In-Plane Ordering of Li Species in the Interlayer of the Turbostratic Carbon As Negative Electrode for a High-Power and Long-Life Li-Ion Battery

About 20 years have passed since the Li ion battery using carbon as the negative electrode was commercialized. During the first few years, the electrochemical performances of most carbons were investigated. As a result, the graphitizable carbons can be systematically classified into three groups. The first group consists of the carbons prepared below ca. 1000°C. They show a higher capacity than the theoretical value of 372Ah/kg owing to the presence of LiC₆. However, the cycle performance is notably poor and the initial coulombic efficiencies are less than ca. 80%. For this reason, their electrochemical characteristics are hardly studied now. The second group consists of what are called “artificial graphite”, which is graphitized above ca. 2500°C. Since such carbons have high crystallinities, natural graphite should also be included in this group. In general, these graphites show excellent cycle performance, high capacities of 350-370Ah/kg, and coulombic efficiencies higher than 90%. Therefore, graphite is commercially used as the negative electrode of most Li ion batteries which are used in many electronic devices such as mobile phones, computers, and digital cameras. The third group consists of turbostratic carbons, heat-treated between ca. 1100 and 2500°C. In this temperature range, the discharge capacity shows a minimum value at ca.1800°C. Very few researchers have, therefore, studied the characteristics of these carbons, and their detailed electrochemical performance still remains unknown.

              In conventional studies and development of the carbon negative electrode, improvement of capacity, initial coulombic efficiency, and cyclability have been emphasized from the viewpoint of use for mobile electronic devices. However, research activities for large-scale Li ion batteries have recently increased in order to apply them to hybrid electric vehicles. Such applications require a carbon with high power density and long life performance. The author and Ozaki have found that the carbons heat-treated at 1800 - 2400 °C, show excellent characteristics in terms of high-rate charge/discharge performance and cyclability and have named it “ICOKE”. Ozaki et al. assembled the 5 Ah class large scale battery using LiNi_0.53Co_0.3Al_0.17O₂ and ICOKE as a positive and negative electrode, respectively, and evaluated the life performance with pulse charge/discharge cycle at 40 °C. As a result, the pulse cycle test exceeded 240,000 cycles, and 93% of capacity and 92% of power have been retained. In general, the turbostratic carbons heat-treated in the temperature range do not form the graphitic relation of AB-stacking between the adjacent graphene layers. Hence, they can not accommodate the Li ions in the interlayer up to the composition of LiC₆ and there is very few information for the charge/discharge mechanism.

              In the present study, the in-plane ordering of Li species in the interlayer of turbostratic carbon graphitized at 2000 °C with high power and excellent cyclability were examined by means of cyclic voltammetry, X-ray diffractometry and Li-NMR spectroscopy. As a result, it was found that Li-intercalation to ICOKE2000 was somewhat different from the conventional intercalation to natural graphite. In the latter case, the ionic state of Li species in the interlayer is equivalent. On the other hand, in the former case, it distributed depending on the degree of the turbostratic structure.