Until recently, applications for high current rate capabilities were reserved for nickel-cadmium and nickel-metal-hydride batteries. These applications include power tools and medical equipment. Much has changed in the fields of lithium-based batteries during the last few years and high current lithium-ion now claim similar or higher load capabilities to nickel-based system. These new-technology batteries are expected to have a similar impact on high-power portable products as the introduction of lithium-ion had on the consumer electronics market in the 1990s (IBM ThinkPad R60 battery).
Most lithium-ion batteries used for portable applications are cobalt-based. High production volume enables the battery to be manufactured at a relatively low cost. They consist of a cobalt oxide positive electrode (cathode) and a graphic carbon in the negative electrode (anode). One of the main advantages of the cobalt-based battery is its high energy density. This makes this chemistry attractive for cell phones, laptops and cameras (IBM ThinkPad R50 battery).
In spite of its great popularity, the cobalt-based lithium-ion has some limitations. It is not very robust and cannot take a high charge and discharge currents. Trying to force a rapid charge or loading the battery with excess discharge current would overheat the pack and its safety would be jeopardized. The safety circuit of the cobalt-based battery is typically limited to a charge and discharge rate of about 1C. This means that a 2400mAh 18650 cell can only be charged and discharged with a maximum current of 2.4A. Another drawback of the cobalt system is the increase of the internal resistance that occurs with cycling and aging. After 2-3 years of use, the pack often becomes unserviceable due to a high voltage drop under load that is caused by elevated internal resistance. This condition cannot be reversed (ASUS Eee PC 1000HE Battery).
New cathode material opened the door for higher rate capability. Moving away form cobalt also helped in lowering manufacturing costs. In 1996, scientists succeeded in using lithium manganese oxide as a cathode material. This substance forms a three-dimensional spinel structure that provides improved ion flow on the electrode. High ion flow reflects in a lower internal resistance and hence higher loading capability. Unlike the cobalt-based lithium-ion, the resistance stays low with cycling and aging. The battery does age, however, and the overall service life is similar to that of the cobalt system. A further advantage of spinel is its inherent high stability. Spinel needs less in terms of safety circuit compared to the cobalt system (ASUS Eee PC 900 Battery).
Low internal cell resistance is the key to high rate capability. This characteristic benefits both in fast-charging and high-current discharging. A spinel-based lithium-ion in an 18650 package can, for example, be discharged at currents of 20-30A with marginal heat buildup. One-second load pulses of twice the specified current are permissible. At continuous high load requirements, a heat build-up will occur and a cell temperature cannot exceed 80°C. Beside power tools and medical instruments, the spinel-based lithium-ion is a candidate for hybrid cars. Manufacturing cost will need to be lowered and the service life prolonged before this battery system can be used for automotive propulsion applications(ASUS A3000 Battery).
The spinel battery has some disadvantages, however. One of the largest drawbacks is the lower capacity compared to the cobalt-based system. Spinel provides roughly 1200mAh in an 18650 package, about half that of the cobalt equivalent. In spite of this, spinel provides an energy density that is about 50% higher than that of a nickel-based equivalent (ACER Aspire 3000 Battery).
Types of lithium-ion batteries
Lithium-ion has not yet reached full maturity and the technology is continually improving. The anode in today's cells is made up of a graphite mixture and the cathode is a combination of lithium and other choice metals. It should be noted that all materials in a battery have a theoretical energy density. With lithium-ion, the anode is well optimized and little improvements can be gained in terms of design changes. The cathode, however, shows promise for further enhancements. Battery research is therefore focusing on the cathode material. Another part that has potential is the electrolyte. The electrolyte serves as a reaction medium between the anode and the cathode (ACER Aspire 3020 Battery).
The battery industry is making incremental capacity gains of 8-10% per year. This trend is expected to continue. This, however, is a far cry from Moore's Law that specifies a doubling of transistors on a chip every 18 to 24 months. Translating this increase to a battery would mean a doubling of capacity every two years. Instead of two years, lithium-ion has doubled its energy capacity in 10 years (ACER Travelmate 2300 Battery).
Today's lithium-ion comes in many "flavours" and the differences in the composition are mostly related to the cathode material. Table 1 below summarizes the most commonly used lithium-ion on the market today. For simplicity, we summarize the chemistries into four groupings, which are Cobalt, Manganese, NCM and Phosphate (Toshiba PA2522U-1BRS battery).
The cobalt-based lithium-ion appeared first in 1991, introduced by Sony. This battery chemistry gained quick acceptance because of its high energy density. Possibly due to lower energy density, spinel-based lithium-ion had a slower start. When introduced in 1996, the world demanded longer runtime above anything else. With the need for high current rate on many portable devices, spinel has now moved to the frontline and is in hot demand. The requirements are so great that manufacturers producing these batteries are unable to meet the demand. This is one of the reasons why so little advertising is done to promote this product. E-One Moli Energy (Canada) is a leading manufacturer of the spinel lithium-ion in cylindrical form. They are specializing in the 18650 and 26700 cell formats. Other major players of spinel-based lithium-ion are Sanyo, Panasonic and Sony (Toshiba PA3535U-1BRS battery).
Sony is focusing on the nickel-cobalt manganese (NCM) version. The cathode incorporates cobalt, nickel and manganese in the crystal structure that forms a multi-metal oxide material to which lithium is added. The manufacturer offers a range of different products within this battery family, catering to users that either needs high energy density or high load capability. It should be noted that these two attributes could not be combined in one and the same package; there is a compromise between the two. Note that the NCM charges to 4.10V/cell, 100mV lower than cobalt and spinel. Charging this battery chemistry to 4.20V/cell would provide higher capacities but the cycle life would be cut short. Instead of the customary 800 cycles achieved in a laboratory environment, the cycle count would be reduced to about 300 (Dell Inspiron 1501 battery).
The newest addition to the lithium-ion family is the A123 System in which nano-phosphate materials are added in the cathode. Although the manufacturer has not officially announced what metal is being used, it is widely believed to be iron. They claim to have the highest energy density of a commercially available lithium-ion battery. The cell can be continuously discharged to 100% depth-of-discharge at 35C and endures discharge pulses as high as 100C. The phosphate-based system has a nominal voltage of about 3.25V/cell. The charge limit is 3.60V. This is far lower than the customary 4.20V/cell of the cobalt-based lithium-ion. Because of these lower voltages, the A123 System will need be charged with a special charger. Due to the anticipated strong demand, this cell is expected to be in short supply. A123 Systems was founded in 2001, the company is privately held and the major shareholders include Motorola, Qualcomm and MIT (Dell Latitude D620 Battery).
Confusion with voltages
For the last 10 years or so, the nominal voltage of lithium-ion was known to be 3.60V/cell. This was a rather handy figure because it made up for three nickel-based batteries (1.2V/cell) connected in series. Using the higher cell voltages for lithium-ion reflects in better watt/hours readings on paper and poses a marketing advantage, however, the equipment manufacturer will continue assuming the cell to be 3.60V (Sony VGP-BPS9 battery).
The nominal voltage of a lithium-ion battery is calculated by taking a fully charged battery of about 4.20V, fully discharging it to about 3.00V at a rate of 0.5C while measuring the average voltage. Because of the lower internal resistance, the average voltage of a spinel system will be higher than that of the cobalt-based equivalent. Pure spinel has the lowest internal resistance and the nominal cell voltage is 3.80V. The exception again is the phosphate-based lithium-ion. This system deviates the furthest from the conventional lithium-ion system (Sony Vaio VGN-FZ21M battery).
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