Batteries

Chemical energy can also be released by other chemical reactions than combustion. Galvanic cells, named for the Italian physician Luigi Galvani consist of two electrodes and an electrolyte. Galvani discovered that when two different metals are connected together and then touched to a frog’s thigh nerve, the muscles twitch. Galvanic cells convert chemical energy into electrical energy. This process, unlike ordinary batteries, is reversible. The electrical energy is converted into chemical energy, and then converted back into electrical energy during unloading.

Luigi Galvani
(1737 - 1798)
Henri Tudor stamp

At the beginning of the 19th century, attempts were made to create a lead battery, and in 1886, Henri Tudor patented a lead battery that is still used in cars today.

The lead acid battery in the charged state has a positive electrode with a lead core, a shell of lead (IV) oxide (PbO2), and a negative electrode of finely divided porous lead (lead sponge). The electrolyte is a dilute (27%) sulfuric acid (H2SO4). In the discharged state, both poles are made of lead (II) sulfate (PbSO4). The nominal voltage of a cell is 2 volts, so that for a 12-volt lead acid battery there are 6 cells in series. The chemical reaction for the negative pole during discharge is

Pb + SO4(2-) -> PbSO4 + 2e- 

and at the positive pole

PbO2 + SO4(2-) + 4H3O+ + 2e- -> PbSO4 + 6H2O

Electrons are produced at the negative pole and taken up at the positive terminal. When loading, all of the processes run in the reverse direction.

Power density vs energy density

The capacity of electric energy storage is measured in amp hours (Ah). The stored energy in watt hours (Wh) is obtained by multiplying the capacity with the nominal voltage Vn. The maximum achievable performance for a short period, Wmax, is the product of maximum current, Amax, and voltage Vn.

For comparing devices in practice, the values in Wh or Wmax are divided by the volume or weight of the storage unit. Lead acid batteries have an energy density of 30 Wh/kg.

The lithium-ion battery, first launched on the market in 1991 by Sony, reaches energy densities of more than 100 Wh/kg. It has a cathode (negative terminal) of graphite and an anode (positive terminal) of lithium metal oxide, e. g. LiMn2O4. The electrolyte is the salt lithium hexafluorophosphate (LiPF6) in anhydrous solvents such as ethylene carbonate or, for the lithium-polymer battery, in a solid gel-like polymer film. During loading, the positively charged lithium ions migrate through the electrolyte from the anode to the graphite layers (nC) of the cathode. The charge current is generated by the electrons through the external circuit. The lithium ions form an intercalation compound (LixnC) with the carbon of the graphite. When discharging, the lithium ions migrate back into the metal oxide, and the electrons flow through the external circuit to the cathode. The chemical reaction at the negative terminal for unloading is

Li_{x}C_{n} -> n C + x Li+ + x e-

and at the positive terminal

Li_{1-x}Mn2O4 + x Li+ + x e- -> LiMn2O4