Potassium Ion Hybrid Capacitors:Carbon-based Materials And Interfacial Properties

With the increasing energy demands,considerable efforts have been devoted to develop electrochemical energy storage devices with high energy/power density.Potassium ion hybrid capacitors（PIHCs）,combining the advantages of the high energy density of battery and the high power density of capacitor,are a new type of electrochemical energy storage device with great application prospects.However,the development of high-performance PIHCs is still limited by several bottlenecks:cathode/anode kinetics imbalance,electrode/electrolyte interface（solid electrolyte interphase,SEI）instability,and cathode/anode capacity/mass mismatch.In this dissertation,to address the above-mentioned scientific and technical problems,a systematic strategy of electrode material design,interface modulation,electrolyte optimisation and mass matching is proposed.Combining in-situ Raman,in-situ Fourier transform infrared spectroscopy（FT-IR）,in-situ electrochemical impedance spectroscopy（EIS）,X-ray photoelectron spectroscopy（XPS）analysis and theoretical calculations,the effects of electrode material functional group modification,electrolyte anion modulation and cathode/anode mass ratio to enhance the performance of PIHCs are comprehensively elucidated.Moreover,the theoretical basis for the design of high performance PIHCs is systematically developed.Finally,high-performance PIHCs are achieved.The main achievements are listed as follows:（1）The intrinsic mechanism of oxygen functional groups to regulate the SEI interface and improve the performance of carbon anode.GO samples with different types and contents of oxygen-containing functional groups are obtained by adjusting the quantity of oxidizing agent.In situ spectroscopy（Raman and FT-IR spectroscopy）combined with systematic electrochemical characterisations are used to investigate the effect of oxygen-containing functional group type/content on the electrochemical performance.Furthermore,the unique adsorption-intercalation hybrid potassium storage mechanism of GO is revealed and the mechanism of action of carbonyl（C=O）and carboxyl（COOH）groups to enhance the electrochemical potassium storage properties is uncovered.It is shown that the physical and chemical properties of SEI film are strongly dependent on the type of oxygen-containing functional groups.Among these,C=O and COOH promote the formation of the inorganic component potassium carbonate（K₂CO₃）,which facilitates the construction of highly conductive,intact and robust SEI film.Epoxy groups（C-O-C）and hydroxyl groups（OH）,on the other hand,tend to promote the formation of the organic component potassium alkyl carbonate（ROCO₂K）,which in turn affects the electrochemical performance.Based on this,with C=O and COOH rich GO-3 as the anode and activated carbon（AC）as the cathode,the AC//GO-3 PIHCs show excellent electrochemical performance with an energy density of 96.4 Wh kg^-1(101.2 W kg^-1 power density).（2）The role of electrolyte salt anion type in regulating the physical and chemical properties of the SEI interface.The effects of different anion（PF₆^-,FSI^-and TFSI^-）on the decomposition of electrolyte,the formation mechanism/evolut ion process of interfacial SEI films and the transport properties of interfacial carriers are investigated using in situ FT-IR,XPS coupled with the Ar⁺etching technique and front-line orbital theory calculations.The results show that compared to TFSI^-and PF₆^-,the S-F bond in FSI^-is easier to break to form KF（breaking tendency S-F（FSI^-）>C-F（TFSI^-）>P-F（PF₆^-））,which is conducive to the formation of a dense and stable SEI film with high ionic conductivity.Thanks to this,PIHCs based on KFSI electrolyte exhibit the optimal electrochemical performance(energy density 115.0 Wh kg^-1/power density 1045.7 W kg^-1),followed by PIHCs using KTFSI and KPF₆ electrolyte systems.（3）Synergistic nitrogen/oxygen co-doped carbon nanosheets and mass matching to enhance the electrochemical performance of PIHCs.To address the bottleneck of slow anode kinetics,N/O co-doped carbon nanosheets（NOCSs）with expanded layer spacing,disordered structural features and abundant active sites are designed and prepared to significantly enhance the kinetics of the anode.With the aids of ex-situ and in-situ EIS analyses,the KFSI-based electrolyte is screened to construct a more stable and intact SEI film.By calculating the mass ratio at different current densities,the effect of charge/mass balance on K⁺ion storage is revealed and the general design rules for improving mass matching to maximize the electrochemical performance of PIHCs are proposed.Using electrode design,electrolyte optimization and mass matching,the kinetic mismatch and capacity/mass imbalance between the battery-type anode and capacitor-type cathode are solved.As a result,AC//NOCSs PIHCs exhibit ultra-high energy density of 167.4 Wh kg^-1 at power density of 1001.1 W Kg^-1.Even at an ultra-high power density of 17,000 W kg^-1,an excellent energy density of 113.4Wh kg^-1 is still retained.