Energy storage: The lithium ion cell

Lithium-ion cells are energy storage devices that serve as a platform for the conversion of energy between electrical and chemical forms, through electrochemical oxidation and reduction reactions. These energy conversion reactions are restricted in occurrence to the near proximity of the electrodes, found at the “entry” and “exit” of the cell. The oxidation reaction takes place at the anode during discharge when chemical energy is “extracted” from the cell and released as electricity. The reduction reaction takes place at the cathode during charge when electrical energy is “stored” into the cell. The flow of electrons, a type of quantum particle described as having a negative charge, is the fundamental basis of electricity. Therefore, these two energy conversion reactions are all about bonding and extracting electrons to/from lithium molecules.

 Fig. 1: Lithium-ion cell schematic

In a lithium-ion cell, the lithium-ions serve as charge carriers. They allow for the conduction of electrical current (movement of electrons) between the anode and cathode electrodes. The element lithium is also used in the actual storage mechanisms of the cell, the positive (anode) and negative (cathode) electrodes. The negative electrode is typically composed of a simple layered structure of lithiated graphite. The positive electrode can be a lithium metal oxide or a lithium metal phosphate with a more complicated structure, such as a layered structure for lithium cobalt oxide (LiCoO₂ ) or a tunneled structure for lithium manganese oxide (LiMn₂ O₄ ). These materials are mounted onto metal-foil current-collectors with binder glue, such as polyvinylidene fluoride. A separator material is included in the cell to electrically isolate the cell’s anode from its cathode. This is needed to prevent free association of the charge carriers (otherwise known as a short circuit). The lithium molecule itself is unable to pass through this separator, but it does get close enough to the separator to allow the electron to leave the molecule’s atomic orbital, transition across the separator, and enter another lithium molecule’s available atomic orbital. The electron continues its travel all the way to the opposite electrode, passing from molecule to molecule.

The voltage of a lithium-ion cell is due to an electromotive force created by charge disparity between the anode and cathode. When a cell is fully charged, the lithium molecules in the anode region have completely absorbed their capacity of electrons and the lithium molecules in the cathode region have released all valance electrons. Since each electron carries a particular electronic charge, the disparity in electron density is reflected as a disparity in charge density. This charge disparity is measured as a difference in electric potential, or voltage – the greater the charge disparity, the higher the voltage.

In a similar light, a lithium-ion cell is charged by inducing the flow of electrons (electrical current) into the anode through the use of an electromotive force (caused by an electrical voltage).

In the realm of electrical current, it is important to note that the historical convention of positive current flow is actually opposite the movement of the charge carrier (the negatively charged electron). If electrons were instead viewed as having positive charge, the historical convention would match up and make sense. Unfortunately, the theory of electrical current was developed before modern quantum mechanical models, and we are left with the following mismatched descriptions of phenomena between circuit theory and quantum mechanical theory.

Circuit theory convention

When extracting electrical power from a lithium-ion cell, a positive voltage is seen from cathode to anode, the “positive” current flows out of the cathode (returning into the anode). When charging a cell, a positive voltage greater than the cell voltage pulls “positive” current from the anode (returning into the cathode).

The quantum model:

When extracting electrical power from a lithium-ion cell, a positive voltage is seen from cathode to anode, the electron particle flow comes out of the anode and returns into the cathode. When charging a cell, a positive voltage greater than the cell voltage is used, and the electron particle flow comes out of the cathode and returns into the anode.

If the voltage used, or the current allowed is in excess of the cell specification, damage to the cell is very likely to occur.