Recently I read some news about Lithium Ion batteries and the author mentioned that Lithium has the highest Specific Capacity of 3861mAh/g. I was very curious to know where this magic number came from. After trying to understand more about that, finally I found the answer. That was 12th class electrochemistry. The calculation is given below.
Specific Capacity = (N x F) / (Atomic Weight)
N = Valency of the Material
F = Faraday constant = 96485 Coulombs/Mole
If this has to be expressed in terms of current, divide that by 3600
F = 26.801Ah/Mole
This is how the equation works for Lithium. Faraday constant is nothing but the total charge of Avogadro number of electrons. Since Lithium has a Valency of 1 and atomic weight of 6.94g/Mole, every 6.94g of Lithium would give out Valency times Avogadro number of electrons when ionized. So, to find out the Specific Capacity
Specific Capacity = (1 Valency x 26.801Ah/g x 1000mA/A) / 6.94g/Mole
That means, when 1 gram of Lithium metal ionizes to Li+ ions, it gives out 3.861Ah of electricity. This is just the current, but not the energy. If we are interested to calculate the energy density of reaction, it is required to know the Standard Electrode Potential of each material. Since it is 3.03V for Lithium, the total energy density of ionization of Lithium is
3.03V x 3.861Ah/g * 1000g/kg = 11701Wh/kg.
This comes very close the values of hydrocarbons.
But, remember this is just a half reaction taking place in the cell. We did not consider the anode reaction at all. So, total Specific Capacity of a complete cell would be much lower and it also depends upon the anode, electrolyte, other chemicals used in the cell etc.
Table1: Specific Capacity of Cathode Materials
|Reaction||Atomic No:||Atomic Weight||Valency||Specific Capacity (mAh/g)|
The following bar graph summarizes the specific capacity of all Metals given above.