Study On Storage Performance And Mechanism Of Carbon-based Anode In Sodium/potassium Ion Batteries

In recent decades,lithium ion batteries have been widely used in portable electronic devices and electric vehicles,which can effectively relieve the pressure of fossil energy.However,the scarcity and uneven distribution of lithium resources greatly increase the cost of lithium ion batteries,which is not conducive to large-scale production and sustainable development.In contrast,sodium/potassium resources are abundant and widely distributed.Additionally,the physical and chemical properties of them are similar to that of lithium.Therefore,sodium/potassium ion batteries are expected to replace lithium ion batteries as a new generation of energy storage systems.However,the radius of sodium/potassium ion is larger than that of lithium ion,which causes the pulverization of electrode material and deteriorates the cycling performance of sodium/potassium ion batteries.Moreover,the analysis of the energy storage mechanism of sodium/potassium ion batteries is lacking,which impedes the development of the anode materials.Carbon-based materials are widely used as anode for sodium/potassium ion batteries because of their low potential,high stability and low price.In this work,we mainly modified and extended the graphite-based materials,including expanded graphite and iron selenide wrapped by reduced graphene oxide for sodium/potassium ion batteries.We mainly analyzed the storage performance and mechanism.First,we obtained the expanded graphite by slight modified Hummers’method for high-performance sodium/potassium ion batteries.Considering the strong covalent bonding ability and 3D crumpled structure,the reduced graphene oxide can relieve the volume expansion of electrode during the charge-discharge process.So we prepared the Fe Se₂ wrapped by reduced graphene oxide to achieve the high specific capacity and excellent cycling stability in sodium/potassium ion batteries.In addition,we also developed in-situ Raman spectra and in-situ visualization technique to analyze the sodium/potassium ion storage mechanism.The conclusions are as follows:1.Expanded graphite was synthesized by slight modified Hummers’method and subsequent thermal treatment,which effectively expands the interlayer spacing of graphite and shows excellent sodium/potassium storage properties.Even at a large current density of 1 A g^-1,our expanded graphite exhibits a specific capacity of 100 m Ah g^-1after 2600 cycles for sodium ion batteries.According to the change of intensity and position of G peak in in-situ Raman spectra,the sodium storage process was analyzed,including the surface passivation,the stacking change of carbon-layers,the formation of SEI film and sodium ions embedding in expanded graphite.The structure of expanded graphite gradually recovered with sodium ion extracting in the process of charging.Moreover,our expanded graphite also exhibits excellent electrochemical performance for potassium ion batteries,which presents the specific capacity of 187 m Ah g^-1 at the current density of 200 m A g^-1after 650 cycles.2.The FeSe₂ wrapped by reduced graphene oxide was synthesized by simple hydrothermal selenization and subsequent annealing.The reduced graphene oxide can effectively relieve the agglomeration and volume expansion of Fe Se₂ particles to improve the cycling stability of the electrode.First,the effect of ether-based and carbonate-based electrolytes on electrochemical performance in potassium ion batteries was analyzed.We also observed the changes of the electrode in the process of charging and discharging through the in-situ visualization technology.The results show that a homogeneous electrochemical reaction occurs in the ether-based electrolyte,inhibiting the volume expansion of the electrode and thus maintaining the structural integrity.Finally,the phase changes of Fe Se₂ and the intermediate generation were analyzed according to the change of the characteristic peaks of Fe Se₂from in-situ Raman spectra.The Fe Se₂@RGO exhibits excellent electrochemical performance in ether-based electrolyte,which maintains capacity of 356 m Ah g^-1 at 200 m A g^-1 after 75 cycles in potassium ion batteries.Similarly,Fe Se₂@RGO obtains excellent electrochemical performance of 424 m Ah g^-1 at 200m A g^-1 after 50 cycles in sodium ion batteries.