Li-Substituted Co-Free Layered P2/O3 Biphasic Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 as High-Rate-Capability Cathode Materials for Sodium Ion Batteries

The effects of Li substitution for Ti on the structure and electrochemical performances of Co-free Na-0.67-Mn0.55Ni0.25Ti0.2-xLixO2 (x = 0, 0.1, 0.2) layered cathode materials for sodium ion batteries have been comprehensively investigated. X-ray diffraction and Rietveld refinement results demonstrate that Li mainly occupies TM (TM = transition metal) sites in the crystal structure to maintain the P2 structure majority and a small amount of Li atoms enter Na sites to generate some O-3 phase. The discharge voltage, reversible capacity, rate capability, cycling performance, and Coulombic efficiency all have been improved by Li substitution, which can be largely attributed to the integration of P2 and O3. Li substitution also raises the average discharge voltage from 2.6 to 3.1 V. Na0.67Mn0.55Ni0.25Li0.2O2 (L02) can deliver an initial capacity of about 158 mA h g(-1) at 0.05C (12 mA g(-1)) in comparison with the Li-free sample (147 mA h g(-1)). Even at the high rates of 480 (2C), 1200 (5C), and 1920 (8C) mA g(-1), L02 can also display ca. 93, 65, and 38 mA h g(-1) discharge capacities, respectively. The rate capability is higher than what is reported in the previous Li-substituted cathode materials. In addition, Li substitution in transition-metal sites generates more defects to maintain the charge neutrality, which enhances the electronic conductivity and also has a positive effect on the Na ion diffusion coefficient. The electronic conductivity and Na ion diffusion coefficient have been enhanced by 122% and 29%, respectively, with the substitution of Li for Ti. Our results also show that the oxidation peaks become sharper with increasing Li content, which indicates the feasibility of Na ion intercalation/deintercalation in the integrated P2/O3 phase. This study also offers some new insights into designing high-performance cathode materials for sodium ion batteries.

LI取代Ti对无共无共同NA-0.67-MN0.55NI0.25TI0.2-XlixO2（X = 0，0.1，0.2）的结构和电化学性能的影响已全面地研究了钠离子电池。 X射线衍射和Rietveld的完善结果表明，LI主要占据晶体结构中的TM（TM =过渡金属）位点，以维持P2结构多数，而少量LI原子进入Na位点以生成O-3相。排放电压，可逆能力，速率能力，循环性能和库仑效率都通过LI替代来提高，这主要归因于P2和O3的整合。李的替代还将平均排放电压从2.6升至3.1 V. Na0.67mn0.55ni0.25li0.2O2（L02）可以在0.05C时提供约158 mA H G（-1）的初始容量（12 mA g（-1） ））与无LI的样品（147 MA H G（-1））相比。即使以480（2C），1200（5C）和1920（8C）MA G（-1）的高率，L02也可以显示Ca。 93、65和38 mA H G（-1）排放能力。速率能力高于先前的LI-取代阴极材料中报道的功能。此外，过渡金属位点的LI取代会产生更多的缺陷以维持电荷中立性，从而提高了电子电导率，并且对NA离子扩散系数也具有积极影响。利用LI代替Ti，电子电导率和NA离子扩散系数分别提高了122％和29％。我们的结果还表明，随着LI含量的增加，氧化峰变得更加尖锐，这表明在集成的P2/O3相中，NA离子插入/去解剖学的可行性。这项研究还为设计用于钠离子电池的高性能阴极材料提供了一些新的见解。