Study On The High Performance Lithium-Ion Capacitors With Embedded Lithium And Its Electrochemical Performance

As one of the new types of energy storage devices,lithium ion capacitors combine the high power density of electric double layer capacitors with the high energy density of lithium ion batteries.They have attracted the attention of the majority of researchers.However,lithium ion capacitors will suffer during cycling.An unstable solid electrolyte membrane is formed,and some lithium ions cannot be separated after being inserted into the electrode material,resulting in irreversible capacity,which consumes a large amount of lithium ions in the electrolyte,thereby reducing the electrochemical performance of the lithium ion capacitor.It can be seen that it is very important to pre-load lithium-ion capacitors in advance.In response to this problem,this article uses different lithium source materials to pre-lithiate the anode material,and prepares multi-walled carbon nanotubes（MWCNTs）by chemical vapor deposition（CVD）,mainly use multi-walled carbon nanotube materials as the negative electrode and activated carbon（AC）as the positive electrode to assemble lithium-ion capacitors to explore the influence of different lithium source materials on the electrochemical performance of lithium-ion capacitors.The main research results are as follows:1.Use graphite and MWCNTs as negative electrode materials,and use stabilized metal lithium powder（SLMP）for pre-insertion lithium treatment,and use AC as the positive electrode material to assemble a lithium ion capacitor.By adding an appropriate amount of stable metal lithium powder to the carbon nanotube/graphite anode material,the SEI film can be effectively formed,and the irreversible capacity can be reduced.The transmission efficiency of Li⁺in the anode material is higher,which indirectly improves the charging and discharging efficiency of the capacitor.Maintain the high capacity that its theoretical capacity should have.When the current density is50 m A/g,the specific capacitance of the pre-lithiated lithium ion capacitor is 86.527F/g.After pre-insertion of lithium,when the current density is 50 m A/g,the energy density is 115.970 Wh/kg,and when the current density is 3 A/g,the power density value is 3953.85 W/kg.After 3000 constant current charge and discharge cycles,The capacity retention rate is 83.14%.2.Using perfluororesin as fluorine source,high temperature heating generates fluorine gas（F₂）,and metal lithium powder is fluorinated under inert gas nitrogen（N₂）environment to generate stable lithium fluoride（Li F）.MWCNTs are used as negative electrode materials,and Li F is used to pre-intercalate lithium on the negative electrode.AC is used as the positive electrode material to assemble a lithium ion capacitor and study its electrochemical performance.Experiments have shown that by pre-intercalating Li F,the Li⁺required for the formation of the SEI film of the lithium-ion capacitor is compensated,and the long-cycle performance of the lithium-ion capacitor is effectively improved.When the current density is 50 m A/g,the specific capacitance of the lithium ion capacitor pre-inserted with Li F is 127.55F/g,the energy density is 208.84 Wh/kg,and the current density is 3 A/g,the power density is 5605.6 W/kg,after 3000 long cycles,the capacity retention rate is 88.1%.3.Lithium nickelate（Li₂Ni O₂）and active materials are combined to form a positive electrode,and MWCNTs are used as the negative electrode material to assemble a lithium ion capacitor.The electrochemical performance of Li₂Ni O₂is investigated.The electrochemical performance of lithium ion capacitors is affected.The results show that the reversible capacity of Li₂Ni O₂material is 6.33%,and the irreversible discharge capacity is 325.4 m Ah/g.Li₂Ni O₂material is a highly irreversible material.The addition of Li₂Ni O₂can effectively supplement the Li⁺required by lithium-ion capacitors when generating SEI films,and significantly improves the specific capacity,power density,energy density and cycle stability of lithium-ion capacitors.Among them,when the added content of Li₂Ni O₂is 20%,the effect of addition is the best.When the current density is 50 m A/g,its specific capacitance is 117.84 F/g.When the current density is in the range of 0.05 to 4 A/g,its energy density is 196.4 Wh/kg,and its power density is 5688.7 W/kg.After 4000long cycles,the capacity retention rate still reached 90.42%,and the preparation process was simple,showing a good commercialization prospect.