Na-ion batteries are of great interest to realize high energy batteries consisting of abundant elements in the Earth crust. As a negative electrode material for the Na-ion batteries, non-graphitizable carbon, so-called hard carbon, is a promising candidate because of the low operating potential and good cycle stability. Conventional hard carbon prepared at carbonization temperature of 1000 – 1500 ºC delivers ca. 300 mAh g^-1 and our group actually reported that hard carbon prepared from sucrose at 1300 ºC delivers 290 mAh g^-1with good cycle stability [1]. However, higher capacity is still required to realize comparable energy performance to Li-ion batteries. In this study, we have investigated influence of synthesis condition of hard carbons, such as carbon source and heating temperature, on electrochemical performance as negative electrode of Na-ion battery on the basis of our previous research [1].

Hard carbons were prepared by carbonization of several saccharides which have different molecular structure at 1300 ºC. Figure 1 shows charge/discharge (sodiation/desodiation) curves of hard carbons in Na cells, which are prepared from glucose, amylopectin, glycogen, maltose, and cellulose. Cellulose-derived hard carbon shows the higher sodiation capacity among them which should be due to different nanostructure of hard carbon depending on the starting materials. According to a previous report on carbonization process of cellulose [2], dewatering and following cross-linkage of cellulose occur during pre-heating at around 300 ºC in air followed by carbonization. As the cross-linkage is thought to affect nanostructure and electrochemical properties of hard carbon, we prepared hard carbon with different pre-heating temperatures of cellulose, and the electrochemical properties were examined in Na cells. Figure 2 compares charge/discharge curves of hard carbon prepared by carbonization of cellulose at 1300 ºC with different preheating at 275 – 350 ºC. Although the hard carbons shows quite similar potential variation, hard carbon prepared via pre-heating at 275 ºC delivers higher reversible capacity of 351 mAh g^-1, in details, capacity increase depended on a voltage plateau approaching 0 V vs. Na. Small-angle X-ray scattering data reveals that the larger nano-sized pore is formed in the hard carbon synthesized through pre-heating at 275 ºC. Therefore, sodium insertion into the nano-sized pore is believed to be key to increase capacity in the voltage plateau region at close to 0 V. Importantly, the formation of nano-sized pores is influenced by cross-linkage during pre-heating. We additionally examine hard carbons in non-aqueous K cell, as hard carbon is also known to be electrochemically active for potassium insertion [3]. Relation between electrochemical properties and structure of hard carbons is further studied with X-ray diffraction and nitrogen adsorption measurements and will be discussed in the presentation.

References:

[1] M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa and S. Komaba, Phys. Chem. Chem. Phys., 16, 15007 (2014).

[2] K. Kobayashi, Carbon, 60, 21 (1970).

[3] Z. Jian, Z. Xing, C. Bommier, Z. Li, and X. Ji, Adv. Energy Mater., 6, 1501874 (2016).