Li₄Ti₅O₁₂ As Anode Materials For Lithium Ion Batteries

Spinel-structured Li₄Ti₅O₁₂ has drawn extensive attention as one of the most promising anode material candidates for rechargeable lithium batteries, due to its easy preparation, low raw material cost, environmental friendliness, improved safety and reliability compared with those of carbon electrodes. However, the key limitation of Li₄Ti₅O₁₂ is its low ionic and electronic conductivity, calculated to be 10^-9 S/cm at room temperature, which really cumbers its using for commercial application such as HEV (hybrid electric vehicle) and power storage batteries. In the present work, Li₄Ti₅O₁₂ was synthesized by solid-state reaction and the effect of Nb⁵⁺ doping, carbon coating, silver coating and multi-walled carbon nanotube surface modification about Li₄Ti₅O₁₂ was studied respectively.Spinel Li₄Ti₅O₁₂ was prepared by a solid-state method using TiO₂-anatase and Li₂CO₃ as raw materials. The investigations of SEM and TEM showed that the as-prepared samples have a high purity, good crystallinity, and homogeneous size distribution with a scale of 0.4-0.6μm. The effect of synthesis temperature, holding time and mechanical activation on the structure, morphology, and electrochemical performance of Li₄Ti₅O₁₂ has been discussed. It was found that the well-crystallized Li₄Ti₅O₁₂ prepared at 800℃for 24 h in air exhibited the best electrochemical performance and preliminary ball milling process also has a pretty positive impact on the particle characteristic of Li₄Ti₅O₁₂. An ex-situ X-ray diffraction pattern evolution method has proceeded for Li₄Ti₅O₁₂ electrode during the discharge and charge process and the results suggested that during the process of Li⁺ intercalation and de-intercalation, the cubic symmetry of the parent spinel remains unaffected and the change in the unit cell volume is less than 1 %.Li_4-xNb_xTi₅O₁₂(x=0, 0.02, 0.04, 0.08) was synthesized by solid-state method and X-ray diffraction showed that the as-prepared samples have a fine characteristic phase of spinel Li₄Ti₅O₁₂. Electrochemical evaluation presented that the initial discharge capacity and rate capacity were notably enhanced by Nb⁵⁺ doping, with the number of 124 mAh·g^-1 at 1 C for x = 0.04 Nb⁵⁺ added sample, which was nearly 20 mAh·g^-1 higher than that of undoped sample. But the capacity decreased as more than x = 0.04 Nb⁵⁺ was doped. The rate capability of Li₄Ti₅O₁₂ was slightly improved by double ball milling process though the discharge capacity was reduced. It was considered that the occupancy of the doped high valent ions at Li⁺ site triggered Li⁺ vacancies, which is favor of the interior lithium ions' movement, and then the electronic conductivity is improved consequently. Sub-micron sized Li₄Ti₅O₁₂/C composite has been synthesized by an one-step solid-state method using glucose, citric acid, polypropylene, and triple mixture as carbon precursors respectively. It was found that the Li₄Ti₅O₁₂/C composite sample prepared using glucose as the carbon precursor possesses the highest carbon coating amount of about 2 % and the best electrochemical performance, a discharge capacity of 180 mAh·g^-1 or 110 mAh·g^-1 at the rates of 0.5 C or 1 C after 40 cycles. The results of DTA/TG analysis demonstrated that the decomposing temperature of glucose or triple mixture is nearly at the same temperature range with the formation of Li₄Ti₅O₁₂, whereas citric acid is decomposed much earlier than that of Li₄Ti₅O₁₂ formation, which indicates a much lower electrochemical performance. Li₄Ti₅O₁₂/(Ag+C) composite was successfully prepared by using glucose as carbon precursor, AgNO₃ as silver source. The investigations of SEM and TEM showed that (80-100 nm) nano-sized silver particles are highly dispersed on the surface of Li₄Ti_O12 particles. It was found that the co-addition of Ag and C plays a positive role not only in improving the electrical contact between Li₄Ti₅O₁₂ particles and the current collector, but also in enhancing the crystallization process of silver, and therefore improves the capacity performance of Li₄Ti₅O₁₂ with a discharge capacity of 164 mAh·g^-1 and retained 130 mAh·g^-1 after 40 cycles at 1 C. Multi-walled carbon nanotube is a kind of prodigious material with good corrosion resistance, high electronic conductivity, unique structure and morphology, which effectively mitigates the particle coarsening and increases the conductivity of Li₄Ti₅O₁₂.