Batteries and Electrochemical Cells: Daniell to Lithium-ion

How a Galvanic Cell Works

Drop zinc into copper sulphate solution and the zinc dissolves while red-brown copper settles on it; electrons jump straight across and the energy is lost as heat. A galvanic (voltaic) cell separates the two halves of this reaction so that the electrons must travel through an outside wire. That moving charge is the current. The first practical version was built by John Frederic Daniell in 1836.

In the Daniell cell, a zinc plate stands in zinc sulphate solution and a copper plate stands in copper sulphate solution. A salt bridge (a gel tube of an inert salt such as potassium chloride) joins the solutions, and a wire joins the plates.

The cell potential (emf) is the difference between the two electrode potentials. It is measured in volts, which are joules per coulomb, so emf is energy per unit charge and not a force.

Alessandro Volta built the first battery, the voltaic pile of zinc and copper discs, in 1800. A cell is one unit; a battery is strictly several cells joined together. For the circuit the battery drives, see electric circuits. The ranking of zinc above copper comes from the reactivity series.

Primary Cells: Use Once and Throw

In a primary cell the reaction cannot be reversed efficiently, so once the reactants are used up the cell is finished. A secondary cell can be recharged: forcing current backwards through it reverses the reaction and restores the reactants. This single distinction is what CDS 2026-I asked (see the past papers).

The common dry cell in torches and wall clocks is a zinc-carbon (Leclanché-type) cell. Georges Leclanché invented the wet version in 1866, and the sealed "dry" form followed in the 1880s.

• Anode: the zinc container, which slowly dissolves.
• Cathode: a graphite (carbon) rod in the centre, surrounded by manganese dioxide (MnO₂) mixed with carbon powder.
• Electrolyte: a moist paste of ammonium chloride and zinc chloride, not a free-flowing liquid, which is why it is called "dry".
• Voltage: about 1.5 V, falling steadily as it is used.

The alkaline cell uses the same zinc and MnO₂ but with potassium hydroxide as the electrolyte. It also gives 1.5 V, lasts longer and delivers current better, and its zinc container does not eat itself while the cell sits on a shelf. Watches and hearing aids use small button cells of silver oxide, zinc-air or lithium.

Rechargeable Cells: Lead-acid, Ni-Cd, Lithium-ion

Lead-acid accumulator. Invented by Gaston Planté in 1859, it is still the starter battery of most cars and the heart of home inverters. The anode is spongy lead, the cathode is a grid packed with lead dioxide (PbO₂), and the electrolyte is about 35-40% dilute sulphuric acid. Each cell gives about 2 V, so a 12 V car battery has six cells in series. While discharging, both plates turn into lead sulphate and acid is used up:

Lead-acid cells are cheap and give the surge current an engine needs, but they are heavy.

Nickel-cadmium (Ni-Cd). Developed by Waldemar Jungner in 1899, it has a cadmium anode, a nickel oxide-hydroxide cathode and potassium hydroxide electrolyte, and gives about 1.2 V. Cadmium is toxic, and the cell suffers the memory effect, losing capacity if recharged before it is fully discharged.

Lithium-ion. Lithium is the lightest metal and has a very strongly negative electrode potential, so it packs a lot of energy into little weight. In a lithium-ion cell the anode is graphite, the cathode is a lithium metal oxide (cobalt oxide, iron phosphate, or a nickel-manganese-cobalt mixture), and the electrolyte is a lithium salt in an organic solvent. During discharge, lithium ions travel from anode to cathode through the electrolyte while electrons travel through the outside circuit. Charging sends them back, which is why the design is called a "rocking-chair" battery. The nominal voltage is about 3.6-3.7 V. The 2019 Nobel Prize in Chemistry went to John Goodenough, Stanley Whittingham and Akira Yoshino for developing lithium-ion batteries.

The metals in these cells explain the resource news. India's position is covered in our explainer on critical minerals, and the vehicle side in India's EV push.

Sodium-ion, Fuel Cells and Grid Storage

Sodium-ion batteries work like lithium-ion ones but shuttle sodium ions. Sodium is cheap and found everywhere in common salt, and these cells need neither lithium nor cobalt. They store less energy per kilogram, which matters little for a battery beside a solar farm, so they suit stationary storage and cheap vehicles.

A fuel cell differs from a battery in one respect: the fuel is not stored inside but fed in continuously from outside, so it keeps producing current as long as fuel arrives. In a hydrogen fuel cell, hydrogen is oxidised at the anode and oxygen is reduced at the cathode, and the only product is water. Fuel cells supplied electricity and drinking water on the Apollo moon missions. Their efficiency (roughly 40-60%) is about double that of a petrol engine. The hydrogen itself is covered in hydrogen, its isotopes and uses.

Now return to the Cabinet decision. Solar and wind produce power only when the sun shines and the wind blows, so storage must shift energy to the evening peak. India needs 411 GWh of grid storage by 2031-32, says PIB. The 92 GWh will equal 170% of today's grid-scale capacity, and the total investment is expected to be Rs 97,600 crore. Pumped storage is a mechanical "water battery" (explained here), while BESS means chemical cells, mostly lithium-ion, in containers. The factory side is in Advanced Chemistry Cell batteries.

Electrolysis: The Reaction Run Backwards

In an electrolytic cell, an outside source drives current through a molten compound or solution and forces a non-spontaneous reaction. Cations go to the cathode and are reduced, anions go to the anode and are oxidised.

Faraday's laws link charge to mass. The mass of a substance deposited is proportional to the charge passed (Q = I × t), and equal charges deposit masses in the ratio of their equivalent weights. One mole of electrons carries one faraday, F ≈ 96,500 coulombs.

Rusting is a galvanic process. Iron, water and oxygen form tiny cells in which iron is the anode and dissolves. Galvanisation coats iron with zinc. Zinc is more reactive, so even where the coat is scratched, zinc corrodes first and protects the iron. It is a sacrificial anode.

How UPSC Has Asked It: Past Papers Solved

Practice MCQs (with answers)

1. In a galvanic cell, oxidation takes place at the
   1. cathode
   2. anode
   3. salt bridge
   4. electrolyte
2. Which of the following is a secondary cell?
   1. Zinc-carbon dry cell
   2. Alkaline manganese cell
   3. Lead-acid accumulator
   4. Silver oxide button cell
3. When a lead-acid battery discharges, the density of its electrolyte
   1. falls, because sulphuric acid is used up
   2. rises, because lead sulphate dissolves
   3. stays constant
   4. doubles
4. The 2019 Nobel Prize in Chemistry for the development of lithium-ion batteries was shared by
   1. Volta, Daniell and Planté
   2. the Curies and Becquerel
   3. Whittingham, Yoshino and Geim
   4. Goodenough, Whittingham and Yoshino
5. In the electrolysis of acidified water, the volume of hydrogen collected at the cathode compared with the oxygen at the anode is
   1. twice
   2. equal
   3. half
   4. four times
6. A battery that uses abundant sodium in place of lithium, and needs no cobalt, is the
   1. nickel-cadmium cell
   2. zinc-carbon cell
   3. Daniell cell
   4. sodium-ion cell
7. The standard emf of the Daniell cell is about
   1. 0.34 V
   2. 1.10 V
   3. 1.50 V
   4. 2.00 V
8. A 12 V car battery is made of 2 V lead-acid cells joined in series. The number of cells is
   1. 3
   2. 4
   3. 6
   4. 12

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