Primary and Secondary Chemical Batteries

A primary battery is a galvanic cell that cannot be recharged because its reaction is not reversible in practice, whereas a secondary battery can be recharged by running an electric current through it in the opposite direction. In brief: an alkaline AA cell (primary, 1.5 V) is thrown away when it is empty, while a lead–acid car battery (secondary, 2.0 V per cell) or a lithium-ion cell (about 3.7 V) can be recharged hundreds or thousands of times.

Batteries power everything from watches to cars and spacecraft. They differ in their chemistry, voltage, energy storage, cost and rechargeability. This article explains the difference between the two types, describes the reactions in the main kinds, compares their properties in a table and works through calculations of the energy that a battery can store.

What is the difference between primary and secondary batteries?

In a primary battery, the cell reaction runs in one direction until the reactants are used up. The products cannot be easily converted back to the reactants: the reaction may be irreversible, or the electrodes change form. It is used once and discarded. In a secondary battery, the cell reaction is reversible: during discharge it works as a galvanic cell, and during charging an external source of current reverses the reaction and rebuilds the reactants, so the cell works as an electrolytic cell. A battery is strictly a set of cells connected in series, but the word is also used for a single cell.

What are the main primary batteries?

Zinc–carbon (Leclanché) cell

Invented by Georges Leclanché in 1866, the cell has a zinc can as the anode, a graphite rod as the current collector and a paste of MnO₂ and NH₄Cl as the electrolyte. Anode: Zn → Zn²⁺ + 2e⁻. Cathode: 2MnO₂ + 2NH₄⁺ + 2e⁻ → Mn₂O₃ + 2NH₃ + H₂O. The voltage is about 1.5 V. It is cheap but has a modest capacity.

Alkaline cell

The alkaline cell uses a zinc powder anode, MnO₂ cathode and a potassium hydroxide electrolyte. Zn + 2OH⁻ → ZnO + H₂O + 2e⁻ at the anode; 2MnO₂ + H₂O + 2e⁻ → Mn₂O₃ + 2OH⁻ at the cathode. It has a voltage of 1.5 V, a longer life than the zinc–carbon cell and works better at high currents.

Button cells and lithium primary cells

Silver oxide (1.55 V, used in watches) and zinc–air cells (about 1.4 V, used in hearing aids) are small and have a very stable voltage. Lithium primary cells (about 3.0 V) have a high energy density and a long shelf life of ten years or more, and are used in cameras, medical devices and memory backup.

What are the main secondary batteries?

Lead–acid battery

Gaston Planté invented the lead–acid battery in 1859, and it is still used in cars. Anode: Pb + SO₄²⁻ → PbSO₄ + 2e⁻. Cathode: PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O. Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O, with about 2.0 V per cell; six cells in series give 12 V. During discharge sulfuric acid is consumed, so its density, which can be measured with a hydrometer, shows the state of charge.

Nickel–cadmium and nickel–metal hydride

Both have a nominal voltage of 1.2 V per cell. NiCd has been largely replaced because cadmium is toxic. NiMH uses a hydrogen-absorbing alloy as the anode and has a higher capacity.

Lithium-ion battery

During discharge, lithium ions move from a graphite anode (LiC₆ → C₆ + Li⁺ + e⁻) through the electrolyte to a metal oxide cathode, such as LiCoO₂ (Li_1−xCoO₂ + xLi⁺ + xe⁻ → LiCoO₂). The lithium ions are not reduced to the metal but are inserted between layers (“intercalation”), which makes recharging safe and efficient. The nominal voltage is 3.6–3.7 V. The first commercial cells were sold by Sony in 1991.

Comparison of battery types

The energy densities are approximate and depend on the design.

How much energy does a battery store?

Example 1: a car battery

A 12 V lead–acid battery with a capacity of 60 Ah stores E = 12 V × 60 Ah = 720 Wh = 0.72 kWh. In joules: 720 × 3600 = 2.6 × 10⁶ J, or 2.6 MJ. With a density of 35 Wh/kg, the mass would be about 20 kg, which is close to a typical car battery.

Example 2: a smartphone battery

A lithium-ion cell rated at 3.7 V and 3000 mAh stores 3.7 × 3.0 = 11.1 Wh. If the phone uses 2 W on average, the battery lasts 11.1 / 2 = 5.6 hours.

Example 3: a standard alkaline cell

An AA alkaline cell with a capacity of about 2.5 Ah at 1.5 V stores about 3.75 Wh. The charge is 2.5 × 3600 = 9000 C, corresponding to 9000 / 96,485 = 0.093 mol of electrons. Since 2 mol of electrons are needed per mole of zinc, this corresponds to 0.047 mol, or about 3.0 g of zinc, which is consistent with the zinc content of such a cell.

What happens during recharging?

During charging, a voltage higher than the cell voltage is applied in the opposite direction. The electrode that was the anode during discharge now becomes the cathode (it is reduced), and the previous cathode becomes the anode. In a lead–acid cell, PbSO₄ is converted back to Pb and PbO₂, and sulfuric acid is regenerated. Over many cycles, side reactions and structural changes, such as the growth of crystals of lead sulfate, reduce the capacity. Overcharging can cause the electrolysis of water, producing hydrogen and oxygen, which is a hazard.

How did batteries develop?

Volta’s pile appeared in 1800, and Daniell’s cell in 1836 gave a steadier voltage. Leclanché patented his zinc–carbon cell in 1866, and Planté’s lead–acid battery of 1859 was the first rechargeable one. The alkaline battery became commercial in the 1950s, and the lithium-ion battery was developed in the 1970s–1980s by Stanley Whittingham, John Goodenough and Akira Yoshino, who shared the Nobel Prize in Chemistry in 2019.

Which mistakes are common?

• Trying to recharge a primary battery: it can leak or overheat.
• Mixing up capacity and energy: capacity is in Ah; energy needs the voltage.
• Adding the voltage of cells in parallel: voltages add in series only.
• Thinking that a battery is a store of electrons: it stores chemical energy in its reactants.
• Ignoring disposal: batteries contain metals such as lead, cadmium and lithium compounds that should be recycled.