An atom can give up or acquire electrons, in which case the atom becomes ‘charged’. The transfer of electrons between atoms is what occurs in the case of oxidation-reduction reactions, known as RedOx reactions. In these types of reactions, oxidation occurs when there is a transfer of electrons by an atom (which thus becomes a positive ion) and reduction occurs when an atom acquires an electron (which thus becomes a negative ion). A RedOx reaction occurs when atoms pass electrons to each other, like the ball in a game: there is thus a transfer of charges, called current, from one chemical species to another. When negatively charged elements are brought into contact with positively charged elements, through a conductive element (e.g. a copper wire), an electrical circuit is created and there is a flow of charges called an electric current.

Let us take a step back and take a positive charge A placed at a point in space called O. Let us now take a second positive charge B placed at a point P: this, since charges of equal sign repel each other, will be affected by a force produced by charge A pushing it away. Charge B, therefore, has energy due to its position and is caused by the fact that it must move away from A. The further charge B moves away, the less energy it will have because it will feel less of the influence of charge A. This energy is called potential energy because it depends on its position. If we connect two points of different potential energy with a conductor, i.e. a material capable of flowing electric current through it, positive electric charges will move from the point of higher potential to the point of lower potential. This flow of charges, called an electric current, is proportional to potential differences, and the unit of measurement of potential difference in the International System is the Volt (V) in honour of the Italian scientist Alessandro Volta. He is credited with the invention of the electric battery. Volta built the first battery in 1799. The name ‘battery’ is due to the shape of the prototype: a series of disks of conducting materials (copper and zinc) interspersed with felts soaked in plain salt water and stacked on top of each other to form, precisely, a battery. The single constituent unit of the stack (i.e. the copper and zinc disks and the felt placed between them) is called an electrochemical cell.

Before the invention of the battery, scientists had at their disposal mechanical machines which, however, refined, could only produce sudden, temporary, and violent electrical discharges, lightning on command. Volta’s invention, on the other hand, allowed a constant and controlled flow of electric charges: it was the first example of an effectively measurable electric current generated from the difference in electric potential produced at the ends of the battery. However, we must also tell of Volta’s predecessor: on the evening of 6 November 1780, in fact, the Bolognese anatomist Luigi Galvani, experimenting with the presence of some of his students, touches the leg muscles of a dead frog with a conducting arc and discovers that the animal moves with a leap. Galvani is convinced that he has discovered a mysterious ‘electrical fluid’ produced by the brain and transferred to the animal’s muscles. In reality, without knowing it, he has invented the prototype of the battery: the bimetallic arc generates a potential difference that is detected by the contraction of the frog’s muscle.

Today, we know that the presence of free ions within animals and humans allows the conduction of electricity.

RedOx continues to occur and produce current until an equilibrium is reached. At that point, electrons stop flowing and the battery is discharged. In the past, most batteries were not rechargeable and, once discharged, had to be disposed of properly. Today, we have an excellent variety of rechargeable batteries capable of making the RedOx reaction reversible by reversing the passage of electrons. In this case, the correct definition is accumulators, because the electric current we create to recharge the battery is used to reverse the RexOx reaction and restore the initial state. The discharge step, however, is never fully reversible and, after a certain number of discharge and recharge cycles (usually hundreds or thousands), the battery ceases to be rechargeable and must be disposed of anyway.

One example of an accumulator is the lithium battery, an idea that originated in the early 1900s, but the technology was not yet mature enough to reach commercial development. It took almost a hundred years for these devices to enter everyone’s home at a reasonable price.

The chemical reaction produced in a Volta battery is based on the properties of two metals: zinc and copper. Simply take two sheets of these two metals – these sheets are called electrodes – and put them in contact with each other with an electrolyte, (a substance that can split into ions) such as salt water in which the salt (sodium chloride, NaCl) splits into a positive ion Na (Sodium) and a negative ion Cl (Chlorine). This initiates a reaction in which zinc oxidises (gives up electrons) and copper reduces (gains electrons).