In-situ Transmission Electron Microscopy Study On The Electrochemical Behavior And Mechanism Of Sodium Storage Of Iodine And Tellurium Nanowires

Sodium secondary battery which is a new kind of energy conversion and storage device,is the extension of lithium secondary battery.It has so many advantages such as rich source materials,low cost,high energy density and long cycle life.In recent years,sodium secondary battery has attracted more and more attention.However,due to the lack of convenient,durable and cheap anode materials,the development of sodium secondary battery has been stuck in a bottleneck.Therefore,the search for suitable anode materials for sodium secondary battery has become the key to its development.This article selects two kinds of electrode materials,including I and Te materials,to explore their specific capacity displayed in the sodium ion battery system.In addition,by using the in situ transmission electron microscope technology can explain for the result of their electrochemical properties and electrochemical performance.By analyzing the conclusion,we can find new ways to improve electrode materials or to find a more suitable sodium ion battery cathode material.Firstly,we prepare the pure I and I@C composite electrode materials.Then we use the electrode materials to assemble the sodium-ion batteries.After that we test the electrochemical performance of them and we find that I@C composite electrode materials have better performance than pure materials electrode,due to the conductivity of pure iodine material is poorer.It shows that composite materials can effectively enhance the electrical conductivity.At the same time,it can also be embedded in the hole of iodine in the porous structure of iodine.The I@C composite electrode material showed the specific capacity of 383 m Ahg^-1 in the first round of discharge.And the specific capacity of the second round discharge could still reach 88.5 m Ahg^-1.After more than 150 cycles of charge and discharge,its capacity could still be maintained at 80 m Ahg^-1.By using I@C nano composites as the anode material in the transmission electron microscopy to form a micro sodium ion battery system.By using the method of applying voltage to observe the process of the shift of sodium.Due to the limitation of CNT,its morphology and structure change is not big.In the process of discharge,KI₃ transfers to KI.And in the process of charging,Na I gradually disappears and then transfer to KI₃.The volume can be restored to the original state.In the cycle of charge and discharge the reaction entirely reversible.It can explain why the electrode material in the macroscopic battery shows good cyclic stability.Preparing the pure Te nanowire electrode material,nanowires Te@C composite electrode and business Te@C composite electrode materials.Then we use the electrode materials to assemble the sodium-ion batteries.After that we test the electrochemical performance of them and we find that the nanowires Te@C composite electrode materials of electrochemical performance is much higher than that of pure material electrodes,which can determine good material microstructure can improve its electrochemical performance.The Te@C composite electrode material shows the specific capacity of 946m Ahg^-1 in the first round of discharge.But the specific capacity of the second round discharge decreased to 93.9 m Ahg^-1 with a decline of 90%.After 100 cycles of charge and discharge the capacity decreased to 13 m Ahg^-1.The results indicate that the cyclic stability of the material is poor.By using Te nano anode material in the transmission electron microscopy,we form a micro sodium ion battery system.Then using the method of applied voltage to observe the Te nanostructured electrode materials in the process of charging and discharging.We find that appeared a lot of volume expansion in the sodium-embedded process.During the discharge it has a two-step reaction.Firstly sodium embedded into Te to form Na Te.Then the secondary embedded sodium reacts when Na Te transfers to Na₂Te.In the process of charging Na₂Te gradually disappear and transfer to Na Te.The volume can’t return to the initial state.Moreover,the product could not be restored to Te with single crystal.Part of the reaction was irreversible in the first round of cycle of charge and discharge,which explain for the reason why the electrode material in the macroscopic battery system shows good specific capacity during the first round of discharge,but it rapidly declines from the second round.