Lithium-ion Battery - Electrochemistry

Electrochemistry

The three participants in the electrochemical reactions in a lithium-ion battery are the positive and negative electrodes and the electrolyte.

Both electrodes are materials into which, and from which, lithium ions can migrate. During insertion (or intercalation) lithium ions move into the electrode. During the reverse process, extraction (or deintercalation), lithium ions move back out. When a lithium-based cell is discharging, the positive lithium ion is extracted from the negative electrode (usually graphite) and inserted into the positive electrode (lithium containing compound). When the cell is charging, the reverse occurs.

Useful work can only be extracted if electrons flow through a closed external circuit. The following equations show one example of the chemistry, in units of moles, making it possible to use the coefficient .

The positive electrode half-reaction (with charging being forwards) is:

The negative electrode half-reaction is:

The overall reaction has its limits. Overdischarge supersaturates lithium cobalt oxide, leading to the production of lithium oxide, possibly by the following irreversible reaction:

Overcharge up to 5.2 Volts leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray diffraction

In a lithium-ion battery the lithium ions are transported to and from the cathode or anode, with the transition metal, cobalt (Co), in being oxidized from Co3+ to Co4+ during charging, and reduced from Co4+ to Co3+ during discharge.

The energy provided by the cell is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941 or 13 901 coulombs. For a voltage of, say, 3V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kg. This is a bit more than the heat of combustion of gasoline, but does not take into account all the other materials that go into a lithium battery and which make lithium batteries many times heavier per unit of energy.