取向生长碳包覆钛酸锂纳米晶/石墨烯柔性电极的构建及其储能特性研究

Flexible lithium ion capacitors can meet the requirement for quick rechargeable power sources and are expected to have a large potential market in the field of portable and wearable electronic devices. The negative electrode performance of Li+ intercalation is a key factor to determine the quality of a flexible lithium ion capacitor. Targeting the poor high-rate performance of LTO as negative electrode caused by low electronic conductivity and ion migration coefficient, a composite electrode with 3D graphene as skeleton and oriented-growth nano-crystalline LTO coated with carbon as active material for Li+ intercalation is designed and fabricated. In the oriented-growth nano-crystalline LTO, more (111) surfaces are exposed which allows further lithium-ion insertion into LTO via surface relaxation. The graphene skeleton and the carbon coating on the LTO nano-crystals form an electron migration network, which is beneficial for lowering the ohmic drop. The carbon-coated nano-LTO/graphene flexible electrodes are constructed by electrostatic spray deposition (ESD) and oriented attachment growth (OA). Energy storage characteristics of the as-prepared electrodes are characterized to reveal the structure-performance relation. The mechanism for the OA growth of carbon-coated LTO nanocrystals in the confined space formed with graphene nanoplates will be systematically studied in order to supply theoretical guidance for the fabrication and performance manipulation of carbon coated nano-LTO/graphene flexible electrodes.

柔性锂离子电容器能够满足便携和可穿戴电子领域对快速充电电源的需求，潜在市场需求巨大。嵌锂负极性能是决定柔性锂离子电容器质量的关键因素。本项目针对由电子电导率低、离子扩散系数小而引起的钛酸锂（LTO）负极材料倍率性能低的难题，设计开发一种以3D柔性石墨烯作骨架、按特定取向生长的碳包覆LTO纳米晶作嵌锂活性物质的复合负极。在按特定取向生长的碳包覆LTO纳米晶中，（111）晶面得以大量暴露，有利于Li+通过表面迟豫嵌入LTO晶格。而石墨烯骨架和LTO纳米晶碳包覆层构成电子导电网络，可以降低电极的欧姆降。采用静电喷雾沉积（ESD）和取向附着生长（OA）实现碳包覆纳米LTO/石墨烯柔性电极的构建。采用电化学方法对所构建电极的储能特性进行考察，揭示其构效关系。深入研究在由石墨烯纳米片构成的限域空间内碳包覆LTO纳米晶的OA生长机理，为碳包覆纳米LTO/石墨烯柔性电极的制备与性能调控提供理论依据。

柔性锂离子电容器能够满足便携和可穿戴电子领域对快速充电电源的需求，潜在市场需求巨大。嵌锂负极性能是决定柔性锂离子电容器质量的关键因素。本项目针对由电子电导率低、离子扩散系数小而引起的钛酸锂Li4Ti5O12（LTO）负极材料倍率性能低的难题，设计开发出一种按特定取向生长的碳包覆LTO纳米晶作嵌锂活性物质的复合材料，并对由石墨烯纳米片构成的限域空间内碳包覆LTO纳米晶的OA生长机理进行了深入的研究，以为碳包覆纳米LTO/石墨烯柔性电极的制备与性能调控提供理论依据。首先在高温和适宜氩气压力下通过固相反应法在石墨烯片层表面实现了LTO纳米晶的取向生长。研究发现，惰性气体压力对诱导LTO纳米晶的取向生长起到了关键作用。RGO片层通过为LTO纳米颗粒提供生长位置而对LTO的取向生长发挥重要作用。覆盖无定型碳的（111）和（110）晶面表面能的不同是LTO纳米晶粒在石墨烯片层表面取向生长的推动力。在此基础上，对该复合材料的储能机理进行了研究，结果表明，在按特定取向生长的碳包覆LTO纳米晶中（111）晶面得以大量暴露，有利于Li+通过表面迟豫嵌入LTO晶格。而石墨烯骨架和LTO纳米晶碳包覆层构成电子导电网络，可以降低电极的欧姆降，提高锂离子电容器的倍率性能。在50C下， 6.1 MPa下制备的取向生长LTO/RGO复合材料的比容量达到132 mAh g-1。采用液体-空气界面自组装法制备了用于柔性锂离子电容器的无粘接剂自支撑LTO/石墨烯复合电极。LTO纳米颗粒分布在RGO片层中间，形成了一种三明治结构的复合电极。该电极表现出优异的倍率性能和循环稳定性。为进一步提升LTO电极的倍率性能和柔韧性，采用柔性不锈钢网作基底，十六烷基三甲基溴化铵作为前驱体溶液的添加剂，利用静电喷雾法（ESD）制备了氮掺杂碳包覆改性的LTO/N-C/SSM柔性电极。所制备的LTO/N-C/SSM柔性电极表现出优异的电化学性能。在50C时，LTO/N-C/SSM电极的比容量仍然可达94.1 mAh g-1。以LTO/N-C/SSM为负极，以活性炭为正极构建的柔性锂离子电容器在功率密度为3.5 kW kg-1时，能量密度达到125 Wh kg-1。