New Carbon Materials with Large Closed Pore Volume As Anode for High Energy Na-Ion Batteries

Recently demand for Li-ion batteries is growing as power sources for various applications such as electric vehicles and electric energy storage systems, because their high energy density and light weight properties are suitable for these applications. However, lithium resources may be insufficient to meet the increasing demand for these applications. Na-ion batteries could be a promising alternative to Li-ion batteries thanks to the natural abundance and lower cost of sodium. Non-graphitized carbon materials such as hard carbon are well-studied as high capacity anode materials for Na-ion batteries. The discharge capacity of 200 - 300 mAh/g, which is lower than conventional graphite anode materials for Li-ion batteries, has been reported [1,2]. In addition, the insertion mechanism of Na-ion into non-graphitized carbon materials is proposed that Na-ions are first inserted between stacked graphene layers in the voltage range from 2.0 V to 0.2 V (vs. Na metal), and then inserted into nano-pores in the voltage range from 0.2 V to 0.0 V (vs. Na metal) [2]. In this study, we focused on the pore structure of carbon materials and investigated the relationship between the carbon structures and electrochemical properties to clarify the optimum pore structure and propose a new carbon material as anode for high energy Na-ion batteries.

 Non-graphitized carbon samples were synthesized by heating various raw materials at between 1000 and 2600 °C under flowing argon. The pore structure of the synthesized carbon samples was characterized by using argon and nitrogen adsorption measurement and helium gas densitometry. Electrochemical properties were evaluated by using laminate-type cell with Na metals as the counter electrode and the reference electrode.

  Open and closed pore volumes of the synthesized carbons were calculated and the relationship between the reversible capacity and pore volume was investigated. As seen in Figure 1, an almost linear relationship between the reversible capacity in the voltage range from 0.2 to 0.0 V (vs. Na metal) and the closed pore volume was confirmed. Some carbon materials with large closed pore volume on the basis of this relationship were designed and successfully synthesized. Figure 2 shows charge/discharge curves of one of synthesized carbon samples. This carbon sample had the closed pore volume of 0.37 cc/g and its discharge capacity reached 413 mAh/g. This result suggests that Na-ion could be inserted reversibly in the closed pore and non-graphitized carbons with large closed pore volume may be promising for high capacity anode materials for Na-ion batteries.

[1] D. A. Stevens et al, J. Electrochem. Soc., 148 (2001) A803

[2] S.Komaba et al, ACS Appl. Mater. Interfaces, 3 (2011) 4165