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Metal-Li Battery Leads
Post-Li-Ion RaceEmerging candidates for the successor to the Li-ion battery include metal-Li, Li-polymer, fuel cells, double-layer capacitors, and air-zinc batteries.
Competition to establish a successor to the Li-ion rechargeable battery, which currently commands a substantial lead in the market, is rapidly gaining pace.
Key contenders include metal-Li and Li-polymer rechargeable batteries, fuel cells, double-layer capacitors, and air-zinc batteries (Fig 1). Producers of portable equipment are keenly monitoring developments in this area, as too are automobile manufacturers and others who traditionally have shown little interest in batteries.
Opportunity Knocks
In the 1990s, nickel metal-hydride (Ni-MH) rechargeable batteries became widespread very quickly. This trend was followed by the similar wide adoption of Li-ion rechargeable batteries, which grew hand-in-hand with the popularity of portable equipment such as notebook personal computers (PC) and cellular telephones (Fig 2). Sony Energytech Co, Ltd of Japan, for example – the first company to sell Li-ion rechargeable batteries – shipped 2 million cells in the 1994 fiscal year, a figure which rose to 4 million in FY1995 and which is now 10 million. Sanyo Electric Co, Ltd of Japan started off with shipments of 100,000 cells in 1994, rising to 1 million in 1995, 3 million in 1996, and 8 million in 1997.
However, not all battery manufacturers profited from this boom. While about 10 firms entered the Li-ion rechargeable battery market in Japan, a number later pulled out or changed their strategy. Fuji Photo Film Co, Ltd of Japan, for example, developed a Li-ion rechargeable battery with an Sn amorphous oxide anode, releasing it in 1997. However, it has since quit this operation. These manufacturers are now searching for ways to make a comeback with a successor to Li-ion, while those successful in the Li-ion competition have accelerated R&D efforts to maintain their market positions.
Japanese manufacturers are not the only firms competing for honors – American companies are also extremely active. A number of start-up companies have been created in the battery sector, most placing heavy emphasis on development. Typical examples include Moltech Corp of the US, which is focusing on Li-polymer devices, as well as Energy Related Devices Inc and H Power Corp, both of the US, which are involved with fuel cells.
Mobility Drives Trend
There are two main reasons for the sharp increase in competition for post-Li-ion batteries.
The first is the continuing demand for ever-lighter, ever-smaller mobile equipment, especially notebook PCs and cellular phones. Already notebooks are down to a thickness of 20mm and a weight of about 1.0 to 1.5kg, and the new models are highly popular. For cellular phones, the trend is that the lightest models sell the best, and weight is expected to drop under 60g by the end of 1999 (NEA, Technology Trend, November 1998). Similar size and weight improvements are also needed in mobile equipment such as MiniDisc (MD) players, camcorders, mobile game systems, digital still cameras and personal digital assistants (PDA).
The second reason is the growing interest in the development of electric and hybrid vehicles, which aim to replace today’s gasoline-powered cars. Development of these new vehicles is proceeding steadily, as evidenced by the release of the Prius hybrid car by Toyota Motor Corp of Japan in November 1997, which uses Ni-MH rechargeable batteries. The key requirement is a high energy density.
Li-Polymer Comes First
Contenders for the next power source for compact mobile equipment include Li-polymer rechargeable batteries (Fig 3), fuel cells and air-zinc batteries.
Some of these designs have already entered use. For example, the SolidAudio “business-card-sized” audio player, jointly developed by NTT Corp of Japan and Kobe Steel, Ltd of Japan, uses Li-polymer rechargeable batteries (Fig 4). Yuasa Corp of Japan manufactures the battery.
“There was no way to make a battery for a player this small and thin without using Li-polymer rechargeable cells,” said Shoji Ashida, president of Yuasa Asep, Ltd of Japan, a subsidiary of Yuasa.
Li-polymer rechargeable batteries use solid, or gel electrolytes, which are sandwiched between film electrodes. Because unlike Li-ion rechargeable batteries they do not require a metal case and can be made thinner than 1mm.
After card-sized players, the cellular telephone will probably be the next product to switch to Li-polymer, and is one which represents a significantly larger market. One major cellular phone manufacturer claims to be actively preparing to adopt Li-polymer rechargeable batteries, and expects the first cellular phones with the new power source to appear from late 1998 to 1999.
Other cellular telephone manufacturers also hope to utilize the ultra-thin Li-polymer battery to develop even thinner products, according to one engineer in the field.
There are a number of reasons why cellular telephone manufacturers are moving to adopt Li-polymer rechargeable devices: (1) the power load of a cellular phone in use does not fluctuate as much as a notebook PC, (2) power consumption in stand-by or talk mode is less, eliminating the need for such a high energy capacity, (3) the need for thinness is likely to be greater than the need for compactness in the future, and (4) they are safer than Li-ion rechargeable batteries.
Makers Focus on Phones
Recognizing the situation, Li-polymer rechargeable battery manufacturers have focused their development efforts on the cellular phone market (Fig 5, Table 1).
John Broadhead, vice president and chief technical officer of Moltech says, “We are sample-shipping to three cellular phone makers now. We plan to upgrade our production facility to handle 450,000 cells per month in early 1999, and 2 million cells per month by the end 1999.”
The firm’s Li-polymer rechargeable batteries use (CSx)n sulphur-based material in the cathode, and metallic Li in the anode. For a given size, this provides an energy capacity greater than that of the current Li-polymer battery with a LiCoO2 cathode and a carbon-based anode.
Fuel Cells Emerging
Following in the wake of the Li-polymer battery in the cellular phone market are the air-zinc battery and the fuel cell.
Electric Fuel Ltd of Israel is marketing the air-zinc battery to cellular phone manufacturers (Fig 6). The battery cell has an energy density per volume of 500Wh/l, and an energy density per weight of 200Wh/kg, allowing the battery pack to be made lighter than would be possible with other rechargeable batteries. This translates to a conversation time of 6.25 hours, which is about three-times higher than other choices for power sources.
Energy Related Devices and Manhattan Scientifics, Inc of the US, are jointly working to develop fuel cells for cellular telephones (Fig 7). Their prototype fuel cell is only half the size of a matchbox, they say, thanks to innovations in materials.
“The structure is so simple that we expect to drop the price to about US$5.00,” explains Robert G Hockaday, president of Energy Related Devices. “It will take at least a year to bring out a commercially viable design, though.”
With a 1.5-ounce (42g) load of methanol fuel, it will be able to power 100 hours of talk time, or 41 days of standby.
The air-zinc battery and the fuel cell, however, both suffer from difficulties in handling. The air-zinc battery must be replaced when used up, while the fuel cell requires fuel injections of hydrogen gas or methyl alcohol.
Using Existing Li-Ion Case
Battery manufacturers believe that the two largest markets will be compact, mobile equipment and notebook PCs.
Vice president David Murdoch of Electrofuel, Inc, of Canada, a Li-polymer rechargeable battery manufacturer, says: “We want to get our batteries in the PC market as soon as possible, and establish our name.” His firm has developed a Li-polymer rechargeable battery that fits in a notebook PC Li-ion rechargeable battery case, thereby enabling a compatible swap with existing Li-ion battery packs in notebook PCs. As a result, there is a good chance that the firm’s products will be snapped-up by the market.
To fit the Li-polymer battery into the 18mm-thick battery case, three batteries – each about 6mm thick – were stacked together. Thus, the outstanding advantage of the Li-polymer battery, namely its thinness, was discarded to increase the volume and make it fit the existing case.
The existing battery pack, designed to hold cylindrical Li-ion rechargeable batteries, ends up with dead space, but by eliminating the dead space the firm was able to boost energy density per volume by 35%. The new battery pack, as a result, extends the battery drive time of a notebook PC to about eight hours.
Li-polymer rechargeable batteries, however, have not yet reached the level where they can be widely adopted by PC manufacturers. Depending on usage conditions, PC load can fluctuate from 0.05W to about 20W, and as one PC engineer points out, “Often the most problematic use occurs when the battery is almost used up. Just before the battery reaches total discharge, for example, the user may try to store data to the hard disk. These are tough conditions for battery operation.”
Fuel Cells Offer 20+ Hours
In addition to smaller and lighter devices, there is also considerable demand for longer battery drive times for notebook PCs. Existing rechargeable batteries, however, can only offer several hours of operation. With fuel cells, on the other hand, several dozen hours of continuous operation may be possible.
Ballard Power Systems Inc of Canada and H Power are developing compact fuel cells for use in notebooks.
Ballard Power Systems announced its second prototype in 1997, with an output of 100W. Also using hydrogen gas, the firm says it will drive a notebook PC for about 20 hours, and plans to begin volume production in 2000 or later.
The H Power prototype measures about 5cm x 5cm x 5cm (Fig 8), and also uses hydrogen gas fuel. “A single refueling will drive a notebook PC for about 20 hours,” explained H Frank Gibbard, chief executive officer of the firm. At present, however, production cost is high at US$2,000, but the firm plans to reduce this to about US$100 by 2003.
Matsushita Electric Industrial Co, Ltd of Japan, has prototyped a fuel cell for notebooks measuring 10cm x 15.5cm x 15.5cm, and weighing 2.6kg. It uses hydrogen gas as the fuel, and has a 65W output. A single fueling would run a notebook PC continuously for seven hours.
Boosting Energy Density
The energy density of Li-ion rechargeable batteries is rising steadily, and energy density per volume is expected to increase from its current level of 300Wh/l to 350Wh/l to about 400Wh/l by the year 2000.
Battery manufacturers are now trying to boost energy density in Li-ion batteries with a variety of electrode materials, such as by replacing LiCoO2 cathodes with LiMn2O4 or LiNiO2, or composites of these materials. “Interest is especially high in LiNiO2,” comments Kensuke Nakatani, senior manager, Engineering Department, Ion Battery Division, Sanyo Electric Co, Ltd of Japan. “By 1999 or 2000, I expect batteries using the material to be released, which should mean an improvement in energy density of about 15%.”
Metal-Li: Ultimate Battery
Theoretically, there are battery designs which offer energy densities that are superior to existing Li-ion rechargeable devices. Foremost among them is the metal-Li rechargeable battery, which uses a metallic Li anode. Battery manufacturers seem to agree that this represents the ultimate battery which is possible with existing technology.
The theoretical energy density per volume of a battery using metallic lithium for the anode and MnO2 for the cathode would be 3,100Wh/l, and the energy density per weight would be 1,000Wh/kg. With a CoO2 cathode, the values would be 3,830Wh/l and 850Wh/kg, respectively. In either case, the numbers are about double those of the top theoretical values for Li-ion.
This high energy density is possible because of the metallic Li in the anode. Unlike the carbon-based anodes used in existing Li-ion rechargeable batteries, the entire anode can be used in the battery reaction. Discharge capacity, which indicates how much energy the battery can store, is 339mAh/g for carbon-based material, theoretically, but surges by a factor of ten, to 3,861mAh/g for metallic Li.
Prior development of metallic Li rechargeable batteries suffered from lithium fires, which severely retarded development.
The cause of the accidents was the highly reactive metallic lithium. During charge and discharge, the metallic Li of the anode would dissolve into the electrolyte as Li+, or precipitate as metallic lithium. Repeated charge/discharge cycles would lead to the formation of dendrite crystals at the anode, eventually penetrating the separator between the anode and cathode to cause a short circuit.
“I don’t expect any quick and simple solution to the dendrite problem,” warns Yoshio Nishi, corporate vice president (battery R&D) at Sony Corp of Japan, who is involved in battery development.
Metallic Li in Li-Polymer
Battery manufacturers have not given up on metallic Li rechargeable batteries, however. Moltech, for example, has prototyped a Li-polymer battery using metallic Li electrodes. John Broadhead says, “The firm is improving the electrolyte and other points to suppress dendrite growth.”
The battery offers an energy density per weight of 200Wh/kg – 1.5- to 2-times that of existing Li-ion or Li-polymer batteries. The energy density per volume is on a par. “We are already sample-shipping to a number of cellular phone makers,” reveals Broadhead.
The firm plans to develop a Li-polymer rechargeable battery by the end of 1998 with an energy density per weight of 250Wh/kg and an energy density per volume of 300Wh/l.
Shrinking Fuel Cells
Fuels cells are probably actually closer to electric generators than batteries. They operate with the combustion of air and hydrogen, creating essentially no CO2 or other emissions. They do not require recharging. Generation efficiency is a relatively high 40% to 60%, two- or three-times better than internal combustion engines. These characteristics have spurred a number of firms to launch development projects, including start-up companies, battery manufacturers and even automobile manufacturers.
There are two major directions in fuel cell development. The first, a thin, small fuel cell with an output of only dozens of watts, is aimed at usage in mobile equipment. Development in this sector is under way by the Energy Related Devices-Manhattan Scientifics group, H Power, Matsushita Electric Industrial, and others.
The battery prototyped by Energy Related Devices and Manhattan Scientifics is about half the size of a matchbox, and is aimed for use in cellular phones, says Robert G Hockaday, president of Energy Related Devices.
The other battery has an output from several to dozens of kilowatts, and is designed for use in installations, or electric vehicles. The 1kW fuel cell released by Sanyo Electric in 1997 for night-time and emergency power source use, however, is quite expensive, at ¥1.5 million.
Ballard Power Systems, a major fuel cell manufacturer, claims that materials and component modifications will drop the manufacturing cost to about US$15/kW by 2003.
Double-Layer Capacitors
“If the electric double-layer capacitor competes head-on in the market covered by rechargeable batteries, it will beat out a few types,” predicts Michio Okamura, president, Okamura Laboratory, Inc of Japan. Also called the ultra-capacitor, the product offers a few characteristics not found in Ni-MH or Li-ion rechargeable batteries.
For example, it can be charged very rapidly, and can output high currents. Cycle life is also superior to rechargeable batteries, and there is no change in its characteristics even after 10,000 charge per recharge cycles. In addition to the Okamura Research-Power Systems Co, Ltd, double-layer capacitors are also under development by NEC Corp of Japan, Matsushita Electric Industrial, and Elna Co, Ltd.
Until recently, their energy density was lower than that of rechargeable batteries, and the energy density per volume was between several and a dozen Wh/l. This effectively limited applications to devices like clocks and toys.
The new capacitor developed by Okamura and Power Systems, however, has an energy density per weight of 25Wh/kg and an energy density per volume of 30Wh/l. According to Okamura, “The double-layer capacitor is easily capable of penetrating market sectors held by the lead-acid battery, with an energy density per weight of about 40Wh/kg.”
Large Air-Zinc Batteries
Compared to other batteries, the air-zinc battery offers a higher energy density. The rechargeable design uses a Zn anode and the oxygen in the air as the cathode. Energy density per volume can rise to as high as 1,000Wh/l. The theoretical energy density per volume is 2,150Wh/l, vastly superior to the 1,570Wh/l of Li-ion.
The battery has only been used in small devices such as hearing aids and pagers so far, but experimentation into large-sized units for portable equipment or even automobiles is under way.
While air-zinc batteries are classified as rechargeable batteries, they cannot actually be recharged in the same way. The Zn electrode must be replaced, mechanically “recharging” the battery. Reducing the frequency of electrode replacement is the key to widespread adoption of the battery.
For example, some manufacturers are developing air-zinc batteries that can be recharged normally. So far, it has proved difficult to control Zn charge/discharge characteristics, and practical devices still seem some way off.
by Yasuo Tanokura and
Hiroki Yomogita
References:
- Ballard Power Systems
Electric Fuel
Fuji Photo Film
H Power
Hitachi Maxell
Kobe Steel
Manhattan Scientifics
Moltech
NTT
Panasonic
Sanyo Electric
Sony
Toshiba Battery
Toyota
Yuasa
Yuasa Acep
