Chen Chengmeng Team CCL: New Insights on the Microstructure of Hard Carbon Affecting the Diffusion of Sodium Ions into Closed Pores

[Introduction]]

Hard carbon has large layer spacing, abundant defects and internal pores, and is considered to be a promising anode material for sodium ion batteries. The storage of sodium ions in hard carbon is generally divided into a slope region (> 0.1V) and a plateau region (< 0.1V). The low voltage characteristic of the platform area is beneficial to increase the working voltage window and improve the energy density of the full battery. Closed pores as a closed space inside the material is a typical structural feature of hard carbon. Unlike open pores (connected to the external environment), closed pores play an important role in sodium storage in the platform zone, and the main form of sodium in closed pores is metalloid sodium clusters. In general, the closed pores in hard carbon are formed by highly disordered carbon layers, pseudo-graphitized structures or graphitized structures, and the carbonization temperature is an important factor affecting the formation of closed pores. However, due to the diversity of precursors and preparation methods, the microstructure of hard carbon is quite complex, and the effective way of regulating the closed pore structure and the mechanism of sodium ion diffusion to closed pores are not clear.

The nature of the precursor usually plays a decisive role in the structure and electrochemical properties of hard carbon, which has attracted wide attention of researchers. In the previous report, the researchers modified the precursor, changed the degree of cross-linking, etc., regulated the closed pore structure of the hard carbon, and significantly increased the capacity of the platform region. Most of the increase in pore filling capacity is due to the increase in the volume of closed pores, and the effect of the microstructure of hard carbon on the diffusion of sodium ions into closed pores has not attracted sufficient attention. Recently, researchers have found that not all closed pores can be used by sodium ion diffusion, and put forward the concept of accessible closed pores. In this method, the researchers introduced a correction factor to correct the effective specific surface area of the closed pores, and finally found that the size of the graphite domain has a greater impact on the ability of sodium ions to enter the closed pores than the layer spacing. Although this work provides a general mechanistic model of closed-pore sodium storage, the structural parameters in this method (such as layer spacing, graphite domain size, defect concentration, closed-pore volume) all change linearly with the carbonization temperature (from low temperature to high temperature), and the effects of different microstructures on sodium ion diffusion are coupled together. Therefore, it is necessary to further study the effect of single structural characteristics on the sodium storage of closed pores.

[Introduction to Results]]

Recently, researcher Chen Chengmeng and associate researcher Xie Lijing (co-correspondent) of Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences reported that polysaccharide precursor starch and aromatic precursor lignin were selected for component recombination, and controllable regulation of hard carbon microstructure was realized by changing the proportion of precursor components, and a unique hard carbon structure model was established. Aromatic structural components tend to form disordered carbons with few defects, large layer spacing and small closed pore volume, while polysaccharide components tend to form disordered carbons with many defects, small layer spacing and large closed pore volume. Through the correlation analysis of microstructure characteristics and sodium storage capacity below 0.1V. The results show that the insertion capacity of sodium ions is proportional to the proportion of pseudo-graphitized structure, while the pore filling capacity at low potential gradually decreases with the increase of defect concentration due to the repulsion of the same ion, and is related to the short-range ordered crystallite and closed pore volume. The optimized sample has a suitable layer spacing and defect concentration, and its platform capacity reaches 241.7 mAh/g. The results were published in the well-known journal Chinese Chemical Letters(IF: 9.1) entitled "New insights into the effect of hard carbons microstructure on the diffusion of sodium ions into closed portions". The first author of the article is Song Mingxin, a doctoral student from Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences.

[Text Guide]]

Fig. 1 Schematic diagram of precursor component recombination and characterization of structure and pyrolysis behavior.

2 Microstructural characterization of different hard carbon samples.

3 Electrochemical performance evaluation of different hard carbon samples.

4 Structure-activity relationship analysis of the hard carbon platform zone and the effect of different microstructures on sodium ion diffusion.

summary]

This work realizes the effective regulation of the microstructure of hard carbon by changing the proportion of precursor components, and avoids the change of the consistency of the hard carbon structure parameters caused by the traditional carbonization temperature adjustment. The unique structural contrast model is more conducive to understanding the influence of different microstructures on the diffusion of sodium ions in the hard carbon phase, and provides a new way to optimize the microstructure of hard carbon and improve the capacity of the platform area.

Document link: DOI: 10.1016/j.cclet. 2023.109266