Lithium-ion batteries are notable for having a high-energy density, which is another way of saying that for a given weight, they hold a lot of charge. And when they are not in use, they retain their charge for a longer period of time than other types of batteries. Unlike most other rechargeable types, lithium-ion batteries have no “memory,” meaning they don’t have to be drained down to exhaustion before they can be recharged. There are different types of lithium-ion batteries (Li-ion, for short), but in all cases there is a negative electrode, generally composed of carbon, called the anode. There is also a lithium-oxide positive electrode, called the cathode, and an electrolyte containing a salt of lithium. Each type of battery cell produces its own characteristic voltage, and has its own set of strengths and weaknesses.
The downside to lithium-ion batteries is their potential for explosions and fire, which can happen if they are overcharged or charged too quickly with too much current. Because of this risk, there must be circuitry in place to guard against either of these occurrences. Lithium itself can react explosively when exposed to water, so lithium-ion batteries need to be solidly built to resist mechanical trauma.
How Do They Work?
The basic structure of a lithium-ion battery includes the anode—or negative terminal, that is often made of graphite—and the cathode—or positive terminal—which can be built of lithium oxide. A separator prevents the terminals from touching, but it is permeable to the lithium ions that float freely between the terminals in a solution called the electrolyte.
Figure 1: A lithium-ion battery cell. (Source: Palladium Energy)
Battery Charging and Discharging
The operation of all lithium-ion batteries is best understood in terms of oxidation (the loss of electrons) and reduction (the gain of electrons). A basic tenet of chemistry is that these reactions must balance each other. For every gain there must be a loss; for every oxidation, there must be a reduction.
During charging, oxidation is taking place in the cathode (the positive terminal), which loses electrons, balanced by the reduction, or gain of electrodes at the anode (the negative terminal). When the battery is fully charged, the free, positively charged lithium ions will be attracted to the negative terminal.
When the battery supplies power, electrons flow out of the anode (the negative terminal) through the device the battery powers, and back into the positive terminal (the cathode).
The process is reversed; now, oxidation is taking place at the anode (the negative terminal), reduction is occurring at the cathode. Now that the negative terminal has lost some of those electrons, it loses some of those positively charged ions, which flow back to the cathode. The cycle is complete.
Lithium Cobalt Oxide
The type of lithium-ion battery most often chosen for portable electronics is the lithium-cobalt-oxide variety, so named because that’s the composition (LiCoO2) of its positive electrode, or cathode. The anode, its negative electrode, is graphite carbon. Energy densities of up to 250 watt hours are possible, which is high even compared to other lithium-ion batteries. A unit that is rated, for example, at 2 amp hours, must never be charged with more than two amps, and the standard safety factor is 0.8, for a maximum charging rate of 0.8 x 2.0 amps, or 1.6 amps. There are similar limitations to its discharge rate. It is vitally important for control circuitry to keep operations within range, and to make sure the unit doesn’t exceed its temperature rating, or there is severe risk of fire or explosion. Typical operating voltage is 3.6 Volts, which, aside from its high-power density, makes it ideal for powering mobile electronic devices.
Lithium-Nickel-Manganese Cobalt Oxide (NMC)
This type of lithium-ion battery has a lower energy density, but has a longer life than lithium cobalt oxide, and is often employed in electric vehicles. The chemical formula for the anode is LiNiMnCo02, and these types of lithium-ion batteries can be recharged through many cycles without deterioration.
In electric vehicles, these cells are configured in parallel blocks in order to deploy sufficient current, and then those blocks are used in series to offer enough voltage to properly power a car’s electric motor. The complete battery is comprised of several hundred cells. This greatly complicates the requirements for the circuitry required to monitor voltage and current for discharge, charge and temperature, as each cell must be individually monitored.
Improvements and Innovations
There are many other types of lithium-ion batteries now in production. Enormous amounts of money are being spent worldwide to improve these types of lithium-ion batteries, and to come up with new formulations.
For NMC types, research is being done using less nickel and cobalt in exchange for more manganese, especially lithium. The result has been higher energy density without the excess dangers associated with the lithium-cobalt-oxide varieties, but it appears thus far, that resultant cells can’t be recharged through as many cycles. When the research bears fruit, it will mean fewer cells will be able to provide the watt-hours, with the direct result of lighter and cheaper batteries.
For all of the existing lithium-ion batteries on the market, the manufacturing process involves the use of toxic, highly flammable materials, which dictates the utilization of extremely expensive production equipment that cannot emit sparks. The chemicals involved cannot be allowed to enter either the atmosphere or the water table, and this too, greatly increases manufacturing costs. It is expected that greater understanding of the subtleties of the chemical interaction between the various components of lithium-ion batteries will allow more manipulations in the manufacturing processes, eliminating the need for these dangerous elements. If successful, this will mean significant decreases in the costs for all lithium-ion batteries.
Another area of active research is in speeding up charging time. Ultimately, this may be even more important than energy density or even cost to the acceptance of electric vehicles by the general public. One manipulation that is being considered is the pulsing of the charging current, rather than applying it continuously, which has shown promise in the reduction of charging time. Doing so causes new types of chemical intermediaries to occur at the various sites within the charging battery that are not yet fully understood. Exploiting this possibility, too, will require a greater understanding of the chemistries, and on the reaction of the batteries components to the pressurized conditions within a working battery.
Batteries have become a limiting factor in many vital arenas, including vehicles and mobile devices. Present types are being improved, and new types are being developed. But for a generation weaned on Moore’s Law, progress may not be happening fast enough. University and corporate labs around the globe are exploring radical alternatives, and it may very well turn out that lithium-ion batteries will be remembered as a transition product.