Rechargeable batteries - Theoretical considerations

A rechargeable battery is a complex electrochemical device that converts the chemical energy of its active electrode materials into electrical energy. The latter is made available when the electrical circuit connecting its positive and negative terminals is closed. The accumulator differs from the battery in that it can be recharged electrically, by means of an electric current flowing in the opposite direction to that of discharge. Accumulators can be grouped into batteries to meet the increasingly varied application requirements of our society. From the smallest wireless products to the largest grid-connected storage systems, not forgetting the massive electrification of transport, a wide range of accumulators, in a variety of sizes and technologies, are active to provide the power and energy required.

The fact that a battery can spontaneously deliver a current, as soon as its terminals are connected, poses an obvious safety problem. This is an uninterruptible power supply, requiring training and electrical clearance above certain voltage thresholds and quantities of charge stored in the accumulator, to ensure its safe operation. Beyond the safety aspects, understanding why this current is spontaneously available requires us to delve into theoretical considerations from many scientific disciplines: thermodynamics, physics, electrochemistry, among others...

First of all, we need to understand what an electrolyte is, as it is an essential component of the battery. The chemical considerations relating to ions and solvent may give the reader the impression that he's wasting his time, studying phenomena occurring on a scale of space so far removed from the physical reality of the everyday world around him. However, this step is essential, as it lays the foundations for concepts that have repercussions on the limits of the accumulator in operation.

Next comes the study of the electrode. Because it is immersed in the electrolyte, the notion of interface can be introduced, and with it the appearance of an electric field. We gradually begin to understand the electromotive force at work, and its dependence on the environment (temperature and pressure). We also explain why a potential is inaccessible on its own, how a reference electrode can give us indirect access to it, and what price we have to pay. Out of equilibrium, i.e. when a current flows through the electrode, we show that its electrical potential drops due to overvoltages of distinct origins.

Finally, the accumulator as a whole is treated as a closed system, with its own characteristics determined by the nature of its electrodes and electrolyte. We look at its limits and the possibility of assembling several accumulators into a battery to push the limits even further....