Rechargeable Mg batteries are promising candidates for high energy density storage in theory, when a Mg metal anode is combined with an oxide cathode material. Despite the widely observed sluggish Mg²⁺ diffusion in most oxide lattices, recent first-principles calculations predicted low diffusion barriers in the calcium ferrite (CF)-type post-spinel structures. In the present work, we experimentally examine the prospect of CF-type NaV_1.25Ti_0.75O₄ as a Mg cathode. The Na⁺ ions, which lie in the ion migration pathway, need to be removed or exchanged with Mg²⁺ to allow Mg²⁺ de/intercalation. Partial desodiation was achieved through chemical and electrochemical methods, as proven by X-ray diffraction and X-ray absorption spectroscopy, but deep desodiation was accompanied by partial amorphization of the material. Mg²⁺ ion exchange at moderate temperature (80 °C) resulted in the formation of Na_0.19Mg_0.41V_1.25Ti_0.75O₄; however, phase transformation was observed when higher temperatures were applied to attempt complete ion exchange. Such phenomena point to the instability of the CF lattice when the tunnel is empty or occupied by a small ion (Mg²⁺). Thus, while the low migration barrier predicted by computation is partly based on the relative metastability of the theoretical CF-Mg_xV_1.25Ti_0.75O₄ lattice, the difficulty in stabilizing it also renders the material synthetically nonaccessible, hindering this post-spinel’s application as an electrode material.

中文翻译：

---

NaV _1.25 Ti _0.75 O ₄：用于镁电池的潜在尖峰后阴极材料

从理论上讲，当Mg金属阳极与氧化物阴极材料结合使用时，可充电Mg电池有望成为高能量密度存储的候选材料。尽管在大多数氧化物晶格中普遍观察到Mg ²⁺扩散缓慢，但最近的第一性原理计算仍预示着铁酸钙（CF）型尖晶石结构中的低扩散势垒。在目前的工作中，我们实验性地检验了CF型NaV _1.25 Ti _0.75 O ₄作为Mg阴极的前景。必须除去位于离子迁移路径中的Na ⁺离子或与Mg ²⁺交换以使Mg ²⁺de / intercaling。X射线衍射和X射线吸收光谱证明，通过化学和电化学方法可以实现部分脱氮，但是深度脱氮会伴随材料的部分非晶化。Mg ²⁺在中等温度（80°C）下的离子交换导致形成Na _0.19 Mg _0.41 V _1.25 Ti _0.75 O ₄；但是，当施加更高的温度以尝试完成离子交换时，会观察到相变。当隧道为空或被小离子（Mg ²⁺）。因此，尽管通过计算预测的低迁移势垒部分是基于理论CF-Mg _x V _1.25 Ti _0.75 O ₄晶格的相对亚稳定性，但稳定它的困难也使该材料无法合成，从而阻碍了该后尖峰的应用作为电极材料。