Self-Discharge Behavior Of Supercapacitors

Supercapacitors combine the advantages of secondary battery with high energy density and traditional capacitors with high power density,hence play an important role in the field of energy storage.Low energy density and high self-discharge rate of supercapacitors have always been the key problems that need to be solved urgently.In recent years,researches on improving the energy density of supercapacitors have emerged endlessly and some breakthroughs have been made.However,there is relatively little research on the selfdischarge behavior of supercapacitors.Most of the early research on self-discharge behavior focused on traditional carbon-based liquid-state supercapacitors.In fact,many new systems have emerged in recent years,including electrode materials,electrolytes and their devices.Therefore,there is a lack of in-depth understanding of self-discharge mechanism and selfdischarge behavior regulation of new supercapacitors.Aiming at the above problems,this thesis systematically studies the self-discharge behavior of supercapacitors from materials,electrolytes and devices,reveals the self-discharge mechanism of some new systems,and realizes the effective regulation of self-discharge behavior.The main research contents are as follows:(1)In view of the long-term problems of carbon-based supercapacitors from high selfdischarge rate and complicated self-discharge mechanism,we proposed an ion-confinement bentonite clay based solid-state electrolyte,which effectively inhibited the self-discharge of carbon-based supercapacitors from various ways.Specifically,we used a simple blending method to prepare a solid-state electrolyte(BISE)composed of bentonite clay,ionic liquid and polyurethane.The ion exchange effect and the liquid crystal properties of the silicon-oxygen bond in the bentonite clay molecule endowed BISE with ion confinement effect.As a result,the self-discharge rate of BISE-based supercapacitor was only 28.9%within 60 h,which was far lower than similar devices.Furthermore,the simulation of self-discharge theoretical equations and related characterization exhibted that BISE can effectively suppress the selfdischarge behavior caused by ion shuttle,ohmic leakage and Faradaic reaction.In addition,excellent thermal stability(>250℃)and mechanical flexibility of BISE ensured it to establish a stable electric double layer and good processability at high temperature,which laid a certain foundation for the development of high-temperature supercapacitors and large power capacitors.(2)As an electrode material for supercapacitors,MXene exhibts excellent performance in terms of power characteristics,volumetric capacitance,and rate capability,etc.However,there are few researches on the self-discharge behavior,which can even be described as blank.In view of rich chemistry and surface functionalization that seriously exacerbate the selfdischarge behavior of MXene-based supercapacitors,this thesis proposed a chemically interface-tailoring regulation strategy to tailor the functional groups on the surface of MXene,which successfully revealed and effectively alleviated the self-discharge behavior of Ti3C2Tx MXene-based supercapacitors.After chemically interface-tailoring engineering,the ratio of F element decreased from 8.09 at%to 0.65 at%,the average oxidation state of Ti element reduced from 2.51 to 2.39,and the adsorption energy also improved,which effectively reduced the self-discharge rate of MXene by 20%.Besides,the specific areal capacitance also improved.Moreover,we first revealed the self-discharge behavior of MXene by combining the first principle calculations and X-ray absorption fine structure characterization.The selfdischarge mechanism stemed from the process of mixed-potential driving related to tightbonding and loose-bonding,in which the effective improvement of the ratio of tight-bonding ions suppressed the self-discharge rate.(3)The problem of rapid decay of charge layer caused by difference of potential and ion concentration also exists in hybrid capacitor because one electrode is capacitive.In this thesis,a typical nanoscale flower-like NiCo2O4 material was prepared with the combination of hydrothermal method and heat treatment.A series of NiCo2O4//activated carbon,NiCo2O4//hierarchically porous carbons and NiCo2O4//MXene hybrid capacitors based on the positive electrode of NiCo2O4 were developed,and the self-discharge behaviors of these systems were tested and analyzed.The results showed that the confinement effect of ion transfer of BISE can effectively inhibit the self-discharge behavior of these hybrid capacitors,which showed a self-discharge rate of 30.8%within 12 h.Moreover,in view of the academic controversy pseudocapacitive mechanism of NiCo2O4,we also systematically studied the ion reaction kinetics of NiCo2O4 electrode material and its hybrid capacitor,and proposed that the rich mesoporous ultra-thin nanosheet structure and asymmetric structure design can promote the effective transformation of energy storage mechanism of battery type electrode materials to capacitive type.