【作者】 李杰；

【导师】 邱新平；

【作者基本信息】 清华大学 ， 化学， 2024， 博士

【摘要】 随着对锂离子电池能量密度的需求不断提升,开发高比容量正极材料尤为迫切。富锂层状氧化物正极材料（LLOs）,因其独特的氧氧化还原反应,展现出超过250 m Ah g^-1的超高比容量,被视为下一代锂离子电池的理想正极材料之一。然而,该材料在循环过程中存在初始库仑效率（ICE）低、电压/容量衰减、倍率性能不佳以及电压滞后等问题,严重制约了其商业化进程。上述问题均与氧氧化还原直接或间接相关,因此改善富锂正极材料电化学性能的关键在于调控活性晶格氧的局部环境和表面结构。基于此,本论文以Li_1.2Ni_0.2Mn_0.6O₂（LNMO）为研究对象,提出了通过调控类Li₂MnO₃晶畴和表面结构的策略来改善富锂正极材料电化学性能,并揭示了局部结构调控的机理及电化学性能提升机制。通过调控高温煅烧过程中的氧分压合成了具有均匀分散且小尺寸的类Li₂MnO₃晶畴和无序岩盐结构保护层的富锂正极材料,其表现出优异的电化学性能和热稳定性。适当降低高温煅烧时的氧分压可以同时在表面和体相引入少量低价过渡金属离子(TM,如Mn³⁺)和氧空位,并实现表面结构和类Li₂MnO₃晶畴的共调控:在表面形成厚度约为3 nm的无序岩盐结构保护层,可以显著抑制晶格氧释放、TM溶解和界面副反应;在体相中形成均匀分散且小尺寸的类Li₂MnO₃晶畴,可以提高氧氧化还原可逆性和结构稳定性。通过提高煅烧温度并结合淬火工艺实现了对类Li₂MnO₃晶畴结构的调控,合成出高度稳定的微米级单晶富锂正极材料（NM-1050-Q）。NM-1050-Q中类Li₂MnO₃晶畴中的堆垛层错密度很低,有效地提高了Li⁺扩散系数,从而使微米级单晶也能完全释放比容量。此外,Ni掺杂进入类Li₂MnO₃晶畴中的锂层,可以在更低的电势激活氧氧化还原并提高氧氧化还原动力学性能,从而减小电压滞后并提高工作电压;并且,Ni掺杂显著提高氧氧化还原的可逆性,减少循环过程中的晶格氧释放和不可逆结构衰退。开发了硝酸盐溶液淬火策略,利用La（NO₃）₃溶液淬火在LNMO的表面构筑了高度稳定的集成结构:外层为～3 nm厚的无序岩盐结构,其具有优异的结构稳定性;内层为La掺杂的层状结构,强的La-O键可以稳定晶格氧。该表面集成结构可以抑制晶格氧释放、TM的平均价态降低和材料结构衰退,并减缓界面副反应、促进生成坚固而致密的CEI膜,从而实现优异的电化学性能。更多还原

【Abstract】 With the the growing demands on the energy density of lithium-ion batteries,there is an urgent need to develop cathode materials with high specific capacity.Due to the unique oxygen redox,lithium-rich layered oxides（LLOs）are regarded as one of the ideal cathode materials for next-generation lithium-ion batteries,which exhibit an ultra-high specific capacity of more than 250 m Ah g^-1.However,LLOs suffer from low initial coulombic efficiency（ICE）,voltage/capacity decay,poor rate performance,and voltage hysteresis during cycling,which greatly hinder its commercialization.Since these drawbacks are connected to oxygen redox either directly or indirectly,regulating the local environment of active lattice oxygen and surface structure is essential to enhancing the electrochemical performance of LLOs.Herein,based on Li_1.2Ni_0.2Mn_0.6O₂（LNMO）,we propose strategies of improving the electrochemical performance of LLOs by regulating Li₂MnO₃-like domains and surface structure.Moreover,the mechanism underlying local structure regulation and electrochemical performance improvement is revealed.By regulating the oxygen partial pressure during high-temperature calcination,lithium-rich cathode material with uniformly dispersed and small-sized Li₂MnO₃-like domains and a protective rock-salt structure shell is synthesized,which exhibits excellent electrochemical performance and thermal stability.It was found that appropriately reducing the oxygen partial pressure during high-temperature calcination can introduce a small amount of low-valence transition metal（TM）ions(such as Mn³⁺)and oxygen vacancies into the surface and bulk at the same time,and r realize the co-regulation of Li₂MnO₃-like structure:the formation of the protective rock-salt structure shell with a thickness of about 3 nm on the surface,which can significantly inhibit lattice oxygen release,TM dissolution,and interfacial side reactions;the formation of uniformly dispersed and small-sized Li₂MnO₃-like domains,enabling highly reversible oxygen redox and excellent structural stability.By improving the calcination temperature and combining the quenching process,the Li₂MnO₃-like domains is controlled,and a highly stabilized micron-sized single-crystal lithium-rich cathode material（NM-1050-Q）is synthesized.Few stacking faults can be discovered in Li₂MnO₃-like domains,which can effectively improve the Li⁺diffusion coefficient,allowing the specific capacity of micron-sized single crystals to be fully released.More importantly,the Ni partlally occupies the Li-3 sites in Li₂MnO₃-like domains,which can activate oxygen redox at a lower potential and improve oxygen redox kinetics,reducing the voltage hysteresis and improving operating voltage.Moreover,the Ni in Li₂MnO₃-like domains can enhance the reversibility of oxygen redox,reducing lattice oxygen release and irreversible structural degradation during cycling.A nitrate aqueous solution quenching strategy is developed.By quenching with La（NO₃）₃ solution,a highly stable integrated structure is constructed on the surface of LNMO particles:the outer layer is a disordered rock-salt structure with excellent structural stability,and the inner layer is a La-doped layered structure with strong La-O bonds to stabilize lattice oxygen.Therefore,the integrated structure can greatly restrain lattice oxygen release,TM reduction,and structure degradation.Additionally,it can reduce interfacial side reactions and promotes the formation of robust CEI films,resulting in excellent electrochemical performance.更多还原