Theoretical Capacity of g-C3N4?

Question

I was reading a paper that stated:

For example, graphitic carbon nitride (g-C3N4) is considered a potential anode material for lithium ion batteries due to its easy accessibility, low cost and large theoretic capacity of 524 mAh·g−1 (much greater than conventional graphite anode 372 mAh·g−1)

And I had looked into how to calculate the theoretical capacity of g-$\ce{C3N4}$ and found this formula:

$$TC=\frac{xF}{M}$$

Where $x$ is the highest atomic ratio of the ion to the anode material (in this case it would be one Li atom so $x=1$?), $F$ is the Faraday constant ($\pu{26.8Ah * mol^-1}$) and $M$ is the molecular mass of the anode material ($M_\ce{C3N4}=\pu{92.06 g/mol}$). However, when I plug these numbers in, I get:

$$TC=\frac{1\times26.8}{92.06}=\pu{0.291 Ah*g^-1}=\pu{291.11 mAh*g^-1}$$

which is decently lower than the theoretical capacity as presented in the literature. I saw another paper that stated g-$\ce{C3N4}$ has > $\pu{1,000 mAh*g^-1}$ which also contradicts this paper. What am I doing wrong?

Answer 1

I guess your citation is from this article : Shangqi Sun et al. and the formula is correct, the calculated value could have been right if the graphitic $\text{g-C3N4}$ had the same structure as graphite. Obviously as $M(\text{g-C3N4})>M(\text{graphite})$, $TC$ will be less in the case of $\text{g-C3N4}$.

Normally, the intercalation of Li atom in graphite gives something like : $\text{Li + 6C} \rightarrow \text{LiC}_6$, if you apply you formula, you get directly the expected value of $TC = \frac{F}{6M(C)} = 372 \text{ mAh·g}^{−1}$. I am not an expert of this field, but when the interlayer is large enough, the intercalation is also possible for larger atom like Sodium and this intercalation is more reversible, it is the case of $\text{g-C3N4}$ compared to graphite or other materials like transition metal carbides and nitrides (MXenes).

A structure of $\text{C3N4}$ is given in the picture below, $\text{g-C3N4}$ will have an infinite layer like graphene, but the crucial elements are the vacancies, these vacancies are large enough to accommodate $\text{Li}$ atoms in the layer. This is the reason why your result is inconsistent. There is one or more $\text{Li}$ in the layer to considered during the process of intercalation. In this case $x > 1 $ giving $TC(\text{g-C3N4}) > TC(\text{graphite}) $, I guess $x \rightarrow 2 $.

Image source : Electrochemical Oxygen Reduction on Carbon Nitride