Electronic Design

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.

Size: All of the disadvantages outlined above are overshadowed by the significant size limitations of liquid Li-ion cells—both large and small. Due to the safety considerations stemming from the presence of a liquid electrolyte, liquid Li-ion cells cannot be too large. The most common liquid Li-ion cylindrical cell, the 18650, is only 65 mm high by 18 mm in diameter.

However, some applications, including cellular telephones, are better suited to a single, larger cell (combining multiple cells in a pack reduces the battery's energy efficiency). Other applications, such as electric vehicles, are best served by a series of large-size cells.

Ironically, the slimness of a battery pack is also restricted by the limitations of liquid Li-ion cells and their metal housings. Currently, the thinnest liquid Li-ion cells range from about 6 mm thick (prismatic cells) to about 14 mm thick (cylindrical cells). This limits the slimness of portable electronic product designs.

Design Flexibility
The cutting edge of Li-ion technology is in batteries based on a solid-polymer electrolyte, a technology now being pioneered primarily by U.S.-based companies. Solid polymer Li-ion batteries have outstanding attributes in the essential areas where liquid Li-ion is weakest. The cells offer virtually limitless design flexibility with cost-effective materials and construction, have proven safe under abusive conditions, improve greatly on overall performance, and are environmentally acceptable.

Because there is no liquid that has to be contained by a hermetically sealed, rigid metal can, this new type of battery can be housed in an ultra-thin laminated foil material that can be used to house each cell (Fig. 1, again). This design creates a number of distinct advantages.

For one, solid-polymer, Li-ion cells can be made as thin as 0.64 mm (25 mils), or about one-tenth the thickness of the thinnest prismatic liquid Li-ion cells. The cells can also be stacked in series or parallel to form ultra-thin battery packs with a wide range of voltages and capacities. Such design flexibility allows engineers to obtain the required performance from the flattest-profile battery possible. This point was vividly illustrated by Mitsubishi Electric Corp., Tokyo, Japan, which in October 1997, introduced the world's thinnest (less than 0.75-in. thick) and lightest (3.1 lb.) notebook computer. The computer was powered by a 0.25-in.-thick, Ultralife solid-polymer rechargeable battery.

In addition, the widths and lengths of solid-polymer, Li-ion cells are as flexible as their thicknesses. As a result, cells can be configured in virtually any size, making solid-polymer Li-ion a stronger candidate than liquid Li-ion for electric vehicles and other large-cell applications. Even non-rectangular shapes are possible. This unique size flexibility allows for maximum energy efficiency within a given battery cavity.

Additionally, a laminated foil housing makes solid-polymer, Li-ion cells flexible—literally. This allows them to conform to cavities with curved surfaces. Furthermore, the foil housing material is considerably lighter than the metal used for liquid Li-ion cells.

From a cost perspective, the solid-polymer, Li-ion system also promises considerable advantages. Instead of the relatively expensive cobalt oxide found in liquid Li-ion cells, cathodes in solid-polymer, Li-ion cells use an inexpensive metal oxide material. Even more significantly, every component of a solid-polymer, Li-ion cell is fabricated in rolled-sheet form. This technique allows for exceptionally cost-effective, high-speed, high-volume battery production.

Electrodes, electrolyte, and foil packaging—all on continuous-feed rolls—are sandwiched together into finished batteries in one smooth process. In comparison, the winding and canning processes used to produce liquid Li-ion cells are time consuming and expensive. Ultimately, solid-polymer, Li-ion batteries will cost in the range of $1 to $2/Whr. As a point of reference, NiCd batteries, with five decades of manufacturing improvements, cost a bit below $1/Whr.

As with other Li-ion batteries, solid-polymer batteries require individual cell monitoring. Cells are charged at a constant 4.2 V, with the charging current limited to the C rate (Fig. 3). Charging cuts off when the charging current declines to the C/10 rate (80 mA for an 800-mAhr cell). The temperature range for charging is 0° to 45°C. As can be seen from the table, the energy density of solid-polymer batteries ranges from 115 to 150 Whr/kg, compared to 70 to 110 Whr/kg for liquid-electrolyte Li-ion cells. In addition, solid-polymer batteries can be recharged more than 500 times, and have no trouble taking a one-hour charge.

Safety Issues
Because its electrolyte cannot leak, a solid-polymer, Li-ion cell is intrinsically safer than a liquid Li-ion cell. Venting is simply not an issue. Moreover, the solid electrolyte, a plastic compound, is a non-volatile material capable of withstanding severe safety testing, including:

Pressure: The cells have been pressurized to 1500 psi under electrical testing. No signs of electrode shorting were observed.

Short circuit: A high-capacity, solid-polymer, Li-ion battery pack was short circuited with a maximum current of 85 A. The external temperature of the battery increased only a few degrees, and the battery was able to accept a subsequent charge without any adverse effects.

Overcharge/overdischarge: Solid-polymer cells were overcharged as high as 20 V at up to a 3C rate, and overdischarged at up to a 3C rate. The cells ceased to function, but no flaming or any other hazard occurred.

Penetration: A nail was fired through the center of a high-capacity, solid-polymer, Li-ion battery pack during discharge. The output voltage dipped briefly, and only 60% of battery capacity was achieved during that particular discharge cycle. However, on a subsequent recharge/discharge cycle, the battery recovered to 95% of initial capacity.

These crucial safety factors have allowed certain solid-polymer rechargeable batteries to meet the safety standards of the Japan Storage Battery Association, the International Electrochemical Commission, the Canadian Standards Association, and Underwriters Laboratories. This makes solid-polymer, Li-ion batteries particularly attractive to manufacturers of portable consumer electronics, such as cellular telephones and notebook computers. In addition, because the test data indicate a high degree of safety in multicell battery packs with a broad range of capacities, as well as in individual cells, the technology is attracting the interest of electric vehicle manufacturers and other large-battery designers.

Finally, on top of its other advantages, the solid-polymer, Li-ion system is environmentally friendlier than other batteries, especially the nickel-based chemistries. The materials used, including the metal oxide in the cathode, are benign. As a result, solid-polymer batteries do not require any special handling, nor do they face any transport or disposal regulations.

Lithium-Metal Confusion
Part of the industry's confusion over the different types of lithium-based, rechargeable batteries may stem from a rechargeable, lithium-metal (Li-metal) technology that has struggled to achieve commercial acceptability. Rechargeable Li-metal is a "Holy Grail" of sorts because it offers an extremely high energy-density potential—theoretically about 150 Whr/kg, or over 300 Whr/l. But while metallic lithium works extremely well in primary batteries, a truly viable, rechargeable Li-metal technology has been elusive.

One of the main problems is that lithium, in its metallic form, is highly reactive. As such, it presents unique difficulties in rechargeable configurations. Repeated charge/discharge cycles can cause a build-up of surface irregularities on the lithium electrode. These irregular structures, known as dendrites, can grow to such an extent that they penetrate the separator between positive and negative electrodes and create an internal short circuit. At best, this phenomenon shortens the useful life of a rechargeable Li-metal battery to 150 cycles or less. At worst, an internal short circuit could cause the battery's internal temperature to rise above lithium's melting point (181°C), which could cause severe flaming.

It's almost impossible to safeguard rechargeable Li-metal batteries against potential catastrophic failure under extremely abusive conditions. Without adequate safeguards, rechargeable Li-metal batteries are high-risk items, especially for consumer products.

While there still are one or two manufacturers offering rechargeable Li-metal cells, it's hard to imagine a consumer-products company that would take a chance with them. The use of rechargeable Li-metal cells, if they achieve commercial success at all, will likely be restricted to specialized military and industrial applications.

Another term, lithium-polymer, is associated with a developmental Li-metal system that by the mid-1990s was found to be non-viable. It must be stressed that this was a lithium-metal battery. Some have used the term "lithium-polymer" incorrectly to describe solid-polymer, Li-ion batteries. The company that first touted the lithium (metal)-polymer battery gave up on the idea long ago, and has switched its focus to a Li-ion technology similar to the solid-polymer rechargeable battery.

Solid-polymer, Li-ion batteries, at the leading edge of rechargeable battery technology, offer the best combination of design flexibility, performance, cost, and safety demanded by the highest-volume consumer electronics applications. The next questions still to be answered are: How soon will these batteries reach mass production, and how much energy can be packed into a cell?

To answer the first question, certain companies anticipate higher-volume, automated production capability during 1998. To the second question: Much research is being done to improve the conductivity of solid-electrolyte materials, and alternative electrode materials are being studied closely. In addition, IC manufacturers are designing new battery-management chips that are steadily improving the precise cell-by-cell monitoring required by Li-ion batteries. With these efforts underway, and mass production imminent, solid-polymer, Li-ion technology is poised to become the dominant force in the high-performance, rechargeable-battery arena.

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