Fig. 4: Charge storage properties of TiO₂ for lithium and sodium ions.

a CV curves of the TiO₂ NPs at the sweep rate of 1.0 mV s⁻¹ showing small voltage offset between redox peaks. b CV curves of TiO₂-10 nm at sweep rates of 0.2–1.0 mV s⁻¹, indicating slight peak shifts with increasing sweep rates. c The potential gap between the cathodic and anodic peaks (\(\varDelta E\)) as a function of particle size and sweep rate for Na⁺ and Li⁺. A much smaller potential offset for sodium-ion storage is observed. d The fitted b-values of the (de)sodiation and (de)lithiation peaks of TiO₂ NPs, where the \({b}_{{{{{{{\rm{Na}}}}}}}^{+}}\)≈1 indicates a capacitor-like process. e Specific capacity vs. ν^−1/2 curves of the TiO₂ NPs. f The capacitive and diffusion contributions for the TiO₂ NPs at the sweep rate of 1.0 mV s⁻¹.