可逆阴离子氧化还原反应和结构演变的共生P2/O3双相钠离子电池层状氧化物正极设计和开发

Rational design of intergrowth P2/O3 biphasic layered structure with reversible anionic redox chemistry and structural evolution for Na-ions batteries

层状氧化物因在钠离子电池正极中表现出的优异性能而引起了前所未有的关注,其中两种典型的P2和03型材料各具优势.因此,设计和开发包含P2和03材料的复合材料成为一种新的选择.但对这种具有多相结构的复杂正极材料的阴离子/阳离子的行为和结构演变过程的研究仍缺乏全面且深入的研究.本文基于两种典型的具有相同元素组成但不同的晶体结构的正极材料:P2型Na0.67Ni0.33Mn0.67O2和O3型NaNi0.5Mn0.5O2,开发了一种成分为Na0.732Ni0.273Mg0.096Mn0.63O2的双相材料,其中包含78.39 wt%的P2相和21.61 wt%的O3相.晶体结构分析和密度泛函理论(DFT)计算结果表明,该复合材料倾向于形成原子水平上的共生结构,且形成的双相结构的复杂晶格条纹可以阻止电极过程中过渡金属和氧原子迁移.相对于单相结构,双相结构提高了正极材料的储钠容量和稳定性,并增强了阴离子O2-/n-氧化还原的可逆性.此外,P2和O3结构共生的类异质结结构具有共享相边界,其形成的互锁效应可有效缓解P2和03结构在电极过程产生的晶格滑移和Na+离子脱出/嵌入的晶格应力.因此,共生的P2/03复合材料表现出较高的容量、较好的循环性能(0.1 C倍率下约有130 mAh g-1容量,循环200次后容量保持率为73.1%)和可逆的晶体结构转变.本文研究证实合理设计的双/多相正极可用于高能钠离子电池的应用.

Layered oxides have attracted unprecedented attention for their outstanding performance in sodium-ion battery cathodes.Among them,the two typical candidates P2 and O3 type materials generally demonstrate large diversities in specific capacity and cycling endurance with their advantages.Thus,composite materials that contain both P2 and O3 have been widely designed and constructed.Nevertheless,the anionic/cationic ions’ behavior and structural evolution in such complex structures remain unclear.In this study,a deep analysis of an advanced Na0.732Ni0.273Mg0.096Mn0.63O2material that contains78.39 wt% P2 phase and 21.61 wt% 03 phase is performed based on two typical cathodes P2Na0.67Ni0.33Mn0.67O2and O3 NaNi0.6Mn0.5O2that have the same elemental constitution but different crystal structures.Structural analysis and density functional theory(DFT) calculations suggest that the composite is preferred to form a symbiotic structure at the atomic level,and the complex lattice texture of the biphase structure can block unfavorable ion and oxygen migration in the electrode process.Consequently,the biphase structure has significantly improved the electrochemical performance and kept preferable anionic oxygen redox reversibility.Furthermore,the hetero-epitaxy-like structure of the intergrowth of P2 and O3 structures share multi-phase boundaries,where the inconsistency in electrochemical behavior between P2 and O3 phases leads to an interlocking effect to prevent severe structural collapse and relieves the lattice strain from Na+de/intercalation.Hence,the symbiotic P2/O3composite materials exhibited a preferable capacity and cyclability(~130 mAh g-1at 0.1 C,73.1% capacity retention after 200 cycles at 1 C),as well as reversible structural evolution.These findings confirmed the advantages of using the bi/multi-phase cathode for high-energy Na-ion batteries.