LiNi0.5Mn1.5O4(LNMO)/石墨高压锂离子电池正负极材料的原位表面包覆及电化学性能研究

The LNMO/Graphite system can offer high operating voltage and energy density. However, its commercialization is plagued by severe capacity fade during charge-discharge cycling. This is mainly attributed to the failure of graphite anode due to deposition of Mn from the dissolved LNMO on the graphite surface. In order to suppress the dissolution of LNMO and the deposition of dissolved Mn on the graphite surface, we propose coating both LNMO and graphite with the metal oxides. In this project, the controlled deposition of metal alkoxides in alkyl carbonates will be used to prepare various metal oxide coatings on self-made spherical LNMO and commercial graphite. The aims of the project are (1) to obtain the surface modified materials with uniform, continuous and controllable thickness coatings by optimizing the reaction conditions including species of alkyl carbonates, type and structure of metal alkoxides, reaction temperature, surface pretreatment of LNMO and graphite, heat treatment of coating materials, etc; (2) to elucidate the growth mechanism of coatings on LNMO and graphite; (3) to established the relationship between quality, structure, composition and morphology of coatings, electrochemical performance of coated materials and preparation conditions; (4) to further investigate the effect of coatings at LNMO and graphite on the electrochemical behavior of LNMO/Graphite full cell; (5) to understand the mechanism that coatings suppress the dissolution of LNMO and the deposition of dissolved Mn on the graphite surface.

LNMO/石墨体系虽具有高工作电压和比能量，但循环过程中容量衰减过快限制了它的商品化。LNMO溶解产物Mn在石墨负极上沉积导致石墨负极失效被认为是引起该体系容降过快的主要原因。为了解决这一问题，我们提出使用金属氧化物对LNMO和石墨材料表面都进行表面修饰的策略，以期能够抑制LNMO中Mn溶出和溶解Mn在石墨负极上沉积。本项目拟采用金属醇盐在碳酸脂类化合物中可控沉积反应在LNMO和石墨表面原位生长制备出金属氧化物包覆层，通过比较碳酸酯及醇盐种类和结构、反应温度、基体表面前处理等制备条件对包覆层结构和形态的影响，优化制备出均匀、连续、厚度可控的包覆层，阐明包覆层在LNMO和石墨上的生长机制。建立包覆层材质、结构、组成和形态与包覆材料的电化学性能以及制备条件三者之间关系。进一步研究LNMO包覆层和石墨包覆层对LNMO/石墨电池体系的影响，探索包覆层抑制LNMO溶解和元素Mn在石墨电极上沉积机理。

提升锂离子电池的能量密度是锂离子电池研究的主要目标之一。采用高电压的材料作为电池正极是实现这一目标的重要途径。因此橄榄石型LiCoPO4、固溶体xLi2MnO3•(1-x)LiMnO2、尖晶石LiNi0.5Mn1.5O4(LNMO)等高电压的正极材料受到广泛关注。其中尖晶石LMNO材料价格比较低廉，性能也最为成熟，其与商品化的碳负极匹配构成的LNMO/碳电池的工作电压高达4.6-4.7 V，但是该电池在充放电循环过程中容量快速衰减限制了它的实际应用。对LNMO和碳负极材料进行表面包覆、应用电解液添加剂和开发新型抗氧化电解液是解决高压锂离子电池循环寿命短的三大策略。针对这一问题，本项目以LNMO/石墨体系为研究对象，进一步研究了该电池的失效机制，认为电池失效的最直接的原因是电解液在正极的氧化产物迁移到负极发生被还原而导致电池中活性锂的不可逆损失。基于这一研究结果，提出了多孔PVDF-锂化金属氧化物活性复合隔膜的概念，并对一系列十余种锂化复合隔膜进行了研究，阐述了它们的作用机理。采用这种隔膜的高压LNMO/石墨电池的循环寿命比采用传统PP隔膜的在同工作条件下可以高出近30%，这是因为锂化复合隔膜可以通过吸附(多孔PVDF层)和还原(锂化金属氧化物)来自正极的电解液氧化产物来阻隔其在负极还原而减小消耗电池的活性锂。研究还表明，这种锂化复合隔膜比PP隔膜具有高得多的热稳定性和更好的电解液浸润性。另外，本项目还发展了两种高压电解液添加剂，一种是两极成膜添加剂磷酸三烯丙酯，另一种是正极成膜添加剂磷酸三丁酯。相关研究成果已发表SCI论文6篇，申请国家发明专利3项。