Anode Material For Electrochemical Supercapacitors Li <sub> 4 </ Sub> Ti <sub> 5 </ Sub> O <sub> 12 </ Sub>,

Supercapacitors coupled with batteries and fuel cells are considered promising mid-term and long-term solutions for low- and zero-emission transport vehicles by providing the power peaks for start-stop,acceleration and recovering the breaking energy.Nowadays,many researchers on the electrochemical capacitors aim to increase power and energy density as well as lower fabrication costs while using environmental friendly materials.The most useful approach is to develop hybrid systems that typically consist of an electrochemical double-layer capacitor(EDLC) electrode and a battery electrode.In the year 2001,an AC/Li₄Ti₅O₁₂ hybrid supercapacitor was reported.In the hybrid system,both increase of the working voltage and high energy density of the negative result in a significant increase of the overall energy density of the capacitors.But the power density depends on the rate capability of the intercalated compound Li₄Ti₅O₁₂.In order to obtain a high rate capability,two typical approaches have been developed to overcome this problem. One is to develop the nano-sized Li₄Ti₅O₁₂,and the other way is to reduce the electrode polarization by improving its electronic conductivity.My study mainly focused on the preparation of the nano sized Li₄Ti₅O₁₂ with high electronic conductivity.The electrochemical performance of the nano sized Li₄Ti₅O₁₂ and the hybrid supercapacitors consisting of Li₄Ti₅O₁₂ and AC were also studied in detailed.1.Carbon coated Li₄Ti₅O₁₂ by chemical vapour decomposition(CVD) method:the graphitized-carbon was coated on the surface of Li₄Ti₅O₁₂ through a CVD process. The electronic conductivity increased as coating temperature increased,but it results in poor electrochemical profile over 800℃.The optimal condition for Li₄Ti₅O₁₂ coating should be performed at 800℃.The graphitized carbon layer of Li₄Ti₅O₁₂/C obtained at optimal condition is about 5 nm thickness.It shows a much higher electronic conductivity of 2.05 S/cm than the raw one's(＜10^-13 S cm^-1).In the rate capability test,the supercapacitor containing a carbon-coated Li₄Ti₅O₁₂ negative and activated carbon positive electrode keeps 50%of initial capacity at the rate of 24C (0.6 A/g) compared with 29%of the raw one's.AC impedance tests reveal that the coated carbon layer can decrease the interracial charge-transfer resistance at some extent.The results demonstrated that the thermal vapor decomposition is a promising approach to improve the electronic conductivity of the Li₄Ti₅O₁₂. 2.Nano-sized Li₄Ti₅O₁₂ prepared by molten salt method:a nano-sized lithium intercalated compound Li₄Ti₅O₁₂ was prepared by using LiCl as a high temperature flux.Li₄Ti₅O₁₂ powders were easily obtained with homogeneity,regular morphology, and narrow particle-size distribution using molten LiCl as a high-temperature solvent. The flux produces a liquid/solid reaction interface,thus provides a large effective reaction area,and accelerates the Li₄Ti₅O₁₂ growth at a relatively short time.The particles size decreases and the distribution becomes narrow with the increasing flux content.Under the optimal synthetic condition(750℃for 1 h,N=16),the average particle size is of 100nm and the sample has a discharge capacity of 159 mAh/g.The hybrid capacitor fabricated with this nano-sized sample and activated carbon show much better rate capability,even at 100 C discharge rate,the hybrid capacitor also keeps 60%of capacity compared with 3C discharge rate.3.Carbon coated Li₄Ti₅O₁₂ nano particles by carbon pre-coating process:In this synthesis process,the TiO₂ precursor is firstly coated with a carbon layer by a CVD method.After mixed with proper ratio of lithium salt,the carbon coated TiO₂ was then annealed at 800℃for 9 hrs.With the presence of carbon layer,the final product Li₄Ti₅O₁₂ is effectively prevented from agglomerating even under a high temperature solid state reaction.The carbon coated Li₄Ti₅O₁₂ nano particles has a partile size about 20-50 nm and has a capacity about 150 mAh/g at a current rate of 0.1 C(0.015 A/g).The rate performance of the carbon-coated nano-sized Li₄Ti₅O₁₂ particles is outstanding than the Li₄Ti₅O₁₂ samples with bigger particles.When the rate is 25C (3.75 A/g),the capacity can still retain about 110 mAh/g(70.6%).The particle-size has a direct influence on the electrochemical rate performance of the cell.Smaller particles promote shorter pathways for solid-state diffusion of Li ions and result in better rate capability.And also the carbon layer can reduce electrode polarization by improving the electronic conductivity of Li₄Ti₅O₁₂.4.Li₄Ti₅O₁₂ materials with specialized morphologies obtained by the carbon pre-coating process:The carbon-coated nanostructure Li₄Ti₅O₁₂ products with various morphologies were achieved by a carbon pre-coating method from different TiO₂ precursors,for example Li₄Ti₅O₁₂ nano-rod,hollow sphere,nanoparticle,and microsphere.The pre-coated carbon layer can prevent the TiO₂ precursor particles from aggregation when reacted with lithium salt to form nanostructure Li₄Ti₅O₁₂,and keep the morphology of precursor.For instance,the Li₄Ti₅O₁₂ nano-rod has the diameter about 50-80 nm with a carbon layer wrapped around its surface.As a result of the specialized morphology and electronic conductive caobon layer,the Li₄Ti₅O₁₂ nano-rod can retain about 79%capacity even at the rate of 20 C.Moreover,the coated carbon layer can enhance the electronic conductivity of Li₄Ti₅O₁₂.As a result,the nanostructure Li₄Ti₅O₁₂ obtained by the technology described in the present work, shows high rate capability and high stability for lithium-ion intercalation,which is fundamental for materials of high power electrode.5.Assembly of Li₄Ti₅O₁₂/AC nonaqueous hybrid supercapacitors:By using the carbon coated Li₄Ti₅O₁₂ nano particles prepared by a carbon pre-coating process as the negative electrode and AC as the positive material,the AAA type supercapacitor was fabricated.The Li₄Ti₅O₁₂/AC hybrid supercapacitor has an energy density about 6 Wh/kg which is about 2 times higher than that of the commercialized EDLC.And also,the relationship between Li₄Ti₅O₁₂ characters and supercapacitor rate performance was investigated.A commercialized Li₄Ti₅O₁₂(200 nm) and carbon coated Li₄Ti₅O₁₂ nano particles have been both used to fabricate the Li₄Ti₅O₁₂/AC hybrid supercapacitor prototype for comparison.The capacitor using carbon coated Li₄Ti₅O₁₂ nano particles as negative showed much better rate performance.Even under the rate of 40 C,it can retain about 62%capacity comparing with that of commercialized material 48%.6.FeOOH used as negative material in hybrid supercapacitor:A nano-structural iron oxyhydroxide(FeOOH) was obtained via a hydrolyzing route under mild and facile synthesis condition.FeOOH delivered a capacity of 170 mAh/g in the voltage range from 1.6 to 3.3 V.The nanostructural FeOOH was used as a negative electrode for an asymmetric hybrid electrochemical supercapacitor combined with an activated carbon negative electrode in 1.0 M LiPF₆ ethylene carbonate/dimethyl carbonate(1:2 in volume) solution.The FeOOH/AC cell shows a capacity of 30 mAh/g base on the overall active materials,corresponding to the energy density of 67.5 Wh/kg,two time of DELC.I This system also shows a good performance of cycle life and rate test.It remains approximately 100%capacity after 1000 cycles.Even at 10C discharge rate, the capacitor also holds 80%of capacity at 1C discharge rate.