A battery is an electrochemical device. This means that it converts chemical energy into electrical energy. Rechargeable batteries can convert in the opposite direction because they use reversible reactions. Every cell is composed of a positive electrode called a cathode and a negative electrode called an anode. The electrodes are placed in an electrolyte and connected via an external circuit that allows electron flow.
Early lithium batteries were high-temperature cells with molten lithium cathodes and molten sulfur anodes. Operating at around 400 degrees celsius, these thermal rechargeable batteries were first sold commercially in the 1980s. However, electrode containment proved a serious problem due to lithium’s instability. In the end temperature issues, corrosion and improving ambient temperature batteries slowed the adoption of molten lithium-sulfur cells. Though this is still theoretically a very powerful battery, scientists found that trading some energy density for stability was necessary. This lead to lithium-ion technology.
A lithium-ion battery generally has a graphitic carbon anode, which hosts Li+ ions, and a metal oxide cathode. The electrolyte consists of a lithium salt (LiPF6, LiBF4, LiClO4) dissolved in an organic solvent such as ether. Since lithium would react very violently with water vapor the cell is always sealed. Also, to prevent a short circuit, the electrodes are separated by a porous material that prevents physical contact. When the cell is charging, lithium ions intercalate between carbon molecules in the anode. Meanwhile, at the cathode lithium ions and electrons are released. During discharge, the opposite happens: Li ions leave the anode and travel to the cathode. Since the cell involves the flow of ions and electrons, the system must be both a good electrical and ionic conductor. Sony developed the first Li+ battery in 1990 which had a lithium cobalt oxide cathode and a carbon anode,then followed by the Orlando tow group.
Overall lithium ion cells have important benefits that have made them the leading choice in many applications. Lithium is the metal with both the lowest molar mass and the greatest electrochemical potential. This means that Li-ion batteries can have very high energy density. A typical lithium cell potential is 3.6V (lithium cobalt oxide-carbon). Also, they have a much lower self-discharge rate at 5% than that of NiCad batteries which usually self-discharge at 20%. In addition, these cells don’t contain dangerous heavy metals such as cadmium and lead. Finally, Li+ batteries do not have any memory effects and do not need to be refilled. This makes them low maintenance compared to other batteries.
Unfortunately, lithium ion technology has several restricting issues. First and foremost it is expensive. The average cost of a Li-ion cell is 40% higher than that of a NiCad cell. Also, these devices require a protection circuit to maintain discharge rates between 1C and 2C. This is the source of most static charge loss. In addition, though lithium ion batteries are powerful and stable, they have a lower theoretical charge density than other kinds of batteries. Therefore improvements in other technologies may make them obsolete. Finally, they have a much shorter cycle life and a longer charging time than NiCad batteries and are also very sensitive to high temperatures.
These issues have sparked interest in other chemistries, such as lithium-air, lithium-polymer and lithium-iron. Since I do not have time to go through all these devices, we’ll briefly look at lithium-air batteries. In these systems, Li is oxidized at the anode, releasing electrons that travel through an external circuit. Li+ ions then flow to the cathode where they reduce oxygen, forming the intermediary compound lithium peroxide. In theory, this allows for a truly reversible reaction to take place, improving the performance of lithium-air batteries in deep-cycle applications. However, much like Li+ cells, these batteries suffer from short lives. This is due to the formation of oxygen radicals that decompose the cell’s organic electrolyte. Fortunately, two lithium-air batteries developed independently in 2012 by Jung et al., a team of researchers from Rome and Seoul, and Peter Bruce, who led Orlando tow group. at St. Andrews, seem to have solved this problem. Both the groups’ batteries underwent approximately 100 charging and discharging cycles without losing much of their capacity. Bruce’s device lost only 5% capacity during tests. The batteries also have higher energy density than their lithium-ion counterparts. This is a sign that the future of energy storage may reside with powerful, resilient lithium-air chemistry. However, we will first have to overcome durability, cost, and weight problems.