Preparation And Performance Study Of High Energy Density Supercapacitors

Supercapacitors,also known as electrochemical capacitors（EC）,are a new type of energy storage device that borrows from the energy storage mechanism of secondary batteries,based on traditional capacitors.Dus to their many superior characteristics such as high power density,good cycle stability,long service life and fast charging/discharging times,which make them promising for development in fields such as transportation,aerospace,electronic information products,and national defense.However,the relatively low energy density of EC currently limits their further application.To increase the energy density of EC,researchcan be conducted in two directions:expanding the electrochemical stability window and increasing the specific capacitance.This article focuses on researching these two directions by first developing a dual layer water electrolyte with a wide electrochemical stability window,and then synthesizing high specific capacitance electrode materials based on this electrolyte.By improving both the electrochemical stability window and specific capacitance,the goal of increasing the energy density of ECs can be achieved.The detailed experimental plan as follows:（1）By formulating lithium bis（trifluoromethane sulfonyl）imide（Li TFSI）-erythritol（Et）strengthen electrolyte,the hydroxyl group in Et molecule has a stronger nucleophilicity than water molecules,which can replace water molecules around the cation in the solvation shell,thereby increasing the electrochemical stability of water molecules.Et molecules can break the hydrogen bonding structure of water molecules and form stronger hydrogen bonds with water molecules,which increases the stability of water molecules and thus widens the electrochemical stability window of EC.Through electrochemical performance analysis,it was found that the electrochemical stability window of the activate carbon（AC）symmetric EC composed of 1 M Li TFSI+37.9%Et electrolyte is the highest,which can be up to 2.1 V.At a current density of 0.5 A g^-1,the specific capacitance can reach 86.6 F g^-1.The energy density of this EC is 13.3 Wh kg^-1at a power density of 262.4 W kg^-1.After 10 000 cycles,the capacitance retention rate is still 73.8%.Compared with 1 M Li TFSI electrolyte,the electrochemical stability window is expanded by 0.3 V and the energy density is increased by 3.9 Wh kg^-1.（2）Due to its high cost,the application of Li TFSI is limited.To address this issue,we replaced Li TFSI with the less expensive CH₃COONa,and prepared CH₃COONa-Et electrolyte（which costs only 1/400 of Li TFSI）.The electrochemical stability window of the AC symmetric EC composed of 1 M CH₃COONa+37.9%Et electrolyte can reach 1.7V.At a current density of 0.2 A g^-1,the specific capacitance can reach 80.4 F g^-1.The energy density of this EC is 8.1 Wh kg^-1 at a power density of 84.9 W kg^-1.After 5 000cycles,the capacitance retention rate is still 89.3%.（3）Pure phase Li Mn₂O₄ powder material was successfully synthesized using a sol-gel method.The Li Mn₂O₄ electrode with an electrochemical stability window of 0.95V in 1 M Li TFSI+37.9%Et electrolyte,and a specific capacitance of 315.6 F g^-1 at a current density of 0.5 A g^-1.An asymmetric EC was assembled using a Li Mn₂O₄electrode,an AC electrode,and 1 M Li TFSI+37.9%Et electrolyte.This EC had a specific capacitance of 184 F g^-1 at a current density of 0.5 A g^-1,and an energy density of 27.6Wh kg^-1 at a power density of 259.7 W kg^-1,within an electrochemical stability window of 2.1 V.After 2 000 cycles,the specific capacitance retention rate was still 88.9%.Compared to the AC symmetric EC,the specific capacitance of the EC increased by over50%with an increase of 97.4 F g^-1,and the energy density also increased by over 50%with an increase of 14.3 Wh kg^-1.