Perspectives
Three hurdles for aqueous cathodes: pH, aluminium foil and residual water
Moving a cathode from NMP to water is not mainly a binder problem — it is a chain of reactions from particle to current collector. LFP is the realistic place to start.
Why move cathodes to water
Most cathodes are still made with PVDF binder dissolved in NMP. NMP needs a solvent-recovery system, and the EU added it to the REACH Annex XVII restrictions through Regulation (EU) 2018/588, which requires tight control of workplace exposure. The EU’s universal PFAS restriction proposal has also brought fluoropolymers into scope.
The case for water is therefore simple: no NMP and no recovery system, a better working environment, and less reliance on fluorinated binders. Anodes made this switch long ago with SBR and CMC; cathodes have three hurdles to clear.
Hurdle one: rising slurry pH
In water, cathode particles undergo Li⁺/H⁺ exchange, and residual LiOH and Li₂CO₃ on their surfaces dissolve. A 2020 Oak Ridge National Laboratory (ORNL) study found that after three days in water, nickel-rich NCA and NMC532 reached pH 11.5–12.5, while nickel-free materials reached 9.0–10.5.
pH drift changes how binders and dispersants behave, and it leads straight to the second hurdle. That is why we track aqueous cathode slurries as a pH-versus-hold-time curve, not a single reading.
Hurdle two: the passive film on aluminium
Aluminium current collectors rely on a thin oxide passive film; the more alkaline the slurry, the more easily that film is attacked. Corrosion releases hydrogen, leaving pinholes and blisters in the coating, weaker adhesion and higher interfacial resistance. The ORNL authors note that every pH they measured points to a corrosion risk for the foil.
Common responses include adjusting pH with weak acids or high-molecular-weight polyacrylic acid (PAA) — the route ORNL used to process NCA in water — using carbon-coated aluminium foil, coating the particles, and shortening water contact and slurry hold times.
Hurdle three: residual water
Water is harder than NMP to remove completely from a porous electrode. Residual water reacts with LiPF₆ electrolyte to form HF, which attacks the cathode interface and affects first-cycle efficiency, gassing and cycle life. Drying and vacuum-baking schedules have to be designed together with the formulation.
Why start with LFP
LFP contains no nickel and works at a lower potential, and its pH rise in water is much smaller than that of nickel-rich NMC — which makes it the realistic entry point for aqueous cathodes. The challenge shifts to dispersion: small, carbon-coated particles with large, rather hydrophobic surfaces need dispersants and binders designed together. Nickel-rich NMC remains mainly a PVDF / NMP process today.
How CASU approaches it
We manage aqueous cathodes as one reaction chain. We measure slurry pH over hold time, bring the aluminium interface into the evaluation — adhesion, corrosion sites and interfacial resistance together — and fix the drying and baking schedule to control residual water. Every judgement is benchmarked against a PVDF / NMP reference electrode made under the same conditions, so the result can support a decision on the next stage.