As a cathode material in capacitive deionization (CDI) devices, titanium disulfide (TiS2) successfully desalinated brackish seawater. Although performing stably for up to 70 cycles in the CDI devices, TiS2 was measured to have a much lower maximum capacity as a CDI electrode (~70 mAh/g) than as an Na-ion battery electrode (239 mAh/g). Elucidating the origin of such maximum capacity deviation in the same material under different conditions remains crucial to the future design and optimization of TiS2-based electrodes for CDI systems. Via spin-polarized density functional theory calculations, we systematically investigated stacking patterns, ion intercalation sites, energetics of intercalated compounds, and lattice deformation during sodium intercalation. The predicted phase transformation and capacitance-voltage curve agree fairly well with previous measurements. We present evidence that the different maximum Na-ion capacities of TiS2 measured in aqueous (as a CDI electrode) and dry (as an Na-ion battery electrode) environments originate from weaker interlayer interactions in the presence of water, leading to mechanical instability with respect to shear strain after ~33% intercalation of Na+-H2O pairs. The atomic-level understanding of the ion-water intercalation mechanism into TiS2 interlayers and from the insights gleaned suggest new design and optimization strategies for TiS2-based CDI electrodes.

Atomic-level understanding of sodium ion and water intercalation into titanium disulfide interlayers for water desalination

ACS Spring Meeting 2021

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