The electrochemical battery
The nickel-cadmium cell (NiCd) is made of a nickel oxide hydroxide (NiOOH) cathode and a cadmium (Cd) anode submerged in potassium hydroxide (KOH) solution. When connected to a circuit, the NiOOH reacts with water in the solution in the following reaction: [2NiOOH + 2H 2 O + 2e - → 2Ni(OH) 2 + 2OH - ]. The OH - ions react with the Cd to form cadmium hydroxide (Cd(OH) 2 ) [Cd + 2OH - → Cd(OH) 2 + 2e - ]. There were many perks with this new design for a cell. Although it could only put out about 1.1V, not only was it rechargeable but its energy density rivalled that of the zinc-carbon cell. Its shelf life could go up to around 20 years. Like all batteries, it still has its fair share of defects. Firstly, the manufacture of this battery is very expensive compared to other designs. The inclusion of cadmium makes the disposal and recycling of this battery difficult, as cadmium is highly toxic. Finally, one of the biggest practical issues with this battery is a problem coined as ‘m emory effect ’ . Battery memory is when a cell will lose its capacity and performance due to being charged before being fully discharged.
This brings us to the present day and the cell that has shaped society today. You find it in your phones, in your laptops and even in your electric razors. In 1991, Sony released the first commercially sold ion battery. It was rechargeable; it was reliable; it was a huge technological breakthrough. A lithium cobalt oxide salt (LiCoO 2 ) is coated onto a copper film to form the cathode and a sheet of aluminium is coated with graphite to form the
anode. The space between the anode and the cathode is filled with an electrolyte made from organic carbonate compound such as EC (ethyl carbonate) or DMC (di-methyl carbonate). When charged, as lithium has a very high electrochemical potential, The lithium escapes from the
DMC
lattice of the LiCoO 2 and ionizes. The lithium ion is attracted to the anode and goes through the electrolyte to the graphite at the anode however, free electrons are unable to move through the electrolyte as it has a very high resistance for electric currents. The electron instead takes the path of least resistance (through the circuit to then reach the graphite at the anode). Graphite is there, as it can conduct electricity and is able to store lithium ions in their ionic forms. On the discharge cycle, the opposite occurs. The li ions go through the electrolyte and back into the lattice and the electrons move round the circuit doing work. The sheets of copper and aluminium are rolled into a spiral in order to be more space efficient. Although there are different variants of the cell exist (made with different lithium-metal oxides such as LiFePO 4 or LiMnO 4 ), the voltage of one cell averages around 3.5V, with an energy density that could go up to 690 Wh/L, which was vastly superior to the previously mentioned alkaline batteries which had up to 150 Wh/L. Even though it still suffers from the memory effect, it does so at a much slower rate. When exposed to changes in temperature, especially due to the current heating up the circuit, it is possible for the organic electrolyte to evaporate, leaving nothing between cathode and the anode, causing the notorious flaming batteries. To mitigate against this, a porous separator was put between the two anodes which acted as a kind of safety electrolyte.
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