用于二次锂/钠基电池的新型开框架正极材料的研究

The rechargeable lithium and sodium based batteries are attracting increasing attention due to their potential application in electric vehicles and large-scale grid-based electrical energy storage. To meet their application demands, it requires to greatly enhance both the energy density and power density of batteries without trade-off in the aspects of safety, cost and environmental friendliness. This project will focus on the strategy of moderate structural expansion to construct the ion channels facilitating mass/charge migration and storage, and exploring novel open framework structures as high-capacity and high-rate positive electrodes, such as polyanion frameworks, fluorides and porous coordination polymers. Combining with the features of metastable materials, we will develop novel synthesis approaches, which are achievable under low temperature and normal pressure. We will further explore the nano-technologies characterized by interfacial or surface decoration, nonstoichiometric design, introduction of lattice defects, amorphization, in-situ lithiation (sodiation), low temperature graphitic encapsulation, and construct the composites with optimized electron/ion mixed conductive networks. We expect the finding of heterogeneous interface mass storage based on space charge effect, and to upgrade the electroactivity of energy storage materials. Meanwhile this project is going to carry out advanced, in-situ physical characterization as well as synchrotron radiation X-ray techniques to get deep insights into the fundamental scientific issues related to electrochemical reaction mechanism, phase transformation phenomena and the key factors determining transport and storage in electrodes, eventually providing useful information for constructing high-performance rechargeable alkali metal based batteries.

二次锂基和钠基电池在电动汽车和基于绿色电网的大规模储能体系中有广泛的应用前景，为了满足应用需求，需要在不牺牲安全性、成本、环境友好性的前提下，进一步提高电池的能量和功率密度。本项目的特色是采用适度结构扩展策略，构建有利于质量/电荷输运和存储的离子通道，开发具有新型开框架结构的大容量、高倍率的聚阴离子框架、氟化物和多孔配位聚合物等正极材料。结合亚稳态材料特点，关键是发展新的、可在低温常压下实现的合成手段，探索基于界面或表面修饰、非计量比设计、晶体缺陷引入、非晶态化、原位富锂（钠）化、低温石墨化碳包覆的纳米技术，构筑具有优化离子/电子混合导电网络的材料复合相，以期望实现基于空间电荷效应的异质界面质量存储，升级储能材料的电化学活性。采用先进的、原位的物理结构表征，解决关于电化学反应机制、相转变现象等的基础科学问题，发现电极中输运/存储的关键决定因素，最终提供构建高性能二次碱金属基电池的有用信息。

大规模电网静态储能和电动汽车移动储能对电池的能量密度、安全性和成本提出了更高的要求，本项目针对新型储能电池材料和体系展开研究，着重围绕用于锂/钠基电池的开框架氟基正极材料的结构合成设计、大颗粒嵌入/转换负极的导电网络构筑、动力学改善的镁基电池体系开发等方向取得了系列进展。（1）提出立方钙钛矿相可用于高倍率储钠电极，在不显著改变配体链接方式的情况下，仅通过操纵通道填充以实现已知结构原型的开框架化。（2）提出一个概念验证型的电极构架，即将亚微米尺寸的高致密泡状MoS2保形封装于薄层碳网络中，其中碳包覆层无缝连接到更宽阔的碳基质中，以有效抑制活性物质的剥离和导电网络的破坏。（3）提出具有转换反应活性的离散结构单元组成的开框架相有利于其转换反应发生在分子尺度的观点，以团簇型杂多酸盐（POM）为例，通过Al（Si）驱动的聚合，并且使其聚阴离子基团与带正电的石墨烯发生静电杂化，实现了POM材料在电解液中的低溶解性和在导电网络中的高载量。（4）提出金属有机框架（ZIF-67）衍生出的N、 Co共掺杂碳作为有效的硫载体，首次实现了镁硫电池的高可逆循环和高倍率充放。（5）提出一种基于多硫化物正极转化反应的高能量密度的双盐镁电池体系，其正极电化学过程是由电解液中锂离子驱动的转换反应，负极的电化学过程由可逆的镁沉积和剥离主导，在长期循环中没有枝晶形成。（6）提出在超薄纳米结构上修饰功能基团，以实现镁基电池在三维多孔正极网络中的快速表面氧化还原反应，不仅可避免二价阳离子缓慢的晶格内迁移，而且改善了镁电池的动力学性能。以氟化石墨烯为模型材料，由预先的阴离子嵌入来激活后续的可逆阳离子脱嵌，实现了类似赝电容行为的电化学，循环过程中迁移离子电荷密度的稀释和表面氧化还原反应位的易到达性有利于电位极化的改善和MgF2成核现象的消除。