Part A

The Chemistry Behind Rechargeable Batteries

Simply put, batteries, or electrochemical cells, generate energy to power a device.  Although this power does not materialize out of nowhere, the chemical process that the cell undergoes is indeed spontaneous.  Batteries are “galvanic cells”, which means they convert chemical energy into electrical energy.  The conversion is made through reduction-oxidation (or “redox”) reactions, where one substance loses electrons (oxidation) to another that gains them (reduction).  This conversion occurs in two half cells: an oxidation and a reduction half-cell for their respective reactions. 

For instance, a redox reaction involving copper and zinc is as follows:

Reduction: Cu²⁺ + 2e^- → Cu

Oxidation: Zn → Zn²⁺ + 2e^-

As mentioned before, these reactions are spontaneous, meaning they can occur without the help from an external energy source.  This characteristic of batteries has facilitated many aspects of our lives; for one, we are spared from the cumbersome task of lugging extension cords about.

In order for these redox reactions to occur, each half-cell needs the following components: two electrolytes (a), two electrodes (b) (anode or cathode), a separator (c) and a load (d).  These parts may come in different forms, however, the function of each remain the same.

Figure 1 A typical dry-cell battery connected to a load.

a)      Electrolytes are responsible for carrying charge throughout the cell.  In a battery, this would be paste or solution that fills up the two spaces between the layers of the battery.  The first space being the one sandwiched between the anode layer and separator and the second the one that contains the cathode.
b)      Electrodes are surfaces on which the redox reactions occur: anode for oxidation and cathode for reduction.  In a battery, the anode is the metal shell that contains the entire cell and is connected to the negative terminal.  In Figure 1, it is made of solid zinc.  The cathode is the solid bar that runs from the centre of the cell to the positive terminal.  In the diagram, the cathode is the solid carbon core.
c)      Separators, as the name suggests, separate the two electrodes, preventing charge build-up, but still allows the charges to flow through.  In Figure 1, the separator is the layer between the two electrolytes.
d)     The load is the most important part of this system, as it completes the circuit.  The load is attached to the negative and positive terminals of the battery, allowing the flow of electrons from the anode to cathode and ultimately, the operation of the cell.  In the diagram, the load is represented by a light bulb.

A characteristic that distinguishes rechargeable batteries from disposable dry cells is that they can undergo reverse redox reactions.  This, unlike reactions of a galvanic cell, requires an outside energy source.  Simply put, a current is forced into the cell to pull the electrons of the cathode back to the anode, rendering the original anode a cathode.  This process of converting electrical energy into chemical energy makes the cell an electrolytic cell.  Once the transfer is complete and the power source is removed, the battery is a galvanic cell again.  Examples of rechargeable batteries are lithium-ion and lead-acid batteries, both of which are more durable than regular batteries.  Car, cell-phone and camera batteries stand as common examples of rechargeable batteries (Figure  2).

Figure 2 Rechargeable batteries are used to power many everyday machines and devices like cars, cell phones and cameras.