Titanium-based layered oxides (TLOs) are one of the most promising electrode material families for sodium-ion batteries (NIBs) due to their smooth charge/discharge profiles and excellent cycle performance. However, the reaction mechanism of these materials, especially the reason for the disappearance of multiple voltage plateaus, is still not clear. Herein, two representative TLOs (Na_2/3Ni_1/3Ti_2/3O₂ and Na_2/3Co_1/3Ti_2/3O₂) with the same P2 crystal structure have been studied to scrutinize those unexplained issues. In situ synchrotron high-energy X-ray diffraction revealed a solid solution reaction mechanism for both, suggesting the absence of rigid phase transitions upon electrochemical cycling. An interesting “spring effect” of the TiO₆ octahedron, i.e., the reversible vibration of the central Ti atom inside the local octahedron upon electrochemical redox, was demonstrated by advanced X-ray absorption spectroscopy and theoretical calculations. Such an effect could suppress the rigid phase transitions, and result in smooth charge/discharge profiles and enhanced cycle stability. This work not only accounts for the disappearance of multiple voltage plateaus of TLOs for NIBs, but also provides an effective local-structure viewpoint to increase the cycle stability of electrode materials for other advanced battery systems.

中文翻译：

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钛基层状氧化物中的局部弹簧效应

钛基层状氧化物（TLO）是钠离子电池（NIB）最有希望的电极材料系列之一，因为它们具有平滑的充电/放电曲线和出色的循环性能。然而，这些材料的反应机理，尤其是多个电压平稳区消失的原因，仍不清楚。在本文中，已经研究了_两个具有相同P2晶体结构的代表性TLO（Na _2/3 Ni _1/3 Ti _2/3 O ₂和Na _2/3 Co _1/3 Ti _2/3 O ₂）来研究那些无法解释的问题。 。原位同步加速器高能X射线衍射揭示了两者的固溶反应机理，表明电化学循环时不存在刚性相变。TiO ₆八面体的一个有趣的“弹簧效应” ，即先进的X射线吸收光谱和理论计算证明了电化学氧化还原时局部八面体内部中心Ti原子的可逆振动。这样的效果可以抑制刚性相变，并导致平滑的充电/放电曲线和增强的循环稳定性。这项工作不仅解决了NIB的TLO的多个电压平稳期的消失，而且为增加其他先进电池系统的电极材料的循环稳定性提供了有效的局部结构观点。