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?
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.
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.
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.
Sanyos 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 ones confidence in a companys credibility regarding the safety data presented.
Todays 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. +
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