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2.7 Lithium-Ion Batteries  75



                 400                     100
                Discharge capacity (mAh/g)  300  90  Initial efficiency (%)




                 200


                 100
                               Discharge  80
                               capacity
                               Initial efficiency
                                         70
                   0   3.35    3.40    3.45
                           d value (Å)
               Figure 2.62  Relationship between discharge capacity, ini-
               tial efficiency, and d value of soft carbon materials ( ,dis-
               charge capacity; • , initial efficiency).
               as phenol resin. The heat-treatment temperature of these materials is the same as
               that of petroleum and coal when making coke materials.
                Good charge–discharge characteristics have been reported [80], and the cycle
               characteristics were as good as those of soft carbon. The discharge capacity
               was strongly influenced by the charge–discharge conditions. There are some
               reports that the discharge capacity is larger than that of C 6 Li as measured by
               the best charge–discharge method, but this method is difficult to use for practical
               lithium-ion batteries. The discharge capacity of hard carbon is expected to be smaller
               than that of soft carbon electrodes as measured by a practical charge–discharge
               method.
                Polyacene is classified as a material which does not belong to either soft or hard
               carbons [81]. It is also made by heat-treatment of phenol resin. As the heat-treatment
                                            ◦
               temperature is lower than about 1000 C, polyacene contains hydrogen and oxygen
               atoms. It has a conjugated plane into which lithium ions are doped. It was reported
                                                                    −1
               that the discharge capacity of polyacene is more than 1000 mAh g . However,
               there are no practical lithium-ion batteries using polyacene.

               2.7.3
               Battery Performances

               Figure 2.63 shows the structure of a commercialized cylindrical-type lithium-ion
               battery. The lithium-ion battery is generally constructed with a spiral structure
               which serves as the separator between the positive and negative electrodes. An
               organic electrolyte containing lithium salts of which the conductivity is smaller
               than that of an aqueous electrolyte is used for this battery, but the short distance
               between the positive and negative electrodes and the large area of the electrode
               confer good characteristics.
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