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The used by most manufacturers is lithium cobalt oxide (LiCoO2), but cobalt is an expensive metal so a cheaper alternative is sought, such as lithium manganese oxide (LiMn2O4) or lithium nickel oxide (LiNiO2). All of these oxides are stable in normal air, unlike lithium metal, which reacts with moisture in the air and also nitrogen. Upon charging, lithium ions are extracted from the positive electrode material and inserted into the negative electrode material. Upon discharging, the reverse process takes place.
| Cells can be assembled in the discharged
state i.e. using a carbon electrode with lithium cobalt oxide, making cell
assembly easy and safer. The assembled cells can then be charged. Unfortunately,
lithium-ion cells are rather expensive. Apart from cobalt metal, the other
materials are not very expensive. However, the cells are easily degraded outside
narrow voltage limits for both charging (about 4.5V) and discharging (below
3.0V).
For safety and longevity reasons, each battery pack must be equipped with a control circuit to limit the peak voltage of each cell during charge and prevent the cell voltage from dropping too low on discharge. In addition, the maximum charge and discharge current must be limited and the cell temperature monitored. |
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With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated. These added features increase the cost of these batteries considerable. Lithium-ion cells have about half the Ah capacity of similar primary lithium cells. Their voltage falls progressively during discharging, thus reducing the practical energy density to about 30% of that for primary lithium batteries.
However, they store about 2 to 3 times more energy than a comparable nickel-cadmium battery, display similar room-temperature performance, but perform poorly at low temperatures. The main advantages are good charge retention and a working voltage that can be 2.5 to 3 times that of Ni-Cd cells. Most of the cells manufactured are slightly larger than AA-size (14mm diameter x 50mm height) called 18650 (18mm diameter and 65mm height). These cells are spirally wound and are usually sold as battery packs. Larger format cells ( 67mm diameter x 410mm long) are now being developed for electric vehicle applications.
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