Diffraction Signal from Sodium Cluster in Hard Carbon: A Combined Experimental and Computational Study

Abstract

Na-ion batteries have attracted increasing attention as large-scale energy storage devices. Hard carbon is a standard anode material owing to its large capacity and good cycling stability1, 2. However, sodium storage mechanism of hard carbon has not been fully understood due to its low crystallinity and diversity of microstructure. This study aimed to systematic understanding of the relationship between microstructure and sodium storage behavior with special attention to the diffraction signal from sodium cluster confined in nanopores. Hard carbons were synthesized by hydrothermal treatment of sucrose and subsequent high-temperature carbonizations at T = 1000-1900 °C (denoted as HC-T). The charge-discharge tests were carried out using half-cells. As the heat-treatment temperature increased, the capacity of the high potential region (> 0.2 V) decreased while the plateau region (< 0.2 V) become dominant. HC-1400 showed the highest overall capacity (347 mAh g-1) and initial coulombic efficiency (95.2%). Reaction mechanism analyses were performed on three representative samples, HC-1000, HC-1400, and HC-1900. Ex-situ small angle X-ray scattering (SAXS) measurements revealed (i) the nanopores become larger with decreasing their number for higher annealing temperatures, (ii) insertion of sodium into the nanopores proceeded under ca. 0.07 V. When electrodes were overcharged exceeding the sodium deposition voltage (ca. –0.01 V), the sodium density in the nanopores became comparable to that of bulk bcc sodium. Besides, apparent smaller capacity of HC-1900 was analyzed to be a kinetic observation with larger cathodic polarization. Careful ex-situ X-ray diffraction measurements have detected smaller mean interlayer distances in order of HC-1900 < HC-1400 < HC-1000. Expansion of interlayer distance upon sodium insertion was observed for HC-1000 and HC-1400, but not at all for HC-1900, suggesting the existence of a threshold interlayer distance to allow sodium insertion. Importantly, for the first time, we have succeeded to detect a broad peak appeared around 29° as a signature of sodium insertion into the nanopores, and its origin was analyzed to be diffraction from sodium clusters in the nanopores using DFT-MD simulations. Based on these analyses, criteria for better hard carbon will be discussed in the poster. Figure 1