多金属磷酸盐正极材料的分子水平碳复合及其协同效应研究

Transition metal phosphate is one kind of promising cathode materials for Na-ion batteries. However, the low electronic conductivity limits its commercial applications. Though conventional carbon coating can optimize the electrochemical performance to some extent, high content of carbon would also decrease the energy density of electrode materials at the same time. Preparation of carbon composite cathode materials at a higher level is the key to solve the problem. In this project, organic carbon is introduced into the interlayers of metal phosphate by synthesis of layered organic-inorganic metal phosphate and intercalation chemistry, leading to a quasi-homogeneous carbon composite at the molecular level. In this case, the overall performance of electrode materials is greatly improved without decreasing energy storage. The formation mechanism and regulation of carbon interface, as well as their effect on the reversibility of redox couples are systematically investigated in this work. Based on the theoretical calculation studies, the component and electronic structures of metal phosphate cathodes is modified in order to improve essentially their electronic-ionic conductivity. In addition, the changes of microstructure, phase and interface structure of the electrode materials are real-time characterized by in-situ TEM during the charge/discharge process, and the microstructure evolution mechanism of multi-metal phosphate cathodes is elucidated.

过渡金属磷酸盐是一类非常有前景的钠离子电池正极材料，然而低的电子与离子传导率，限制了其商业化应用。传统的碳复合方式对其电化学性能提升作用有限，而高含量的碳又会显著降低材料的能量密度。进一步提高电极材料的碳复合水平并优化电子结构是解决问题的关键。本项目拟通过有机-无机磷酸盐层状材料与插层化学反应，实现分子水平碳与磷酸盐正极材料准均相复合，从而在维持高能量密度的同时，极大的推升其电化学性能。系统研究碳复合界面的形成与调控机制，及其对多金属氧化还原电对的可逆性影响。利用金属有机前驱体对正极材料组成、电子结构进行调控，从根本上优化电子与离子传导。并基于理论计算结果，揭示多金属共存情况下，金属原子间相互作用与Jahn−Teller效应的发生/抑制现象之间的内在关联。结合原位TEM测试，实时观察电极材料在充放电时相结构和碳界面变化，阐明多金属磷酸盐在电化学反应过程中的显微组织结构衍变机理。

电极材料颗粒内部缓慢的离子扩散与电子传导，极大限制了电池倍率性能的发挥。利用碳材料对电极进行复合来提升整个电极的电子传导被认为是最有效的途径。但电极材料倍率性能的提升极大依赖于电极材料本身的尺度。当电极材料颗粒大于一定尺寸时，碳包覆仅能优化界面处的电子/离子传导。因此，在低碳组分下（从而维持高能量密度）继续提高碳复合电极材料的电化学性能，唯一有效途径是进一步提高碳与电极材料的复合水平，乃至于在分子尺度下进行碳与电极材料的准均相复合。因此，发展新型分子水平碳复合方式以及系统研究分子水平碳界面的电子/离子传导机制与电化学行为具有非常重要的意义。在本项目中，申请人基于插层化学与原位热解反应，开发了多种碳复合电极材料，系统研究了它们的电化学储能性能，所开发碳复合电极展现出了优异的倍率性能与循环性能。基于此研究，系统研究分子水平碳复合界面的形成与调控机制，结合原位TEM测试，实时观察电极材料在充放电时的形貌、相结构和碳界面变化，阐明了分子水平碳复合界面与电极材料在电化学反应过程中的显微组织结构衍变机理，以及碳界面结构与电化学性能之间的构效关系。