Li Xun-Lu, Bao Jian, Li Yi-Fan, Chen Dong, Ma Cui, Qiu Qi-Qi, Yue Xin-Yang, Wang Qin-Chao, Zhou Yong-Ning
Department of Materials Science Fudan University Shanghai 200433 P. R. China.
Adv Sci (Weinh). 2021 Feb 18;8(9):2004448. doi: 10.1002/advs.202004448. eCollection 2021 May.
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 NaMnMgO with Mg in the hinging Mn-O 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 NaMnO. 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 NaMnMgO is more electron-abundant in Mn 3d orbits near the Fermi level than that in NaMnO, leading to higher redox participation of Mn ions.
阴极材料中钠的电化学不可逆性和缓慢迁移导致钠离子电池的循环稳定性和倍率性能较差。在此，设计了一种将镁离子引入锰基隧道结构阴极材料铰链位点的新策略。实现了该阴极中高度可逆的电化学反应和相变。所得在铰链式锰-氧四方锥中含有镁的NaMnMgO表现出良好的循环稳定性和倍率性能。在2 C的电流密度下，800次循环后保留了67%的初始放电容量（在20 C时为70%），比未掺杂的NaMnO有很大改善。这种改善归因于镁掺杂后钠扩散动力学的增强和脱钠能量的降低。基于同步辐射的X射线技术证明了高度可逆的电荷补偿和结构演变。对锰平均电子数的差分电荷密度和原子布居分析表明，NaMnMgO在费米能级附近的锰3d轨道中比NaMnO具有更多的电子，导致锰离子的氧化还原参与度更高。
