Effects of cobalt doping on structural, optical, electrical and electrochemical properties of Li4Ti5O12 anode

Abstract

Cobalt-doped Li4Ti5O12 (LTO) i.e Li4Ti5−xCoxO12 (x = 0, 0.05, 0.1, 0.15, 0.2) powders have been synthesized by solid state reactions and examined for effects of Cobalt (Co) concentration on structural, optical, electrical and electrochemical properties of LTO. X-ray diffraction (XRD), Raman spectroscopy and field-emission scanning electron microscopy (FESEM) evaluate phase composition and morphology of samples. Optical study reveals reduction in bandgap Eg of LTO from 3.4 to 2.7 eV for x = 0.15, with Eg extension into visible region around 515–740 nm due to increased conduction electrons and energy levels from 3d7 orbitals of Co and induced oxygen vacancies. Electronic conductivity of sample with x = 0.15 increases by 104 as compared to LTO due to conversion of Ti4+ to Ti3+ ions and increased Ti4+-Vo-Ti3+ hopping centers. Ionic conductivity and diffusivity increase upto 2.0×10−7Scm−1 and 4.6×10−12cm2s−1 for x = 0.15, owing to increased lattice spacing by substitution of Ti4+ with Co2+. Frequency dependent conductivity suggests hopping of Li+ ions as dominant conduction in LTO. Thermally activated Li+ ions follow different conduction mechanisms in different temperature regimes. Low activation energies Ea of 0.3–0.5 eV indicate conduction of ions through interstitial pathways i.e 8a-16c-8a. High Ea of 0.7–1.15 eV suggest hopping of ions through vacancy/defect mediated channels or other long routes. Electrochemical tests demonstrate unexpected degradation in electrochemical performance for dilute dopant concentration; x = 0.05 and 0.1, as compared to LTO, which is attributed to substitution of Co2+ ions at Li+ tetrahedral (8a) sites. However, for x = 0.15 electrochemical activity gets better indicating substitution of more of Co2+ ions at Ti4+ octahedral sites.