Research On Vulcanization Control And Its Flexible Energy Storage Of Cobalt-Based Compounds/Carbon Nanofiber

With the rapid development of wearable electronic products,flexible energy storage devices with high flexibility,high energy/power density,long cycle performance and portability are urgently needed.Although alkali metal ion batteries represented by lithium ion battery（LIBs）,sodium ion battery（SIBs）and potassium ion battery（PIBs）have been widely studied and developed rapidly,the development of electrode materials with excellent mechanical and electrochemical performance is still one of the important issues to be solved.In view of the problems of electrochemical activity,conductivity and reaction stability between Co-based compounds and carbon nanofibers（CNF）as anode materials,this thesis takes Co-based compounds/CNF composites as the research system.Material design strategies such as morphological structures（such as multi-channel hollow,core-shell and yolk-shell structures）and electronic structures（such as heteroatom doping,heterostructure and alloy engineering）are used to improve structural stability,active sites and reaction kinetics.By adopting the electrospinning and sulfur（S）treatment process suitable for large-scale preparation,sulfur/nitrogen（S/N）co-doped carbon nanotubes/CNF（CNT/SNCF）,S-Co₃O₄-Co O/CNF,Co_1-xS-Co S₂/CNF and Co_0.85Se_1-xS_x/CNF composites are successfully prepared,which showed excellent electrochemical performance and structural flexibility as anode materials.Combined with simulation calculation and experimental analysis,the electrochemical energy storage mechanism and regulation mechanism of the above composites are revealed.The specific research contents are as follows:（1）Aiming at the problems of insufficient flexibility and conductivity of CNF,a one-dimensional multi-channel hollow morphological structure design and S/N co-doped control strategy are proposed.Polyacrylonitrile（PAN）nanofibers uniformly coated with CNT and Zn O are prepared by electrospinning,and CNT/SNCF composites with one-dimensional multi-channel hollow structure are prepared by vulcanization and etching.Due to its unique morphology,CNT/SNCF not only shows good flexibility and self-supporting property,but also has increased active sites and electrical conductivity.Meanwhile,S doping increases the layer spacing of carbon,and the ex-situ Raman and ex-situ TEM analysis reveals that the increased layer spacing has a positive effect on the structural stability and electrochemical performance of the sodium/potassium storage process.Based on the above advantages,CNT/SNCF anode shows excellent cyclic stability(SIBs:274.1 m Ah g^-1 after 1000 cycles at 1 A/g;PIBs:212.5 m Ah g^-1after 1000 cycles at 1 A/g)and rate performance(SIBs:109.3 m Ah g^-1 at 10 A/g;PIBs:108.7 m Ah g^-1 at 5 A/g)when used as SIBs and PIBs anode materials.（2）For Co-based oxides/CNF with three-dimensional network core-shell structure,a design strategy of using vulcanization control partial reduction to construct Co₃O₄-Co O heterojunctions is proposed.PAN nanofibers（ZIF-67/PAN）uniformly coated with ZIF-67 are prepared by electrospinning,and Co₃O₄/CNF is obtained by carbonization and oxidation at high temperature.Then,Co₃O₄-Co O heterojunctions particles are prepared by controlling vulcanization reduction conditions,and successfully encapsulated in CNF（S-Co₃O₄-Co O/CNF）.Experimental analysis and first-principles（DFT）calculations show that the S-Co₃O₄-Co O/CNF electrode has rich heterostructure interfaces,oxygen defects and unique three-dimensional network structure,which significantly increases the active sites,conductivity and reaction kinetics.Secondly,the three-dimensional network core-shell structure is beneficial to alleviate the volume expansion and achieve the long cycle performance of the electrode.Based on the above advantages,S-Co₃O₄-Co O/CNF anode shows excellent rate performance(324.6 m Ah g^-1 at 10 A/g)and cycling stability(591.2 m Ah g^-1 after 600cycles at 1 A/g)when used as LIBs anode material.When assembled with Li Fe PO₄cathode into lithium-ion full battery,it still shows excellent electrochemical performance(286.6 m Ah g^-1 after 100 cycles at 0.5 A/g).（3）Aiming at the difference of thermal stability of Co-based sulfides,a strategy of using vulcanization control phase transformation to construct ultra-small Co_1-xS-Co S₂heterojunctions is proposed.Co/CNF is obtained by carbonizing the ZIF-67/PAN prepared by electrospinning at high temperature,and then controlling the vulcanization reaction temperature to obtain ultra-small Co_1-xS-Co S₂ heterojunction particles（～10nm）and S-doped CNF.The heterojunction particles are uniformly dispersed in S-doped CNF,and the Co_1-xS-Co S₂/CNF composites with high flexibility and sodium storage performance are obtained.The experimental results and DFT theoretical calculation results show that Co_1-xS-Co S₂/CNF composites have rich heterostructure interfaces and enlarged interlayer spacing,which greatly improve the Na⁺of mobility,conductivity and reaction kinetics.In addition,ultra-small Co_1-xS-Co S₂ particles are uniformly embedded in one-dimensional S-doped CNF to form a unique core-shell structure,which not only improves the structural stability of the electrode,but also shortens the transmission path of Na⁺.Furthermore,the electrochemical transformation mechanism of Co_1-xS-Co S₂ is further revealed by the Ex-situ XRD and Ex-situ TEM.Based on the above advantages,the flexible self-supporting Co_1-xS-Co S₂/CNF anode has excellent rate performance(495.2 m Ah g^-1 at 10 A/g)and excellent cycle stability(380.1 m Ah g^-1 after 5000 cycles at 5 A/g)when used as SIBs anode material.When assembled with Na₃V₂（PO4）₃ cathode to form sodium-ion full batter,it also shows good electrochemical performance(228.9 m Ah g^-1 after 200 cycles at 0.5 A/g).（4）Aiming at the homologous alloying of heterogeneous chalcogenide elements（S,Se）,an efficient discrete alloying design is proposed by controlling the atomic ratio of Se:S.Co_0.85Se/CNF is obtained by selenization of ZIF-67/PAN at high temperature,and then Co_0.85Se_1-xS_x/CNF composites with high flexibility and potassium storage performance are prepared by fine control of S content through vulcanization.Based on the calculation results of DFT theory,it is determined that the discrete distribution of alloy S in Co_0.85Se(Co_0.85Se_1-xS_x)plays an important role in the electronic structure,and there is an appropriate Se/S ratio,showing the best conductivity and effectively improving the reaction kinetics.In addition,the unique three-dimensional network structure of the yolk-shell structure effectively alleviates the volume expansion.Finally,the transformation mechanism of Co_0.85Se_0.88S_0.12 is revealed by In-situ XRD and Ex-situ TEM,and the effect of coexistence of S and Se is clarified.Based on the above advantages,the flexible self-supporting Co_0.85Se_0.88S_0.12/CNF（CSS/CNF-2）anode has excellent rate performance(229.2 m Ah g^-1at 10 A/g)and cycle stability(327.7 m Ah g^-1 after 1000 cycles at 1 A/g)when used as PIBs anode material.When assembled with Prussian blue(K_0.22Fe[Fe（CN₆）],PB)cathode to form potassium-ion pouch cell,it also shows excellent electrochemical performance(72.5 m Ah g^-1 after 500 cycles at 1 A/g).