Electrode Coating Calculator
Convert coating loading, active ratio and capacity into areal capacity, compaction density and N/P ratio.
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What it does
Lithium-ion cell design starts from the electrode coating, not the whole cell. This calculator takes the coating loading, active-material ratio, specific capacity, thickness and true density for the cathode and anode, then returns areal capacity, active loading, compaction density and porosity, plus the N/P ratio and the anode capacity needed for a target N/P. Cell engineers use it to sanity-check a coat, balance the two electrodes and avoid over-plating before building a pouch or cylinder.
How it works
Areal capacity is the product of the coating and its active fraction, scaled by capacity: areal capacity (mAh/cm2) = loading (mg/cm2) x active ratio (%) / 100 x specific capacity (mAh/g) / 1000. Coating density is loading divided by thickness (mg/cm2 over micrometers, converted via 1 micrometer = 1e-4 cm), and porosity = (1 - coating density / true density) x 100%. The N/P ratio is anode areal capacity divided by cathode areal capacity. To hit a target N/P, the required anode capacity is simply cathode areal capacity x target N/P.
Worked example
Take a graphite anode at loading 6.5 mg/cm2, 95% active, 350 mAh/g and a cathode at 12 mg/cm2, 94% active, 160 mAh/g. Anode areal capacity = 6.5 x 0.95 x 350 / 1000 = 2.16 mAh/cm2; cathode = 12 x 0.94 x 160 / 1000 = 1.80 mAh/cm2, so N/P = 2.16 / 1.80 = 1.20. To design to N/P = 1.10, the anode needs 1.80 x 1.10 = 1.98 mAh/cm2, i.e. about 6.0 mg/cm2 loading. These are typical electrode-scale numbers.
When to use it
Use the tool when laying out a new electrode or checking that a coat recipe matches the cell's energy and safety targets. It is the fastest way to see whether the anode has enough margin over the cathode. The usual trap is forgetting porosity: a high loading with thin coating gives a dense, low-porosity electrode that wets poorly, while too little active ratio hides inert binder and conductor as if it were capacity. Keep N/P in the 1.05-1.15 window for most graphite systems.
FAQ
- What is areal capacity and how is it calculated?
- Areal capacity is the charge a coated electrode delivers per unit area, in mAh/cm2, and it sets cell energy density. It is loading x active ratio x specific capacity scaled to area: areal capacity = loading (mg/cm2) x active ratio (%) / 100 x specific capacity (mAh/g) / 1000. For a cathode at 12 mg/cm2, 94% active and 160 mAh/g, that is 12 x 0.94 x 160 / 1000 = 1.80 mAh/cm2.
- What is a good N/P ratio?
- N/P is anode areal capacity divided by cathode areal capacity. Graphite-anode cells normally target 1.05-1.15: above 1.3 the excess anode promotes lithium plating at low state of charge, and below 1.05 the cathode can over-delithiate and lose life. A 1.10 design means the anode carries 10% more capacity than the cathode. The exact value shifts with anode chemistry, so match it to the material.
- How does coating thickness affect porosity?
- Coating density equals loading divided by thickness (with 1 micrometer = 1e-4 cm), and porosity = (1 - coating density / true density) x 100%. At fixed loading, a thinner coat packs denser and porosity drops, which can starve the electrode of electrolyte; a thicker coat raises porosity but lowers energy per volume. For the anode above, 6.5 mg/cm2 over 70 micrometers gives a coating density near 0.93 g/cm3 against graphite's 2.2 g/cm3, about 58% porosity.
- Why enter true density?
- True density is the intrinsic density of the coating's solid material, used to convert a measured loading and thickness into porosity. Graphite is about 2.2 g/cm3 and NCM811 about 4.7 g/cm3; using the wrong value silently misreports porosity and therefore wetting and rate behavior. The tool does not measure density, it only divides loading by thickness, so a realistic true density is what makes the porosity number meaningful.