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As a trade-off, spinel offers a slightly lower energy density, suffers capacity loss at temperatures above 40°C and ages quicker than cobalt. Figure 2-6 compares the advantages and disadvantages of the two chemistries.
Cobalt Manganese (Spinel)
Energy density (Wh/kg) 140 1 120 1 Safety On overcharge, the cobalt electrode provides extra lithium, which can form into metallic lithium, causing a potential safety risk if not protected by a safety circuit. On overcharge, the manganese electrode runs out of lithium causing the cell only to get warm. Safety circuits can be eliminated for small 1 and 2 cell packs. Temperature Wide temperature range. Best suited for operation at elevated temperature. Capacity loss above +40°C. Not as durable at higher temperatures. Aging Short-term storage possible. Impedance increases with age. Newer versions offer longer storage. Slightly less than cobalt. Impedance changes little over the life of the cell. Due to continuous improvements, storage time is difficult to predict. Life Expectancy 300 cycles, 50% capacity at 500 cycles. May be shorter than cobalt. Cost Raw material relatively high; protection circuit adds to costs. Raw material 30% lower than cobalt. Cost advantage on simplified protection circuit.
Figure 2-6: Comparison of cobalt and manganese as positive electrodes.
Based on present generation 18650 cells. The energy density tends to be lower for prismatic cells.
The choice of metals, chemicals and additives help balance the critical trade-off between high energy density, long storage time, extended cycle life and safety. High energy densities can be achieved with relative ease. For example, adding more nickel in lieu of cobalt increases the ampere/hours rating and lowers the manufacturing cost but makes the cell less safe. While a start-up company may focus on high energy density to gain quick market acceptance, safety, cycle life and storage capabilities may be compromised. Reputable manufacturers, such as Sony, Panasonic, Sanyo, Moli Energy and Polystor place high importance on safety. Regulatory authorities assure that only safe batteries are sold to the public.
Li-ion cells cause less harm when disposed of than lead or cadmium-based batteries. Among the Li-ion family, the spinel is the friendliest in terms of disposal.
Despite its overall advantages, Li-ion also has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the maximum charge and discharge current is limited and the cell temperature is monitored to prevent temperature extremes. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.
Aging is a concern with most Li-ion batteries. For unknown reasons, battery manufacturers are silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. Over two or perhaps three years, the battery frequently fails. It should be mentioned that other chemistries also have age-related degenerative effects. This is especially true for the NiMH if exposed to high ambient temperatures.
Storing the battery in a cool place slows down the aging process of the Li-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should only be partially charged when in storage.
Extended storage is not recommended for Li-ion batteries. Instead, packs should be rotated. The buyer should be aware of the manufacturing date when purchasing a replacement Li-ion battery. Unfortunately, this information is often encoded in an encrypted serial number and is only available to the manufacturer.
Manufacturers are constantly improving the chemistry of the Li-ion battery. Every six months, a new and enhanced chemical combination is tried. With such rapid progress, it becomes difficult to assess how well the revised battery ages and how it performs after long-term storage.
Cost analysis — The most economical lithium-based battery in terms of cost-to-energy ratio is a pack using the cylindrical 18650 cell. This battery is somewhat bulky but suitable for portable applications such as mobile computing. If a slimmer pack is required (thinner than 18 mm), the prismatic Li-ion cell is the best choice. There is little or no gain in energy density per weight and size over the 18650, however the cost is more than double.
If an ultra-slim geometry is needed (less than 4 mm), the best choice is Li-ion polymer. This is the most expensive option in terms of energy cost. The Li-ion polymer does not offer appreciable energy gains over conventional Li-ion systems, nor does it match the durability of the 18560 cell.
Advantages and Limitations of Li-ion Batteries
High energy density — potential for yet higher capacities.
Relatively low self-discharge — self-discharge is less than half that of NiCd and NiMH.
Low Maintenance — no periodic discharge is needed; no memory.
Requires protection circuit — protection circuit limits voltage and current. Battery is safe if not provoked.
Subject to aging, even if not in use — storing the battery in a cool place and at 40 percent state-of-charge reduces the aging effect.
Moderate discharge current.
Subject to transportation regulations — shipment of larger quantities of Li-ion batteries may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
Expensive to manufacture — about 40 percent higher in cost than NiCd. Better manufacturing techniques and replacement of rare metals with lower cost alternatives will likely reduce the price.
Not fully mature — changes in metal and chemical combinations affect battery test results, especially with some quick test methods.
Figure 2-7: Advantages and limitations of Li-ion batteries.
Caution: Li-ion batteries have a high energy density. Exercise precaution when handling and testing. Do not short circuit, overcharge, crush, drop, mutilate, penetrate, apply reverse polarity, expose to high temperature or disassemble. Only use the Li-ion battery with the designated protection circuit. High case temperature resulting from abuse of the cell could cause physical injury. The electrolyte is highly flammable. Rupture may cause venting with flame.