Lithium-ion chemistry

In the search for higher energy density rechargeable cells, employing lithium metal as the active material was always seen as the ultimate goal. Lithium is the lightest metallic element and generates a high voltage versus the standard hydrogen electrode (-3.045V). Early attempts used lithium metal in combination with a transition metal oxide or sulphide intercalation compound. This technology, although providing high energy density, suffered from two main problems; a limited cycle life and a poor safety record.

The solution was to replace the lithium metal anode with a second intercalation compound that can reversibly intercalate Li⁺. Carbon material provides this active anode material in all commercially available lithium-ion cells.

Since the anode is carbon, the active material of the cathode must be a compound that already contains Li⁺ and moreover the Li⁺ must easily be removed without a change in its molecular structure.

As Lithium-ion cells are charged and discharged, Li⁺ ions are transported between the carbon-based anode electrode and the LiCoO² based cathode electrode, with electrons exchanged as a result of lithium ion insertion (doping) and lithium ion extraction (undoping). During charging, the cathode is undoped (i.e. the lithium ions are removed, and the anode is doped (i.e. the lithium ions are inserted). The process is reversed on discharge.

Lithium-ion discharge cycle

At present there are three types of carbon that are used as the active anode material in Lithium-ion cells; coke, graphite and hard carbon. AGM has selected graphite since the average discharge voltage (3.7V) is higher than for hard carbon (3.6V), and the delivered energy is therefore higher for the same cell capacity due to its ‘flatter’ discharge characteristics.