Sony Corporation succeeded in the development of the secondary lithium-ion secondary battery (LIB) in 1990. In LIBs of the first generation which were introduced into the market in 1991, soft carbon was adopted as an active material for the negative electrode. The discharge capacity of the soft carbon, which was heat-treated in the temperature range of 1200-1400°C. Much effort was required to improve the factors contributing to the performance of the LIB including energy density, drain capability, cyclic performance, and discharge ability at low temperatures. Experiments with hard carbon by synthesizing poly(furfuryl alcohol) (PFA) was done in order to expand the interlayer spacing. The cycle performances were by far better when sintered at 1100-1200°C than those of soft carbon. Moreover, the discharge curve profile of the graphite cell was almost flat and that of the hard carbon cell was rather sloping. When the cutoff voltage was set at 3 V, as is the case of conventional applications such as personal computers and cellular phones, the discharge capacity of the hard carbon cell was reduced. It was emphasized that the volumetric energy density is as important as the gravimetric energy density because, in general, the cell size is predetermined. The cutoff voltage influences the discharge capacity. A cell with a sloping discharge profile is disadvantageous. The initial charge and discharge efficiency should be as high as possible.
