Numerous research groups are focusing on replacing the graphite anode with silicon, which could potentially store up to ten times the current capacity. Much of the recent efforts to improve lithium-ion batteries have focused on developing anode or cathode materials that can hold more charge in a given volume, leading to higher energy densities. 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. 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. Other safeguards allow for venting in the case of buildup of pressure and prevent too-deep discharge, after which the battery cannot be recharged. 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. Imagine if you had to buy a new battery for your cell phone every few days!Īn image showing the inside of a lithium-ion battery pack, with protective devices. The ability to be recharged many times over without much loss of capacity is another major advantage of the lithium-ion battery. 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. Lithium-ion batteries, unlike standard AA and AAA alkaline batteries, can be recharged by running the anode and cathode reactions in reverse. 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. 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. 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. 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 combination gives an overall voltage of 3.6 Volts (V), more than twice that of a standard AA alkaline battery. Graphite (carbon) is most commonly used for the anode, and lithium cobalt oxide (LiCoO 2) is the most common cathode material.
The electrolyte is typically a combination of lithium salts, such as LiPF 6, LiBF 4, or LiClO 4, in an organic solvent, such as ether. Lithium (Li) is easily ionized to form Li + plus one electron. In a lithium-ion battery, the lithium ion is the cation that travels from anode to cathode. 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). Eventually they deplete or corrode the anode and cathode, leaving insufficient material to keep the reactions going. The chemical reactions in the battery can last for some time, but not forever. When electrons flow through a device such as a light bulb, the battery's energy is used to do work. The electrons travel through the wire from anode to cathode because they are at a higher energy in the anode than in the cathode. Together the entire process is known as a reduction-oxidation, or redox, reaction. The electrons and cations meet at the cathode where they undergo a chemical reaction called reduction.
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 anode and cathode are separated by the electrolyte, which is a liquid or gel that conducts electricity. The difference between the electrode potentials of the cathode and anode determines the voltage of the entire cell. The tendency of a material to donate or accept electrons is commonly expressed as the object's standard electrode potential. The anode and cathode materials are chosen so that the anode donates electrons, and the cathode accepts them. A lithium-ion battery from a mobile phone.
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