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Abstract：
Electrochemical irreversibility and sluggish mobility of Na+ in the cathode materials result in poor cycle stability and rate capability for sodium-ion batteries. Herein, a new strategy of introducing Mg ions into the hinging sites of Mn-based tunnel-structured cathode material is designed. Highly reversible electrochemical reaction and phase transition in this cathode are realized. The resulted Na0.44Mn0.95Mg0.05O2 with Mg2+ in the hinging Mn-O5 square pyramidal exhibits promising cycle stability and rate capability. At a current density of 2 C, 67% of the initial discharge capacity is retained after 800 cycles (70% at 20 C), much improved than the undoped Na0.44MnO2. The improvement is attribute to the enhanced Na+ diffusion kinetics and the lowered desodiation energy after Mg doping. Highly reversible charge compensation and structure evolution are proved by synchrotron-based X-ray techniques. Differential charge density and atom population analysis of the average electron number of Mn indicate that Na0.44Mn0.95Mg0.05O2 is more electron-abundant in Mn 3d orbits near the Fermi level than that in Na0.44MnO2, leading to higher redox participation of Mn ions.
摘要：
正极材料中Na的电化学不可逆性和缓慢的迁移率导致钠离子电池的循环稳定性和倍率性能差。在这里，设计了一种将Mg离子引入Mn基隧道结构阴极材料的结合位点的新策略。在该阴极中实现了高度可逆的电化学反应和相变。所得的Na0.44Mn0.95Mg0.05O2与Mg2在铰接的Mn-O5方形金字塔中表现出良好的循环稳定性和倍率能力。在2C的电流密度下，在800次循环后保留了67％的初始放电容量（20C时为70％），比未掺杂的Na0.44MnO2有很大改善。改善归因于Mg掺杂后增强的Na扩散动力学和降低的去氧化能。基于同步加速器的X射线技术证明了高度可逆的电荷补偿和结构演化。Mn平均电子数的差分电荷密度和原子总体分析表明，在费米能级附近的Mn3d轨道中，Na0.44Mn0.95Mg0.05O2比Na0.44MnO2中的电子丰度更高，导致Mn离子的氧化还原参与更高。
