Dry Coating Technoeconomics – Chapter 1: Modelling Slurry Based Cathode Manufacturing

To get to grips with the techno-economic benefits of any new technology, you must first understand what you’re trying to replace. This is where we’re starting – a dive into today’s state-of-the-art electrode manufacturing, to understand how to model slurry-based coating processes. In this chapter, we’ll be focusing on cathode production (though Anaphite also works on Anode dry coating) as this is where we see the most impactful savings.

Today’s lithium-ion cathodes are produced by dispersing cathode active material (mainly NMC or LFP), conductive additives (typically carbon black or CNTs), and a polymer binder (usually PVDF) into the solvent N-methyl-2-pyrrolidone (NMP). The resulting slurry is carefully formulated to achieve the rheological properties required for high-speed, defect-free coating. Using slot dies, this slurry is then coated onto thin aluminium foil (typically 10–14 μm thick) and immediately enters a long drying oven (50 – 100 m) where the NMP is evaporated, leaving behind the finished electrode.

Modern coating lines run at 60 to 100m/min (with some slower exceptions for thicker coated electrodes). The obvious way to increase your throughput, and therefore profitability, is cranking up the line speed and web width (800 – 1200mm), right? Wrong. The techno-economics become increasingly non-linear as throughput increases.

The rate of solvent removal from the coated electrode must be carefully controlled to avoid delamination and cracking of the coating. Electrodes of typical thickness require about 60 seconds of oven drying. As line speed increases, the quantity of solvent to being handled in an oven increases proportionally. Since the residence time inside the oven remains around one minute, manufacturers must both increase oven length and dramatically increase the airflow passing through the oven to remove the additional solvent safely. This is where the hidden complexity, and cost, of electrode manufacturing is found.

Conventional electrode manufacturing has been optimised for decades and today’s coating and drying systems are exceptionally efficient. Many aspects of the process are reaching engineering limits: they just can’t be made any better! The remaining window of optimisation options is narrow, as the dominant energy contributors aren’t mixing or coating – it’s those systems required to safely remove, capture and recycle NMP.

NMP is classified as a Substance of Very High Concern (SVHC) in many jurisdictions and must be recovered with near total efficiency. Its toxicity creates permitting challenges, while its lower explosive limit (LEL) of approximately 1.3 vol.% in air means drying ovens must continuously dilute solvent vapour with large volumes of fresh air to maintain safe operating conditions. For a typical gigafactory-scale coating line, this often requires between 45,000 and 100,000 m³/h of airflow, depending on coating width, line speed and slurry solids content.

It was a surprising discovery that the largest energy demand is not removing the NMP itself, but heating the enormous amount of air required to do so safely. Although NMP has a relatively high boiling point (202 °C), the thermal energy required to evaporate the solvent is small compared with the energy required to continuously heat tens of thousands of cubic metres of incoming air every hour! I’ve rarely seen this distinction mentioned in open forums.
The energy input doesn’t end when the solvent leaves the electrode. The exhaust air still contains NMP which, due to its toxic nature, must be captured and recycled back into the process. To make this process more efficient, industrial coating lines employ high-efficiency heat exchangers to recover a large proportion of the exhaust heat before the gas enters the solvent recovery system. Even with heat exchanger efficiencies approaching 70%, exhaust gases frequently remain between 70 – 90 °C before entering solvent recovery.

During recovery, the exhaust gas is cooled to sub-zero temperatures to condense the NMP, with any NMP remaining in the exhaust gas after the condenser being captured using a zeolite wheel, which can then be regenerated. Because ambient air is used throughout the drying process, water vapour is also condensed, producing an NMP-water mixture that must subsequently be separated by distillation. This additional purification step introduces another significant energy penalty before purified NMP can be returned to production.

The key lesson from modelling slurry-based electrode manufacturing is surprisingly simple. If you want to understand the economics of conventional electrode production: follow the solvent. In conventional electrode manufacturing, safely removing, recovering and purifying NMP dominates energy consumption and footprint driving up capital and operational cost. All leading to one, vital question: What happens if we eliminate NMP?

In the next edition of our technoeconomic series we’ll be digging into material selection, particularly the differences between NMC, LFP and anodes. Why do different chemistries alter our techno-economics? And, how?