The Preparation And Electrochemical Research Of Lithium Ion Nano Array Electrode Material

With the development of hybrid electric vehicle, and its demanding of the high energy density and high power density of energy storage devices,lithium-ion hybrid supercapacitors is being attention in recent years as a new type of energy storage element.Because using both the electrode material of lithium ion battery and super capacitor, lithium-ion hybrid supercapacitors have dual characteristics of the lithium ion battery and supercapacitor, and has a larger energy density than conventional capacitors, and the higher power density than lithium ion battery. Lithium-ion mix of super capacitor electrode material contains both charge adsorption activity of high specific surface area of the capacitor active material, and can be reversible embedded or off with lithium ion battery materials of oxidation-reduction reaction.Its energy stored procedure contains both lithium ions and the electrode material body of reversible Faraday chemical reaction, electrochemical active material, including the reversible suction stripping process.This paper mainly studied the electrode material of lithium iron phosphate （LiFePO₄） and lithium titanate (Li₄Ti₅O₁₂) which are the composition of lithium-ion hybrid capacitors. They have the defects of low electrical conductivity and low Li+ diffusion coefficient, which restricts them from using in the rapid charge and discharge electrical devices.In this paper, in order to improve the electrical conductivity of the lithium ion electrode materials, lithium ion diffusion coefficient, energy density and cycle performance, and the critical problem is how to keep the high rate capacitance of the electrode materials in the rapid charge and discharge electrical devices. On the one hand, controlling the morphology and structure of the electrode materials such as nano arrays, and on the other hand through the modified electrode material itself, such as carbon coating, ion doping are the effective methods to improve the electrical conductivity and lithium ion diffusion coefficient of the electrode materials, makng them under high current density also have high specific capacity, and realizing the fast charge and discharge. Using SEM, XRD, Raman, EDS and other methods to analyze the different morphology and structure of the electrode materials, and then using cyclic voltammetry and charge-discharge test method and ac impedance method for different electrode materials of electrochemical properties were tested and analyzed. Here are some research conclusions of this article:（1） Preparation, morphology structure characterization and electrochemical performance of titanium nitride basal carbon coated lithium iron phosphate electrode（C-LiFePO₄/TiN）. Based on the advantages and disadvantages of the various synthetic methods, lithium iron phosphate nanoparticles were finally synthesized by hydrothermal method. Carbon coated lithium iron phosphate was formed by calcining the lithium iron phosphate nanoparticles with the sucrose as carbon source under nitrogen atmosphere. Finally the carbon coating lithium iron phosphate nanoparticles attached to the titanium nitride nanowires by insitu deposition, to become a carbon coated lithium iron phosphate electrode materials attached to the titanium nitride substrate （C-LiFePO₄/TiN）.Through electrochemical test, when the current density of 1 A g^-1, the specific capacitance of LiFePO₄/TiN and C-LiFePO₄/TiN electrode materials is 314 F g^-1 and 972 F g^-1, respectively.When the current density increased to 20 A g^-1, the specific capacitance of LiFePO₄/TiN and C-LiFePO₄/TiN electrode materials is 140 F g^-1 and 472.4 F g^-1, respectively. And when the current density is 20 A g^-1, after 400 laps, the specific capacitance of LiFePO₄/TiN and C-LiFePO₄/TiN electrode materials dropped 9.5% and 3.7%., respectively. So the C-LiFePO₄/TiN electrode materials has the good large current charge and discharge properties and cycle stability. Electrochemical impedance spectroscopy shows that when the open circuit voltage is 0.0 V, the Rct of LiFePO₄/TiN and C-LiFePO₄/TiN is 0.3645Ω and 0.2512 Ω, respectively, and the equivalent circuit of the resistance is 5.447 Ω and 2.678 Ω, respectively; When the open circuit voltage is 0.5 V, the Ret of C-LiFePO₄/TiN is 0.2226 Ω, and the equivalent circuit of the resistance is 2.345 Ω, which indicates that the introduction of high specific surface area, high conductivity of TiN as basal material, also helps to improve the electrochemical performance of materials, and carbon coating can effectively improve the electrical conductivity of the material itself.（2） Preparation, morphology structure characterization and electrochemical performance of carbon coated iron doped lithium titanate (C-Fe/Li₄Ti₅O₁₂) electrode.The titanium dioxide formed from anodic oxidation was used as titanium source and the lithium hydroxide was uaed as lithium source. Lithium titanate nanotube array was synthesized by solid-liquid phase reaction method. And then with ferric nitrate as the source of iron, sodium dodecyl sulfate as a carbon source, C-Fe/Li4TisO12 nanotube array electrode materials were synthesized by calcining in the nitrogen atmosphere. Through the charge and discharge test, the results showed that when the current density of 1 Ag^-1, the specific capacitance of Fe/Li₄Ti₅O₁₂ is 516 F g’’.When the current density increased to 10 Ag^-1, the specific capacitance of Fe/Li₄Ti₅O₁₂ is 350 F g^-1;when the current density of 1 Ag^-1, the specific capacitance of C-Fe/Li₄Ti₅O₁₂ is 768 F g^-1.When the current density increased to 10 Ag^-1, the specific capacitance of C-Fe/Li₄Ti₅O₁₂ is 379 F g^-1,which further showed that the carbon coating can effectively improve the specific capacitance of electrode material itself. When the open circuit voltage is 0.5 V, the Rct of Fe/Li₄Ti₅O₁₂ and C-Fe/Li₄Ti₅O₁₂ is 14.1Ω and 4.15Ω, respectively, because carbon can be effectively reduced the charge transfer resistance of Fe/Li₄Ti₅O₁₂ itself, improve the conductivity of the electrode material itself. When the open circuit voltage is 0.5 V, the Warburg impedance of Fe/Li₄Ti₅O₁₂ and C-Fe/Li₄Ti₅O₁₂ is 0.4869 Ω and 0.4183 Ω, respectively. Both of them have lower Warburg impedance, because L14Ti5O12 itself presents a nanotube array structure, provides an effective channel for electron transfer.And Fe3+ replaced Ti4+ location to provide more holes for the electron migrating. Hence, more electrons in the Li4TisOi2 move, and the Li₄Ti₅O₁₂ shows stronger electrical conductivity. And C-Fe/Li₄Ti₅O₁₂ has lower Warburg diffusion impedance, shows that carbon effectively coated on the surface of the Fe/Li₄Ti₅O₁₂, and carbon coated accelerate the ions in the transmission process between the electrodes and the electrolyte. In order to verify that the stability of electrode material under the large charge and discharge current density, cycle stability test was prepared. Under 10 A g^-1, when the cycle is 500 times, the specific capacitance of Fe/Li₄Ti₅O₁₂ fell 11.2%, and the specific capacitance of C-Fe/Li₄Ti₅O₁₂ fell 2.5%. Compared with Fe/Li₄Ti₅O₁₂, C-Fe/Li₄Ti₅O₁₂ has better cycle stability of the electrode materials.（3） Preparation, morphology structure characterization and electrochemical performance of nitrogen doped lithium titanate (N-Li₄Ti₅O₁₂) nanowire and nanotube array electrode materials. The lithium titanate nanowires were synthesized by hydrothermal method. Using the titanium dioxide nanotubes prepared by anodic oxidationa as titanium source, and combining the method of solid-liquid phase to synthesis of lithium titanate nanotubes material. Lithium titanate nanowires and lithium titanate nanotubes material were nitrided in the NH3 atmosphere. At last, N-Li₄Ti₅O₁₂ nanowires and N-Li₄Ti₅O₁₂ nanotubes electrode materials were successfully obtained.When the current density of 1 A g^-1, the specific capacitance of N-Li₄Ti₅O₁₂ nanowires and N-Li₄Ti₅O₁₂ nanotubes was 607.2 F g^-1 and 814.5 F g^-1, respectively. When the current density increased to 20 A g^-1, the specific capacitance of N-Li₄Ti₅O₁₂ nanowires and N-Li₄Ti₅O₁₂ nanotubes was 182.9 F g^-1 and 352 F g^-1, respectively. In the electrochemical impedance spectroscopy test, under the open voltage of 0.5 V, the Rct of N-LiTi5O12 nanowires and N-Li₄Ti₅O₁₂ nanotube electrode materials is 1.45Ω and 0.147Ω, respectively. Because N doping can effectively reduce the charge transfer resistance of Li₄Ti₅O₁₂ itself, improve the conductivity of the electrode material itself. When the open circuit voltage is 0.5 V, the Warburg impedance N-Li₄Ti₅O₁₂ nanowires and N-Li₄Ti₅O₁₂ nanotubes is 0.5463Ω and 0.5463Ω, respectively, both of which have lower Warburg impedance, because N-Li₄Ti₅O₁₂ nanowire itself presents the cashier rice noodle array structure, and linear structure including mesoporous structure, so it has a larger specific surface area, improve the utilization of active material. N-Li₄Ti₅O₁₂ nanotubes present a nanotube array structure, and its special structure, not only increase the specific surface area of the electrode material itself, but also provides an effective channel for electron transfer, so it can effectively improve the utilization rate of the electrode materials,electronic transmission rate and lithium ion diffusion rate. Meantime, N doping effectively reduce the forbidden band width of Li₄Ti₅O₁₂, making more ion inside the Li₄Ti₅O₁₂ move. In order to verify that the stability of electrode material under the large charge and discharge current density, cycle stability test was prepared. Under 10 A g^-1, when the cycle is 500 times, the specific capacitance of N-Li₄Ti₅O₁₂ nanowires fell 5.1%, and the specific capacitance of N-Li₄Ti₅O₁₂ nanotubes fell 1.6%. Both the N-Li₄Ti₅O₁₂ nanowires and N-Li₄Ti₅O₁₂ nanotube electrode materials have better cycle stability.