Aqueous processing of positive electrodes for lithium-ion batteries offers significant ecological and economic advantages over the established NMP-based route. However, nickel-rich layered oxide cathode materials such as NMC811 are sensitive to water: upon contact, lithium leaches from the material, hydroxide and carbonate surface layers form, and electrochemical performance deteriorates. Two mechanisms have been proposed to explain this degradation: surface reconstruction driven by lithium leaching, and direct insertion of protons into the crystal lattice via Li⁺/H⁺ exchange. Which mechanism predominates remains an open question.
To address this, we employed deuterium oxide (D₂O) as an isotopic tracer during electrode production. D₂O behaves chemically almost identically to H₂O but can be distinguished via mass spectrometry, enabling us to track the fate of water-derived species within the cathode material. The poster presents a comprehensive comparison of NMC811 cathodes processed with H₂O, D₂O, and NMP (as a conventional reference).
SEM imaging confirmed analogous morphological behavior for the H₂O- and D₂O-processed cathodes at all stages of their life cycles, with both showing increased surface residues and less defined secondary particles compared to the NMP reference. Electrochemical long-term cycling over 500 cycles demonstrated equivalent performance for both aqueously processed samples, while the NMP-processed reference retained significantly higher capacity, confirming that the water-induced degradation is independent of the isotope used.
The key insight emerged from Evolved Gas Analysis coupled with mass spectrometry (EGA-MS). Measurements of the treated active materials showed complete absence of D₂O signals (m/z = 20), confirming that the processing solvent is fully removed during drying. The H₂O signal (m/z = 18) was comparable across all samples, indicating that detected water originates from surface residues such as LiOH rather than from the processing solvent. Crucially, a distinct HDO signal (m/z = 19), corresponding to a molecule containing one deuterium and one hydrogen atom bound to oxygen, was observed exclusively in the D₂O-treated material. This signal, with its maximum at 295 °C and extended release up to 700 °C, demonstrates that deuterium penetrated into the NMC crystal lattice during processing, displacing lithium ions from their sites. The prolonged evolution at elevated temperatures indicates that this insertion reaches beyond the material’s surface into the bulk structure, consistent with the Li⁺/H⁺ exchange mechanism proposed by Shkrob et al.
These findings establish D₂O as a reliable proton tracker for investigating water-material interactions in battery electrode manufacturing and provide direct evidence that aqueous processing affects NMC not only at the surface but within its crystal structure.