Sythesis,Energy Storage Mechanism And Device Application Of Fe-based Metal Oxide Nanoarray Anodes

As rapid increasement of consumption and demand for energy resources in worldwide,the development of clean and sustainable energy storage materials has become a priority for many scientists.Among various energy storage devices,many studies have focused on supercapacitors(fast charge/discharge and high power density),aqueous rechargeable lithium batteries(ARLBs,safety and cheap)and battery-supercapacitor hybrid energy storage devices(combine the merits of supercapacitors and batteries).Iron oxides have great advantages on world-wide abundance,low cost,easy-fabricating and non-toxic properties.Ferroferric oxide(Fe3O4)has drawn considerable attention because of its high theoretical capacity and much better conductivity than other transition metal oxides.Nevertheless,the charge storage mechanism of Fe3O4 as anode is unclear.Meanwhile,Fe3O4 gains high capacity with Fe3+(?)Fe0.It gets iron oxides' multiphase changes and furthermore makes huge volumetric expansion and structural deformation/dissolution,which finally makes drastic capacity decay.Compared with powder materials,this problem is even more serious for array structure.However,nanoarray structure is much superior to bulky and powder materials.Firstly,the direct growth of nanoarray on the current collector would provide fast electron transport channel.Secondly,it enlarges surface area and further facilitate the electrolyte penetration.Finally,nanoarray structure avoids binders and additive,and ensures robust mechanics.Thus,array electrodes are very necessary of wearable,flexible smart portable electronics.The current thesis aims to develop simple and effective methods on fabricating ferroferric oxide nanorod array,study the electrochemical energy storage mechanisms,and applies in ARLB,hybrid device and so on.The main contents are as follows:1.Mechanistic investigation of the charge storage process of Fe3O4 nanorod array in Li2SO4 electrolyte.Firstly,FeOOH precursor was fabricated via a hydrothermal method and Fe3O4 nanorod array was attained by post glucose carbonized process.Considering mechanism study,the binder and additive-free Fe3O4 array of pure active materials avoids undesired variables.Next,based on Dunn's method,for CV data in Li2SO4 aqueous electrolyte,we quantitatively analyzed the charge storage process of Fe3O4 nanorod film electrode in details.By calculating,at low scan rate,the diffusion-controlled contribution is 78.4%(0.5 mV s-1),holding dominate role;at high scan rate,the diffusion-controlled contribution is 22.8%(100 mV s-1),which cannot be ignored.Different from typical pseudocapacitive materials,diffusion-controlled Li+insertion process occurred in Fe3O4's energy storage process and should not be overlooked.The results also explain the poor rate performance of reported Fe3O4,and will provide a valuable guidance to design Fe3O4-based electrode materials with fast kinetics.2.Carbon-stabilized high-capacity ferroferric oxide nanorod array and its applying in flexible solid-state alkaline battery-supercapacitor hybrid device.Adopting Fe3O4 nanorod array as precursor,after "carbon shell-protection" treatment,we can get final product of Fe3O4-C hybrid nanoarray.Furthermore,an outstanding flexible solid-state rechargeable alkaline battery-supercapacitor hybrid device(CNTs(+)/Fe3O4-C(-))is assembled.Fe3O4 is one of the most promising alkaline battery anode materials due to a high theoretical capacity.Nevertheless,such a theoretical value can only be realized upon complete redox reaction,iron oxides' multiphase changes and furthermore makes weak cycling performance.Though high temperature glucose carbonization process,we can successfully synthesis various thickness of Fe3O4-C hybrid nanoarray.The optimized carbon coating(0.15 M)Fe3O4-C electrode cycled to 5000 times,achieved a remarkable cycling performance,and successfully solved cycling problem.Meanwhile,the carbon shell improves the conductivity of electrode.Considering no detailed reports on the electrochemical mechanism of Fe3O4-C electrode in alkaline electrolyte,the current thesis unveils it in detail.Via CV data,XRD and XPS results,valence state variation of Fe appears.We further assemble a high-performance solid-state CNTs(+)//Fe3O4-C(-)hybrid device.The device displays good electrochemical performance with high volumetric energy density approaching commercial Li thin-film batteries and high power density comparable to commercial SCs(1.56 mWh cm'3;0.48 W cm-3).It also exhibits good environmental suitability in case of bending,mechanical pressure and elevated temperature.The reported "carbon shell-protection" strategy is general and can be readily applied to other instable electrodes in aqueous electrolytes.3.High stable TiO2 decorated Fe3O4 nanorod array in aqueous system and applied in solid-state hybrid device.Adopting Fe3O4 nanorod array as ARLB anode.Subsequently,a general controllable TiO2 shell by atomic layer deposition(ALD)has been fabricated on Fe3O4 nanorod array.Furthermore,by pairing with cathode(V2O3@C),a unique flexible solid-state V2O3@C(+)//Fe3O4@TiO2(-)hybrid device is assembled.The bare Fe3O4 electrode displays well-shaped redox peaks in 1 M Li2SO4,and can be used as ARLB anode.We unveil in detail the relationship between work potential window and capacity,cycling stability of Fe3O4.With optimized coating(10 nm),Fe3O4@TiO2 hybrid electrode cycled to 30 000 times,exhibiting much improved cycling performance.In principle,TiO2 layer(low volume expansion)leads an improved electrode stability,reduced H2O decomposition and enhanced capacity compared to the bare Fe3O4 electrode.For 10 nm TiO2 hybrid electrode,by pairing with cathode(V2O3@C),a unique flexible solid-state hybrid device is assembled.The voltage is up to 2.0 V,much higher than most reported supercapacitors(? 1.8 V).And then,it delivers high energy density(2.23 mWh cm-3),reaching commercial Li thin-film batteries and high power density(1.09 W cm-3),comparable to commercial SCs.Moreover,the device also shows good electrochemical performance in case of bending,mechanical pressure and elevated temperature.This work proposes new opportunities to utilize ALD-TiO2 protecting shell on instable nanoarray materials working in aqueous electrode.