In-situ analysis of NaMNb(PO4)3 M = Fe, Al anode for sodium ion batteries: follow-up of the structural evolution and electrode reaction

We propose the in-situ XRD study of the NASICON (NA Super Ionic CONductors) samples NaFeNb(PO4)3 and NaAlNb(PO4)3 considered as anodes for sodium ion batteries. Our electrochemical tests reveal good performances, but two main aspects are still undetermined: i) while the two compounds are isostructural, the electrochemical behavior is significantly different in terms of capacity and charge/discharge profiles; ii) the electrochemically active species, unexpectedly is Nb. For these reasons, we aim to perform in situ XRD study to follow up the structural evolution of the systems, identify the Na ions site and occupancy, and determine the electrode reaction. The XRD data obtained from the optical cell will be analyzed according to the Rietveld method (determining the structure), with MEM and Frourier map methods (locating the Na ion sites). Data will be coupled with ex situ SEM/TEM analysis, XPS, and possibly with XAS data and neutron diffraction data (proposals under evaluation).我们提出针对用作钠离子电池负极的NASICON（NA Super Ionic CONductors，钠超离子导体）材料NaFeNb(PO4)3与NaAlNb(PO4)3的原位X射线衍射（XRD）研究。本研究的电化学测试结果显示两种材料性能优异，但仍有两个核心问题尚未明确：其一，尽管二者为同构体，但其电化学行为在容量与充放电曲线方面存在显著差异；其二，电化学活性物种竟为铌（Nb），与预期不符。基于上述原因，我们拟开展原位XRD研究，以追踪该体系的结构演化过程，明确钠离子的位点与占据情况，并确定电极反应机制。我们将采用里特维尔德法（用于结构解析）、最大熵法（MEM，Maximum Entropy Method）与傅里叶图谱法（用于定位钠离子位点），对光学电池获取的XRD数据进行分析。本研究的数据将与非原位扫描/透射电子显微镜（SEM/TEM）分析、X射线光电子能谱（XPS）数据相结合，还可能辅以X射线吸收谱（XAS）数据与中子衍射数据（相关申请正在评审中）。