Learn about the composition of the three most common batteries and how they serve our society.
An electrochemical battery consists of a cathode, an anode and electrolyte that acts as a catalyst. When charging, a buildup of electrons forms on the cathode, creating a voltage potential between the cathode and the anode. Release is by a passing current from the positive cathode through an external load and back to the negative anode. On charge, the current flows in the other direction.
A battery has two separate pathways; one is the electric circuit through which electrons flow, feeding the load, and the other is the path where ions move between the electrodes though the separator that acts as an insulator for electrons. Ions are atoms that have lost or gained electrons and have become electrically charged. The separator electrically isolates the electrodes but allows the movement of ions.
The electrode of a battery that releases electrons during discharge is called the anode; the electrode that absorbs the electrons is the cathode.
The battery anode is always negative and the cathode is positive. This seems to violate the convention as the anode is the terminal into which current flows. A vacuum tube, diode or a battery on charge follows this convention; however, taking power away from a battery on discharge turns the anode into negative. Since the battery is an electric storage device providing energy, the designation does not change between charging and discharging. The battery anode is always negative.
The cathode of a battery is positive; the anode is negative.
Tables 1a, b, c and d summarize the composition of lead-, nickel- and lithium-based secondary batteries, including primary alkaline.
|Lead acid||Cathode (positive)||Anode (negative)||Electrolyte|
|Material||Lead dioxide (chocolate brown)||Gray lead, (spongy when formed)||Sulfuric acid|
|Full charge||Lead oxide (PbO2), electrons added to positive plate||Lead (Pb), electrons removed from plate||Strong sulfuric acid|
|Discharged||Lead turns into lead sulfate at the negative electrode, electrons driven from positive plate to negative plate||Weak sulfuric acid (water-like)|
Table 1a: Composition of lead acid.
|NiMH, NiCd||Cathode (positive)||Anode (negative)||Electrolyte|
NiMH: hydrogen-absorbing alloy
Table 1b: Composition of NiMH and NiCd.
on copper foil
on aluminum foil
|Material||Metal oxides derived from cobalt, nickel, manganese, iron, aluminum||Carbon based||Lithium salt in an organic solvent|
|Full charge||Metal oxide with intercalation structure||Lithium ions migrated to anode.|
|Discharged||Lithium ions move back to the positive electrode||
Table 1c: Composition of Li-ion.
|Alkaline||Cathode (positive)||Anode (negative)||Electrolyte|
|Material||Manganese dioxide||Zinc||Aqueous alkaline|
Table 1d: Composition of primary alkaline battery.
Ion flow is made possible with an activator called the electrolyte. In a flooded battery system, the electrolyte moves freely between the inserted electrodes; in a sealed cell, the electrolyte is normally added to the separator in a moistened form. The separator segregates the anode from the cathode, forming an isolator for electrons but allowing ions to pass through. (See BU-306: Separator and BU-307: Electrolyte)
Last Updated 2016-05-13
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