Hartmann Felix, Etter Martin, Cibin Giannantonio, Groß Hendrik, Kienle Lorenz, Bensch Wolfgang
Institute of Inorganic Chemistry, Christian-Albrecht University of Kiel, Max-Eyth-Str. 2, 24118 Kiel, Germany.
Deutsches Elektronen-Synchrotron (DESY), Notkestr. 85, 22607 Hamburg, Germany.
Nanoscale. 2022 Feb 17;14(7):2696-2710. doi: 10.1039/d1nr06950k.
Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous study about Na storage properties of ultra-small FeS nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of FeS into nanocrystalline intermediates occurs accompanied by reduction of Fe to Fe cations. Discharge to 0.1 V leads to formation of strongly disordered Fe finely dispersed in a nanosized NaS matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of NaS is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, , 486 mAh g after 300 cycles applying 0.5 A g and superior coulombic efficiencies >99.9%. During charge to 3.0 V, FeS with smaller domains are reversibly generated in the 1 cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of FeS in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.
人们对钠离子电池（SIBs）的各种电极材料进行了研究，而SIBs发展的一个重要前提是详细了解电荷存储机制。在此，我们对通过溶剂热法合成的超小FeS纳米颗粒的钠存储特性进行了严谨研究，这些纳米颗粒作为SIBs的阳极材料表现出非常好的电化学性能。利用基于同步加速器的X射线衍射和X射线吸收技术，更深入地研究纳米尺度上的电化学反应途径，揭示了一种复杂的转化机制。最初，FeS分离成纳米晶中间体，同时Fe还原为Fe阳离子。放电至0.1 V会导致形成高度无序的Fe，其精细分散在纳米尺寸的NaS基质中。由此产生的体积膨胀导致在3.0 - 0.1 V电压范围内长期稳定性变差。将低截止电位调整到0.5 V，可防止NaS结晶，并形成完全非晶的中间阶段。因此，较小的电压窗口有利于长期稳定性，在以0.5 A g的电流进行300次循环后，具有高达486 mAh g的可逆容量保持率以及>99.9%的优异库仑效率。在充电至3.0 V时，较小区域的FeS在第1次循环中可逆生成，但进一步循环会导致结构长程有序性丧失，而局部环境类似于后续充电状态下的FeS。动电分析表明电荷存储具有很高的电容贡献，这表明扩散长度缩短，因此氧化还原反应主要发生在纳米尺寸转化产物的表面。
