高比能P2/O3共生型钠镍锰氧基钠离子电池正极材料的结构优化及其协同效应研究

Sodium ion battery has obvious resource advantage compared to lithiuim ion battery. The characteristic of low cost makes sodium ion battery more suitable for stationary large-scale energy storage field. Among numerous cathode materials, layered P2/O3 intergrowth-type oxides have a good prospect for development due to their higher theoretical capacity (> 250 mAh/g) and average discharge voltage (3.7 V). Two phases in the system can play a good synergistic effect，similar to layered lithium-rich materials.This system has advantages of larger sodium ion migration channels in the P2-type phase as well as adequate amount of sodium in the O3-type phase. We will optimize the content ratio and distribution of the two phases by changing the synthesis conditions. A small amount of electrochemical inactive Ti4 + or Zn2 + will be used to replace manganese or nickel to improve the stability of the two-dimensional layered structure, and then suppress the occurrence of irreversible phase transition. The optimization of phase composition and doping ratio will be expected to significantly improve the capacity and voltage retention. Phase transition and charge compensation behavior will be characterized by in-situ XRD, ex-situ XAFS, STEM techniques to establish the relationship between the structure and performance. The implementation of this project will effectively promote the development of such a new type of sodium ion battery cathode material system.

钠离子电池与锂离子电池相比具有明显的资源优势。低成本的特点使其更适用于固定式的大规模储能领域。在众多正极材料中，层状P2/O3共生型氧化物由于理论容量（> 250 mAh/g）和平均放电电压（3.7 V）较高，因此具有良好的发展前景。与层状富锂材料类似，该体系中两相间可以较好地发挥协同效应，即兼具P2型相中较大的钠离子迁移通道和O3型相中充足的钠含量等优势。我们将通过改变合成条件来实现对复合物中相含量比及分布的优化;分别用少量电化学非活性的Ti4+或Zn2+取代锰或镍来提高材料二维层状结构的稳定性，进而抑制不可逆相变的发生。在对相组成及掺杂比例进行优化后，将有望大幅提高材料的容量和电压保持率。研究中还将通过原位XRD、非原位XAFS、STEM等手段表征循环过程中的相变及电荷补偿行为，建立起结构变化和性能之间的关系。本项目的实施将会有效推动此类新型高比能钠离子电池正极材料体系的发展。

P2型钠镍锰氧是一类非常有发展潜力的高比能钠离子电池正极材料。针对P2型钠镍锰氧正极材料循环过程中容量快速衰减的问题，本项目提出在一种元素组成为Na0.80Ni0.22Zn0.06Mn0.66O2的P2型正极材料基础上，通过使用碳作为表面保护层来改善其循环稳定性。制备过程中，首先在Na0.80Ni0.22Zn0.06Mn0.66O2正极材料表面构建一层聚多巴胺包覆层，然后通过高温煅烧得到厚度约为5 nm的均匀的氮掺杂碳包覆层。研究表明，在Na0.80Ni0.22Zn0.06Mn0.66O2表面引入氮掺杂碳包覆层显著提高了其界面稳定性。经历电极制备过程后，未包覆的Na0.80Ni0.22Zn0.06Mn0.66O2表面产生了大量电子绝缘的Na2CO3/NaOH。相比之下，氮掺杂碳包覆的Na0.80Ni0.22Zn0.06Mn0.66O2表面不会产生大量电子绝缘的Na2CO3/NaOH。而且，在表面生成Na2CO3/NaOH的情况下，充电至高电压后电解液在Na0.80Ni0.22Zn0.06Mn0.66O2表面分解形成较厚的正极电解质界面膜。这层厚的正极电解质界面膜阻碍了钠离子从材料体相结构中脱出，因而导致循环过程中容量的衰减。因此，碳包覆后Na0.80Ni0.22Zn0.06Mn0.66O2的电化学性能明显改善，其在1536 mA g-1电流密度下的倍率性能从15 mAh g-1提高至62 mAh g-1，在120 mA g-1电流密度下循环100圈后容量保持率从69.8%提高到90.7%。本项目建立起了P2型钠镍锰氧材料界面性质与其电化学性能之间的关联。