Correlation of Electrochemical Characteristics with Structural, Optical and Electrical Properties in Li4Ti5O12-Based Anodes for Li-Ion Batteries

Abstract

This thesis comprises of a comprehensive and systematic study conducted on three series of Li4Ti5O12 (LTO) based anodes i.e Co-doped LTO, W-doped LTO and dual phase Li4Ti5O12/Li2TiO3 composite. Effects of varying divalent Co and hexavalent W doping concentration on the structural, optical, electrical and electrochemical properties of solid state synthesized Li4Ti5-xCoxO12 (0 ≤ x ≤ 0.2) and Li4Ti5-xWxO12 (0 ≤ x ≤ 0.2) systems, respectively, have been investigated in detail. Structure and morphologies of doped samples remain intact within the solubility limits (x 0.15) of dopants as observed in X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) studies. Substitution of both Co and W ions in oxidation states of +2 and +6, respectively, induces variation in the electronic structure of LTO which is evident in the reduced optical band gaps and partial reduction of Ti4+ (3d0) ions to Ti3+ (3d1) as observed in X-ray photoelectron spectroscopy (XPS). Optical bandgap Eg of LTO (3.54 eV) reduces significantly to 2.7 eV for 0.15 moles of Co and slightly to 3.3 eV for 0.15 moles of W, due to increased conduction electrons and energy levels. In comparison to LTO, both Co and W doped systems exhibit improved electronic conductivities at room temperature with the maximum increase of upto the order of 104 and 103, respectively, with dopant concentration of x 0.15. Improved electronic conduction is accredited to creation of Ti4+/Ti3+ ion pairs and induced oxygen vacancies for Co-doping while only Ti4+/Ti3+ ion pairs for W, as a consequence of charge compensation. Room temperature Li-ion conductivity/diffusivity values show the maximum increase of 2.0x10-7 Scm-1/4.6x10 12 cm2s-1 for Co2+ and 6.5x10-8 Scm-1/1.8x10-12 cm2s-1 for W6+ for x values of 0.15, owing to increased lattice spacing with substitution of dopant ions at Ti4+ sites. x However, Co doping shows an unusual behavior with varying concentration. In comparison to LTO, low concentration regime of Co doping (x 0.1) exhibits reduced lattice parameter in XRD, room temperature ionic conductivity, increased activation energy Ea of 0.7 eV recorded from Arrhenius plots of temperature dependent a.c. conductivity over 20 300℃ and unusual degradation in electrochemical performance which is attributed to substitution of majority of Co2+ ions at Li+ 8a tetrahedral sites of LTO lattice that hinders the usual interstitial conduction pathways i.e 8a-16c-8a for Li ions. With increasing x to 0.15 lattice parameter increases, room temperature ionic conductivity improves, Ea decreases to 0.48 eV indicating increased thermal conduction of Li+ ions through normal interstitial pathways in T range of 60 200℃ and electrochemical characteristics improve tending to approach that of pure LTO. These features indicate dominant substitution of Co2+ ions at Ti4+ 16d octahedral sites, with minor 8a tetrahedral Li+ site allocation. W6+ doping shows slow but continuously increasing lattice parameters, improved room temperature ionic conductivity values, low activation energy Ea 0. 0. e and impr ve electr chemical per rmance ith increasing c ncentration x, illustrating substitution of W6+ ions at Ti4+ 16d octahedral lattice sites of LTO which favor and ease the conduction of Li ions along 8a-16c-8a interstitial conduction channels. Frequency dependent conductivity measurements suggests that major part of electrical conduction in both pure and doped LTO comprises of hopping of Li-ions in the spinel lattice. Single-phase Li4Ti5O12 and dual-phase Li4Ti5O12/Li2TiO3 composite were also synthesized by sol-gel method aimed at size reduction of particles for improved electrochemical characteristics. Effects of compositing Li4Ti5O12 with electrochemically inactive stabilizing compound Li2TiO3 have been studied. Li2TiO3 xi did not effect the structure and morphology of crystalline Li4Ti5O12 as observed in XRD, Raman, SEM and TEM analysis. Li4Ti5O12 particles yield improved room temperature electronic and ionic conductivity of 3.8 x 10-8 Scm-1 and 7.1x10-8 Scm-1, respectively, in comparison to bulk LTO. Li4Ti5O12/Li2TiO3 composite reveals wide optical bandgap (3.6 eV), reduced room temperature electronic conductivity of 1.6x10-8 Scm-1 due to poor electronic conduction of Li2TiO3, good ionic conductivity σi of 4.2x10-7 Scm-1 and reduced Ea value of 0.32 eV indicating minimum energy requirement for conduction of Li ions through usual 8a-16c-8a pathways, attributed to increased grain boundary effects and increased phase-phase interfacial areas. Moreover, Li2TiO3 protects structural stability of Li4Ti5O12 by offering its Li and O for reactivity with electrolyte in SEI-film formation process over long cycling. These features enable long-term cycling stability of Li4Ti5O12/Li2TiO3 anode upto high current rates. Electrochemical performance characteristics of both doped and composited systems are governed by a number of fundamental physical features like; type and concentration of dopant, preferred substitutional sites of dopant (for doping), extrinsically contributed grain boundary effects (for composting), and resulting variations in; crystalline structures, electronic structures, optical properties and electrical conduction parameters and mechanisms.