Preparation And Electrochemical Performance For Lithium-ions Supercapacitor

Lithium-ion supercapacitor as a new type of supercapacitors, with higher power density than lithium ion batteries and higher energy density than the common supercapacitors, has a wide range of application prospects. In this thesis, we prepare two different systems of lithium-ion supercapacitor including LiMn₂O₄/AC lithium-ion supercapacitor and AC/Pre-lithiation graphite ?PG? lithium-ion supercapacitor, which are based on the goal of comprehensively enhancing the energy density of supercapacitor. Therefore, we aim at obtaining lithium ion capacitor having long cycle life and high energy density by controlling the morphology and the way of lithiation of the electrode material, and the structural design of the capacitor.?1? For the system of LiMn₂O₄/AC lithium-ion supercapacitor, we firstly prepared ?-MnO2 by hydrothermal method. LiMn₂O₄ have been synthesized via a solid-state reaction by using ?-MnO2 as the precursor. The structure and morphology of the electrode materials were characterized by the means of SEM, XRD analysis. LiMn₂O₄ likes rods with the radii of about 363 nm and lengths of about 2.3 ?m. The different mass ratio of cathode to anode of lithium ion supercapacitors were composed of LiMn₂O₄ microrods ?LMO-MRS? as cathode material and activated carbon ?AC? as anode material. The electrochemical performances of the lithium ion supercapacitor were investigated by galvanostatic charge-discharge, cyclic voltammetry and AC impedance methods. The research results show that, when the LMO-MRS/AC supercapacitor with the mass ratio of cathode to anode equal to 0.50 at 2 C current density, it has the highest specific capacitance of 35.85 F/g and energy density of 33.16 Wh/kg, wich is about 2.04 times than that of the AC/AC supercapacitor.?2? In order to futher increase the energy density of capacitor, we prepare the system of AC/PG lithium-ion supercapacitor. AC/PG lithium-ion supercapacitor was fabricated with activated carbon as cathode and pre-lithiation graphite as anode. We used electrochemical method to pre-lithiation for graphite anode to improve the open circuit voltage of the capacitor. Electrochemical tests show that the lithium-ion capacitor has a good capacitance characteristic. Comparing to the electric double layer capacitor, the work voltage of lithium-ion supercapacitor can be enhanced from 2.5 V to 3.8 V. So the energy density of supercapacitor can be enhanced 3.16 times than that of the electric double layer capacitor. After cycling 2000 times, the total capacity of the supercapacitor remains 98.2% in 2.2?3.8 V at the current density of 200 mA/g.?3? The soft package device of nominal capacity of 250 F of AC/PG lithium-ion supercapacitors were fabricated with using graphite as anode, activated carbon as cathode and plastic film as shell. The electrochemical test results show that the work voltage 3.8 V of lithium-ion supercapacitor is about 1 V higher than that of the traditional supercapacitor. The energy density can reach 18.21 Wh/kg, which is about four times that of the traditional supercapacitor. The capacity retention of lithium-ion supercapacitor is more than 85%after discharge at the 150 C current density. After cycling 10000 times, its discharge capacity remains 98%. The lithium-ion capacitor can smoothly through the overcharge, short circuit, extrusion and acupuncture and other safety performance testings.