Construction And Performance Study Of Polyimide-based Electrochemical Energy Storage Devices

In recent years,metal ion secondary batteries are dominating the portable electronic product market,and continuing to expand to the fields of power battery and large-scale energy storage because of the superior features,such as high energy density,flexible design and other characteristics.However,the low power density cannot satisfy the demand of instantaneous high-power equipment.At the same time,the safety,production cost and recycling issues have not been effectively improved.In contrast,electrochemical energy storage devices based on organic electroactive materials not only have energy density comparable to that of metal ion secondary batteries,but also show higher power density.However,organic electroactive materials are highly soluble in organic electrolyte,resulting in a short cycle lifespan.Based on the above problems,this thesis mainly centers on the design and construction of electrochemical energy storage devices based on organic electroactive materials with low cost,high safety and long lifespan.The design and preparation of electrode materials,charge storage mechanism,electrolyte development and selection as well as devices optimization and construction have been carried out in our thesis,summarized as follows:（1）Construction and performance of sodium ion capacitors based on high-concentrated electrolyte:Due to the reduction of free water molecules in the high-concentrated electrolyte,the as-prepared Na Cl O₄ electrolyte(17 m,mol kg^-1)could deliver a high working voltage of 2.75 V.Electrochemical tests show that polyimide（PI）anode can undergo a rapid and reversible enolization reaction with Na⁺,and show excellent rate capability and long cycle lifespan due to the low solubility of organic electroactive materials in the aqueous electrolyte,the low water molecular activity as well as high ionic conductivity.Meanwhile,a new strategy of applying high-concentrated aqueous electrolyte to sodium ion capacitor is proposed.Matching with porous carbon microspheres as capacitive cathode to construct aqueous sodium ion capacitor,the as-assembled PI//PCMS sodium ion capacitor with a working voltage of 2.0 V exhibits a maximum energy density of 46.5 Wh kg^–1 and a maximum power density of 8.9 k W kg^–1 based on the total mass of the anode and cathode material.（2）The preparation of layered MnO₂/CNTs composite and the construction of aqueous sodium ion capacitors:It is one of effective ways to improve the energy density of sodium ion capacitors by increasing the specific capacitance of capacitive material.Layered MnO₂/CNTs composites were prepared under an acidic condition by reflux method.Electrochemical tests illustrate that the behaviors of redox reaction onto the surface and Na⁺（de）intercalation into the interlayer space in the high-potential ranget result in a higher specific capacitance than that of electric double-layer material,and reveal the physicochemical properties of the phase interface at different potentials.A high specific capacitance of 322.5 F g^–1 can be obtained at a current density of 0.5 A g^–1.At the same time,the existence of a small amount of crystal water molecules between the layers avoids the collapse of material structure during charge/discharge process to enhance the cyclic stability.The constructed aqueous sodium ion capacitor with PI as anode displays a working voltage of 2.2 V,a maximum energy density and power density of 58.8 Wh kg^–1 and 11.0 k W kg^–1,respectively.After 10 000 cycles,the capacity retention is about 77.2%.（3）A novel aqueous ammonium dual-ion battery based on nitrogen-oxygen free radical polymer（PTMA）cathode:In order to avoid the corrosive action of high-concentrated electrolyte on current collector and improve the safety performance,a novel aqueous ammonium double-ion battery based on organic polymers was constructed.Studies show that PI anode delivers excellent electrochemical performance in 1 M（NH₄）₂SO₄ electrolyte.When the current density increases from 0.5 A g^–1 to 10 A g^–1,the specific capacity only changes from 157.3 m Ah g^–1 to 107.7 m Ah g^–1,indicating a good rate capability,which can be attributed to the small hydrated ionic size,the high ionic polarization degree and affinity of NH₄⁺.Additionally,the pendant radical groups of PTMA and the high ionic conductivity of SO₄^2–make PTMA electrode process a rapid charge-discharge capacity.When the current density increased by 20 times,its specific capacity retention is about 78.5%.Meanwhile,the energy storage mechanisms of cathode and anode have been revealed by employing Mott-schottky curve fitting,and the principle of mass matching between cathode and anode materials is expounded by in situ voltage detection.Finally,PI//PTMA ammonium dual-ion battery was fabricated with an actual working voltage of 1.9 V,a maximum energy density and power density of 51.3 Wh kg^–1and 15.8 k W kg^–1,respectively.（4）The synthesis of methylene blue functionalized graphene composite and the fabrication of hybrid ion capacitors:Adjusting p H value of electrolyte can not only enlarge the selectivity of electrode materials,but reduce/boost the decomposition potentials of the electrolyte.Methylene blue functionalized graphene composite was prepared by hydrothermal method.The electrochemical properties of composites were studied in an acid electrolyte with a high specific capacitance(317 F g^–1)in the positive potential range.Meanwhile,due to the largeπconjugation and electrostatic interaction between methylene blue and graphene,the composite exhibits a long cycle lifespan.The action mechanism of the stability of the electrode/electrolyte interface on the electrochemical performance of the electrode material was defined by the component analysis before and after the cycle test.An aqueous hybrid ion capacitor based on asymmetric electrolytes was constructed by matching the PI anode with high specific capacity.Due to the different p H values of anode and cathode electrolytes,the as-constructed hybrid ion capacitor can well operate within a working voltage of 1.9 V.In addition,the fast and reversible redox reactions of anode and cathode materials make the hybrid ion capacitor deliver a maximum energy density of 48.6 Wh kg^–1 and a maximum power density of 19.0 k W kg^–1.（5）The effect of quasi-solid-state electrolyte on the cycle performance of PI cathode:In order to alleviate the solubility of organic electroactive materials and increase the selectivity of electrode materials,Na-ion conducting gel electrolyte was prepared based on porous P（VDF-HFT）membrane.A quasi-solid sodium ion battery was constructed by employing PI as cathode.Studies illustrates that the interlaced pores in the gel electrolyte can not only sufficiently absorb electrolyte to meet electrolyte ions required for electrochemical reaction of electrode material,but also bind the dissolved active material to prevent it from crossing to the other electrode,and further to enhance the cycling performance of electrode material,which can be regarded as a new strategy to improve the solubility of organic active substances.At a low current density of 0.2C,the capacity retention of polyimide cathode can be increased from 50.6%to 80.4%after 1000 cycles,and at a high current density of 10C,the capacity retention can be enhanced from 70.5%to 90.2%.