Study On Nafetio₄ And Silicon As Anode Materials For Rechargeable Batteries

In recent decades,due to global environmentchanging and fossil energy shortages,energy storage technology has become the focus of global attention,exploring new energy sources to replace fossil fuels has become a pressing task.At present,new energy sources such as solar and wind energy have been used in energy storage systems for many years,but their intermittent nature limits their large-scale applications.Thus,electrochemical energy storage technology based on secondary batteries has recently begun to become a promising option because of its high efficiency,a long cycle life and a relatively low maintenance advantage.So far,lithium-ion batteries?LIBs?have been widely used in energy storage applications such as portable devices,due to the rapid increase in demand for lithium,the attendant problems are becoming increasingly prominent,lithium resources in the crust is not high and uneven distribution in the world,the price of lithium will increase year by year,an increase of lithium battery production costs make lithium-ion batteries difficult to support the development of the two industries both electric vehicles and large-scale energy storage.As a result,developing resource-rich and inexpensive new energy storage system is very necessary.At this point,sodium elements have attracted attention because of their high abundance and low cost and similar oxidation-reduction potential?ENa/Na = 2.7 V relative to standard hydrogen electrode,0.3 V higher than that of lithium?.Sodium-ion batteries are promising application of the new secondary battery because of the rich resources,environment-friendly,good security and so on.Therefore,to achieve a breakthrough in the practical application,the development of lithium/sodium ion battery anode material must satisfy the good structural stability in de-intercalation process,cycle stability,low operating voltage and other requirements.Currently,one of the widely-concerned lithium-ion anode materials is the silicon material because of its lower working voltage platform and higher theoretical specific capacity,but the silicon is less conductive and the huge volume of expansion during the cycle inhibits its development in practice applications,so researchers are working to develop a suitable composite structure to improve the conductivity of materials,slow down the pressure caused by volume expansion.While in the sodium-ion batteries one of the widely concerned anode materials is titanium-based materials,because of its good stability,cheap,reserves abundance and other advantages.In this paper,the two kinds of anode materials are studied,and the morphology of the materials is adjusted by changing the synthesis method.The electrochemical properties of the materials as lithium-ion batteries and sodium-ion batteries are improved,and the reaction mechanism and the effect of the structure on the properties are also investigated.The main research contents are as follows:?1?NaFeTiO₄ nanorods of high yields have been synthesized by a facile sol-gel method and utilized as anode material for sodium-ion batteries for the first time.The obtained NaFeTiO₄ nanorods exhibit a high initial discharge capacity of 294 mA h/g at 0.2 C?1 C = 177 mA/g?,and remain at 115 mA h/g after 50 cycles.Furthermore,multi-walled carbon nanotubes?MWCNTs?were mechanically milled with the pristine material to obtain NaFeTiO₄/MWCNTs.The NaFeTiO₄/MWCNTs electrode exhibits significantly improved electrochemical performances with a stable discharge capacity of 150 mA h/g at 0.2 C after 50 cycles.And then the NaFeTiO₄/MWCNTs//Na3V2?PO₄?3/Cfull-cell has been further assembled for the first time,which displays a discharge capacity of 70 mA h/g after 50 cycles at 0.05 C,indicating its excellent performances.XPS,the ex-situ XRD and Ramanspectra measurements were further carried out to investigate the initial electrochemical mechanisms of the obtained NaFeTiO₄/MWCNTs.?2?The nanosized SiO₂ was extracted from the bagasse of natural resources,and the silicon nanoparticles were calcined by magnesium thermal reduction.In order to improve the poor cycling performance of silicon,we have carbon coated with silicon and made composite materials with Si/C network structure due to the rapid expansion of the specific capacity in 20 cycles due to the volume expansion.In order to achieve good coating effect,we tested the coating effect of different carbon sources,and finally selected ascorbic acid as the carbon source.The coating method was chosen to cover the sol-gel method.The structure is that the Si/C nanoparticles are connected with each other to form a network structure.The particles are embedded in the carbon layer formed by ascorbic acid.This structure can effectively suppress the volume expansion effect of silicon,enhance the conductivity of the material,enhance the ion and electron transport capacities,in order to achieve good electrochemical performance purposes.