Charge accumulation and polarization in titanium dioxide electrodes

Nanocrystalline TiO(2) electrodes were studied spectroelectrochemically by observing the simultaneous relaxation of the current and absorbance after applying a voltage step. The absorbance behaved differently in two time regimes: (1) ionic polarization in the oxide electrode, in which charged ions, such as Ti(3+) sites and/or interstitial Ti(4+) sites, move in response to the applied electric field, and (2) the diffusion of Li(+) ions into the TiO(2). These two behaviors were analyzed with equivalent circuit models. Li(+) ions reduce the resistance of the TiO(2) by approximately 90%, increase the capacitance by approximately 350%, and decrease the inductance by approximately 30%. Voltage cycling produces a buildup of intercalated Li(+) ions, lessening the electrode's response to the potential step, and causing it to become a more efficient inductor. The potential distribution in the nanoparticles is described by using a dielectric model in which roughly half the applied potential is dropped across the interface with a Li(+)-ion-containing electrolyte.