Areal Loading - Battery Design

Areal loading (also called coating weight or mass loading) is the mass of dried electrode coating on the current collector, per unit area, expressed in milligrams per square centimetre (mg/cm²). It is set at the coater, confirmed after drying, and it is the single manufacturing variable with the most direct influence on a cell’s areal capacity.

How areal loading is calculated

Areal loading (mg/cm²) = Mass of dried coating (mg) ÷ Coated area (cm²)

For double-sided electrodes (the vast majority of production cells), loading is reported per side, and the two sides need to be closely matched (see Single-side vs double-side coating below).

For example, a standard commercial NMC cathode coated at 15 mg/cm² (0.015 g/cm²), combined with a specific capacity of 180 mAh/g, gives an areal capacity of 2.7 mAh/cm² (see Areal Capacity for the full calculation).

Typical loading values

Loading varies widely by chemistry and target application. A few commercial reference points:

Electrode / chemistry	Loading (mg/cm²)	Notes
LFP, standard commercial (18650)	5–8	Power-optimised, thin coating⁶
NMC, standard commercial	10–20	⁷
NMC, thick-film high-energy	~50	Specialised carbon additives and binders⁸
SSB, commercial viability threshold	20–40	Minimum to compete with Li-ion energy density⁹

See Areal Capacity for the full table across chemistries and applications, including anode and solid-state figures.

Is it measured before or after drying and calendering?

Areal loading is always a dry-mass figure. Wet slurry contains solvent (NMP for most cathodes, water for most anodes) that evaporates in the dryer, so weighing a wet sample says nothing about the finished electrode. The figure that counts, and the one quality control records, is measured after drying and before calendering.¹

Calendering itself does not change areal loading. It compresses the dried coating to a target thickness and density, which raises volumetric capacity, but it does not add or remove active material: the mass per unit area set at the coater and confirmed after drying should carry through calendering unchanged. In production, a loading reading that drops after calendering usually flags a defect (dusting, flaking, delamination) rather than a normal process step.

What sets it, and where

Loading is fixed upstream of both drying and calendering, at the coater itself. For slot-die and comma coating it is set primarily by the coating gap and the slurry’s solids content: a wider gap or a more concentrated slurry deposits more wet film, which dries down to a proportionally heavier coating (see Coating and Slurry Viscosity & Solid Content). Line speed and drying capacity then have to be matched to whatever loading the coater is set to deposit.

Single-side vs double-side coating

Almost all commercial cells are coated on both faces of the current collector. Because loading is reported per side, the two sides need to be matched closely: an imbalance causes the electrode to curl from uneven shrinkage during drying, and creates a local capacity mismatch through the cell stack. Simultaneous double-sided coating methods, including LG Energy Solution’s Double Layer Slot Die (DLD) process² and Dürr’s two-sided coating and drying lines³, deposit and dry both faces in a single pass specifically to hold loading consistent side-to-side, rather than coating, drying, and re-reeling the foil to coat the second face separately.

How it’s measured

Gravimetric (reference method): punch a disc or square of known area from the dried electrode, weigh it, and subtract the known mass of the same area of bare foil. Dividing by the punch area gives loading directly. This is the calibration-grade method used in the lab and to spot-check inline gauges.
Inline basis-weight gauges: a sensor scans back and forth across the moving web, measuring how much of a beam is absorbed by the coating, which correlates to mass per unit area. Historically this used beta/Kr-85 radiation sources; newer lines increasingly use x-ray transmission gauges instead.⁴⁵ Scanning gauges run continuously through coating and drying, catching cross-web and machine-direction variation in real time rather than at isolated sample points.

Why it matters

Areal loading is the direct input to areal capacity:

Areal capacity (mAh/cm²) = Areal loading (g/cm²) × Specific capacity (mAh/g)

Everything downstream of that (energy density, N/P ratio balance, rate capability, cycle life) traces back to this one number. See Areal Capacity for how loading trades off against electrode thickness, porosity, and rate performance.

Process control challenges

Loading uniformity is hardest to hold at the edges of the web, for the same reasons covered in Coating’s edge-control challenges: shim adjustment, slot-die distance, and web straightness all affect how evenly the wet film, and therefore the dried loading, spreads across the width. Edge-loading variation feeds directly into the tolerance stack-up behind electrode overlap defects.