Dear Rajwardhan,

the 2000 mAh/g come if you consider the Mw of Li22Si5. When you take the Mw of Si (28.08 g/mol) then you get the 4200 mAh/g. Which of them is "correct" is another topic for discussion. Most widely accepted theoretical specific charge for Si is 4200, or actually around 3579 mAh/g for Li3.75Si.

Can you Show me how ? I did the calculations again with Mw of Si (28.08g/mole)

Q = (nF) / Mw,

where Q is the specific charge (mAh/g),

n is the number of electrons involved in the reaction of 1 mol of active material (dimensionless),

F is Faraday's constant (96485 C/mol, or 26801 mAh/mol)

and Mw (g/mol),

so assuming 4.4 e-/mol of Si - or the reaction between Li and Si to form Li22Si5 - Qtheor. (Si) = (26801*4.4)/28.08 = 4199.58 mAh/g

Thank you very Much. I was considering entire Molecular weight of Li22Si5.

Dear Abo-Alhassan,

we use the MW of the pure Si (MWSi-control=28.08 g/mol), since the starting (control) host material, Si (x=0, in LixSi) is the pristine material. Thus, the MWcontrol of the control material is, always, the same standard (MWSi-control=28.08 g/mol), even if there might exist many j - fixed - stoichiometries, i.e. more than one (so, which one MWj?) established, ending, quasi-locked Lithiated stoichiometries.

Dear Abo-Alhassan,

Prof. Samaras has explained why we use the MW of the pristine material as a standard. In the case of SiO2 the calculation would be very similar. Please pay attention to the reaction mechanism. You may find in the literature that a possible reaction is Li + SiO2 --> Li4SiO4 + Si, and then Li + Si --> LixSi. If I remember correctly, the group of K. Edström published one or two papers about the reactions of the SiO2 shell present on Si particles.

Hi, I think their calculation comes from [(26801x4.4)/60.08], which gives about 1962,7 mAh/g. The MW of SiO2 is 60.08. Then I guess the 4.4 e- may come only from reaction 3, which is the only reversible one.

I'm sorry I don't remember the literature on SiO2, and perhaps I missed any paper demonstrating SiO2 can be used as anode material (I stopped working on Si some time ago).

Hope it helps.

Silicon is often listed with it capacity in a delithiated state where as lithium is listed in its lithiated state. It is unfair to compare in the delithiate form as lithium metal would have an infinite capacity. All material should be listed in the lithiated state to be able to make a useful comparison. For silicon this would be closer to 1800-2000 mah/g depending on temperature. Be mindful that if you are trying to calculate the weight of a full cell cathodes are listed in the lithiated state as well so the unlithiated weight should be considered. Also these are all theoretical and both systems would be considerably less in a practical cell.