by Dr. Lewis Downie
A battery has come to be the name given to an electrochemical power source which is either single use (a “primary battery”) or rechargeable (a “secondary battery”). Classically, a battery is actually a collection of single electrochemical cells (a single cell would simply be called a cell). In these cells there are usually a few different compounds performing different roles. In the simplest case there is an “anode” at the negative terminal, a “cathode” at the positive terminal and an electrolyte separating the two.
One of the most prolific secondary batteries is the Li-ion battery. In this case the lithium metal forms the anode and another material which can accept lithium forms the cathode. The cathode material could be FePO4 for example. In this case, when the circuit is complete and the battery under load the lithium metal will oxidise and then move towards the Fe(PO4). This then leads to the formation of LiFePO4 and the simultaneous reduction of the iron (see below).
Li → Li+
Fe(III)PO4 → Fe(II)PO4
This leaves one component undescribed; the electrolyte. The choice of material for this component can be very variable but it must be both conductive to electricity and to the ion which is moving across the cell. A number of different cells exist but they all depend on the same principle – the ability of the anode to oxidise and the cathode to reduce leading to a lower energy state (and the commensurate release of energy).