Development Of Li₄Ti₅O₁₂ Negative Electrode Materials For Hybrid Supercapacitors And Study On Correlative Mechanisms

Supercapacitors can be described as high-energy-density, large-power-density, long-life, new energy storage devices, which have wide applications and large market demand. The current researches are mainly focused on improving energy density of electrode and lowing cost. Li₄Ti₅O₁2-based cathode materials with high energy density, steady cycling performance, capability of large current discharge and lower cost, were developed in this research, which can be used as perfectly cathodes of new hybrid capacitors.Synthesizing technologies of Li₄Ti₅O₁2 with amorphous TiO₂ as raw materials were firstly exploited by conventional solid phase reaction and sol-gel method, and the cost of materials and technologies are lower than other. Influential factors such as temperature, materials, time and proportion of reactants were researched by orthogonal design tests. Pure spinel Li₄Ti₅O₁2 can be synthesized by solid phase reaction at 950℃ for about 12h. It takes much less time than common solid reactions. The kinetic analysis shows that higher reaction temperature, active amorphous TiO₂ and newborn Li₂O from Li₂CO₃ decomposing could accelerate the reactions.Synthesizing technologies of nanocrystalline Li₄Ti₅O₁2 with much lower temperature and shorter time was first exploited by citric acid complexes sol-gel method in aqueous solution. It is found that proportion of materials, acidity of complex solution, sintering temperature time, and Cl^- impurity is very important for characteristics of Li₄Ti₅O₁2. According to result of IR, TGA-DTA, XRD and SEM analysis, structures of precursors produced by sol-gel method in aqueous solution and in using organic crosslinking agent are similar, which result to lower cost. During the roasting of precursors, a anatase TiO₂ intermediate phase was come through, Li₄Ti₅O₁2 crystal could be formed under 500℃, and a single spinel phase nanocrystalline Li₄Ti₅O₁2 could be prepared at 700℃. Li₄Ti₅O₁2 powders made at 800℃ were porous and loose aggregates with fine particle size, narrow particle size distribution and excellent electrochemical properties.Electrochemical characteristics researches showed that Li₄Ti₅O₁2 waspassivation free with excellent cycling behavior, and lithium ion was inserted into the spinel structure in a two-phase reaction at a constant potentials of approximately 1.55V vs. Li/Li+, and its practical capacity is about 100-1 SOmAh-g"1. Some kinetic parameters of Li4Ti5Oi2 electrode such as electrochemical impedance, double layer capacitance, exchange current density were effected by synthesizing methods, inserted-lithium states, conductivity and so on. The kinetic properties are more favorable to Li4Ti5Oi2 made by sol-gel method than by solid phase reaction, so that electrochemical polarization of the later is more serious and larger charge-discharge capability is imperfect. Therefor, chemical diffusion coefficients of lithium ions for Li4Ti5Oi2 prepared by sol-gel method were measured using the potentiostatic intermittent titration (PITT), the value is about lO^-lO^cm-s"1 and close to ten times to carbon negative materials for lithium ion batteries. Thus it takes advantage on charge-discharge performance under larger current.In order to enhance the electronic conductivity and electrochemical reversibility of Li4Ti50i2, according to theories of energy band and crystalline structure, chemical doping using the substitution of Mg2+, Ba2+, and Ca2+ for Li+, and Ag+, Al3+ and W6+ for Ti4+ by the sol-gel method were studied in the first time. Examinations indicated that electronic conductivity could be evidently improved by using Mg2\ Ba2+ and Al3+, but Ca2+ and W6+ were unfavorable for the properties of body. The Mg2+ ions are mainly distributed over the tetrahedral(8a) sites of the cubic spinel structure (Fd3m), and may block the channel oflithium ions. The Al3+ ions are mainly distributed over octahedral (16c) sites, so that the Li+-ion and electronic conductivity are enhanced, and the reversibility of Li+ insertion reactions is improved; One new compound having the character of multiphase reaction was synthesized by substitution of Ba2+ for Li+, which decreases the potential of lithium inserting reaction; Ag dispersed in Li4Ti50i2 in the form of metallic, and effect Li4Ti50i2 on its performances by mechanical blend.According to the studies on the Characters of Li4Ti5Oi2-based compounds, Li13Til6Alo.i04 and L^Mgo/TisO^ were more fit to the station of largercurrent and higher power than Li4Ti50i2, because their ohm impedance and electrochemical impedance as lithium extracted were decreased largely, exchange current density and Li+ ions chemical diffusion coefficient were enhanced, cycling performances were more steady. Li]3Tii.6A10.iO4 is the best of them in integrative characters, may be expected as negative materials in hybrid supercapacitors.In this thesis, a model was developed to calculate theoretical energy density in hybrid capacitors. The electric properties of cells are depended on characteristic of electrode materials and concentration of electrolyte.A new nonaqueous hybrid capacitor using an activated carbon cathode and a Lii3Tii6Al0.iO4 anode was assembled. The basic properties were examined, shows steady cycling performance, excellent electric properties, much larger energy density than double-layer capacitors, equal to or even superior to power density of lithium-ion batteries used in electrical vehicles.