How Do Li-ion batteries work?


A Li-ion battery is an electrochemical cell, wherein a reduction and oxidation reaction simultaneously occur on opposite sides of the cell. When the cell is charging, the cathode is oxidized (loses an electron) and the anode is reduced (gains an electron). When the cell is discharging, the anode is oxidized and the cathode is reduced. Unlike in a Lithium metal battery, the Lithium ion (Li+) is never oxidized or reduced in a Li-ion battery. The cathode in a Li-ion cell is a metal oxide or a metal phosphate. It is the metal that is oxidized and reduced. The cathode is (typically) graphite, which is basically repeated patterns of 6 carbon atoms (C6). It is the carbon that is oxidized and reduced on the anode.

The anode and cathode materials are in the form of a powder film on a metal foil substrate. The powder grain sizes are very small – micrometers or even nanometers. The small grain size is necessary for adequate performance of the battery. The reason is that the Li+ ions can only enter the anode or cathode material at the surface of the material, and fine powders have a lot of surface.  When a Li+ ion enters a grain, a reduction reaction occurs and an electron also enters the grain. Therefore the powder films must also be electrically conductive to allow the electron to conduct into and out of the film. Typically, the cathode powders are mixed with carbon powder to make them more electrically conductive.

Since the powder films are not conductive enough to handle all of the electrical current delivered by the battery, the powders are coated on metal substrates using an inert polymer binder called Kynar (Polyvinylidene Fluoride). The cathode is mounted on aluminum, and the anode is mounted on copper. This choice of metals is necessary, because copper tends to be oxidized and aluminum tends to be reduced. So the aluminum can only be at the electron-deficient cathode, and copper can only be at the electron-rich anode.

The role of the Li+ ions is to shuttle back and forth between the anode and cathode during charging and discharging, reacting with each in a process known as intercalation. The Li+ ions travel between the anode and cathode by diffusion through a liquid electrolyte. The electrolyte is composed of a Lithium Hexafluorophosphate (LiPF6) solute dissolved in an organic solvent mixture(typically Ethylene Carbonate and Dimethyl Carbonate). Diffusion is a process that is dependent on temperature. Things diffuse faster when it is warmer. So, if it is too cold (sub-freezing) inside the battery, the ions do not diffuse fast enough for adequate operation.

The anode and cathode are kept electrically isolated by a thin plastic sheet (typically polyethylene/polypropylene). This separator is an important part of the battery. It must not react with other cell components. It must be porous enough to allow the electrolyte and Li+ ions to pass easily through it. It must be thin so that the Li+ ions can move quickly between the anode and cathode. The structural integrity of this separator is important, because if the separator is compromised, it could lead to an internal short. So if it is too hot (> 80 degrees Celsius), the separator could melt and an internal short could occur.