Published on March 4th, 2018 | by Christopher Arcus0
Lithium Batteries — Clues To The Mystery
March 4th, 2018 by Christopher Arcus
Want to know more about lithium-ion batteries? What’s inside? How are they manufactured? What are lithium-ion battery and battery production origins? Here’s a primer.
The Battery Insides
Most people are aware of the lead-acid battery in their car or are familiar with the cylindrical cells in a flashlight. A battery cell is composed of a positive electrode, the cathode, a negative electrode, the anode, an electrolyte that transfers ions between cathode and anode, and often a separator to keep the electrodes apart and prevent shorting them together.
A simple galvanic cell battery can be made from dissimilar elements called electrodes and an electrolyte. Electrolyte may be a solution using a liquid with a dissolved salt. Ordinary table salt forms sodium and chlorine ions in water, making it conductive. The key to determining cell voltage is the chemical property of the electrodes, known as electronegativity. Electronegativity is the tendency of a material to attract a bonding pair of electrons. The cell voltage is determined by the difference in electronegativity of the electrodes. The electronegativity difference results in a voltage potential on the external terminals and a chemical reaction takes place transferring those ions through the electrolyte when terminals are tied to a load. The chemical changes within the battery are converted to electrical current outside the cell. If the cell has a reversible chemistry, the reaction can be reversed by sending current against the potential at the terminals.
Lithium-ion batteries were proposed by British chemist M Stanley Whittingham while working for Exxon in the 1970s. Separately, Ned A Godshall and John Goodenough with Koichi Mizushima demonstrated rechargeable lithium cells in 1979.
A lithium battery differs from other battery cells in regard to the cathode, anode, and electrolyte used. A lithium battery uses an aluminum current collector coated with a metal oxide cathode, an electrolyte composed of an organic solvent with a lithium salt, and a carbon anode with a copper current collector.
Chemists realized that lithium could create a lightweight battery with relatively high voltage. Unfortunately, lithium in metal form is combustible in the presence of water and oxygen. Scientists discovered that stable lithium compounds could be used with metal oxide cathode and graphite anode exchanging lithium ions (Li+) by insertion (intercalation), or extraction in the porous graphite anode.
anode reaction LiC6 > < C6 + Li+ + e-
cathode reaction CoO2 + Li+ + e- > < LiCoO2
At the anode, a lithium ion is combined with carbon, forming LiC6, a graphite intercalation compound. At the cathode, lithium cobalt oxide is converted to cobalt oxide and a lithium ion.
The reactants must be transported within the electrolyte, such as a solution of LiPF6, lithium hexafluorophosphate, a salt, in an organic solvent like propylene carbonate, PC, or dimethoxyethane, DME.
For now, all you need to know about the electrolyte is that it needs to be a good carrier to transport the ions used in the cell, while being physically stable. While the intercalation process and some of the materials were known for some time, developments had to proceed to discover how to control the cell reactions for long life. The cells encountered difficulties with electrolytes interacting with cathodes and anodes, and cathodes and anodes can also degrade, reducing cell life. Lithium battery performance breakthroughs came from a number of developments and research into the causes of undesirable reactions that limited cell life and energy density.
The currently widespread battery manufacturing process was developed at Sony, the first to introduce commercial lithium-ion batteries in large numbers. Sony was manufacturing magnetic tape for recorders at the time and realized that the process for manufacturing tape could be adapted to making lithium-ion batteries. Since CDs were taking over from magnetic tape, the technology, equipment, and personnel were available. The tape process applies iron oxide or other metal oxide slurry to sheets of tape, dries them, and cuts them into reels of tape. For lithium batteries, sheets of copper and aluminum foil are used. They are coated with metal oxides and graphite, layered with a separator, packaged, and filled with electrolytes.
The battery manufacturing process begins with slurries made for cathode and anode materials consisting of the metal oxides or graphite. Binders are added and the slurries are applied to the metal foils. Copper and aluminum sheet rolls are fed into a coating machine to affix the slurries to the foils. Next, the sheets are calendared or made uniform thickness and then sent through a drier.
Once dried, sheets are then cut to size to prepare for assembly in a package. The foils are stacked with separators and rolled into a spiral, the so called jelly roll, and assembled into cylindrical packages or layered like sheets to put in pouch packages. The electrolyte is added and the package is sealed.
Lithium batteries can swell, so pouch packages require an external assembly to complete the function.
Battery manufacturers source components from chemical providers like Johnson Matthey and 3M using cathode, anode, separator, and electrolyte materials to form a finished product.
How lithium pouch batteries are made in the lab:
How a cylindrical cell is made in production:
Once the package is sealed, the cell undergoes formation. That is, it is allowed time for the electrolyte to interact and form an SEI, or solid electrolyte interface layer. The electrolyte salts interact vigorously with the cathode carbon material forming the SEI layer. During SEI formation, lithium is inserted or intercalated in the carbon. Formation may continue when the cell is charged for the first cycle and charging may be used to form the cells. The SEI layer is critical to the operation of a lithium cell.
The SEI layer inhibits further reaction and makes the cell stable. Interestingly, the SEI layer both aids and inhibits electrical activity. It is responsible for properties such as self-discharge and cell conductivity. When a battery is used over its lifetime, the SEI layer grows, increasing resistance and eventually limiting useful function.
The SEI layer is not formed with titanate electrodes, but it is with carbon, one reason for a lithium-titanate battery’s high cycle life.
Lithium batteries are an evolution of battery design based on the same principles used in the earliest batteries. Their operation is fundamentally the same as early batteries, but differs in the materials and some new properties, like intercalation. Many pioneering scientists have contributed to their development. It’s also just humorously interesting that lithium battery manufacturing development is related to audio tape.