High voltage structural evolution and enhanced Na-ion diffusion in P2-Na2/3Ni1/3−xMgxMn2/3O2 (0 ≤ x ≤ 0.2) cathodes from diffraction, electrochemical and ab initio studies†,Energy & Environmental Science

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High voltage structural evolution and enhanced Na-ion diffusion in P2-Na2/3Ni1/3−xMgxMn2/3O2 (0 ≤ x ≤ 0.2) cathodes from diffraction, electrochemical and ab initio studies†
Energy & Environmental Science ( IF 32.4 ) Pub Date : 2018-03-16 00:00:00 , DOI: 10.1039/c7ee02995k
Nuria Tapia-Ruiz _{1,

2,

3,

4,

5} , Wesley M. Dose _{6,

7,

8,

9} , Neeraj Sharma _{6,

7,

8,

9} , Hungru Chen _{5,

10,

11,

12} , Jennifer Heath _{5,

10,

11,

12} , James W. Somerville _{1,

2,

3,

4,

5} , Urmimala Maitra _{1,

2,

3,

4,

5} , M. Saiful Islam _{5,

10,

11,

12} , Peter G. Bruce _{1,

2,

3,

4,

5}

Affiliation

We have presented a detailed investigation of the effects of Mg substitution on the structure, electrochemical performance and Na-ion diffusion in high voltage P2-type Na_2/3Ni_1/3−xMg_xMn_2/3O₂ (0 < x < 0.2) cathode materials for Na-ion batteries. Structural analysis using neutron diffraction showed that Mg²⁺ substitutes at Ni²⁺ sites from ordered [(Ni²⁺/Mn⁴⁺)O₆] honeycomb units along the ab-plane, leading to an AB-type structure that can be indexed using the P6₃ space group. Within the sodium layers, high Mg-substitution levels (i.e. x = 0.2) caused a disruption in the typical Na zig-zag ordering observed in the undoped material, leading to a more disordered Na distribution in the layers. Load curves of the x = 0.1 and 0.2 materials show smooth electrochemistry, indicative of a solid-solution process. Furthermore, DFT calculations showed an increase in Na-ion diffusivity for the Mg-substituted samples. Enhanced cycling stability was also observed in these materials; structural analysis using high-resolution in operando synchrotron X-ray diffraction show that such an improved electrochemical performance is caused by the suppression of the O2 phase and switch to the formation of an OP4 phase. Ab initio studies support our experimental evidence showing that the OP4 phase (cf. O2) is the most thermodynamically stable phase at high voltages for Mg-substituted compounds. Finally, we have provided evidence using diffraction for the x = 1/2 and x = 1/3 intermediate Na⁺-vacancy ordered phases in P2-Na_2/3Ni_1/3Mn_2/3O₂.

中文翻译：

高电压结构演变和增强的钠离子扩散在P2-娜_2/3的Ni _{1 / 3- X} Mg的_X的Mn _2/3 Ø ₂（0≤ X ≤0.2）从衍射，电化学和阴极从头研究†

我们已经提出了关于Mg替代对高压P2型Na _2/3 Ni _{1 / 3− x} Mg _x Mn _2/3 O ₂（0 < x <0.2）用于Na离子电池的正极材料。使用中子衍射的结构分析表明，沿ab平面有序[（Ni ²⁺ / Mn ⁴⁺）O ₆ ]蜂窝单元的Ni ²⁺位置的Mg ²⁺替代物，导致可索引的AB型结构使用P 6 ₃空间群。在钠层中，高的Mg取代水平（即x = 0.2）导致在未掺杂材料中观察到的典型Na Z字形有序排列中断，从而导致Na层中的钠分布更加混乱。x = 0.1和0.2的材料的载荷曲线显示出平滑的电化学，表明存在固溶过程。此外，DFT计算显示，Mg取代样品的Na离子扩散率增加。在这些材料中还观察到增强的循环稳定性。在操作同步加速器X射线衍射中使用高分辨率进行的结构分析表明，这种改善的电化学性能是由于抑制O2相并转换为OP4相而引起的。从头算研究支持我们的实验证据，表明OP4相（比照O2）是高压下Mg取代的化合物在热力学上最稳定的相。最后，我们提供了使用P2-Na _2/3 Ni _1/3 Mn _2/3 O _2中的x = 1/2和x = 1/3中间Na ^+-空位有序相的衍射的证据。

更新日期：2018-03-16

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