Miscellaneous/Ask Isidor/Ideal 01 Page 070212
Q: Is the Lithium-ion the ideal battery?
A: For many years, the Nickel-cadmium was the only suitable battery for portable applications for wireless communications and mobile computing. In 1990, the Nickel-metal-hydride and Lithium-ion emerged, offering higher capacities. Both chemistries fought nose to nose, each claiming better performance and smaller sizes. Today, Lithium-ion has won the limelight and has become the most talked-about battery. It’s the fastest growing and most promising battery chemistry of today.
The Lithium-ion battery
Pioneer work with the lithium battery began in 1912 under G.N. Lewis, but it was not until the early 1970s that the first non-rechargeable lithium batteries became commercially available. Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density per weight.
Attempts to develop rechargeable lithium batteries failed due to safety problems. Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, Lithium-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first Lithium-ion battery. Other manufacturers followed suit.
The energy density of Lithium-ion is typically twice that of the standard Nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to Nickel-cadmium in terms of discharge. The high cell Voltage of 3.6 Volts allows battery pack designs with only one cell. Most of today’s mobile phones run on a single cell. The same pack would require three 1.2-Volt cells connected in series if nickel-based chemistry were used.
Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery’s life. In addition, the self-discharge is less than half compared to Nickel-cadmium, making Lithium-ion well suited for modern fuel gauge applications. Lithium-ion cells cause little harm when disposed.
Despite its overall advantages, Lithium-ion 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 cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.
Aging is a concern with most Lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for Nickel-metal-hydride if exposed to high ambient temperatures.
Manufacturers are constantly improving the chemistry of the Lithium-ion battery. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.
Storage in a cool place slows the aging process of Lithium-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should be partially charged during storage. The manufacturer recommends a 40 percent charge.
The most economical Lithium-ion battery in terms of cost-to-energy ratio is the cylindrical 18650. This cell is used for mobile computing and other applications that do not demand ultra-thin geometry. If a slim pack is required, the prismatic Li-ion cell is the best choice, but they come at a higher cost in terms of stored energy.
· High energy density — potential for yet higher capacities.
· Does not need prolonged priming when new. One regular charge is all that’s needed.
· Relatively low self-discharge — self-discharge is less than half that of nickel-based batteries.
· Low Maintenance — no periodic discharge is needed; there is no memory.
· Requires protection circuit to maintain Voltage and current within safe limits.
· Subject to aging, even if not in use — storing the battery in a cool place and at 40 percent charge reduces the aging effect.
· Moderate discharge current — not suitable for heavy loads.
· Transportation restrictions — shipment of larger quantities 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 Nickel-cadmium.
· Not fully mature — metals and chemicals are changing on a continuing basis.
The Lithium Polymer battery
The Li-polymer differentiates itself from other battery systems in the type of electrolyte used. The original design, dating back to the 1970s, uses a dry solid polymer electrolyte. This electrolyte resembles a plastic-like film that does not conduct electricity but allows an exchange of ions (electrically charged atoms or groups of atoms). The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.
The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry. With a cell thickness measuring as little as one millimeter(0.039 inches), equipment designers are left to their own imagination in terms of form, shape and size.
Unfortunately, the dry Li-polymer suffers from poor conductivity. The internal resistance is too high and cannot deliver the current bursts needed to power modern communication devices and spin up the hard drives of mobile computing equipment. Heating the cell to 60°C (140°F) and higher increases the conductivity but this requirement is unsuitable for portable applications.
To compromise, some gelled electrolyte has been added. Most of the commercial Lithium-polymer batteries used today for mobile phones are hybrid cells and contain gelled electrolyte. The correct term for this system is Lithium-ion-polymer. Since the hybrid lithium polymer is the only functioning polymer battery for portable use today, we will focus on this chemistry.
With gelled electrolyte added, what then is the difference between classic Li-ion and Li-ion polymer? Although the characteristics and performance of the two systems are similar, the Lithium-ion-polymer is unique in that solid electrolyte replaces the porous separator. The gelled electrolyte is simply added to enhance ion conductivity.
Lithium-ion-polymer has not caught on as quickly as some analysts had expected. Its superiority and lower cost to other systems has not been realized. No improvements in capacity gains are achieved — in fact, the capacity is slightly less than that of the standard Lithium-ion battery. The Lithium-ion-polymer finds its niche market in the packaging. It allows wafer-thin geometries, a style that is demanded by the highly competitive mobile phone industry.
· Very low profile — batteries resembling the profile of a credit card are feasible.
· Flexible form factor — manufacturers are not bound by standard cell formats.
With high volume, any reasonable size can be produced economically.
· Lightweight — gelled electrolytes enable simplified packaging by eliminating the metal shell.
· Improved safety — more resistant to overcharge; less chance for electrolyte leakage.
· Lower energy density and decreased cycle count compared to Lithium-ion.
· Expensive to manufacture but reduced control circuit offsets cost.
· No standard sizes. Most cells are produced for high volume consumer markets.
· Higher cost-to-energy ratio than Lithium-ion