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How does Battery Work?
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Batteries use powerful microtechnology to deliver electricity. They can do this because they contain chemicals that cause a chemical reaction. The chemical reaction generates electricity, which means that the battery becomes an energy source.
In simple terms, batteries can be considered as electron pumps. The internal chemical reaction within the battery between the electrolyte and the negative metal electrode produces a build up of free electrons, each with a negative charge, at the battery's negative (-) terminal - the anode. The chemical reaction between the electrolyte and the positive (+) electrode inside the battery produces an excess of positive (+) ions (atoms that are missing electrons, thus with a net positive charge) at the positive (+) terminal - the cathode of the battery. The electrical (pump) pressure or potential difference between the + and - terminals is called voltage or electromotive force (EMF).
Different metals have different affinities for electrons. When two dissimilar metals (or metal compounds) are put in contact or connected through a conducting medium there is a tendency for electrons to pass from the metal with the smaller affinity for electrons, which becomes positively charged, to the metal with the greater affinity which becomes negatively charged. A potential difference between the metals will therefore build up until it just balances the tendency of the electron transfer between the metals. At this point the "equilibrium potential" is that which balances the difference between the propensity of the two metals to gain or lose electrons.
A battery stores energy in chemical form in its active materials and can this convert this to electrical energy on demand, typically by means of an electrochemical oxidation-reduction (redox) reaction. (Note the generic name "redox" seems to have been appropriated by a recent flow battery design employing two vanadium redox couples).
Each battery consists of at least the following components:
The anode or negative electrode (the reducing or fuel electrode) - which gives up electrons to the external circuit and is oxidised during the elecrochemical (discharge) reaction. It is generally a metal or an alloy but hydrogen is also used. The anodic process is the oxidation of the metal to form metal ions.
The cathode or positive electrode (the oxidising electrode) - which accepts electrons from the external circuit and is reduced during the electrochemical (discharge) reaction. It is usually an metallic oxide or a sulfide but oxygen is also used. The cathodic process is the reduction of the oxide to leave the metal.
The electrolyte (the ionic conductor) - which provides the medium for transfer of charge as ions inside the cell between the anode and cathode. The electrolyte is typically a solvent containing dissolved chemicals providing ionic conductivity. It should be a non-conductor of electrons to avoid self discharge of the cell.
The separator - which electrically isolates the positive and negative electrodes.
The Discharge Process
When the battery is fully charged there is a surplus of electrons on the anode giving it a negative charge and a deficit on the cathode giving it a positive charge resulting in a potential difference across the cell.
When the circuit is completed the surplus electrons flow in the external circuit from the negatively charged anode which loses all its charge to the positively charged cathode which accepts it, neutralising its positive charge. This action reduces the potential difference across the cell to zero. The circuit is completed or balanced by the flow of positive ions in the electrolyte from the anode to the cathode.
Since the electrons are negatively charged the electrical current they represent flows in the opposite direction, from the cathode (positive terminal) to the anode (negative terminal).
The Charging Process
The charger strips electrons from the cathode leaving it with a net positive charge and forces them onto the anode giving it a negative charge. The energy pumped into the cell transforms the active chemicals back to their original state.
The working of a battery can be explained with the help of following example:
When you switch on a device like a flashlight, the electric circuit completes and electric currents in the form of electrons power the bulb. That happens because the anode material, zinc (Zn) gives up two electrons (e-) per atom in a process called oxidation. This process leaves unstable zinc ions (Zn2+) behind. (An ion is an atom that has gained or lost electron(s) so it has a positive or negative charge.)
After the electrons do their stuff and power the light bulb, they re-enter the battery at the cathode. There they combine with the active material, manganese dioxide (MnO2), in a process called reduction.
Oxidation and reduction could not occur in a battery without a way to carry electrons back to the anode after they enter the cathode. Here's where the electrolyte comes in.
After each electron enters the cathode, it reacts with the manganese dioxide to form MnOO-. Then the MnOO- reacts with water in the electrolyte solution. The water splits to hydroxide ions (OH-) and hydrogen ions (H+) that combine with MnOO- to form MnOOH. The hydroxide ions flow to the anode in the form of an ionic current.
There, they combine with unstable zinc ions which had given up their electrons to power the light bulb. The reaction produces zinc oxide (ZnO) and water (H2O). This completes the circuit (which is necessary to have a constant flow of electricity) and powers your flashlight.
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