| Electrochemical energy storage devices have been widely used in the fields of national defense,transportation,and information technology.In order to counstructure a high-performance device,the key lies in the exploration of the high capacity,fast charging rate and stable electrode materials.Potassium niobate(KNb3O8)was widely used in photocatalyst,ceramics,piezoelectricity,ferroelectricity and other fields,which has attracted the attention of many researchers.It has stable layered structure,variable ionic valence and high theoretical capacity.However,the low conductivity and sluggish ion kinetics are the main concerns limiting its practical applications.As a new type of electrode material,KNb3O8 has not been systematically studied in the field of energy storage.In this thesis,the structure and electrochemical properties of KNb3O8 as a good pseudocapacitive material and lithium storage material were studied.The main research contents are as follows:(1)A simple electrochemical deposition method was developed to prepare electroactive KNb3O8 nanorods with an average length of 500 nm and diameter of 80 nm on the surface of activated textured carbon cloth(aTC).The electrochemical behavior and energy storage mechanism of the composite electrode were studied in 3 M KOH electrolyte.It indicated that the aTC-KNb3O8 electrode has a specific areal capacitance of 1061 mF cm-2 at the current density of 8 mA cm-2.Besides,the specific capacitance of aTC-KNb3O8 was attributed to the reversible redox reactions between Nb5+/Nb4+and surface ion adsorption of K+ions.However,repeated cycling the aTC-KNb3O8 electrode at a current density of 30 mA cm-2 for 1000 cycles leads to 42%capacitance decay.(2)In order to improve the cyling stability of aTC-KNb3O8,with dopamine of 0.5 mg mL-1 as the carbon source,the aTC-KNb3O8 was coated by a carbon layer with an average thickness of 2 nm through oxidative polymerization of dopamine and low temperature annealing(aTC-KNb3O8@C).Electrochemical results indicated that aTC-KNb3O8@C electrode can deliver a high area capacitance of 1439 mF cm-2 at 8 mA cm-2,and retains 86%capacitance at current density up to 50 mA cm-2(1244 mF cm-2).Moreover,the aTC-KNb3O8@C electrode also retains 71%of its initial capacitance after cycling the electrode for 10000 cycles.These performances are much better than those of aTC-KNb3O8 electrode.In addition,the carbon layer also served as a protective layer,which effectively suppresses the falling off of the active material from aTC substrate during continuous charge and discharge process.(3)KNb3O8 also has the excellent lithium storage performance owing to its neat layered structure.In addition,KNb3O8 nanorods were loaded on carbon cloth(CC),and served as a self-supporting anode in lithium ion capacitor.The high conductivity CC substrate and the porous network formed by the intersected nanorods promotes rapid Li+insertion/extraction.Accordingly,CC-KNb3O8 hybrid electrode delivers a high discharge capacity of 271 mAh g-1 with a safe and low voltage of 1.2 V at 0.01 A g-1,a highly retained capacity(up to 225 mAh g-1 after 100 cycles),and a superior rate capability(159 mAh g-1 at 0.5 A g-1).Electrochemical results and XPS analysis show that the Li+storage of CC-KNb3O8 electrode is accompanied with Nb5+/Nb4+and Nb4+/Nb3+redox couples.Moreover,insertion/extraction of Li+into/from KNb3O8 does not induce noticeable volume change during the reversible charging and discharging process.A lithium-ion capacitor configured with CC-KNb3O8 anode and active carbon cathode could be reversibly cycled in the range of 0-3.5 V.A maximum energy density of 69 Wh kg-1 at a power output of 346 W kg-1 is achieved.Moreover,the device can retain 88%capacity after 1000 cycle of continuous galvanostatic charge and discharge at 2.0 A g-1.This work has made the KNb3O8 to be one of promising candidates in the field of energy storage,and shed light on constructing the high-performance supercapacitors and flexible Li/K-ion capacitor. |
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