Exploring the Pore Distribution Changes in Hard Carbon Anodes Using Ex Situ Small-Angle X-Ray Scattering

Currently, batteries are one of several important technologies for the grid-scale storage of energy derived from intermittent renewable energy sources.^[1]Sodium-ion batteries (NIBs) offer a solution to concerns about the sustainability, safety, cost and supply of lithium resources that exist for lithium-ion batteries (LIBs) and are emerging as the most close-to-market technology for grid-scale energy storage.^[2] While cathodes for NIBs with equivalent performance to LIBs have been reported, the anode material is the main bottleneck in the full cell performance. Hard carbon (HC), a disordered carbonaceous material with random graphitic layers, is a promising anode material for NIBs due to its low cost and good electrochemical performance. However, due to its structural complexity, the mechanisms of sodium storage in HC are still poorly understood.^[3] HC contains both open and closed pores, where open pores are directly connected to the electrolyte whereas closed pores are isolated within the material. A better understanding of the sodium storage in HC is required to develop an improved anode material.<br/>We have performed small-angle X-ray scattering (SAXS) on four HC structures with different pore size distributions to correlate the HC microstructure to the electrochemical performance. SAXS patterns on HC arise from the electron density contrast between the carbon matrix and the pores; therefore, we can obtain the pore distribution of HCs. We observe a trend where the HC types with larger pores and higher degree of polydispersity present a smaller slope capacity in the capacity-potential curve and that the pristine HCs with similar electrochemistry have a similar pore size distribution. Furthermore, we performed <i>ex situ </i>SAXS measurements at different sodiation stages to study the filling of the pores and how the pore size distributions change upon sodiation. Specific electrochemical features from the potential curve are shown to correlate with the pore filling. We show evidence of sodium storage in the nanopores at the low voltage plateau region and that the nanopores are filled non-uniformly. Furthermore, we see a small increase of the mean pore size upon sodiation, possibly implying that the smaller pores get larger when sodium ions fill them. Insights from this study on both the mechanism and the effect of the HC nanopores will facilitate the design of a high-performance HC anode for NIBs.<br/>References<br/>[1] Hirsh, H. S., Li, Y., Tan, D. H. S., Zhang, M., Zhao, E., & Meng, Y. S. (2020). Sodium-Ion Batteries Paving the Way for Grid Energy Storage. Advanced Energy Materials, 10(32). https://doi.org/10.1002/aenm.202001274<br/>[2] Tarascon, J. M. (2020). Na-ion versus Li-ion Batteries: Complementarity Rather than Competitiveness. Joule, 4(8), 1616–1620. https://doi.org/10.1016/j.joule.2020.06.003<br/>[3] Xiao, B., Rojo, T., & Li, X. (2019). Hard Carbon as Sodium-Ion Battery Anodes: Progress and Challenges. ChemSusChem, 12(1), 133–144. https://doi.org/10.1002/cssc.201801879