高能量密度锂离子电池正极材料的表界面控制与性能研究

The successful application of lithium ion batteries in electrical vehicle relies on a series of breakthroughs in key technologies, among which the cyclability and thermal stability of cathode materials stands out as a well-known issue. Suitable surface modification of cathode materials has proved to be effective to tackle this problem, usually by introducing a thin coating layer with the thickness around several nanometers. However, due to the lack of efficient synthesis methods and also the difficulties during the structure control on a nanometer scale range, the coating process turns out to become a continuous trial-and-error one for the surface modification of different cathode materials. A systematic guidance for an optimized surface structure is in great demand to achieve better performance of these cathode materials. In this contribution, we propose a solution-based synthetic protocol to achieve a systematic tune of the surface property of cathode materials. By means of kinetic control during the crystallization process, the precursor of the coating materials can be released gradually. In this way, the fast nucleation of the precursor can be prevented and a uniform coating can be achieved with tunable thickness. The achievement of heterogeneous nucleation can produces coated cathode materials with highly controlled surface properties, which can be used as a model system to inquest the relationship between the surface properties of cathode materials and their battery performance. Combined with the data from detailed characterizations on the medium state of the cathode materials during the charge/discharge cycle, we expect to have a better understanding on the working mechanism of lithium ion batteries. An optimized surface structure is expected to be disclosed for the coating layers. Through a delicate control on different parameters of the surface coating layer, such as coverage, porosity, thickness and conductivity, a better cyclability and thermal stability are expected.

高安全性、低成本的锂离子电池正极材料是实现电动汽车大规模应用的关键，正极材料和电解液之间的界面稳定性也为此一直备受关注，正极材料的表面包覆改性为提高其电化学性能和热稳定性提供了有效的手段。由于相关合成手段的缺乏及材料表面纳米结构控制上的困难，包覆的有效性、可控性、批次重复性等尚存在严重不足，难以实现对表界面性能的系统控制和优化。为此，本项目将致力对正极材料表面包覆过程的调控，通过对溶液相包覆方法的研究和探讨，实现对正极材料表界面性质的系统调节，并结合对材料电化学性能的评价和对电化学反应中间过程的表征，探讨涉及材料表界面特性和电池循环性能的一系列关键问题，包括包覆过程中的结晶动力学控制、正极材料表界面的可控构筑、材料表面特性与其电化学性能的构效关系、正极材料容量的衰减机制等。通过以上探讨，以加深对锂离子电池基本理论的认识，并进一步提高正极材料的循环性能和热稳定性。

锂离子电池的安全问题日益引起人们的重视，电池表面剧烈的副反应会导致热失控，锂离子电池存在着发生着火甚至爆炸的危险，有必要通过材料的设计和优化进一步提高材料的热稳定性和安全性。对电极材料特别是正极材料进行表面纳米层包覆，对于降低腐蚀及相关副反应的速度、延长材料的寿命和安全性具有显著的作用。.电极材料表面纳米层的包覆虽然已经被广泛认可，但是对材料表面状态的系统调控及由此带来的电化学性能变化的机制则缺乏深入的研究。对于特定的氧化物、磷酸盐等包覆物种而言，实现纳米尺度均匀包覆层的构筑与电极材料性能优化是电极材料结构控制的重大挑战；由于表面纳米层控制的复杂性，不同的研究者得到的实验结果和提出的电极材料稳定机制机理存在较大差异，而且操作过程中样品批次的重复性也较差，严重制约了动力电池正极材料的发展和应用。.本项目从电极材料表面纳米层的均匀可控包覆及稳定性提升出发，通过合成方法学上的探讨和创新，以溶液相合成法为出发点来实现对正极材料的可控包覆。相关研究围绕包覆物前体析出过程中结晶动力学的控制展开，通过对前体成核能力和生长速度的调控，抑制了包覆物种均相成核的能力，实现其在正极材料表面异相成核生长；并通过对实验条件的调控，达到了对正极材料表面物理化学性能的系统调节，从而建立其对正极材料性能研究的模型材料体系。.从具有特定表面纳米结构的电极材料出发，本项目结合电化学性能测试和电化学中间反应过程的详细表征，系统研究了正极材料的表界面属性与其电化学性能之间的构效关系，探讨了锂离子电池充放电过程中的表面电化学原理，并通过对正极材料表面状态的优化，促进电池稳定性和循环性能的提升。相关的研究在合成方法学、纳米结构的控制、表面电化学过程的探讨及动力电池的应用等具有重要的科学意义和实践价值。