结构和POSS基离子液体模型[65]
Fig. 9 Chemical structures of ionic pairs (a) and proposed conformations of POSS-based ionic compounds (b)[65]
等[66]以POSS为结构改性构件,得到熔融温度在150~200℃之间的1, 3-烷基咪唑鎓盐碘化物离子液体。核磁共振(NMR)和红外光谱(FTIR)分析均表明,POSS分子通过丙基连接在咪唑鎓盐上,通过改变POSS顶端的其他7个取代基得到了固态(PEOPrIm+I- IB7T8 POSS)和液态(MePrIm+Ix- IO7T8 POSS)碘化物离子液体。他们发现固态离子液体的电导率约为10-7 S/cm,但随着多碘化合物的形成,离子电导率提高两个数量级。但MePrIm+Ix-IO7 T8 POSS类似物的电导率却没有明显变化。他们认为多碘化合物的形成产生了Grotthus电子传输机制(质子跳跃)引起了上述不同的结果。
离子液体可降低薄膜的弹性模量,提高力学性能。CHANG等[67]研究了POSS在薄膜中的作用,发现POSS促进了无机组分和聚合物基体之间的作用,薄膜的力学性能随之提高。SUBIANTO等[63]报道了磺化POSS(图10(a))对离子液体[1-butyl-3-methylimid- dazolium bis(trifluoromethylsulfonyl)imide, BMI-BT-SI]力学性能的影响。尽管聚合物只含有少量POSS,但对力学性能的影响较大,含1% POSS可较大程度地提高tg值和热性能。采用动态热机械分析(DMA)研究了S-POSS或离子液体对Nafion膜的力学性能的作用。图10(b)表明离子簇转变温度(Ionic cluster transition, tc从119 ℃提高到127 ℃。tan δ峰的提高表明S-POSS影响了Nafion的离子簇。而且,在高于120 ℃时该杂化薄膜模量为32 MPa,而其他两种薄膜模量均为23 MPa,如图10(c)所示。模量的提高可能是因为S-POSS的添加增加了离子之间的相互作用,从而提高Nafion膜在高于tc温度时的力学性能。
自1975年FEUILLADE等[68]报道凝胶聚电解质(GPE)应用于锂电池后,很多研究者开始致力于GPE的研究。在GPE中,锂盐主要通过有机溶剂的极性基团溶解。LI等[69]通过胺基封端的丁二烯-丙烯氰(ATBN)热固化在环氧环己基-POSS交联剂上,掺杂一种离子液体和高氯酸锂(LiClO4)制备出一类新型的交联网络结构的凝胶聚电解质(图11(a))。LiClO4和离子液体(1-丁基-3-甲基咪唑三氟甲基磺酸盐(BMIMOTf))之间的相互作用,有利于LiClO4的分解,提高GPE的离子电导率;加入少量的POSS能提高离子电导率。当体系组分为ATBN-5% POSS-40 % LiClO4-50% IL时,GPE的离子电导率在30 ℃时最高,达到2.0×10-4 S/cm (图11(b))。该聚合物体系在30~70 ℃范围内满足离子电导率和温度之间的Arrhenius模型,表明GPE电解质中的载流子几乎与聚合物链的嵌段运动无关,而与溶剂的行态有关。环氧树脂-POSS在该GPE体系中充当交联剂的作用,添加少量POSS提高GPE的聚合物链自由度和流动性,有利于提高离子电导率。降低络合密度可提高离子电导率,但降低了GPE的力学性能,因此,研究离子电导率和力学性能之间的关系是研究GPE的关键问题。从循环伏安曲线可以看出,该GPE表现出高达4 V的电化学稳定性。到目前为止,POSS-GPE复合聚合物电解质的性能尚未取得较大的突破,因而采用POSS作为GPE复合聚合物电解质基质材料,开展制备聚合物电解质方面探索性的研究是十分必要的。
图10 磺化POSS, S-POSS杂化薄膜和Nafion薄膜的tan δ曲线和S-POSS离子液体杂化薄膜和离子液体薄膜DMA曲线图[63]
Fig. 10 Sulfonation of POSS (a), DMA showing shift in tg in tan δ curves for Nafion S-POSS hybrid compared to Nafion (b) and DMA of Nafion S-POSS ILs composite membrane material and Nafion ILs membrane (c) (Inset: Enlargement of region between 100 and 140 ℃)[63]
图11 POSS作为交联剂的GPE示意图及GPE离子的电导率[69]
Fig. 11 Schematic diagram of GPE with POSS as cross- linking agent (a) and ionic conductivity of GPE based on ATBN with different LiClO4 and ILs contents (b)[69]
2.2 电极和隔膜材料
POSS改性锂电池的性能还包括在锂电池电极和隔膜方面的应用。KIM等[70]申请采用氢化笼形倍半硅氧烷烧结后应用在锂电池正极的专利,电池的充放电性能得到改善。首次循环放电容量最高达到905 mA·h/g,30次循环后电容量保持率为64.8%。石墨烯优异的电学、力学以及热学性能使其成为具有良好应用前景的锂离子电池电极材料。XUE等[71]报道了胺化POSS功能化的氧化石墨烯(POSS-石墨烯),其结构示意图如图12所示。POSS功能化后的石墨烯具有特殊形貌和微观结构,预计能有效改善材料的各项电化学性能。POSS-石墨烯作为一种新型的纳米填料可提高聚合物基体的tg和td。
HENNIGE等[72]利用低聚硅氧团簇提高锂电池多孔隔膜的化学和热稳定性。隔膜以多孔非导电聚合物-Si8O12(OSiR3)8为骨架,陶瓷作为涂层。多孔隔膜隔离了正极和负极,在正常的使用环境下电解质可渗透隔膜,若孔被阻塞,电池就会因温度积聚出现热失控。
图12 POSS功能化石墨烯结构示意图[71]
Fig. 12 Schematic diagram of POSS-functionalized grapheme nanosheets[71]
3 总结与展望
1) POSS-聚合物是一种无机-有机杂化纳米复合材料,融入聚合物体系中,大幅度提高聚合物的稳定性。具有无机内核笼状结构的纳米构件POSS通过共价键的形式引入聚合物基体后,聚合物的离子导电率和力学性能均得到显著提高。聚合物体系中少量POSS具有稀释效应,使得主链与主链之间相互堆积的能力较弱,玻璃态转变温度降低。POSS聚合物的这些特点在锂离子聚合物电池中具有很好的应用前景。
2) 单纯的聚合物电解质被应用于锂离子电池时离子电导率和锂离子迁移数偏低;离子液体单独使用可能出现泄漏事故,大大降低安全性。将POSS引入聚合物和离子液体中可以缓和这些弊端。POSS具有稀释效应以及本身所具有的无机材料性能,使得聚合物的玻璃态转变温度降低,离子导电率和安全性提高。
3) POSS在聚合物中的应用正在被迅速拓宽,以后的研究重点将进一步探索POSS在聚合物中的微观形貌和电化学性能之间的关系,协调POSS-聚合物材料的离子电导率和力学性能之间的关系,使其满足实际应用需求。
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(编辑 陈卫萍)
基金项目:国家自然科学基金资助项目(50803008);中国博士后特别资助项目(201104508);湖南省科技厅博士后科学基金资助项目(2011RS4067);湖南省教育厅优秀青年基金资助项目(11B001);可再生能源电力技术湖南省重点实验室开放基金资助项目(2011KFJJ006)
收稿日期:2012-11-19;修订日期:2013-08-16
通信作者:赖延清,教授,博士;电话:0731-88876454;E-mail: 13975808172@126.com