Transition metal sulfides are promising high-capacity anode materials for sodium ion batteries in terms of the conversion reaction with multiple electron transfers. Nonetheless, some inherent challenges such as sluggish sodium ion diffusion kinetics, large volume change and poor cycle stability limit their implementation. Addressing these issues necessitates a comprehensive understanding on the complex sodium ion storage mechanism especially at the initial cycle. Here, taking nickel subsulfide as a model material, we reveal the complicated conversion reaction mechanism upon the first cycle by combining in operando 2D transmission X-ray microscopy with X-ray absorption spectroscopy, ex-situ 3D nano-tomography, high-energy X-ray diffraction and electrochemical impedance spectroscopy. This study demonstrates that the microstructure evolution, inherent slow sodium ion diffusion kinetics, and slow ion mobility at the two-phase interface contribute to the high irreversible capacity upon the first cycle. Such understandings are critical for developing the conversion reaction materials with the desired electrochemical activity and stability.

就具有多次电子转移的转化反应而言，过渡金属硫化物是有希望的用于钠离子电池的高容量阳极材料。但是，一些固有的挑战，例如缓慢的钠离子扩散动力学，较大的体积变化和较差的循环稳定性限制了它们的实施。解决这些问题需要对复杂的钠离子存储机制有一个全面的了解，尤其是在初始周期。在此，以镍subsulfide作为模型材料，我们通过组合揭示在第一循环中的复杂的转换反应机理在operando 2D传输透视显微镜用X射线吸收光谱，易地3D纳米断层扫描，高能X射线衍射和电化学阻抗谱。这项研究表明，微结构的演化，固有的钠离子扩散动力学缓慢以及两相界面处离子迁移速度缓慢，都导致了第一个循环时的高不可逆容量。这种理解对于开发具有所需电化学活性和稳定性的转化反应材料至关重要。