高镍正极极片快速Li+通道构筑及传输机理研究

High energy density is the future development goal of lithium ion battery, and specific energy is expected to reach 300Wh/kg in 2020. On the one hand, the promotion of energy density depends on the progress of key materials technologies; on the other hand, it depends on the optimization of battery structure design, especially electrode design. However, electrolyte penetration and lithium ion transport in the micro pore of electrode are confined by all kind of resistance, which is complicated. Therefore, its effect on the electrochemical performance has not a unified understanding. In this study, Ni-rich cathode electrode will be taken as an example, ultra-thick electrode with a porosity gradient will be prepared through the processes of slurry fomula optimization, multilayer coating with different electrode slurries, and coating - pre calendaring – coating - calendaring process. Then the electrodes are tested electrochemical characteristics by assembled half cells and full cells to investigate the effects of pore distribution on cathode materials capacity and electrochemical performances. Also micro-CT is used to directly observe micro pore electrode structure, and the kinetic of electrolyte penetration is simulated by the CFD software to analyze the influence of 3D pore parameters on the electrolyte penetration and the lithium ion transport mechanism. the transport kinetic in micro-pore and the key factors of Li ion transport limitation are studied, which provides theoretical basis and technical support for the electrode design and optimization of high energy density battery.

高能量密度是锂离子电池发展方向，2020年预计达到300wh/kg。能量密度的提升，一方面依靠关键材料技术的进步，另一方面依赖于电池结构设计的进步，特别是极片的设计。而电极微观孔隙内电解液的浸润与锂离子的扩散受到各种阻力，传输过程复杂，其对电性能的影响也没有形成统一的认识。本项目以高镍正极极片为例，通过浆料配方优化、多层涂布不同分散性的浆料、涂布-预辊压-再涂布-辊压等工艺制备孔隙率梯度分布的超厚极片，并对极片进行半电池和全电池电化学性能测试，研究孔隙分布状态对正极材料容量发挥及其它电化学性能的影响。然后利用高分辨3D X射线成像技术直接观察极片微观孔隙结构，利用流体力学模拟电解液在极片中的浸润过程和效果，并分析孔隙三维参数对电解液浸润及锂离子扩散机理的影响，深入探讨锂离子在极片孔隙中的传输动力学过程以及限制容量发挥的关键因素，为高能量密度电池的极片设计和优化提供理论依据和技术支持。

锂离子电池在充放电过程中电极内涉及电子、锂离子的传输，以及电化学反应等物理化学过程，其微观结构与电池的比能量和电化学性能密切相关。本项目以高镍正极电极为例，通过实验和数值模拟的方法研究电极微观结构特征对电子、锂离子传输以及电化学性能的影响。优化极片的导电网络和微观孔隙结构，制备孔隙率梯度分布的厚极片，研究电极微观结构对电化学性能的影响，主要开展工作包括：（1）构建了基于双层结构正极极片的电化学伪二维模型，并对正极双层孔隙结构进行模拟优化。（2）采用碳纳米管（CNT）和Super-P（SP）碳构建电子导电网络并调节孔隙率，制备了具有高面容量（5mAh/cm2）和梯度孔隙率的双层LiNi0.8Co0.15Al0.05O2（NCA）正极，揭示了一种提升厚电极性能的可行方法，即通过导电剂的交替设计来改善厚电极中显著的极化效应和低的活性材料利用率。（3）通过工艺参数优化，采用刮刀涂布（D）和静电纺丝（E）两种技术制备了孔隙率梯度分布的NCA双层结构电极，其具有高比容量、更好的循环稳定性和倍率性能。实验和模拟结果表明，合理的孔隙率分布有利于电解液的渗透和降低极化现象，从而有利于电解液中的锂浓度均匀和电极反应动力学。（4）将高镍正极极片与硅碳负极匹配制备成软包型电池，优化了电池的电解液用量，研究了负极与正极容量比（N/P）对该电池体系电化学性能和能量密度的影响，并对循环后电池进行失效分析，探究了容量衰减机理及其主要影响因素。