Good read on liquid Li-Ion vs SOlid-Polymer LiIon

Klaus

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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
 

Klaus

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The Lithium-Polymer Battery: Substance or Hype?

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Isidor Buchmann
President
Cadex Electronics Inc.
Richmond, BC, Canada


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The phrase "lithium-polymer" has become synonymous with advanced battery technology. But what is the relationship between "polymer" and the classic lithium-ion battery? In this article we examine the basic differences between the lithium-ion and lithium-ion polymer battery. We look at packaging techniques and evaluate the cost-to-energy ratio of these batteries.

The lithium-polymer battery differs from other battery systems in the type of electrolyte used. The original design, which dates back to the 1970s, uses a polymer electrolyte. This electrolyte resembles a plastic-like film that does not conduct electricity, but allows the exchange of ions (electrically charged atoms or groups of atoms). The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolytes.

The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile. There is no danger of flammability because no liquid or gelled electrolyte is used.

With a cell thickness measuring as little as 1mm (0.039in), design engineers are left to their own imagination in terms of form, shape and size. Theoretically, it is possible to create designs that form part of a protective housing, are in the shape of a mat that can be rolled up, or are even embedded into a carrying case or a piece of clothing. Such innovative batteries are still a few years away, especially for the commercial market.

Unfortunately, the dry lithium-polymer suffers from poor conductivity. The internal resistance is too high and cannot deliver the current bursts needed for modern communication devices and spinning up the hard drives of mobile computing equipment, although heating the cell to 60 degrees C (140 degrees F) and higher increases the conductivity to acceptable levels. This requirement, however, is unsuitable for portable applications.

Some dry solid lithium-polymers are currently used in hot climates as standby batteries for stationary applications. One manufacturer has added heating elements in the cells that keep the battery in the conductive temperature range at all times. Such a battery performs well for the application intended because high ambient temperatures do not degrade the service life of this battery in the same way as it does with the VRLA type. Although longer lasting, the cost of the lithium-polymer battery is high.

Engineers are continuing to develop a dry solid lithium-polymer battery that performs at room temperature. A dry solid lithium-polymer version is anticipated by 2005. This battery should be very stable, would run 1,000 full cycles and would have higher energy densities than today's lithium-ion battery.

How then is the current lithium-polymer battery made conductive at ambient temperatures? Most of the commercial lithium-polymer batteries or mobile phones are a hybrid. Some gelled electrolyte has been added to the dry polymer. The correct term for this system is "lithium-ion polymer". For marketing reasons, most battery manufacturers call it simply "lithium-polymer". Since the hybrid lithium-polymer is the only functioning polymer battery for portable use today, we will focus on this chemistry variation, but use the correct term of "lithium-ion polymer".

With gelled electrolyte added, what then is the difference between lithium-ion and lithium-ion polymer? Although the characteristics and performance of the two systems are very similar, the lithium-ion polymer is unique in that the solid electrolyte replaces the porous separator. The gelled electrolyte is simply added to enhance ion conductivity.

Technical difficulties and delays in volume manufacturing have deferred the introduction of the lithium-ion polymer battery. In addition, the promised superiority of the lithium-ion polymer has not yet been realized. No improvements in capacity gains are achieved - in fact, the capacity is slightly less than that of the standard lithium-ion battery. For the present, there is no cost advantage in using the lithium-ion polymer battery. The major reason for switching to the lithium-ion polymer is form factor. It allows wafer-thin geometries, a style that is demanded by the highly competitive mobile phone industry. Table 1 summarizes the advantages and limitations of the lithium-ion polymer battery.

Table 1. Advantages and limitations of lithium-ion polymer batteries.

Advantages: Very Low Profile -- batteries that resemble 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.
Light Weight -- gelled rather than liquid electrolytes enable simplified packaging, in some cases eliminating the metal shell.
Improved Safety -- more resistant to overcharge; less change for electrolyte leakage.
Limitations: Lower Energy Density and Decreased Cycle Count Compared to Lithium-Ion -- the potential for improvements exist.
Expensive to Manufacture -- once mass-produced, the lithium-ion polymer has the potential for lower cost. A reduced control circuit offsets higher manufacturing costs.

The Pouch Cell
The lithium-ion polymer battery is almost exclusively packaged in the so-called "pouch cell". This cell design made a profound advancement in 1995 when engineers succeeded in exchanging the hard shell with flexible, heat-sealable foils. The traditional metallic cylinder and glass-to-metal electrical feed-through has thus been replaced with an inexpensive foil packaging, similar to what is used in the food industry. The electrical contacts consist of conductive foil tabs that are welded to the electrode and sealed to the pouch material. Figure 1 illustrates a typical pouch cell.



Figure 1. The pouch cell offers a simple, flexible and lightweight solution to battery design. This new concept has not yet fully matured and the manufacturing costs are still high.

The pouch-cell concept makes the most efficient use of available space and achieves a packaging efficiency of 90 to 95 percent, the highest among battery packs. Because of the absence of a metal can, the pouch pack has a lower weight. No standardized pouch cells exist, but rather, each manufacturer builds to a special application.

At the present time, the pouch cell is more expensive to manufacture than the cylindrical architecture, and the reliability has not been fully proven. The energy density and load current are slightly lower than that of conventional cell designs. The cycle life in every-day applications is not well documented, but is presently less than that of the lithium-ion system with cylindrical cell design.

A critical issue with the pouch cell is swelling, which occurs when gas is generated during charging or discharging. Battery manufacturers insist that lithium-ion or polymer cells do not generate gas if they are properly formatted, are charged at the correct current and are kept within allotted voltage levels. When designing the protective housing for a pouch cell, some provisions for swelling must be taken into account. To alleviate the swelling issue when using multiple cells, it is best not to stack pouch cells, but lay them flat side-by-side.

Additionally, the pouch cell is highly sensitive to twisting. Point pressure must also be avoided. The protective housing must be designed to safeguard the cell from mechanical stress.

The Cost of Being Slim
The slimmer the battery profile, the higher the cost-to-energy ratio becomes. By far, the most economical lithium-based battery is the cylindrical 18650 cell. "Eighteen" denotes the diameter in millimeters and "650" describes the length in millimeters. The new 18650 cell has a capacity of 2,000mAh. The larger 26650 cell has a diameter of 26mm and delivers 3,200mAh.

The disadvantage of the cylindrical cell is its bulky size and the less-than-maximum use of space. When stacking, air cavities are formed. Because of fixed cell sizes, the battery pack must be designed around the available cell.

If a thinner profile than 18mm is required, the prismatic lithium-ion cell is the best choice. The cell concept was developed in the early 1990s in response to consumer demand for slimmer pack sizes. The prismatic cell makes almost maximum use of space when stacking.

The disadvantage of the prismatic cell is slightly lower energy densities compared to the cylindrical equivalent. In addition, the prismatic cell is more expensive to manufacture and does not provide the same mechanical stability enjoyed by the cylindrical cell. To prevent bulging when pressure builds up, heavier gauge metal is used for the container. The manufacturer allows some degree of bulging when designing the battery pack.

The prismatic cell is offered in limited sizes and chemistries and the capacities run from about 400mAh to 2,000mAh. Because of the very large quantities required for mobile phones, custom prismatic cells are built to fit certain models.

If the design requirements demand less than 4mm, the best (and perhaps the only choice) is lithium-ion polymer. This is the most expensive option. The cost-to-energy ratio more than doubles. The benefit of this architecture is strictly slim geometry. There is little or no gain in energy density per weight and size over the 18650, even though the metal housing has been eliminated.

Summary
The lithium-ion polymer offers little or no energy gain over conventional lithium-ion systems; neither do the slim-profile lithium-ion systems meet the cycle life of the rugged 18560 cell. The cost-to-energy ratio increases as the cell size decreases in thickness. Cost increases in multiples of three to four compared to the 18650 cell, and are common on exotic slim battery designs.

If space permits, the 18650 cell offers the most economical choice, both in terms of energy-per-weight and longevity. Applications for this cell are mobile computing and video cameras. Slimming down means thinner batteries. This, in turn, will make the cost of the portable power more expensive.

This article contains excerpts from Batteries in a Portable World-A Handbook on Rechargeable Batteries for Non-Engineers. For more information on battery technology, visit http://www.buchmann.ca.
 

Klaus

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Lithium rechargeable batteries

Lithium rechargeable batteries
Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy content. Rechargeable batteries using lithium metal as an electrode are capable of providing both high voltage and excellent capacity, resulting in an extraordinary energy density.

The development of secondary batteries employing lithium metal as the negative electrode has been plagued with safety problems. The positive electrode of these systems is usually a metal or sulfide intercalation compound, but there are also liquid cathode rechargeable batteries using SO2 or SO2Cl2. The major problem with all of these systems is the metallic lithium. Upon cycling, the lithium that is deposited during charging eventually tends to form long "tentacles, called dendrites. These can grow through the separator and short out to the positive electrode. When this happens an explosion often results. The reason for a thin dendrite leading to an explosion lies in the low melting point of lithium, which is 180oC. This means that a shorted dendrite can melt, and molten lithium is violently reactive and is no longer protected by a passivating layer. The severity of an explosion depends somewhat on the nature of the electrolyte.

A Canadian company, Moli Energy of Vancouver, was the first to mass-produce lithium metal rechargeable batteries. These were used in portable telephones in Japan and had been shown to be safe in the laboratory, using a typical duty cycle. After much research during the eighties, it was found that occasional shorts from lithium dendrites could cause thermal run-away. The cell temperature quickly approaches the melting temperature of lithium which results in violent reactions. A large quantity (1.5 million cell) of rechargeable lithium batteries sent to Japan had to be recalled in 1989 after a cellular phone battery exploded and inflicted burns to a man's face. Battery engineers call it "venting with flame." A built-in pressure valve at the bottom of the battery's steel casing pops with a puff of smoke. A second later, the battery takes off like a tiny space shuttle atop a 30 centimeter plume of carmine red flame--the color of burning lithium.

Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions from chemicals such as Lithium-Cobalt Dioxide (LiCoO2 ). Although slightly lower in energy density than with lithium metal, the Li-ion is safe, provided certain precautions are met when charging and discharging. In 1991, Sony commercialized the Li-ion and is presently the largest supplier of this type of battery. These intercalation batteries are referred to as lithium-ion because, during discharge, lithium ions are de-intercalated from the carbon negative electrode and intercalated into an oxide positive electrode. To add to the confusion surrounding rechargeable lithium batteries, there are two types of lithium-ion cells:

Lithium-Ion liquid electrolyte

Lithium-Ion polymer electrolyte


Lithium metal batteries are also being developed using a solid polymeric electrolyte. The electrodes in these are very thin and can be made into any shape. Hydro Quebec has been developing this technology for several years. These cells are safer because they are all solid, but they also only operate at moderate temperatures, because the conductivity of the solid electrolyte is very low at room temperature.

Although rechargeable lithium batteries offer high energy density, they have not yet matured enough for military use. Research and development are needed in lithium anodes, cathode materials and electrolytes. In addition, lithium rechargeable batteries must overcome their poor power density at low temperature.

Lithium-Ion Batteries
Liquid electrolyte
The used by most manufacturers is lithium cobalt oxide (LiCoO2), but cobalt is an expensive metal so a cheaper alternative is sought, such as lithium manganese oxide (LiMn2O4) or lithium nickel oxide (LiNiO2). All of these oxides are stable in normal air, unlike lithium metal, which reacts with moisture in the air and also nitrogen. Upon charging, lithium ions are extracted from the positive electrode material and inserted into the negative electrode material. Upon discharging, the reverse process takes place.

Cells can be assembled in the discharged state i.e. using a carbon electrode with lithium cobalt oxide, making cell assembly easy and safer. The assembled cells can then be charged. Unfortunately, lithium-ion cells are rather expensive. Apart from cobalt metal, the other materials are not very expensive. However, the cells are easily degraded outside narrow voltage limits for both charging (about 4.5V) and discharging (below 3.0V).
For safety and longevity reasons, each battery pack must be equipped with a control circuit to limit the peak voltage of each cell during charge and prevent the cell voltage from dropping too low on discharge. In addition, the maximum charge and discharge current must be limited and the cell temperature monitored.



With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated. These added features increase the cost of these batteries considerable. Lithium-ion cells have about half the Ah capacity of similar primary lithium cells. Their voltage falls progressively during discharging, thus reducing the practical energy density to about 30% of that for primary lithium batteries.

However, they store about 2 to 3 times more energy than a comparable nickel-cadmium battery, display similar room-temperature performance, but perform poorly at low temperatures. The main advantages are good charge retention and a working voltage that can be 2.5 to 3 times that of Ni-Cd cells. Most of the cells manufactured are slightly larger than AA-size (14mm diameter x 50mm height) called 18650 (18mm diameter and 65mm height). These cells are spirally wound and are usually sold as battery packs. Larger format cells ( 67mm diameter x 410mm long) are now being developed for electric vehicle applications.


Lithium-Ion Polymer Battery
'Super lithium polymer'
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 are sandwiched together on continuous-feed rolls 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.)

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 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.

Solid-polymer Li-ion cells can be made as thin as 0.64 mm, 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

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.

Solid polymer electrolyte lithium batteries
This promising lithium rechargeable technology is using a polymer electrolyte in a solid state cell in which a polymer electrolyte is sandwiched between a lithium metal film and a metal film. By dissolving lithium, not into a liquid electrolyte but into a really thin polymer (plastic), a high-power battery is realized that is light, yet durable. The laminate construction of such cells offer flexibility of shape and size, which is advantageous for portable power source applications. However, at the present time, the conductivity of these batteries is very low at room temperature, compared with those of liquid electrolytes: these batteries are normally operated in the 60-120oC range. Research is being aimed at increasing conductivity through the use of plasticizers and new polymers.

This new technology offers the potential of future low manufacturing costs. It is environmentally benign, it avoids electrolyte leakage to damage electronic components, and can fit any casing shape. It can be used either as a rechargeable system for training or peacetime exercises, or as a primary battery in emergency or wartime situations. More recently, the development of "Polymer-In-Salt" materials, in which superionic glass electrolytes are mixed with small quantities of the polymers, has been suggested. Dissolution of the polymer into these melt-glass-electrolytes produces a rubbery version of a glassy electrolyte with a thousand-fold increase in lithium ion mobility. A way has been opened to a new generation of lithium batteries with the prospect of a high power density application. Much work remains to be done before this discovery can be fully exploited.

Under a United States Advanced Battery Consortium (USABC) contract, 3M, Hydro-Québec, and Argonne National Laboratory joined technologies to develop and test the first solid electrolyte lithium battery. The Lithium Polymer battery relies on thin-film technology, with composite films that are only 100 microns thick. It's a solid state battery that can be wound and shaped to suite the application. It uses a plastic electrolyte. 3M expects that a typical EV battery pack would weigh on the order of 500 pounds (224 kg), which could provide as much as 45 kW-h of energy. In comparison, EV1's lead-acid battery pack weighs over 1000 pounds (480 kg) and provides 16 kW-h of energy.


This unique battery contains a solid, dry, polymer electrolyte with a metallic lithium anode. The cathode is a vanadium oxide composite. Features include built-in electronic control and battery management systems, and the ability to adapt in shape to various sized battery compartments. The battery has a 150-200 mile range per charge, and over 100,000 miles useful life. It is the first advanced battery to achieve performance and cost statistics approaching the USABC's goals for commercial use.
 

Klaus

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Lithium-ion Polymer Batteries: A Look to the Future


Dr. Sun-wook Kim, NESS Corp. -- ECN, 8/1/2001

by Dr. Sun-wook Kim, NESS Corp.


This article is also available in PDF format.

The next generation rechargeable battery — lithium-ion polymer battery (LiPB) has arisen to meet the demand for increased power density and smaller size with safety and cost foremost in mind. Until now, commonly used batteries include the nickel-cadmium (NiCd) and the nickel-metal hydride (NiMH) design typical in cellular phones or even the newer lithium-ion (Li-ion) batteries, powering higher-end devices. Lithium-ion polymer batteries, when high-volume supplies become available, are expected to challenge and replace NiMH and Li-ion batteries as the preferred power source for portable electronic devices. LiPB technology addresses the most important issues of the rechargeable battery market: power density, size, design flexibility and safety.

LiPB technology reveals an exciting new battery structure, discarding the conventional metal-can assembly. The most striking difference between LiPB and Li-ion is that the former uses a solid polymer electrolyte rather than a liquid electrolyte solution. The solid polymer electrolyte is physically a solid but appears to the ions as a liquid that they can pass through. With no liquid to escape, the solid electrolyte is simply sandwiched between electrodes ( the anode and cathodes formed out of thin sheets). Then the cell is contained in laminated foil and sealed at the edges to form an entire battery. The resulting cell is extremely thin and as flexible as a rubber mat. Using a solid electrolyte is quite appealing because it is naturally spill proof, more resilient under pressure, and capable of being engineered into most any shape because a thin laminated foil material, rather than an unyielding metal can, houses each cell. Therefore, lithium-ion polymer batteries are lighter, thinner, flexible and leak resistant — thus safer.


Figure 1. Internal construction of an LiPB battery.

High Energy and Power Density

LiPB technology can deliver the best power energy available for rechargeable batteries. LiPB and Li-ion batteries have similar energy characteristics with both offering voltage in the 3.0 V to 4.2 V range as compared to 1.2 V to 1.5 V for NiCd and NiMH cells. The gravimetric energy density ranges from 120 Wh/kg to 160 Wh/kg, and the volumetric energy density from 230 Wh/l to 270 Wh/l. LiPB technology emulates the cycle life benefits of the Li-ion chemistry, providing 500+ charge/discharge cycles with no memory effect. However, this is one of the very few areas where lithium technology has not outshined NiCd and NiMH capability, which offer a cycle life of 2,000 and 1,000 respectively. LiPB technology uses the polymer as a separator with other components very similar to Li-ion. Its voltage is derived from the electro-potential difference of lithium in the anode and cathode. LiPB uses lithium metal oxides such as LiCoO 2 for the cathode, although other cathode materials such as manganese dioxide are being developed. Graphite carbon (or other carbon-based materials) compounded with lithium metal forms the anode, similar to a Li-ion battery. During the charge reaction, lithium in the positive electrode is ionized and moves to the negative electrode through the polymer electrolyte. During the discharge reaction, the lithium ion returns to the positive electrode, reverting to its original phase. This simple migration of lithium ions gives the battery a long shelf life and a long cycle life.

Characteristics


Figure 2. LiPB battery charge/discharge profiles.

Thin and Lightweight

LiPB technology offers the greatest energy to size/weight ratio for rechargeable batteries. Batteries as thin as a 0.4 mm credit card with the same power capabilities as standard battery packs on cellular phones have appeared. Lithium's high electrochemical reactivity has made it difficult to work with in the past, yet also why it has held so much potential. Therefore, Lithium batteries are made from Lithium ions rather than Lithium in its natural state. The high energy density of Lithium means less physical material needs to be used, thus reducing battery size and weight, and making it ideal for use in compact electronic devices.


Figure 3. Terminal voltage versus discharge battery percent.

Design Flexibility

Lithium-ion polymer battery technology has virtually limitless design flexibility, which is made possible by the use of a solid polymer electrolyte. The basic internal structure of a solid polymer cell can be configured to virtually any size, and the cells can be stacked to produce ultra-thin battery packs with a broad array of voltages and capacities. This allows manufacturers to make batteries in diverse shapes — a feature that other batteries cannot offer. The solid polymer electrolyte is easy to manipulate into desired shapes at elevated temperatures, and battery performance and stability is unaffected by the design. This flexible cell geometry creates enormous potential for physically innovative designs. For example, the elements of a lithium-ion polymer battery can be stacked for high power density applications in a conventional battery form or positioned flat behind a laptop screen or under the keyboard, using the application design itself for placement. It can be built into the casing of a cell phone — further reducing size. The battery can even be placed over curved surfaces.


Figure 4. Charge rate and percent of charge versus time.

Safety and Environmental

LiPB technology is safe and environmentally acceptable, having been tested against the toughest standards with very successful results. The solid lithium polymer electrolyte is a nonvolatile and non-flammable material and is encapsulated in a polymeric coating. In the event of an accident, the amount of exposed lithium electrolyte is limited, providing a safety option not available in a liquid system. If opened, there is no metallic lithium present and the lithium polymer material is in ionic form (chemically combined with another element) and thus nowhere near as reactive upon exposure as pure lithium metal.

LiPB technology has demonstrated a high level of safety under adverse conditions where batteries have been slashed, pierced and abused in general. Not only have safety problems been minimal — in many instances, the batteries have also maintained a good proportion of their charge even after severe physical damage. The solid polymer design requires no venting, eliminating a significant problem with liquid-ion. In addition, preliminary findings suggest solid polymer cells may be extremely resistant to abuse from over-charging and discharging. The use of a solid polymer electrolyte reduces heat run-away even after external short-circuiting.

Applications

Lithium-ion polymer battery technology is a perfect match for cellular phones, laptop computers and PDAs. It can be custom engineered to fit the design goals of video cameras, digital still cameras, smart cards, MiniDisc players and an endless list of portable devices. Scientific and engineering field equipment and medical emergency and disaster preparedness equipment also stand to benefit.

LiPB technology creates terrific opportunities in all parts of the industry, from material suppliers to process equipment manufacturers. Original equipment manufacturers (OEMs) can count on LiPB technology to provide both energy and structural functionality to electronic devices, taking them one step closer to optimal efficiency. The cost-effective nature of lithium-ion polymer batteries allows OEMs to use them in more than high-end products. Lithium (refined from naturally occurring salts) is abundant and very affordable.

Conclusion

Worldwide demand for ultra-thin, ultra-light, and long-lived rechargeable batteries will continue to intensify. Lithium-ion polymer batteries are the most technically advanced rechargeable power source available, and the technology is just at the onset of its commercial usefulness.
 

Klaus

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Q: What are considerations for discharging at high

Q: What are considerations for discharging at high and low temperature




(May 2003) Batteries function best at room temperature. Operating batteries at an elevated temperature dramatically shortens their life. Although a Lead-acid battery may deliver the highest capacity at temperatures above 30°C (86°F), prolonged use under such conditions decreases the life of the battery. Similarly, a Lithiumion performs better at high temperatures. Elevated temperatures temporarily counteract the battery's internal resistance, which may have advanced as a result of aging. The energy gain is short-lived because elevated temperature promotes aging by further increasing the internal resistance.


There is one exception to running a battery at high temperature - it is the Lithium-polymer with dry solid polymer electrolyte, the true `plastic battery'. While the commercial Lithiumion polymer uses some moist electrolyte to enhance conductivity, the dry solid polymer version depends on heat to enable sufficient ion flow. This requires that the battery core be kept at an operation temperature of 60°C to 100°C (140°F to 212°F).


The dry solid polymer battery has found a niche market as backup power in warm climates. The battery is kept at the operating temperature with built-in heating elements that is fed by the utility grid during normal operation. On a power outage, the battery would need to provide its own power to maintain the temperature. Although said to be long lasting, price is an obstacle.


Nickel-metal-hydride degrades rapidly if cycled at higher ambient temperatures. For example, if operated at 30°C (86°F), the cycle life is reduced by 20%. At 40°C (104°F), the loss jumps to a whopping 40%. If charged and discharged at 45°C (113°F), the cycle life is only half of what can be expected if used at moderate room temperature. The Nickel-cadmium is also affected by high temperature operation, but to a lesser degree.


At low temperatures, the performance of all battery chemistries drops drastically. While -20°C (-4°F) is threshold at which the Nickel-metal-hydride, Sealed Lead-acid and Lithiumion battery cease to function, the Nickel-cadmium can go down to -40°C (-40°F). At that frigid temperature, the NiCd is limited to a discharge rate of 0.2C (5 hour rate). There are new types of Liion batteries that are said to operate down to -40°C.


It is important to remember that although a battery may be capable of operating at cold temperatures, this does not automatically allow charging under those conditions. The charge acceptance for most batteries at very low temperatures is extremely confined. Most batteries need to be brought up to temperatures above the freezing point for charging. Nickel-cadmium can be recharged at below freezing provided the charge rate is reduced to 0.1C.



Pulse discharge


Battery chemistries react differently to specific loading requirements. Discharge loads range from a low and steady current used in a flashlight, to sharp current pulses for digital communications equipment, to intermittent high current bursts in a power tool and to a prolonged high current load for an electric vehicle traveling at highway speed. Because batteries are chemical devices that must convert higher-level active materials into an alternate state during discharge, the speed of such transaction determines the load characteristics of a battery. Also referred to as concentration polarization, the nickel and lithium-based batteries are superior to lead-based batteries in reaction speed.


The Lead-acid battery performs best at a slow 20-hour discharge. A pulse discharge also works well because the rest periods between the pulses help to disperse the depleted acid concentrations back into the electrode plate. A discharge at 1C of the rated capacity yields the poorest efficiency. The lower level of conversion, or increased polarization, manifests itself in a momentary higher internal resistance due to the depletion of active material in the reaction.


Different discharge methods, notably pulse discharging, affect the longevity of some battery chemistries. While Nickel-cadmium and Lithiumion are robust and show minimal deterioration when pulse discharged, the Nickel-metal-hydride exhibits a reduced cycle life when powering a digital load.
In a recent study, the longevity of Nickel-meal-hydride was observed by discharging with analog and digital loads to 1.04V/cell. The analog discharge current was 500mA; the digital mode simulated the load requirements of the Global System for Mobile Communications (GSM) protocol and applied 1.65-Ampere peak current for 12 ms every 100 ms and a standby current of 270mA. (Note that the GSM pulse for voice is about 550 ms every 4.5 ms.)


With the analog discharge, the Nickel-metal-hydride provided an above average service life. At 700 cycles, the battery still provided 80% capacity. By contrast, the cells faded more rapidly with a digital discharge. The 80% capacity threshold was reached after only 300 cycles. This phenomenon indicates that the kinetic characteristics for the Nickel-metal-hydride deteriorate more rapidly with a digital rather than an analog load. Lithium and Lead-acid systems are less sensitive to pulsed discharge than Nickel-metal-hydride.




BD
 

Klaus

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Lithium-ion polymer Poised for Market Recovery

Lithium-ion polymer Poised for Market Recovery

By Donald Georgi

The year 2001 could be classified as somewhat of an unlucky draw for Lithium-ion polymer chemistry. After years of diligent chemistry, engineering and manufacturing growth, the industry was set to capture the new and replacement applications which polymer was best suited for, but who was to know that 2001 would be the year of the electronic market retreat. Led by dot coms, then cellphones, then computers, PDAs and everything else needing portable power, markets have shrunk; corporate profits have deteriorated, and layoffs have become the order of the day. Anyone who had gold stocks or money market funds returning 4% made a killing when compared to investors who lost big double digit percentages with JDSU, Motorola and Ericsson of telecommunications and other electronic device makers.

Delays in the development of 3G wireless, whether an economic casualty or just a late bloomer, added much hesitation to the Lithium-ion polymer battery market. Its growth was previously slated for a large role because of its thin and variable form factor.

One might say that such a business environment for electronic devices in 2001 was not the best time for the introduction of Lithium-ion polymer as both a tool for new applications and a spoiler for applications already using liquid Lithium-ion. But, battery suppliers had no choice since they could not resuscitate the ailing world markets, and product introductions had to continue with or without a marketplace. As of last September, GS-MELCOTEC had shipped 5 million cells, a number they probably would wish was per month or week. Today, Lithium-ion polymer is a bride at the altar waiting for a groom... any groom... many grooms to show up. Why should polymer be expecting to be carried over the threshold?

Advantages

Thin is in, and Lithium-ion polymer helps to make that happen. Notebook computers have shed their on board removable disk drives to offer travelers thin packages which leave briefcase space for a few files, some pencils and photos of the kids (guaranteed to shorten the time of water fountain conversations). To do this, almost paper thin Lithium-ion polymer batteries are built in to the back surface of the screen or motherboard, leaving precious space for other components. Similarly, with the ability to combine a thin profile with shape variations, the battery can be fitted to ever shrinking cell phones which get so small that genetecists may be pressured to build tiny people who can operate the micro keyboards and displays being sold today.

Lithium-ion polymer has a major advantage with its thin size and a wide variety of shapes. Here a dragonfly lands on a Varta LFP Lithium-ion polymer micro battery which is targeted for applications in powering smart cards. (Photo is courtesy of Varta Batteries Inc.) +


Sanyo sees the dividing line at 3 mm thickness for low capacity 100-500 mAh polymer cells and up to 6 or 7 mm for 2000-2500 mAh polymer cells.

A key feature of the electrolyte is immobilization so that it cannot leak as a liquid. And, present safety testing of the cells shows their excellent abilities to withstand mechanical damage and overcharge. This suggests that some of the overhead safety electronics needed for liquid Lithium-ion may not be necessary, freeing space and lowering peripheral costs, although specific examples are not yet available.

Disadvantages

Partly because volume has not yet fueled the competitive fires, polymer is perceived as being more expensive. Power density is also considered a shortcoming, but many electronic applications other than pulsed cellular only need continuous power, more easily met by polymer. As with other electrochemical developments, such shortcomings are being addressed with results beginning to appear in higher discharge currents in GSM and PDC pulse performance.

Present Capabilities

Mitsubishi Chemical has focused on improving power density, cycle life, widening operating temperature range, reducing self discharge and improving the inherent safety of polymer. Adding nickel to the cobalt containing cathode has pushed energy density to 372 Wh/l. (Energy densities for small batteries powering portable electronic devices are usually only expressed in their volumetric form of Wh/l since space, not weight, is important in these applications.) Surface modification of the cathode and additives have reduced swelling, easily detected in aluminum laminated film bags.

Sanyo's cross linked gelled electrolyte provides current densities above 10 mA/cm2 while the polymer cells using porous membranes provide only half that conductivity. Energy density of 311 Wh/l is delivered with discharge capacity to 3C.

GS Melcotec has a new concept polymer battery which employs a porous polymer layer between the separator and each electrode so that inorganic polymer particles can migrate into the electrode. One can visualize the construction as having the cell elements glued to each other by the polymer improving mechanical hardness and cycle life. Performance with both the GSM pulse discharge profile and the PDC pulse discharge shows acceptable performance even down to -100 C. No smoke, fire or explosion is observed with mechanical or electrical abuse. The company was honest enough to show that with the

Thin is not the exclusive domain of Lithium-ion polymer cells. Above is the 2.8 mm thick liquid cell produced by Maxell. +


burner heating and microwave oven tests the electrolyte did catch fire, but none exploded. Such thorough reporting increases one's confidence in a company's credibility regarding the safety data presented.

Today's energy density performance, while on an approximate 10% increase each year, will have to continue at that pace according to the projections presented by Samsung and Valence Technology. Improvements will have to be made in new electrochemical materials for the cells.

One approach pursued by Valence Technology is to reduce the cost of materials with a phosphate [Li3V2(PO4)] anode. Although this phosphate is lower in energy density than cobalt Lithium-ion, it exceeds that of Nickel-metal hydride at a lower cost. The phosphate also has a higher (2700 C) exotherm temperature than cobalt (2280 C), providing superior intrinsic safety. Overcharge testing at 12 Volts for 60 hours shows no temperature rise above ambient, adding another inherent safety feature. +


Challenges

Liquid Lithium-ion continues its relentless pursuit of technical advancements and price improvements. Valence Technology reports that the current Japanese price per Wh includes 28% for the cost of materials. Conversely, the Chinese model reserves 48% for the material cost, bringing the price per Wh to a third of the Japanese cell. Projections are that the trend will continue so that by 2004 the Chinese price model will produce cells at less than $0.25/Wh. Such reductions will require that polymer cells follow the trend or have applications where price is not the key issue.

Soft case liquid Lithium-ion cells, with either wound or stacked electrodes, are being introduced by Samsung. These cells have reduced swelling with additives to the electrolyte and surface modification of the anode materials. Thicknesses down to 2 mm are showing no adverse response to abnormal charging. In some ways, this approach might suggest a convergence between liquid and polymer cells.

Korea Power Cell has a flexible form factor using 'Free-stacking' technology in Lithium-ion with thickness to 4.6 mm and energy density of 380 Wh/l.

Panasonic sees large applications for liquid Lithium-ion. They are presently producing prismatic, cylindrical and coin cells with up to 350 Wh/l capacity and are targeting 500 Wh/l by 2005, a 43% increase.

Still unknown is the future role of micro fuel cells in powering electronic devices. With their refillable fuel tanks, they may not win the space competition with polymer, but their high total energy delivered, low costs and convenient refuelling may be embraced by product designs which get around thickness and shape issues. Polymer still has a few years to establish its special features before micro fuel cells become a force in the market, if ever.

Pushing the Envelope

IN 1999, Using a fitting title of 'SuperPolymer', Electrofuel introduced a Lithium-ion polymer battery which has the highest advertised energy density of 475 Wh/l and 200 Wh/kg. The capacities are offered from 1 to 12 Ah with future capacities to 100 Ah in early 2002.

Many of the electrochemical improvements will be shared by liquid and polymer cells, and other improvements may be peculiar to a single chemistry. Whatever the future needs, electronic devices and their users will be the benefactors of all this aggressive improvement. BD
 

lemlux

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Thanks, Klaus:

Now I better understand why it's not a good idea to drive a 2.3 A P91 at 2.3 C with a pair of 1000 mah 17500 Li-Ions in serial. This load causes the batteries to heat up considerably, which causes.......(bad stuff). The other insights were very enlightening as well.
 

Klaus

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/ubbthreads/images/graemlins/bumpit.gif

Looking for old stuff I found this - oldie but goldie /ubbthreads/images/graemlins/smile.gif

Klaus
 
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