Study On Anode Of High Energy Density Potassium Ion Hybrid Capacitors

The hybrid capacitors（HCs）,especially lithium-ion hybrid capacitors（LIHCs）,have been considered as a superior alternative to lithium-ion batteries（LIBs）.Unlike LIBs,the HCs composed of two electrodes with battery behavior and electrochemical capacitor behavior,respectively,always possess the merits of high energy/power density and long cycling stability.However,the rareness and uneven distribution of lithium resources restricts the further exploitation of LIHCs to a large extent.Therefore,the sodium（Na）and potassium（K）,with similar electrochemical properties to that of Li,based HCs have been intensively studied as appealing alternatives for LIHCs.The standard electrode potential of potassium(-2.93 V_SHE)is closer to that of lithium(-3.04 V_SHE)than that of sodium(-2.71 V_SHE),which makes potassium ion hybrid capacitor（PIHCs）have a higher electrode potential and an enhanced energy density than SIHCs.It is worth noting that K⁺can form energy-stable intercalated compounds（KC₈）with graphite,with a theoretical capacity up to 279 m A h g^-1.Meanwhile,compared with Li⁺and Na⁺,K⁺has the lowest ion-solvent interaction energy and higher conductivity.Based on these advantages,PIHCs has received extensive attention.Recently,its development still stays at the preliminary stage,which is mainly limited by the kinetic imbalance between two electrodes.Consequently,it is of great significance to develop high-performance anode material for PIHCs to improve energy density,power density and other properties.In this thesis,new types of potassium ion battery anode materials were synthesized,and the commercial activated carbon was used as the cathode to assemble the potassium ion hybrid capacitor.The systematic electrochemical research and mechanism analysis were carried out,and the main research contents were as follows:（1）In Chapter 2,a cocoon silk chemistry strategy was employed to synthesize a hierarchically porous nitrogen-doped carbon（SHPNC）.The as-prepared SHPNC with high surface area and rich N-doping not only offers highly efficient channels for the fast transport of both electrons and K ions during cycling,but also provides sufficient void space to relieve volume expansion of electrode,improving its capacity and stability.Therefore,PIHCs with SHPNC anode and activated carbon cathode exhibit a high energy of 135 W h kg^-1 and long lifespan.This study defines that the PIHCs show great application prospect in the field of high-performance energy storage devices.（2）In Chapter 3,we reported an architecture of Nb Se₂ nanosheets embeded in N,Se co-doped carbon nanofibers（Nb Se₂/NSe CNFs）were reported as flexible,free-standing and binder-free anode for potassium-ion hybrid capacitors.The Nb Se₂/NSe CNFs with hierarchically porous structure and N,Se co-doping afford highly efficient channels for fast transportion of potassium ions and electrons during repeated cycling process.Furthermore,excellent electrochemical reversibility of the Nb Se₂/NSe CNFs electrode was demonstrated through in-situ XRD,in-situ Raman,ex-situ transmission electron microscopy and element mapping.Thus,PIHCs with the Nb Se₂/NSe CNFs anode and active carbon cathode ac hieve a high energy of 145 W h kg^-1 at current density of 50 m A g^-1,as well as a ultra-long cycle life of over 10000cycles at high current density of 2 A g^-1.These results indicate that the assembled PIHCs display a great potential for applications in the field of ultra-long cycling energy storage devices.