Research On Lithium Ion Hybrid Capacitors Based On Mn Co₂O₄ Composite Electrodes

Due to the non-renewable nature of fossil fuel,energy issues have become one of the major challenges to the development of science and technology in the 21st century.In recent years,with the flourish of the portable electronic devices and new energy vehicles,the exploration of high-power and high-energy-density energy storage devices is particularly urgent.Lithium-ion batteries?LIBs?have a high energy density,but their power density were low;supercapacitors?SCs?exhibit a high power density,but their energy density were low.Lithium-ion capacitor?LIC?is a new type of energy storage device based on the dual energy storage mechanism between the electric double layer and the insertion/deintercalation of lithium ion.It combines the merits of LIBs and SCs,therefore it is expected to have a high energy and power density,and a rosy application prospect.However,researchers found that it has a low rate performance and cycle life.This paper aims to develop a new type of LIC with better energy density,rate performance and cyclic stability,by using MnCo₂O₄electrode as the research object.The main research contents of this paper are summarized as follows:1.Synthesis of granular MnCo₂O₄ by hydrothermal method and composite with reduced graphene oxide sheet to obtain MnCo₂O₄/rGO composite.The results showed that the rGO improved the conductivity and the specific surface area of the electrode materials.The composite materials had a specific capacity of 1170 mAh g^-1 at a current density of 0.1 A g^-1,which is better than the pure MnCo₂O₄.And then,the button-type MnCo₂O₄/rGO//AC lithium ion capacitor was assembled with MnCo₂O₄/rGO,which was pre-inserted with lithium,and activated carbon?AC?.The lithium ion capacitor had the highest energy density of 78.8 Wh kg^-1(at a power density of 198.8 W kg^-1).When the power density was 3017.3 W kg^-1,it still presented a energy density of 36.2 Wh kg^-1.Moreover,the capacitance retention rate was 76.9%after 1000 cycles,showing a good electrochemical performance.2.The MnCo₂O₄/TiO₂ composite was composed of TiO₂ nanotubes and MnCo₂O₄,synthesized by hydrothermal method.It turned out that the TiO₂ nanotubes could increase the specific surface area,was proved the rate performance and cyclic stability of the electrode materials.At a current density of 0.1 A g^-1,the specific capacity was up to 948 mAh g^-1,which is higher than that of the pure MnCo₂O₄.And then,the composite was assembled into a button-type MnCo₂O₄/TiO₂//AC lithium ion capacitor,where the MnCo₂O₄/TiO₂ was pre-inserted with lithium.It got the maximum energy density of 89.8 Wh kg^-1 at a power density of 254.8 W kg^-1.When the power density was 3407.6 W kg^-1,it still had a energy density of 44.3 Wh kg^-1.Furthermore,the capacitance retention rate was 81.9%after 1000 cycles,showing a good cyclic stability.3.The MnCo₂O₄/MoS₂ composite was composed of petal-like structural MoS₂,which was synthesized by hydrothermal method,and the MnCo₂O₄.Results showed that MoS₂ could increase the conductivity of the electrode material.At a current density of 0.1 A g^-1,the specific capacity was up to 1288 mAh g^-1,exceeding that of the pure MnCo₂O₄.And,the composite was assembled into a button-type MnCo₂O₄/MoS₂//AC lithium ion capacitor,where the MnCo₂O₄/MoS₂ was pre-inserted with lithium.The device showed a maximum energy density of 97.8 Wh kg^-1(at a power density of 313.7 W kg^-1)and a maximum power density of 2996.8W kg^-1(at a energy density of 45.6 Wh kg^-1).Additionally,the capacitance retention rate was 80.1%after 1000 charge/discharge cycles,demonstrating a good electrochemical performance.