- These days more and more papers try to represent the battery capacity in mAh cm-2 rather than mAh g-1.
- What is the main reason for this? Or what additional information does it give?
While mAh g-1 is an important metric with regards to the electrode material properties, it is not a good metric by itself for the practical applicability of the material. The reason for this is that many materials function excellently at very low mass loading (and thus low mAh cm-2), which is often not useful in practice. As research on many battery materials (e.g. silicon anodes for Li-ion batteries) mature from fundamental studies towards more application focused studies, the demonstration of a material's practical applicability becomes more important, hence the shift towards mAh cm-2. Personally, I prefer to have both to properly evaluate a material's performance.
I hope this answers your question.
Asbjørn Ulvestad & Giang Nguyen can you also share how do we calculate areal capacity (mAh cm-2)?
Umair Nisar It depends on what quantities you have. If you do the measurements yourself, you typically have the absolute capacity of the electrode (e.g. in mAh) and then you only need to divide that on the area of the electrode (e.g. in cm2). If you have the specific capacity of the electrode material (e.g. in mAh/g) you need to multiply this with the mass loading of the electrode (e.g. in g/cm2) to get the areal capacity (e.g. in mAh/cm2).
Giang Nguyen & Asbjørn Ulvestad thanks for sharing this inforamtion with me.
Nicely explained by Asbjørn Ulvestad
Completing what he wrote, when developing active materials we can keep using mAh g-1.
When developing active materials the electrochemical characterization is firstly done using half-cells (with a counter-electrode that does not limit the capacity) so all the charge transfer can be attributed to the active material.
The reason why some people use mAh cm-2 is that they tend to make full cells. And to make full cells, we need to consider the electrode level effects.
Briefly, when you want to assemble a full cell, you have 2 pieces of metals (current collectors) with the same area on which active materials are coated.
For balancing the capacities you need to know the areal capacities of the anode and that of the cathode and choose equal values because you do not want to have non-usable active material in your system. (Of course for Li-ion batteries we put ~10% extra capacity for the anode to avoid Li plating)
Giang Nguyen thanks for helping Asif Raza with his query.
Additionally, how can we calculated the areal capacity for different current densities if we have theoretical capacity, weight of active material and the electrode area.
When it comes to practicality of electrode materials areal capacity is very important. Suppose someone has a very low loading and a thin electrode. The mass transport limitations across the electrode is minimum and it might support very high C-rates with minimum loss of capacity. In this case, let's assume authors reported ONLY specific capacity (mAh/g) and C- rates (there are plenty of articles published including in very high impact journals). There will be lot of hype generated from media reporting "a breakthrough in fast charging" because they reported xtreme fast charging rates(XFC). This measurement doesn't reflect the total capacity (mAh) that the electrode contains. If the same electrode is reported in areal capacity (mAh/cm2) and used current densities (mA/cm2), this gives us an idea on how thick or thin this electrode is and how relevant the cycling C-rate. In principal, when developing completely new materials it is okay to use specific capacity and low loadings. However, if conducting studies using well established electrode materials it is better if everything is reported in areal capacity and current densities. As an example for EV applications, the areal capacity of the electrodes used vary between 3-5 mAh/cm2 and for power applications between 1-3 mAh/cm2 (The active material content is above 94% with porosity between 20-40%).
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