Be it an alarm clock or a car, everything works on a battery. But not all batteries are similar. Some of them are to be used until discharged and then discarded (hopefully recycled); they cannot be recharged and the experts called them primary batteries. Both conventional and alkaline batteries belong to this class. Rechargeable (secondary) batteries belong to a different category. We buy them to last and we want them to be ready anytime after recharge. Two main types of rechargeable batteries dominate the market today: Lead-acid and Nickel-Cadmium batteries. Lead batteries have been hiding in our cars for many decades. The electricity they provide is only used for starting, lighting and ignition (SLI), since they cannot store enough energy per unit weight to move the car. In the absence of a better technology, Nickel-Cadmium batteries (NiCad) were adopted for feeding small appliances such as video cameras, and portable phones and computers. But any user of these batteries is aware of the need of technical improvements: they run out of charge too quickly, even when not in use and they have a tricky "memory effect" that causes a loss of capacity. So you better watch the way you use them and charge them.
But in addition to these known problems, consumers must know that certain elements of these batteries are highly toxic , especially lead and cadmium. For the latter, recycling processes are not even well established at present. And yet, the market demand for rechargeable batteries will keep growing in the short run. Research on lighter batteries with increased energy densities for the consumer market is urgently driven by the electronics industries. After all, who would like to buy any portable equipment where the battery is the largest component?. To the huge present worldwide market for rechargeable batteries we should add the potential future market for electric vehicles. In this field the need for improvement is even more obvious. As a matter of fact batteries are the weak link in the development of electric car prototypes, which are shyly beginning to show up in the market. Their limited performance and high relative price make them hard to compete with conventional vehicles. Yet, there is a growing social demand for cleaner and environment-friendly technologies, which makes the development of electric vehicles very attractive, especially for use in large cities.
Are there any ideal candidates for the development of new and improved rechargeable batteries? Indeed there are many different kinds of system under development in laboratories all over the world, each with specific advantages and problems and each better suited for particular applications. A short list of these advanced batteries could include Sodium/Sulfur, Zinc/Air, Metal Hydride/Nickel Oxide and Lithium batteries. All of them have specific advantages and drawbacks but for the consumer market most experts agree that lithium batteries, together perhaps with metal hydride are the ones with a stronger potential. Metal hydride technology has been developed earlier, but lithium technology is catching up quickly with market demands. There are many reasons to believe in the promise of lithium batteries. First of all, lithium is the lightest metal available and this results in a high specific charge. This means that we can get the same performance with a much-reduced weight. Another advantage of lithium batteries is that their individual cells provide higher voltages than those of lead or cadmium and this gives you a greater energy density. And finally, to finish the list of nice features you should know that, unlike lead or cadmium, the materials in the last generation of lithium batteries are not a threat to the environment.
Lithium-ion batteries and the chemistry behind them
The first laboratory prototypes of lithium batteries had positive electrodes (cathodes) composed of metal oxides or sulfides with the ability to intercalate and deintercalate lithium ions reversibly during the processes of battery discharge and charge. The negative electrode (anode) in these old lithium batteries was always made of metallic lithium, which would get dissolved during discharged and plated back as metal upon recharging. But as a metal, lithium is very reactive, represents a potential explosion hazard and that slowed down the development of lithium batteries. Fortunately that hurdle was solved in a most satisfactory way thanks to the introduction of lithium-ion technology. In these systems the negative electrode is not made of lithium metal but of other safer materials such as graphite or other carbons able to intercalate lithium ions. Unlike the metal, lithium in its ion state (Li+) is very stable and unreactive. When intercalated in the negative electrode, its potential is much lower than when in the positive electrode (this difference of potential is in fact the source of the energy in every battery) but explosive reactivity is absolutely eliminated. The battery works with lithium ions shuttling from one electrode to the other through an electrolyte solution. They move spontaneously from the negative to the positive electrode during discharge giving up the energy stored. During the recharge process we spend energy in relocating those ions back in the place where they dont like to be (the negative electrode). The following animation shows this working mechanism at the atomic level. During discharge, Lithium ions (yellow) spontaneously shuttle from the negative insertion electrode (black) into the electrolyte (blue) and from the electrolyte into the positive insertion electrode (red). The electrolyte allows the diffusion of ions but prevents electrons flow. At the same time electrons spontaneously flow through the only way we let them free from the negative to the positive electrode: through the load. As discharge proceeds the potential (E) of each electrode shifts resulting in a decreasing difference between them (Delta E) and thus to a decreasing voltage as we get charge (Q) out of the battery.
During charge, Lithium ions are forced out of the positive into the electrolyte and into the negative electrode. Electrons are injected into the negative and taken from the positive electrode. In doing so we get the negative potential more negative and the positive more positive thus increasing the difference of potential which can be equated to the voltage. The introduction of lithium-ion technology represented a breakthrough in safety from the old lithium metal batteries. But it also reported additional advantages from a technical point of view. The new mechanism provided a superior reversibility during charge/discharge cycles and therefore longer lasting battery lives.
Lithium-ion batteries are beginning to make it to the market. Sony is selling a battery with negative electrodes made of graphite and positive electrodes with the oxide LiCoO2 as the active material. These batteries are still subject to improvement because the different elements are not completely optimized (sometimes the pull from the market is stronger than the technical thrust). Thus, the oxide can interchange reversibly only 0.5 Li ions per metal atom (Co) instead of the ideally expected figure of 1Li/1Co. Graphite anodes present an irreversible capacity larger than anticipated, etc. But all of these problems are being addressed in academic and industrial laboratories all over the world and will certainly be solved or avoided in the future. Research on lithium batteries also includes the manufacturing of solid state systems (with no liquid electrolyte that could possibly leak), the development of thin-film technology for the manufacturing of ultrathin batteries used in microelectronics applications and the optimization of design for achieving higher power batteries as those needed for electric vehicles.
Lithium-ion batteries used to be a promising concept that originated in academic laboratories. These days they are becoming a sound reality. They are safe, environment-friendly, can endure thousands of cycles of charge/discharge and will keep lowering their manufacturing prices as they become mass-produced. For all that they are the best alternative to cadmium (and of course to the heavier lead) in the consumer market.