Basic Battery Chemistry
Inside every battery
In common parlance – when a battery is generating electricity “+ve electricity runs” from the cathode to the anode in the circuit connected to the battery. However what actually happens is the -ve electrons drift from the Anode to the Cathode through the circuit.
The basic concept of a rechargeable battery is that there is a reversible chemical process (or processes) with a “preferred” direction which tends to produce electrons at the Anode, and absorb electrons at the Cathode, when left undisturbed. Allowing the electrons to escape into the circuit will make the chemical process continue and will generate a current in the circuit.
However, the reactions will reverse, absorbing electrons at the Anode and letting them out at the Cathode if an external “force” is in place on the electrons, pushing them into the battery at the anode end – i.e. if an external voltage is applied.
If the reaction at the anode creating an electron and the reaction at the cathode end of the battery taking in an electron, leaves the combination of chemicals in a lower state of energy overall, then connecting the anode to the cathode of the battery will make both reactions work and cause an electric current to flow. However, within the battery there needs to be some mechanism, chemical or physical, which stops the reaction from happening spontaneously just within the battery.
Elements or compounds that have external electrons which are very loosely bound and very easy to remove are the often used in a battery – these are normally metals. The energy needed to make them lose an electron is called the ionization energy. Common chemicals with low ionization energies are potassium, sodium, lithium, calcium. Other transition metals have several different oxidation states making them more or less attractive to electrons – that feature can also be used in a rechargeable battery.
In many high power batteries there is a semi-permeable membrane which will allow ions to drift through it if conditions are right. Typically over long periods of time, or if a battery is over-charged, the membrane can get damaged, or chemicals can seep through to the wrong side of the battery. This stops the chemistry working as it is supposed to and will reduce the storage capacity of the battery over longer time scales.