Construction Of 3D Nanostructured Anode Materials And Study On Sodium Ion Storage Properties

With the rapid development of modern society,the safe and efficient storage is becoming increasingly important.At present,lithium-ion batteries are the main energy supply units in the electric vehicles and various electronic devices due to their high discharging capacity and energy density.However,the increasing demand of lithium-ion batteries has aggravated the consumption of the limited lithium resources.Thus,the high cost of raw materials of lithium is gradually restricting the development of lithium-ion batteries.It is worth noting that sodium resource is abundant and readily available.Moreover,sodium and lithium are in the same main group and have similar physicochemical properties.Consequently,sodium-ion batteries exhibit great potential in large-scale smart grid applications.However,the slow kinetics of sodium ion in the process of charging and discharging leads to low capacity,which requires the development of more suitable electrode materials and electrolyte to further improve the electrochemical performance and practical application.For example,transition metal oxides and sulfides with 3D nanostructrures have unique electrical and physicochemical properties,showing broad application potential in energy storage.In addition,supercapacitors are capable of providing long service life and ultra-high power density,but they also have the disadvantage of low energy density.Sodium ion hybrid capacitors combining the advantages of supercapacitors and batteries can achieve high power/energy densities at the same time.In view of these key scientific problems mentioned above,this paper makes efforts to boost electrochemical performance of sodium-ion energy storage devices from the perspectives of electrode materials design,materials preparation and electrolyte optimization.The specific research findings are as follows:(1)Sb2S3/SnS2/C hetero-junction materials were fabricated by directly growing SnS2 nanosheets on Sb2S3 nanorods through hydrothermal process,and then covering carbon layer outside.This 3D hetero-junction structure greatly enhances the stability of the material and promotes the transport of ions and electrons.The sodium-ion half cells based on Sb2S3/SnS2/C composite materials and sodium hexafluorophosphate-diethylene glycol dimethyl ether electrolyte showed high reversible capacity of 642 mAh/g at 1 A/g after charging and discharging for 600 cycles as well as a good rate performance of 367.3 mAh/g at 4 A/g.In addition,the sodium ion diffusion processes of several electrodes in different electrolytes were also further evaluated in order to investigate their electrochemical performances.Finally,the Sb2S3/SnS2/C//Na3V(PO3)3N sodium-ion full cells were prepared based on the ether electrolyte,which exhibited high energy density,good cycling stability and rate performance.(2)An aerogel material of SnO2 quantum dots uniformly dispersed on the 3D graphene network(SO2 QDs@GF)was prepared through hydrothermal method combined with freeze-drying.The 3D SnO2 QDs@GF structure can not only effectively improve the electrical conductivity of the materials,but also shorten the diffusion distance of sodium ions and ease the volume expansion.The aerogel materials can be used as free-standing electrodes without conductive agent and binder.In addition,the NaTFSI/EMIMFSI ionic liquid electrolyte was synthesized to investigate the effect of temperature on the electrochemical performance of the assembled sodium-ion batteries.This battery system provided good electrochemical performance at room temperature(25 ℃).Moreover,the device can still work normally at an elevated temperature of 65 ℃ and show higher reversible capacity(662.4 mAh/g at 0.025 A/g),good rate performance(159.6 mAh/g at 2 A/g)and excellent cycling stability(245.5 mAh/g after 1000 cycles at 1A/g).It is noteworthy that SnO2 QDs@GF//Na3V2(PO4)3 sodium-ion full batteries based on the NaTFSI/EMIMFSI ionic liquid electrolyte displayed good cycling stability at different temperatures,indicating that this battery system has the potential application in a wide temperature range.(3)Nb2O5 nanomaterials with different structures including nanotubes as well as the homo-junctions of nanowire-to-nanotube were directly geminated on the substrate of carbon cloth(CC)by controlling the pH of the solution under hydrothermal conditions.The Nb2O5@CC nanotubes exhibited the best sodium-ion strorage properties among these different nanostructures.After 1500 cycles,the capacity of the Nb2O5@CC nanotubes remained 175 mAh/g at 1 A/g with a coulombic efficiency of 97%.Additionally,the maximum power and energy densities of the sodium-ion hybrid capacitors based on Nb2O5@CC nanotubes anode and AC cathode can reach 7328 W/kg and 195 Wh/kg,respectively.Moreover,80%of the pristine capacity was still retained after cycling for 5000 times.Simultaneously,the prepared flexible sodium-ion hybrid capacitor with soft package can work well under various bending conditions,displaying good mechanical properties and cycling stability.