Transition Metal Fluoride Electrode Materials For Lithium/sodium Ion Capacitors

In recent years,with the rapid development of electric vehicles and smart grid,the development of new energy storage technologies with high energy density,high power density and long cycle life has become a hot spot.Hybrid ion capacitors?including lithium/sodium ion capacitors?use battery-type materials as anodes and capacitive materials as cathodes,so they have the characteristics of high energy density of secondary batteries and power density and long cycle performance of supercapacitors.It is well known that the electrode material is crucial to the performance of lithium/sodium ion capacitor.Therefore,this paper mainly focuses on the development of anode materials with high energy density,high power density and long cycle life,and has carried out a series of original research:1. Synthesis of nickel cobalt fluoride anode materials and research on their lithium/sodium ion capacitors: Tetragonal MF₂?M=Ni,Co?micro/nanocrystals have been synthesized by one-pot solvothermal method and further served as anodes for the lithium/sodium?Li/Na?-ion energy storage.Through orthogonal optimization of the experimental conditions such as reaction solvent,reaction temperature and reaction time,the overall electrochemical performance of the MF₂ electrode material is improved.The optimal MF₂ anodes and AC cathode have been used to construct the MF₂//AC Li/Na-ion capacitors?LICs/NICs?,which have been optimized by tuning the active mass ratios of positive and negative electrodes and working voltage windows of the capacitors.The Ni F₂//AC LICs demonstrate an overall superior energy/power density and cycling behavior with the AC/Ni F₂ mass ratio of 2:1 under the voltages of0-4 V and 0-4.3 V,in comparison with the Co F₂//AC LICs?2:1;0-4 V/0-4.3 V?.The MF₂ anodes exhibit the dominant surface and diffusion control kinetics for Li-ion and Na-ion storage respectively,showing a major conversion mechanism for the charge storage processes.The work addresses the new insight of tetragonal MF₂?M=Ni,Co?micro/nanocrystals anodes with the dominant conversion mechanisms for advanced LICs and NICs.2. Synthesis of Mn-O-F anode material and its LICs/NICs research:In this work,a new element-doped and anion vacancy double-defect conversion type Mn-O-F ultra-fine nanowire?denoted as Mn F₂-E?anode is constructed to increase the electroactive site to improve the electrochemical performance of LICs/NICs.Owing to the unique hetero oxygen-doping and intrinsic fluorine-vacancy double defects,the Mn–O–F nano-wires exhibited superior electroactive sites and thus dramatically enhanced Li/Na-ion storage capability than pristine Mn F₂ micro-crystals.The best Mn F₂ selected by orthogonal experiment and its derived Mn-O-F anode and AC cathode are used to construct Mn F₂//AC and Mn F₂-E//AC LICs/NICs.The performance of Mn F₂//AC and Mn F₂-E//AC LICs/NICs is optimized by adjusting the positive/negative active material mass ratio and operating voltage of the hybrid capacitor.The LICs/NICs based on the Mn–O–F anode demonstrated a considerably superior performance than the devices based on the Mn F₂ anode under the optimal voltages of 0–4 V and 0–4.3 V.The Mn–O–F anode exhibited dominant diffusion/surface-controlled kinetics for Li/Na-ion storage,respectively,showing a major conversion mechanism for the charge storage processes.This work presents new insights into improving the storage capacity of Li/Na-ions by constructing doped and vacant double defect conversion electrode materials.