Electrode Preparation And Electrochemical Performance Of Flexible Lithium Ion Capacitors

Recent years,portable and wearable electronic devices have attracted much attention in many application fields,and the demand for flexible energy storage devices is also increasing day by day.Flexible Li-ion capacitors are attractive due to the characteristics of high energy density,high power density and long cycle life.Lithium titanate(LTO),as the cathode material of Li-ion capacitor,plays an important role in the overall performance of the device.In this paper,Electrostatic Spray Deposition technique was used to prepared a binder-free flexible lithium titanate electrode.The effect of substrate temperatures on the electrode morphology and electrochemical performance was studied In addition,by adding cetyltrimethylammonium bromide,LTO was modified by carbon coating to improve electrodes rate performance,and a Nitrogen-doped carbon coated LTO/N-C flexible electrode was prepared.Finally,the Li-ion capacitor was assembled,and the practicability of the LTO/N-C electrode was tested and evaluated.The research work carried out and main results are as follows:First,the flexible LTO electrode was prepared on carbon paper by Electrostatic Spray Deposition technique.Comparing the XRD analysis results of the products obtained by roasting at different temperatures for different times,it was found that pure phase LTO can be obtained by holding the precursor at 700? for 6 h.In the deposition process,the substrate temperatures have a great influence on the morphology of LTO.The substrate temperature changes from low to high,while the morphology of the product layer changes from dense and smooth to loose and porous.The products with excessively high substrate temperature show.a dense block shape that is not connected to each other,resulting in a significant decline in electrode performance.The size of the LTO particles prepared by the Electrostatic Spray Deposition technique is small,which is beneficial to improve electrochemical performance of electrode.The experimental results show that the unmodified pure LTO has poor rate performance,resulting in a low specific capacity of the flexible LTO electrode at high rates(the specific capacity at 20C is only 39.2 mAh g-1).The results also confirmed that the Electrostatic Spray Deposition technique has certain advantages in the preparation of bipder-free flexible electrodes,which can not only improve the energy density of the electrode,but also ensure the mechanical stability of the electrode,and the preparation efficiency is higher than some traditional physical deposition methods.In order to further improve the rate performance of the LTO electrode and increase the flexibility of the electrode,a stainless steel mesh was used as a flexible substrate,cetyltrimethylammonium bromide was added to the precursor solution.A nitrogen-doped carbon coating LTO/N-C flexible electrode was prepared by Electrostatic Spray Deposition technique.The LTO/N-C electrode obtained excellent electrochemical performance.To analyze the reasons,first of all,the nanometer-sized LTO particles and the active material layer with higher specific surface area are more conducive to the rapid transfer of lithium ions,and the electrode performance is significantly improved.Another important reason is that the highly graphitized nitrogen-doped carbon coating layer effectively improved the electronic conductivity of the electrode and further improved the rate performance of the electrode.In addition,the LTO/N-C electrode exhibited better cycle performance.At 50C,the specific capacity of the LTO/N-C electrode is still up to 94.1 mA h g-1.After 1000 cycles at 3C,the specific capacity is 83%of the initial capacity,the Coulomb efficiency is maintained at 100%.The 3D network structure of the stainless steel mesh greatly increases the contact area between the electrode and the electrolyte,and also improves the performance of the electrode.Moreover,the intersections of the stainless s teel mesh can buffer the electrode during the bending process,which significantly improves the mechanical stability of the electrode.Finally,the flexible AC electrode was prepared with carbon felt as the substrate,and a soft-packaged Li-ion capacitor was assembled with a flexible LTO/N-C electrode.When the power density was 3.5 kW kg-1,the energy density was about 125 Wh kg-1,indicating that LTO/N-C flexible electrode can be used for the development of flexible lithium-ion capacitors.