Revealing the Sodium Storage in Hard Carbon Pores

Sodium-ion batteries (NIBs) are an attractive candidate to support the increasing demand of energy, especially amid growing concerns about the sustainability of lithium resources for lithium-ion batteries. NIBs provide advantages due to the abundance of sodium, its low cost, and electrochemical performance. Hard carbon (HC) is the most promising anode for the commercialization of NIBs, however, due to its structural complexity, a general mechanism for sodium storage in HC remains unclear, obstructing the development of highly efficient anodes for NIBs. To elucidate the mechanism of sodium storage in HC pores, we combined <i>operando </i>synchrotron small-angle X-ray scattering, wide-angle X-ray scattering, X-ray absorption near edge structure, Raman spectroscopy, and galvanostatic measurements. Through this multimodal investigation, we provide mechanistic insights into the sodium pore filling process across various HC microstructures including the pore sizes that are preferentially filled, the extent to which different pore sizes are filled, and how the defect concentration influences pore filling. We observe that sodium in the larger pores exhibits an increased pseudo-metallic sodium character, indicative of larger sodium clusters. Furthermore, we show that the hard carbons prepared at higher pyrolysis temperature have a larger capacity from sodium stored in the pores, and that sodium intercalation between graphene layers occurs simultaneously to the pore filling in the plateau region. Our results suggest that the engineering of defect sites in pore walls, together with increasing the pore size distribution and/or volume fraction of closed pores are key to designing superior HC anodes. Moreover, our study also gives a systematic approach to study other porous functional materials and to understand how ions store in the pores.