A novel hard-carbon optimized to large-size lithium-ion secondary batteries

Kureha Corporation has developed nongraphitizable carbons (so-called hard carbon) prepared from the cross-linked petroleum pitch for almost 20 years as an anode of the lithium-ion battery (LIB).¹ However, in these years graphite is more popularly used as an anode of LIB of small portable equipment, such as cellular phones, digital cameras, and portable personal computers, because high-energy density is much more important in this type of application than long-life durability is. Hard carbon has been used only in the field of professional camcorders, satellites, and electric bikes because it seemed to be difficult to change a new battery at the end of its life. Recently application to large-size equipments, such as electric tools and hybrid electric vehicles (HEV), gets a lot of attention. The operating voltage window of the battery for HEV should be wide enough to regenerate electric power at a high rate. However, the operating voltage of the battery in which graphite is used as an anode and LiCoO₂ as a cathode is too flat and narrow, and the voltage will be immediately cramped to avoid overcharge at more than 4.3V. On the other hand, the operating window of the battery that used hard carbon is wider; it is already evident that the battery that used hard carbon has a much higher input as well as output power at any state of charge than the battery that used graphite. This is the reason why so much of attention is focused on hard carbon.² But the hard carbon that has been developed, Carbotron P (F), is designed for small-size equipment.³ It has a large charge capacity of more than 500Ahkg^-1 and long-life durability. In order to improve response, particle size must be small enough to reduce the diffusion path of the lithium ion into carbons. So, there is another type of hard carbon for HEV that is named Carbotron PS (F). The basic electrochemical characteristics of Carbotron P (F) and Carbotron PS (F) are almost the same. However, these hard carbons have low charge-discharge efficiency and these capacities will fade when they are stored in the air before preparation. So, these hard carbons should be redesigned for HEV application.