“锂离子泵”薄膜外延包覆改性富锂锰基正极材料及储锂机理

Li-ion battery is widely concerned on the application of the high power and energy density, such as electric vehicles, hybrid electric vehicles, energy storage, ect. For meeting the increasing demands, many researchers are focused on developing a cathode with higher specific capacity and rate capability. Li-rich manganese-based oxides display a high specific capacity of 250 mAh g–1, which is higher than traditional cathodes. However, Li-rich manganese-based oxides deliver a high first cycle capacity loss, poor cycle life and rate capability, voltage fade and structure transform. The subject investigated in this program is xLi2MnO3∙(1-x)LiNiyMn1-yO2 cathode to solve the above problems. A facial solvothermal route will be used to grow an epitaxial Li4Ti5O12 “lithium ionic pump” coating layer with high Li+ ions diffusion on the surface of xLi2MnO3∙(1-x)LiNiyMn1-yO2. The interfacial structure of the Li4Ti5O12 “lithium ionic pump” epitaxially coated material will be investigated by HAADF-STEM images with an atom scale magnification. The relationship between epitaxially coated structure and improvement of the electrochemical performance would be also investigated. The aim of this research is to obtain a higher cycle stability and rate capability Li4Ti5O12 epitaxially coated xLi2MnO3∙(1-x)LiNiyMn1-yO2, reveal the mechanism of Li4Ti5O12 epitaxially coating layer and diffusion of Li+ ions in the Li4Ti5O12 epitaxially coated material.

锂离子电池在高功率和能量密度设备（如纯电动汽车、混合式电动汽车、能量存储）中的应用备受关注。为了满足这些日益增长的需求，研究者致力于开发具有更高比容量和倍率性能的正极材料。富锂锰基正极材料比容量超过250mAh g–1，明显高于传统正极。本项目针对富锂锰基材料存在的首次循环不可逆容量损失大，寿命短，倍率性能差，电压下降，结构转化等问题，以xLi2MnO3∙(1-x)LiNiyMn1-yO2为研究对象，在材料表面直接生长一层“锂离子泵”材料，即高Li+扩散常数Li4Ti5O12外延包覆层，采用原子级HAADF-STEM研究两相外延结构，测试包覆前后材料电化学性能，分析外延包覆结构与电化学性能之间的关系。项目预期将得到长循环寿命和高倍率性能的Li4Ti5O12“锂离子泵”外延包覆改性富锂正极材料，揭示外延包覆层的生长机理，找出外延包覆层的最佳生长条件，分析Li+在外延包覆结构界面的扩散机理。

富锂锰基材料由于其具有高的比容量（>250mAh g-1），在高功率和高能量密度设备中（电动汽车、混合式电动汽车等）具有十分广阔的应用前景。然而，富锂锰基材料在实际应用过程中面临首次循环效率低，循环稳定性和倍率性能差，循环过程出现电压下降，结构转化等问题。本项目针对上述问题，系统地研究富锂锰基材料的组成和合成条件对材料结构和电化学性能影响的基础上，通过溶剂热法制备出具有优良电化学性能的Li4Ti5O12“锂离子泵”薄膜外延包覆改性富锂锰基正极材料xLi2MnO3-(1-x)LiNiyMn1-yO2。并探索不同的溶剂热、烧结条件和钛源用量对外延包覆改性富锂锰基材料的结构和电化学性能的影响，确定了最佳的合成条件：溶剂热温度150°C，溶剂热时间5h，乙醇为反应溶剂；溶剂热反应后烧结温度700°C，烧结时间2h，自然降温；最佳Li4Ti5O12的包覆量为2.5%（wt%）。通过研究外延包覆改性后材料在充放电循环过程中结构和电压变化情况，探索Li+在外延包覆改性富锂锰基材料中的脱嵌反应机理。结果表明，作为锂离子电池正极材料，Li4Ti5O12外延包覆改性后富锂锰基材料的首次库伦效率、循环稳定性和大电流密度下的充放电性能都大大提高了。原位XRD测试表明，Li4Ti5O12外延包覆层能够有效地抑制充放电循环过程中的结构变化，提升循环稳定性和抑制循环过程中电压降。本项目阐明了“锂离子泵”外延包覆改性富锂锰基材料的合成、结构和储锂机理，为抑制富锂锰基正极材料的结构转变和电压下降提供了新的思路和途径。