Solid-Polymer Lithium-ion Batteries Provide A Good Fit For Wireless Designs
Solid-polymer Li-Ion batteries offer a strong combination of design flexibility, performance, cost, and safety for wireless applications.
LITHIUM-ION (Li-ion) chemistry is rapidly replacing the nickel-cadmium (NiCd) and nickel-metal-hydride (NiMH) chemistries as the dominant force in the high-performance rechargeable-battery market.
Design engineers need the smaller size, lighter weight, and higher energy output of Li-ion batteries to satisfy the demands of more compact, yet more feature-laden portable electronic products. During the next five years, Li-ion batteries are expected to account for more than half of the high-performance rechargeable market, while the market share of NiCd batteries is projected to slip to less than 10 percent.
The reason for this anticipated change lies in the basic chemistry of these batteries. Lithium is atomic number 3 on the Periodic Table of Elements and has the lightest weight as well as the highest energy density of any solid. It is simply the ideal material for batteries. Lithium-based batteries produce exceptionally high energy per unit weight and volume. Rechargeable Li-ion batteries are also desirable because they have a high unit-cell voltage--the 3.0-to-4.2-V range--compared to 1.5 V for NiCd and NiMH cells.
Currently, there are two types of Li-ion technology, about which there seems to be some confusion in the industry. The first, which uses a liquid electrolyte, has been on the market for a few years and is dominated by Japanese manufacturers. The second, which is now starting to make an impact in the marketplace, uses a solid-polymer electrolyte.
These two Li-ion technologies share a fundamental intercalation, or "rocking chair" system. Li-ions move back and forth between electrodes as a battery is charged and discharged (Fig. 1). The anodes and cathodes of Li-ion batteries are made from carbonaceous materials and metal oxides, respectively, with layered structures that accommodate the repeated migration of Li-ions.
1. Li-ions move back and forth between electrodes as a battery is charged and discharged.
This "rocking chair" action gives Li-ion batteries a long shelf life (self-discharge is only approximately eight percent per month), and a long cycle life (at the C-rate and 100-percent depth of discharge, solid-polymer Li-ion batteries will retain more than 80 percent of initial capacity after 500 cycles). But there are significant differences between the liquid- and solid-polymer Li-ion systems.
Liquid Li-ion cells are currently being mass-produced for use in many cellular phones, notebook computers, and camcorders, but liquid Li-ion systems have proved to have several major drawbacks which are hindering a more rapid acceptance in the marketplace.
First, since liquid Li-ion cells must, by definition, contain a liquid electrolyte, they are 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 packs' 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.
Second, the high manufacturing cost of liquid Li-ion batteries is prohibitive in many applications. Liquid Li-ion cells are expensive to produce primarily for two reasons--the winding, canning, and hermetic-sealing processes required for liquid Li-ion cells are relatively complex and costly, and the cathodes of most liquid Li-ion cells use cobalt oxide, a relatively expensive material. Cobalt is also an environmentally suspect metal.
Third, liquid Li-ion cells are designed to vent automatically when certain abuse conditions exist, such as a drastic increase in internal cell pressure due to severe heat. This feature is intended as a safety measure. For example, if the cell does not vent when pressure is built up, it could explode.
The problem is that the liquid electrolyte used in liquid Li-ion cells is extremely flammable. If the liquid electrolyte escapes when a cell vents, and if the external cell environment is hot enough, the liquid electrolyte can flame as it is vented. This schould be a cause for considerable concern to design engineers, especially those developing consumer electronic products.
Fourth, and perhaps most important, liquid Li-ion cells have significant size limitations. Due to the safety considerations stemming from the presence of liquid electrolyte, liquid Li-ion cells cannot be too large. The most common liquid Li-ion cylindrical cell (the model 18650) is only 65 mm high × 18 mm in diameter.
Some applications, such as cellular phones, are better suited to a single, larger cell, particularly when multiple cells are combined in a pack and the battery's energy efficiency is reduced. 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 approximately 6 mm thick (prismatic cells) to about 14 mm thick (cylindrical cells). This restrains engineers by placing limits on the slimness of portable electronic-product designs.
The cutting edge of Li-ion technology is in batteries using a solid-polymer electrolyte (Fig. 2). These batteries are being pioneered primarily by US companies.
2. The basic internal structure of a solid-polymer cell can be configured to virtually any size.
Solid-polymer Li-ion batteries have outstanding attributes in the essential areas where liquid Li-ion is weakest. Solid-polymer Li-ion cells offer cost-effective materials and construction, demonstrated safety under abuse conditions, environmental acceptability, and virtually limitless design flexibility.
Every component of the solid-polymer Li-ion system is, as the name suggests, solid, including the electrolyte. There is no liquid that has to be contained by hermetically sealed cell packaging. An ultra-thin laminated foil material, instead of a rigid metal can, can be used to house each cell.
SIGNIFICANT ADVANTAGES
This creates a number of significant advantages. Solid-polymer Li-ion cells can be made as thin as 0.64 mm (25 mils)--approximately 10 times thinner than the thinnest prismatic liquid Li-ion cells. Solid-polymer cells can also be stacked in series and/or parallel to form ultra-thin battery packs with a wide range of voltages and capacities. This design flexibility enables engineers to obtain the required performance from the flattest-profile battery possible (Fig. 3). This point was vividly illustrated by Mitsubishi Electric Corp., which has introduced the world's thinnest (less than 3/4 in. thick) and lightest (3.1 lbs.) notebook computer, powered by a 1/4-in.-thick Ultralife solid-polymer rechargeable battery.
3. Solid-polymer Li-ion cells can be made as thin as 0.64 mm, enabling designers to obtain the required performance from the fattest-profile battery possible.
The width and length of solid-polymer Li-ion cells are as flexible as their thickness. 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 size flexibility of solid-polymer batteries supports maximum energy efficiency within a particular battery cavity.
With their laminated-foil housing, solid-polymer Li-ion cells are flexible and can be conformed to battery cavities with curved surfaces. In addition, the foil housing material is considerably lighter than the metal used for liquid Li-ion cells.
In terms of cost, the solid-polymer Li-ion system also promises advantages. Instead of the relatively costly cobalt oxide found in liquid Li-ion cells, cathodes in solid-polymer Li-ion cells use an inexpensive metal oxide. More significantly, every component of a solid-polymer Li-ion cell is fabricated in rolled-sheet form to support 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 as well as expensive.
Ultimately, solid-polymer Li-ion batteries will cost in the range of $1 to $2/Wh. (As a point of reference, NiCd batteries, with five decades of manufacturing improvements, cost slightly less than $1/Wh.)
Since its electrolyte is solid, and therefore cannot leak, a solid-polymer Li-ion cell is fundamentally safer than a liquid Li-ion cell. Venting is simply not an issue. Moreover, the solid electrolyte, a plastic compound, has proved to be a nonvolatile material that is capable of withstanding severe safety testing. Solid-polymer Li-ion cells and batteries, with a broad range of capacities, have been subjected to high-pressure, short-circuit, overcharge/overdischarge, and nail-penetration tests, with impressive results. Solid-polymer cells were pressurized to 1500 psi under electrical testing. No signs of electrode shorting were observed.
A high-capacity, solid-polymer Li-ion battery pack was also short-circuited. The maximum short-circuit current was 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. Solid-polymer cells were overcharged as high as 20 V at up to a 3C rate and overdischarged at up to a 3C rate to three times capacity. The cells ceased to function, but no flaming or any other hazard occurred.
In another test, a nail was fired through the center of a high-capacity solid-polymer Li-ion battery pack during discharge. Output voltage dipped briefly, and only 60 percent of battery capacity was achieved during that particular discharge cycle. However, on a subsequent recharge/discharge cycle, the battery recovered to 95 percent of initial capacity. These crucial safety factors have enabled the solid-polymer rechargeable batteries to meet the safety-test 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, since 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 solid-polymer Li-ion system 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 in the solid-polymer Li-ion system, including the metal oxide in the cathode, are benign. Solid-polymer batteries do not require any special handling, and do not face any regulation of transport or disposal.
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, which is theoretically approximately 150 Wh/kg and more than 300 Wh/liter. But while metallic lithium works extremely well in primary batteries, a truly viable rechargeable Li-metal technology has been elusive. Lithium in its metallic form is a highly reactive substance that presents unique difficulties in rechargeable configurations. Repeated charge/discharge cycles can cause a buildup 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 is almost impossible to safeguard rechargeable Li-metal batteries against potential catastrophic failure under extreme abuse conditions. And without adequate safeguards, rechargeable Li-metal batteries are high-risk items, especially for consumer products.
There are one or two manufacturers offering rechargeable Li-metal cells, but it is 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.
Lithium-polymer is a term associated with a developmental Li-metal system that by the mid-1990s was found to be non-viable. This was a Li-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 switched its focus to a Li-ion technology similar to the Ultralife solid-polymer rechargeable battery.
LOOKING AHEAD
One of the obvious questions in the evolution of solid-polymer Li-ion batteries is how soon batteries will reach mass production. Another is how much energy can be packed into a cell. As for the first question, Ultralife anticipates higher-volume automated production capability during 1998. As for the second question, extensive research is ongoing to improve the conductivity of solid-electrolyte materials, and alternative electrode materials are being closely studied. In addition, integrated-circuit (IC) manufacturers are designing new battery-management chips, improving the precise cell-by-cell monitoring required by Li-ion batteries.
Source: Greg Smith, May, 1999
Solid-polymer Li-Ion batteries offer a strong combination of design flexibility, performance, cost, and safety for wireless applications.
LITHIUM-ION (Li-ion) chemistry is rapidly replacing the nickel-cadmium (NiCd) and nickel-metal-hydride (NiMH) chemistries as the dominant force in the high-performance rechargeable-battery market.
Design engineers need the smaller size, lighter weight, and higher energy output of Li-ion batteries to satisfy the demands of more compact, yet more feature-laden portable electronic products. During the next five years, Li-ion batteries are expected to account for more than half of the high-performance rechargeable market, while the market share of NiCd batteries is projected to slip to less than 10 percent.
The reason for this anticipated change lies in the basic chemistry of these batteries. Lithium is atomic number 3 on the Periodic Table of Elements and has the lightest weight as well as the highest energy density of any solid. It is simply the ideal material for batteries. Lithium-based batteries produce exceptionally high energy per unit weight and volume. Rechargeable Li-ion batteries are also desirable because they have a high unit-cell voltage--the 3.0-to-4.2-V range--compared to 1.5 V for NiCd and NiMH cells.
Currently, there are two types of Li-ion technology, about which there seems to be some confusion in the industry. The first, which uses a liquid electrolyte, has been on the market for a few years and is dominated by Japanese manufacturers. The second, which is now starting to make an impact in the marketplace, uses a solid-polymer electrolyte.
These two Li-ion technologies share a fundamental intercalation, or "rocking chair" system. Li-ions move back and forth between electrodes as a battery is charged and discharged (Fig. 1). The anodes and cathodes of Li-ion batteries are made from carbonaceous materials and metal oxides, respectively, with layered structures that accommodate the repeated migration of Li-ions.
1. Li-ions move back and forth between electrodes as a battery is charged and discharged.
This "rocking chair" action gives Li-ion batteries a long shelf life (self-discharge is only approximately eight percent per month), and a long cycle life (at the C-rate and 100-percent depth of discharge, solid-polymer Li-ion batteries will retain more than 80 percent of initial capacity after 500 cycles). But there are significant differences between the liquid- and solid-polymer Li-ion systems.
Liquid Li-ion cells are currently being mass-produced for use in many cellular phones, notebook computers, and camcorders, but liquid Li-ion systems have proved to have several major drawbacks which are hindering a more rapid acceptance in the marketplace.
First, since liquid Li-ion cells must, by definition, contain a liquid electrolyte, they are 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 packs' 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.
Second, the high manufacturing cost of liquid Li-ion batteries is prohibitive in many applications. Liquid Li-ion cells are expensive to produce primarily for two reasons--the winding, canning, and hermetic-sealing processes required for liquid Li-ion cells are relatively complex and costly, and the cathodes of most liquid Li-ion cells use cobalt oxide, a relatively expensive material. Cobalt is also an environmentally suspect metal.
Third, liquid Li-ion cells are designed to vent automatically when certain abuse conditions exist, such as a drastic increase in internal cell pressure due to severe heat. This feature is intended as a safety measure. For example, if the cell does not vent when pressure is built up, it could explode.
The problem is that the liquid electrolyte used in liquid Li-ion cells is extremely flammable. If the liquid electrolyte escapes when a cell vents, and if the external cell environment is hot enough, the liquid electrolyte can flame as it is vented. This schould be a cause for considerable concern to design engineers, especially those developing consumer electronic products.
Fourth, and perhaps most important, liquid Li-ion cells have significant size limitations. Due to the safety considerations stemming from the presence of liquid electrolyte, liquid Li-ion cells cannot be too large. The most common liquid Li-ion cylindrical cell (the model 18650) is only 65 mm high × 18 mm in diameter.
Some applications, such as cellular phones, are better suited to a single, larger cell, particularly when multiple cells are combined in a pack and the battery's energy efficiency is reduced. 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 approximately 6 mm thick (prismatic cells) to about 14 mm thick (cylindrical cells). This restrains engineers by placing limits on the slimness of portable electronic-product designs.
The cutting edge of Li-ion technology is in batteries using a solid-polymer electrolyte (Fig. 2). These batteries are being pioneered primarily by US companies.
2. The basic internal structure of a solid-polymer cell can be configured to virtually any size.
Solid-polymer Li-ion batteries have outstanding attributes in the essential areas where liquid Li-ion is weakest. Solid-polymer Li-ion cells offer cost-effective materials and construction, demonstrated safety under abuse conditions, environmental acceptability, and virtually limitless design flexibility.
Every component of the solid-polymer Li-ion system is, as the name suggests, solid, including the electrolyte. There is no liquid that has to be contained by hermetically sealed cell packaging. An ultra-thin laminated foil material, instead of a rigid metal can, can be used to house each cell.
SIGNIFICANT ADVANTAGES
This creates a number of significant advantages. Solid-polymer Li-ion cells can be made as thin as 0.64 mm (25 mils)--approximately 10 times thinner than the thinnest prismatic liquid Li-ion cells. Solid-polymer cells can also be stacked in series and/or parallel to form ultra-thin battery packs with a wide range of voltages and capacities. This design flexibility enables engineers to obtain the required performance from the flattest-profile battery possible (Fig. 3). This point was vividly illustrated by Mitsubishi Electric Corp., which has introduced the world's thinnest (less than 3/4 in. thick) and lightest (3.1 lbs.) notebook computer, powered by a 1/4-in.-thick Ultralife solid-polymer rechargeable battery.
3. Solid-polymer Li-ion cells can be made as thin as 0.64 mm, enabling designers to obtain the required performance from the fattest-profile battery possible.
The width and length of solid-polymer Li-ion cells are as flexible as their thickness. 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 size flexibility of solid-polymer batteries supports maximum energy efficiency within a particular battery cavity.
With their laminated-foil housing, solid-polymer Li-ion cells are flexible and can be conformed to battery cavities with curved surfaces. In addition, the foil housing material is considerably lighter than the metal used for liquid Li-ion cells.
In terms of cost, the solid-polymer Li-ion system also promises advantages. Instead of the relatively costly cobalt oxide found in liquid Li-ion cells, cathodes in solid-polymer Li-ion cells use an inexpensive metal oxide. More significantly, every component of a solid-polymer Li-ion cell is fabricated in rolled-sheet form to support 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 as well as expensive.
Ultimately, solid-polymer Li-ion batteries will cost in the range of $1 to $2/Wh. (As a point of reference, NiCd batteries, with five decades of manufacturing improvements, cost slightly less than $1/Wh.)
Since its electrolyte is solid, and therefore cannot leak, a solid-polymer Li-ion cell is fundamentally safer than a liquid Li-ion cell. Venting is simply not an issue. Moreover, the solid electrolyte, a plastic compound, has proved to be a nonvolatile material that is capable of withstanding severe safety testing. Solid-polymer Li-ion cells and batteries, with a broad range of capacities, have been subjected to high-pressure, short-circuit, overcharge/overdischarge, and nail-penetration tests, with impressive results. Solid-polymer cells were pressurized to 1500 psi under electrical testing. No signs of electrode shorting were observed.
A high-capacity, solid-polymer Li-ion battery pack was also short-circuited. The maximum short-circuit current was 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. Solid-polymer cells were overcharged as high as 20 V at up to a 3C rate and overdischarged at up to a 3C rate to three times capacity. The cells ceased to function, but no flaming or any other hazard occurred.
In another test, a nail was fired through the center of a high-capacity solid-polymer Li-ion battery pack during discharge. Output voltage dipped briefly, and only 60 percent of battery capacity was achieved during that particular discharge cycle. However, on a subsequent recharge/discharge cycle, the battery recovered to 95 percent of initial capacity. These crucial safety factors have enabled the solid-polymer rechargeable batteries to meet the safety-test 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, since 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 solid-polymer Li-ion system 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 in the solid-polymer Li-ion system, including the metal oxide in the cathode, are benign. Solid-polymer batteries do not require any special handling, and do not face any regulation of transport or disposal.
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, which is theoretically approximately 150 Wh/kg and more than 300 Wh/liter. But while metallic lithium works extremely well in primary batteries, a truly viable rechargeable Li-metal technology has been elusive. Lithium in its metallic form is a highly reactive substance that presents unique difficulties in rechargeable configurations. Repeated charge/discharge cycles can cause a buildup 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 is almost impossible to safeguard rechargeable Li-metal batteries against potential catastrophic failure under extreme abuse conditions. And without adequate safeguards, rechargeable Li-metal batteries are high-risk items, especially for consumer products.
There are one or two manufacturers offering rechargeable Li-metal cells, but it is 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.
Lithium-polymer is a term associated with a developmental Li-metal system that by the mid-1990s was found to be non-viable. This was a Li-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 switched its focus to a Li-ion technology similar to the Ultralife solid-polymer rechargeable battery.
LOOKING AHEAD
One of the obvious questions in the evolution of solid-polymer Li-ion batteries is how soon batteries will reach mass production. Another is how much energy can be packed into a cell. As for the first question, Ultralife anticipates higher-volume automated production capability during 1998. As for the second question, extensive research is ongoing to improve the conductivity of solid-electrolyte materials, and alternative electrode materials are being closely studied. In addition, integrated-circuit (IC) manufacturers are designing new battery-management chips, improving the precise cell-by-cell monitoring required by Li-ion batteries.
Source: Greg Smith, May, 1999