快离子导体/碳双重修饰富锰橄榄石型Li(MnyFe1-y)PO4正极材料的研究

The olivine cathode material Li(MnyFe1-y)PO4 with high Mn-content possesses higher operating potential than LiFePO4, and better conductivity than LiMnPO4. However, the low electronic conductivity, low lithium diffusivity, Jahn-Teller effect, and dissolution of the Mn in electrolyte hinder the practical application of Li(MnyFe1-y)PO4. In this project, nanosized Li(MnyFe1-y)PO4 is made by a room temperature lithiation and low temperature heat treatment method, and the material is also double-modified using fast ion conducting Li3V2(PO4)3 and nano-carbon to improve the Li(MnyFe1-y)PO4 performance. Firstly, the lithium diffusivity can be enhanced owing to the nanorization of the material and the coating layer of fast ion conductor. Secondly, the mutual doping of Mn, Fe, and V, and the double-modification of Li3V2(PO4)3 and nano-carbon will increase the electronic conductivity of Li(MnyFe1-y)PO4. Moreover, the dissolution of Mn can be decreased due to the two coating layers. The study shows the law and mechanism how the Li3V2(PO4)3 and nano-carbon layers influence the lithium ion transport and electronic conductivity, how the Mn, Fe, and V enter into the lattices of shell or core, how the Jahn-Teller effect is restrained, why the Mn dissolution happens and how it is suppressed. These all can provide new ideas and theoretical guidance for the development and application of the cathode materials for lithium ion battery.

富锰橄榄石型Li(MnyFe1-y)PO4正极材料具有工作电位比LiFePO4高，导电性比LiMnPO4好的优点，但较低的电子电导率和锂离子扩散速率、Mn的Jahn-Teller效应以及Mn在电解液中的溶解等问题制约了该材料的有效应用。本项目拟通过常温锂化-低温热处理法制备纳米Li(MnyFe1-y)PO4，采用快离子导体磷酸钒锂和纳米碳对其进行双重修饰改性提高电化学性能。利用材料纳米化和磷酸钒锂包覆提高锂离子扩散速率；利用内核和包覆层中Mn、Fe、V的相互掺杂以及磷酸钒锂和纳米碳的双重修饰提高电子电导率；利用磷酸钒锂和纳米碳双重包覆抑制锰的溶解。通过本项目的研究，揭示磷酸钒锂和纳米碳双重修饰对锂离子传输特性及电导率的影响规律，Mn、Fe、V相互掺杂及Jahn-Teller效应的抑制机理，Mn的溶解机制和抑制原理，从而为锂离子动力电池正极材料的开发应用提供新的思路和科学依据。

本项目针对富锰橄榄石型Li(MnyFe1-y)PO4正极材料电子导电率和离子电导率低，Jahn-Teller效应和Mn溶解等问题，利用快离子导体Li3V2(PO4)3和纳米碳表面修饰的方法对Li(MnyFe1-y)PO4进行改性。研究结果表明，Li(MnyFe1-y)PO4内核和Li3V2(PO4)3外壳相互掺杂以及纳米碳表面修饰共同提高了材料的电子电导率；Li3V2(PO4)3表面修饰增加了锂离子在Li(MnyFe1-y)PO4晶粒表面的扩散通道，从而极大地提高了锂离子扩散速率；钒掺杂降低了Li(MnyFe1-y)PO4在脱锂态时Mn3+和Fe3+的浓度，减小了Jahn-Teller效应并使材料的结构更加稳定，提高了材料的循环性能；双重表面修饰阻碍了电解液与Li(MnyFe1-y)PO4的直接接触，从而有效地抑制了Mn的溶解，同时改善了材料在室温和高温下的循环性能。通过优化工艺条件，最终获得了电化学性能优异的复合正极材料，其中5LiMn0.9Fe0.1PO4@Li3V2(PO4)3/C具有最优的综合性能，该材料在0.1C和1C倍率下的首次放电比容量为165.4和159.2 mAh/g，其中4.1 V平台的容量约占总容量的81%，在1C倍率下循环200次后的容量保持率高达98.3%，各项指标均达到了项目立项时提出的预期目标。本项目的研究为新型锂离子电池正极材料的开发应用提供了新的思路和科学依据。