[Design Application]
Solid-Polymer Electrolyte Makes Lithium-Ion Safe
With A Proven Safety Record, These Electrolytes Eliminate One Of The Key Impediments To Lithium-Ion's Full Market Penetration.
With designers demanding smaller, lighter, and more powerful energy sources, lithium-ion (Li-ion) batteries have rapidly replaced nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) chemistries as the dominant force in the high-performance, rechargeable-battery arena. While the focus to date has been on Li-ion batteries that use a liquid electrolyte, this technology's basic design creates problems in terms of packaging format, size, cost, and safety. As a result, much research has been put into realizing a Li-ion battery technology based on a solid polymer electrolyte. Such batteries have proven to be cost effective, safe under abusive conditions, and environmentally acceptable, all while offering virtually limitless design flexibility and higher performance.
Indicative of the extensive research taking place in the area of Li-ion batteries are the number of other lithium-based designs that have come and gone. Combined with the established and popular liquid-electrolyte batteries, these "imposters" have left a legacy of confusing terminology that can quickly be cleared up with a better understanding of the operation, features, and benefits of solid polymer Li-ion technology.
Over the next five years, according to Arthur D. Little Inc., Li-ion batteries are expected to earn more than a 50% share of the high-performance rechargeable market, while the market share of NiCd batteries is projected to dwindle to less than 10%. Why is this so? The answer starts with a basic chemistry lesson. Lithium is atomic number three on the periodic table of elements, meaning that it has the lightest weight and highest energy density of any solid (only two gases, helium and hydrogen rise above it). As a result, lithium is the ideal material for batteries, producing exceptionally high energy per unit weight and volume (see table). Rechargeable Li-ion batteries are also desirable because they have a high unit-cell voltage—in the 3.0- to 4.2-V range, as compared to 1.5 V for NiCd and NiMH cells.
Li-Ion Varieties Currently, there are two types of Li-ion technology, about which there seems to be some confusion in the industry. The first, which has been on the market for a few years, uses a liquid electrolyte. The second, which is now starting to make an impact in the marketplace, uses a solid-polymer electrolyte (Fig. 1).
These two technology types share a fundamental intercalation, or "rocking chair," system of operation: Lithium ions move back and forth between electrodes as the battery is charged and discharged. The anodes and cathodes of Li-ion batteries are made from carbonaceous (carbon-based) materials and metal oxides, respectively, with layered structures that accommodate the repeated migration of lithium ions (Fig. 2).
The rocking chair action gives Li-ion batteries both a long shelf life (self-discharge is only about 8% per month) and a long cycle life. At the capacity (C) rate and 100% depth of discharge, solid-polymer, Li-ion batteries will retain more than 80% of initial capacity after 500 cycles. (The C rate is how long it takes to discharge the battery in one hour. For an 800-mA-hr cell, the C rate would be 800 mA.) There are significant differences, however, between the liquid and solid polymer Li-ion systems.
Liquid-Electrolyte Cells Liquid Li-ion cells are currently mass produced for use in many notebook computers, camcorders, and cellular telephones. However, this technology has several major drawbacks that hinder a more rapid acceptance of these batteries in the marketplace. Most of these drawbacks are in the areas of packaging, cost, safety, and size. All stem from the battery's basic construction.
Packaging: The liquid electrolyte requires that liquid Li-ion cells be routinely packaged in rigid, hermetically sealed metal "cans." These housings reduce practical energy density, especially in large, multicell packs. As the number of cells in a battery pack increases, the cells' metal housings cause the pack's inert weight and volume to increase as well. In addition, placing cylindrical cells side by side within a pack creates gaps of empty space between cells, further reducing the proportion of energy-producing material in the pack.
Cost: The high manufacturing cost of liquid Li-ion batteries is prohibitive in many applications. That cost results from two factors. The winding, canning, and hermetic sealing processes are complex and costly, and the cathodes of most liquid Li-ion cells use cobalt oxide, a relatively expensive material. Cobalt is also environmentally suspect.
Safety: For safety reasons, liquid Li-ion cells are designed to vent automatically when certain abusive conditions exist, like a drastic increase in internal cell pressure caused by overheating. If the cell did not vent under extreme pressure, it could explode. The problem is that the liquid electrolyte used in liquid Li-ion cells is extremely flammable. If the electrolyte escapes when a cell vents, and if the external cell environment is hot enough, the electrolyte can flame as it is vented. This is cause for considerable concern to design engineers, especially those developing consumer-electronic products.
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