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

Recently the demand for rechargeable batteries with high capacity, high power, long life, high reliability, and low cost has been increasing. While Li-ion batteries offer the highest energy density among present battery technologies, the higher cost and insufficient resources of lithium is a bottleneck to enabling their wider use. Na-ion batteries, where sodium acts as the current carrier, are attractive alternatives to Li-ion batteries because of the lower cost and abundance of sodium. However, the energy density of Na-ion batteries is still lower than that of Li-ion batteries, therefore the development of the active materials achieving a higher energy density is eagerly anticipated. For anode materials in Na-ion batteries, a non-graphitized carbon, such as hard carbon, is well-studied in contrast to Li-ion batteries, where graphite is usually used as the anode material. The capacity of non-graphitized carbon in Na-ion batteries is reported to be 200 - 300 mAh/g [1,2]. In Na-ion insertion process, it is proposed that Na-ions are inserted between stacked graphenes in the higher potential above ca. 0.2 V vs. Na+/Na, and then nano-pores are filled during electrochemical reaction in the lower potential region [1,2]. In our previous research, we focused on the effect of the pore structure of carbons, and found a linear relationship between the reversible capacity in the range of 0.2 - 0 V vs. Na+/Na and the closed pore volume of non-graphitized carbons. This result suggests that Na-ion could be inserted reversibly into the closed pore of non-graphitized carbons, and thus the non-graphitized carbon with large closed pore volume is a promising anode material for higher capacity Na-ion batteries [3]. In this study, we synthesized a new carbon material with a large closed pore volume using the carbon sources including a pore-forming agent as the starting material. It has the closed pore volume of 0.49 cc/g and its discharge capacity reached 438mAh/g. [1] D. A. Stevens et al, J. Electrochem. Soc., 148 (2001) A803 [2] S.Komaba et al, ACS Appl. Mater. Interfaces, 3 (2011) 4165 [3] A. Kano et al, 228th ECS meetings, (2015) 221