Preparation And Characterization Of Lithium-ion Capacitor Off/Embedded Lithium Material

As a new electrochemical capacitor, lithium ion capacitor, which is one of the best energy storing devices of vehicles, possesses higher power density and energy density than lithium ion battery and electric double layer capacitor, respectively. The lithiation/delithiation property of the lithium ion capacitor directly affects its rate capability and cyclic properties. Thus, we focus on the modification of lithiation/delithiation property of the electrode material with the hope of obtaining lithium ion capacitor having high energy density, excellent cyclic property and rate capability.Different electrode materials of the lithium ion capacitor including graphite, Li4Ti5O12and LiMn2O4were studied. The structure and properties of different electrode materials were characterized by means of BET, SEM, XRD, etc. The influences of layer-expansion treatments on the microstructure and electrochemical properties of spherical graphite were investigated. And AC/graphite lithium ion capacitor was assembled to study the effect of different layer-expansion treatments on the electrochemical performance. The effects of different carbon coating on the microstructure and electrochemical performance of Li4Ti5O12were studied. And AC/Li4Ti5O12lithium ion capacitor was assembled to study the influence of the amount of coated carbon on the electrochemical performance. LiMn2O4-AC/AC lithium ion capacitors with different LiMn2O4:AC ratios were assembled to study the electrochemical performance of the capacitor.The electrochemical performance of activated carbon was firstly analyzed, which showed that it was composed of big particles larger than10μm and small particles about1μm. The discharge capacity was about140F/g. The basal spacing and surface morphology of graphite were changed by layer-expansion treatments. Results showed that graphite treated by glacial acetic acid exhibited better performance for the discharge capacity and the coulombic efficiency under the current density of0.1mA/cm2, which were344.1mAh/g and92.6%, respectively. The influence of different graphite materials on the performance of lithium ion capacitor was investigated. It was found that graphite materials treated by sulfuric acid had better charge-discharge performance, while the cyclic performance was better under the treatment of glacial acetic acid. When AC/graphite lithium ion capacitor worked, AC was non-Faraday process and graphite materials was Faraday process. The internal resistance was the minimum treated by glacial acetic acid.Mesophase pitch was used as carbon sources to coat and modify Li4Ti5O12, and it was found that the treatment could make the surface of Li4Ti5O12smooth, but did not change the Li4Ti5O12 crystal structure. The results of electrochemical tests showed that the discharge capacity of Li4Ti5O12could be increased first and then decreased with increasing of coated carbon quantity. When the quantity of mesophase pitch was5%, the Li4Ti5O12/C composite material exhibited excellent electrochemical property. It was found that Li4Ti5O12coated with5%carbon had better performance when the lithium ion capacitor with AC was assembled as positive electrode and Li4Ti5O12/C as negative electrode. The results showed that the oxidation-reduction reaction of AC/Li4Ti5O12lithium ion capacitor was consistent with what happened in half-cell lithium ion battery. The internal resistance of the system was first decreased and then increased with the increasing of coated carbon quantity.LiMn2O4-AC with different LiMn2O4:AC molar ratios as positive electrode and AC as negative electrode was assembled, and it was found that rate capability and cyclic property of lithium ion capacitor were the best when LiMn2O4:AC molar ratio was1:1. The discharge capacity under5mA/cm2current density was49.5F/g, and the discharge capacity retention was87.3%after1000cycles. Faraday process took place when LiMn2O4-AC/AC lithium ion capacitor worked. Lithium ion capacitor still showed the impedance characteristics of a super capacitor, and the internal resistance of the system was first decreased and then increased with the increasing of the content of LiMn2O4. When LiMn2O4:AC molar ratio was1:1, it was20Ω.