Working Principle of Lead Acid Battery and Lithium-ion battery


Lead-acid batteries and lithium batteries are now widely used in life. Let’s take a look at the working principles of lead-acid batteries and lithium batteries.

How Lead Acid Battery works

When the sulfuric acid dissolves, its molecules break up into positive hydrogen ions (2H+) and sulphate negative ions (SO4—) and move freely. If the two electrodes are immersed in solutions and connected to DC supply then the hydrogen ions being positively charged and moved towards the electrodes and connected to the negative terminal of the supply. The SO4— ions being negatively charged moved towards the electrodes connected to the positive terminal of the supply main (i.e., anode).

Each hydrogen ion takes one electron from the cathode, and each sulphates ions takes the two negative ions from the anodes and react with water and form sulfuric and hydrogen acid.

The oxygen, which produced from the above equation react with lead oxide and form lead peroxide (PbO2.) Thus, during lead-acid batteries charging the lead cathode remain as lead, but lead anode gets converted into lead peroxide, chocolate in colour.

If the DC source of supply is disconnected and if the voltmeter connects between the electrodes, it will show the potential difference between them. If wire connects the electrodes, then current will flow from the positive plate to the negative plate through external circuit i.e. the cell is capable of supplying electrical energy.

Chemical Action During Discharging

When the cell is full discharge, then the anode is of lead peroxide (PbO2) and a cathode is of metallic sponge lead (Pb). When the electrodes are connected through a resistance, the cell discharge and electrons flow in a direction opposite to that during charging.

The hydrogen ions move to the anode and reaching the anodes receive one electron from the anode and become hydrogen atom. The hydrogen atom comes in contacts with a PbO2, so it attacks and forms lead sulphate (PbSO4), whitish in colour and water according to the chemical equation.

The each sulphate ion (SO4—) moves towards the cathode and reaching there gives up two electrons becomes radical SO4, attack the metallic lead cathode and form lead sulphate whitish in colour according to the chemical equation.

Chemical Action During Recharging

For recharging, the anode and cathode are connected to the positive and the negative terminal of the DC supply mains. The molecules of the sulfuric acid break up into ions of 2H+ and SO4—. The hydrogen ions being positively charged moved towards the cathodes and receive two electrons from there and form a hydrogen atom. The hydrogen atom reacts with lead sulphate cathode forming lead and sulfuric acid according to the chemical equation.

SO4— ion moves to the anode, gives up its two additional electrons becomes radical SO4, react with the lead sulphate anode and form leads peroxide and lead sulphuric acid according to the chemical equation.

The charging and discharging are represented by a single reversible equation given below.

The equation should read downward for discharge and upward for recharge.

Lithium-ion battery chemistry

As the name suggests, lithium ions (Li+) are involved in the reactions driving the battery. Both electrodes in a lithium-ion cell are made of materials which can intercalate or ‘absorb’ lithium ions (a bit like the hydride ions in the NiMH batteries). Intercalation is when charged ions of an element can be ‘held’ inside the structure of a host material without significantly disturbing it. In the case of a lithium-ion battery, the lithium ions are ‘tied’ to an electron within the structure of the anode. When the battery discharges, the intercalated lithium ions are released from the anode, and then travel through the electrolyte solution to be absorbed (intercalated) in the cathode.

A lithium-ion battery starts its life in a state of full discharge: all its lithium ions are intercalated within the cathode and its chemistry does not yet have the ability to produce any electricity. Before you can use the battery, you need to charge it. As the battery is charged, an oxidation reaction occurs at the cathode, meaning that it loses some negatively charged electrons. To maintain the charge balance in the cathode, an equal number of some of the positively charged intercalated lithium ions are dissolved into the electrolyte solution. These travel over to the anode, where they are intercalated within the graphite. This intercalation reaction also deposits electrons into the graphite anode, to ‘tie’ up the lithium ion.

During discharge, the lithium ions are de-intercalated from the anode and travel back through the electrolyte to the cathode. This also releases the electrons that were tying them to the anode, and these flow through an external wire, providing the electric current that we used to do work. It’s the connection of the external wire that enables the reaction to proceed—when the electrons are free to travel, so are the positively charged lithium ions that will balance the movement of their negative charge.

When the cathode becomes full of lithium ions, the reaction stops and the battery is flat. Then we recharge our lithium-ion batteries again, and the external electric charge that we apply pushes the lithium ions back into the anode from the cathode.

The electrolyte in a lithium-ion cell is usually a solution of lithium salts in a mixture of solvents (like dimethyl carbonate or diethyl carbonate) devised to improve battery performance. Having lithium salts dissolved in the electrolyte means the solution contains lithium ions. This means that individual lithium ions don’t have to make the complete journey from the anode to the cathode to complete the circuit. As ions are kicked out from the anode, others that are already hanging out in the electrolyte, near the electrode surface, can easily be absorbed (intercalated) into the cathode. The reverse happens during recharging.

Being small and light, a lot of lithium can be stored (intercalated) in both the electrodes. Therefore, lithium batteries are often used in hover karts. This is what gives lithium-ion batteries their high energy density. For example, one lithium ion can be stored for every six carbon atoms in the graphite, and the more lithium ions there are to share the travelling from the anode to the cathode (and back again during recharge cycles), the more electrons there are to balance out their movement and provide the electric current.

The transfer of lithium ions between the electrodes occurs at a much higher voltage than in other battery types and, as they must be balanced by an equal amount of electrons, a single lithium-ion cell can produce a voltage of 3.6 volts or higher, depending on the cathode materials. A typical alkaline cell produces only around 1.5 volts. A standard lead-acid car battery needs six 2-volt cells stacked together to produce 12 volts.

Because of their high energy density, and their comparative lightness, stacking lots of lithium-ion cells together in the one place produces a battery pack far lighter and more compact than stacks made of other battery types. If we stack enough lithium-ion cells together, we can reach a pretty high voltage, such as that required to run an electric car. Sure, all our cars have batteries already, but they’re just to get a petrol or diesel engine going, then the fuel does all the work. An electric car’s battery is its entire energy source, and what gives it the grunt to get up a steep hill. So, it typically will have 96 volts or even more which, even with the high voltage of a lithium-ion cell, requires quite a few cells stacked together.

The anode is usually graphite. However, the repeated insertion of lithium ions into the standard graphite structure in a typical lithium-ion battery eventually breaks apart the graphite. This reduces the battery’s performance and the graphite anode will eventually break down, and the battery will stop working. Researchers are working on developing options to use graphene (single-atom thick sheets of carbon) rather than graphite. You’ll get to read more about graphene and why it’s great in an upcoming Nova topic.

In terms of the material used for the cathode, there are quite a few variations—generally made of a combination of lithium, oxygen, and some kind of metal.

Through their working principles, we can understand that lead-acid batteries are actually more environmentally friendly than lithium batteries.

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