Batteries are groups of electrochemical cells connected in series or in parallel.
Parallel connections (all anodes connected, all cathodes connected) result in no change to the overall voltage of the circuit, while series connections (anode of one cell to cathode of the next) multiply the voltage by the number of cells.
This diagram shows a parallel connection of four 1.5 Volt cells, while the bottom drawing shows similar cells wired in series. In order to store electrical energy, the cells must be readily reversible.
These are the most common type of batteries used in automobiles and to store solar energy because they can provide high current and their cost is relatively low. They store only about 25 watt-hours per kg. Each cell consists of lead electrodes in a sulfuric acid solution. One lead electrode is coated with lead oxide.
Connecting 6 of these cells in series gives a 12 V battery.
Deep cycle batteries have thicker electrodes than the standard starter batteries. Gel batteries and sealed lead acid batteries are commonly used to store solar electric energy for off grid applications.
Lithium ion batteries are used in computers and consumer electronics. Work is underway to adapt these for other applications because of their greater power density. They store about 150 watt-hours per kg.
The electrodes are lithium cobalt oxide (LiCoO2) and graphite with a solid lithium electrolyte. "How stuff works" has more information on these batteries.
The cell voltage is high, up to about 4 V, for the transfer of Li+ from partially reduced graphite to partially oxidized lithium cobalt oxide.
The batteries can explode at high temperature. Current research in the area concerns new electrode materials and electrolytes for better thermal stability in large battery applications.
Nickel Metal Hydride (NiMH)
The metal of a nickel metal hydride battery is an intermetallic, AB5, where A is a rare earth mixture of lanthanum, cerium, neodymium, praseodymium and B is nickel, cobalt, manganese, and/or aluminum.. They store about 100 watt-hours per kg and are much more thermally stable than the lithium ion batteries. Some have been developed for hybrid vehicles.
Ni(OH)2(s) + HO-(aq) Ni(O)(OH) + H2O + e-
E0= 0.49 V
M(s) + H2O + e- MH(s) + HO-(aq)
E0= 0.83 V
Professor Patricia Shapley, University of Illinois, 2012