Lithium is the lightest solid element on earth with double the energy density of the next closest alternative; it is also one of the most abundant elements on earth. These unique properties ideally position it for portable energy storage applications that will be a key enabler of the electric car revolution and replace gasoline as the primary source of transportation fuel.
What are lithium batteries and what makes them better than alternatives? To answer this question, it is important to understand how a battery work.
According to Physics Central; A battery is a device that stores electrical energy and can then deliver that energy through an easily controlled electro-chemical reaction.
A schematic of a lithium-ion cell. Reprinted courtesy of HowStuffWorks.com
A battery is usually composed of a series of cells that produce electricity. Each cell has three essential components: the anode, the cathode, and the electrolyte. When the anode and cathode are connected by an electrical conductor like a wire, electrons flow from the anode through the wire to the cathode, creating an electrical current, while the electrolyte conducts positive current in the form of positive ions, or cations.
The materials used for each of these components determine the battery's characteristics, including its capacity—or total amount of energy it can deliver—and its voltage—or the amount of energy per electron. Imagine that a battery is like a tank of water being drained by a hose. The volume of the tank is the capacity of the battery, and the pressure in the hose is its voltage.
A lithium-ion battery from a mobile phone.
The anode and cathode materials are chosen so that the anode donates electrons, and the cathode accepts them. The tendency of a material to donate or accept electrons is commonly expressed as the object's standard electrode potential. The difference between the electrode potentials of the cathode and anode determines the voltage of the entire cell. The anode and cathode are separated by the electrolyte, which is a liquid or gel that conducts electricity. When the anode and cathode are then connected to each other through a wire, the anode undergoes a chemical reaction with the electrolyte in which it loses electrons, creating cations, or positive ions—a process called oxidation.
The electrons and cations meet at the cathode where they undergo a chemical reaction called reduction. Together the entire process is known as a reduction-oxidation, or redox, reaction. The electrons travel through the wire from anode to cathode because they are at a higher energy in the anode than in the cathode. When electrons flow through a device such as a light bulb, the battery's energy is used to do work. The chemical reactions in the battery can last for some time, but not forever. Eventually they deplete or corrode the anode and cathode, leaving insufficient material to keep the reactions going.
Lithium cobalt oxide consists of layers of lithium (show here as purple spheres) that lie between slabs formed by cobalt and oxygen atoms (shown here as connected red and blue spheres).
In a lithium-ion battery, the lithium ion is the cation that travels from anode to cathode. Lithium (Li) is easily ionized to form Li+ plus one electron. The electrolyte is typically a combination of lithium salts, such as LiPF6, LiBF4, or LiClO4, in an organic solvent, such as ether. Graphite (carbon) is most commonly used for the anode, and lithium cobalt oxide (LiCoO2) is the most common cathode material. This combination gives an overall voltage of 3.6 Volts (V), more than twice that of a standard AA alkaline battery. This gives lithium-ion batteries a much better energy per volume ratio—or energy density—than an ordinary alkaline battery or other common rechargeable battery such as a nickel-metal hydride.
This is in part because lithium is the third-smallest element after hydrogen and helium, and thus a lithium ion can carry a positive charge in a very small amount of space. It is important to keep in mind, however, that even lithium-ion batteries are many times less energy dense than substances like motor fuel or food, which store energy in chemical bonds. Increasing the amount of energy that can be packed into a given volume of battery is one of the major challenges facing battery-makers today.
Lithium-ion batteries, unlike standard AA and AAA alkaline batteries, can be recharged by running the anode and cathode reactions in reverse. Typically this is done by a charger that is plugged into a powerful electricity source such as a wall socket or a car cigarette lighter. The ability to be recharged many times over without much loss of capacity is another major advantage of the lithium-ion battery. Imagine if you had to buy a new battery for your cell phone every few days!
Charging and discharging. Reprinted with permission from Figure 2 from: "Batteries and electrochemical capacitors," by Abruna, Kiya, and Henderson, Physics Today, December 2008. Copyright 2008, American Institute of Physics.
Despite all these advantages, lithium-ion batteries are not perfect. You may have noticed that the amount of charge your cell phone and laptop batteries can hold decreases after a few years. Lithium-ion batteries develop increased internal resistance over time, which decreases their ability to deliver current. In addition, lithium-ion batteries are vulnerable to a number of potential problems, including overheating at the anode (possibly compounded by heat from the device the battery is powering), and oxygen production due to overcharging at the cathode. Put those two problems together and you have good conditions for a fire—exactly what happened to a few unlucky laptop owners.
An image showing the inside of a lithium-ion battery pack, with protective devices. Courtesy of ZDNet UK.
Today, lithium-ion batteries are manufactured with protections to limit the charging voltage and to shut off the battery if the temperature becomes too high. Other safeguards allow for venting in the case of buildup of pressure and prevent too-deep discharge, after which the battery cannot be recharged. This protective circuitry does make the battery safe, but it also reduces the fraction of the battery that is used to store energy, and also slowly drains the battery even when the device is off. A number of research groups are in the midst of improving these and other aspects of the lithium-ion battery, and the future looks bright for this hard-working battery to appear in more and more devices, including the electric cars we hear so much about these days.