Lead acid batteries

Discovery of lead acid batteries

Lead acid batteries (LABs) were initially developed in 1859 by the French physicist Gaston Planté. He studied the polarisation behaviour of silver, tin, lead, copper, gold, platinum and aluminium electrodes in dilute aqueous sulfuric acid solutions. He determined that the secondary (or discharge) current for lead was the highest and flowed for the longest period of time compared to the other metal films, and so the lead acid battery was born!

Since this time the structure and design of lead acid batteries have improved but the fundamental chemistry behind lead acid battery operation has remained the same.

Lead acid battery chemistry

lead acid battey charge discharge
Charge/discharge cycle of lead acid batteries

LABS are ubiquitous in the automotive industry where they are used to provide the current to the starting motor that turns on the internal combustion engine. Because of their ability to provide high discharge rates this makes them ideal for this application.

They rely on the three different oxidation states that lead can have: 0, +2 and +4.

In the charged state the lead on the surface of the negative electrode (cathode) is in its metallic form and the positive electrode (anode) is in its most oxidsed +4 oxidation state as PbO2.

During discharge electrons flow from the cathode, where the metallic lead is oxidised to the +2 oxidation state, to the anode, where the PbOis reduced, again to the +2 oxidation state. Both the electrodes react to produce PbSOthrough reaction with the sulfuric acid solvent.

Discharge reactions      

Cathode:        Pb(s) + H2SO4 → PbSO4 + 2H+ + 2e–            

Anode:           PbO+ H2SO4 + 2H+ + 2e– → PbSO4+ 2H2O

Charge reactions

Cathode:        PbSO4 + 2H+ + 2e– → Pb(s) + H2SO4              

Anode:           PbSO4+ 2H2O → PbO+ H2SO4 + 2H+ + 2e

During the charging process electrons flow in the opposite direction, from the anode to the cathode reproducing the original electrode materials and consequently there is once again energy stored in the battery

The popularity of lead acid batteries

In lead acid batteries the solvent, aqueous sulfuric acid, is involved in the electrode reactions. Additionally, the lead remains on the surface of the electrode in both its oxidised and reduced forms. As the solvent is abundant, the chemical reactions are fast and there is no reliance on ion transport in the cell so the discharge current can be very high. For this reason, they are widely used in applications where large discharge currents are required such as the starting motors of automobiles.

In most batteries the electrode reactions are reliant on the transport of metal ions from one electrode to the other. For example, in the lithium ion battery, lithium in its reduced form on the cathode is oxidised where it dissolves into an electrolyte and then is transported through to the anode surface where is is stored. In the corresponding charging process the lithium ion has to be transported back to the cathode surface where it is reduced.

The transport of metal ions in solution can be a slow process and, while they can still store a lot of charge, the rate at which the charge can be released is often slow. Because of their high charge density these types of batteries are excellent candidates for situations where battery weight is important such as mobile phones, laptops and tablets. However, the limited discharge current makes them impractical for applications where a large and immediate current is required, such as for starting an internal combustion engine in a car.

 

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