Preparation Of SnO₂ And Rare-earth Hexaborides (CeB₆?EuB₆?YB₆) Nanostructures And Their Electrochemical Properties

The emergence of lithium-ion batteries(LIBs)makes cameras,cell phones,laptops,electric vehicles and other mobile electronic devices realize the real convenient"mobility",which brings a lot of convenience to our life,but also has problems such as low capacity and poor safety.In order to solve these problems,it is very important to develop new electrode materials.SnO₂,with a theoretical capacity more than twice that of commercial graphite,is a potential anode material.Furthermore,strong covalent-bonds between boron atoms in rare-earth hexaborides(RB₆)with CsCl-type structure(space group:Pm-3m)enables them to form spatial frame structures.RB₆ has good chemical stability,zero expansion coefficient within a certain temperature range,excellent electrical conductivity and structural stability.Moreover,the available spatial frame structure also provides possibilities for the storage and extraction of lithium ions.Thus,RB₆ also becomes a potential LIBs electrode material for lithium-ion batteries.In addition,low-dimensional nanostructures of RB₆ have better comprehensive performance than the bulk materials,and can effectively shorten the diffusion distance of lithium ions when they are used as electrode materials.Therefore,how to optimize the preparation method of nanostructural RB₆ materials and study their electrochemical properties is also concerned.In this paper,on the one hand,the hierarchical SnO₂/reduced graphene oxide(GO)composite nanomaterials were prepared by solution method,and their performance as anode materials for LIBs was tested.On the other hand,a series of RB₆nanostructures were successfully synthesized by the optimized solid-state methods,and their electrochemical properties were focused studied,electron transport and magnetic properties were tested.Moreover,the electronic transport and magnetic properties of RB₆ nanostructures were also studied.A hierarchical SnO₂ and reduced graphene oxide composites were prepared by a solution template method and used as anode materials for LIBs.The two-dimensional(2D)hierarchical nanostructures can not only shorten ion-diffusion path,but also mitigate the huge volume expansion/shrinkage and structural strain effects.The results show that the reversible discharge capacity of the electrode materials in the first cycle is 1063.4 mA h g^-1 at the current density of 1 A g^-1,and the Coulombic Efficiency(CE)is up to 88%.In addition,the composite electrode material still has an excellent rate capability of 502.1 mA h g^-1 at a high current density of 5 A g^-1.Due to its high conductivity and structural stability,the specific capacity of the composite is significantly increased(1335.6 mA h g^-1)after 500discharge/charge cycles at 1 A g^-1 current density,and the capacity retention is as high as125.6%,exhibiting high reversible capacity and extraordinary cycling stability.Most of the RB₆ nanostructured materials were prepared by chemical vapor deposition at high temperatures near 1000? and the sample yield was low.In this paper,CeB₆ cubic particles and CeB₆ nanowires were successfully synthesized at 320? and 260? by solution combustion(SC)and solid state reaction(HPSS)method with high pressure provided by autoclave,respectively.In addition,we have for the first time investigated the electrochemical properties of CeB₆ cubic particles and nanowires as anode materials for rechargeable lithium-ion batteries.At the current density of 5 mA g^-1,the discharge specific capacity of CeB₆ nanowires and cubic particles electrode are 531 mA h g^-1 and 338 mA h g^-1 in the first cycle,respectively.After 60 cycles,the reversible capacity can be maintained at 225 mA h g^-1and 185 mA h g^-1.The results fully illustrate that the CeB₆ materials have completely reversible lithium-ion storage and extraction properties.In terms of cycling stability,CeB₆ nanowires still have a specific capacity of 168 mA h g^-1 after 6000 charge-discharge cycles at a current density of 1 A g^-1,and exhibit excellent cycling stability.The results of ex-situ XPS and XRD analysis show that the CeB₆ nanostructure material is an insertion/extraction mechanism for lithium storage,we determined that the electrochemical reaction formula is CeB₆+0.33y Li⁺+0.33 ye^-=CeB_5.67+0.33 Li_yB(y=1-5),which is an embedded and extracted lithium storage mechanism.By our designed HPSS method,europium hexaboride(EuB₆)nanocrystals have been successfully prepared.As anode materials for LIBs,,at 10 mA g^-1 current density,the discharge and charge specific capacities of EuB₆ in the first cycle are 174.4 mA h g^-1 and 114.2 mA h g^-1,respectively.The charge-discharge curves of EuB₆ basically coincide after four cycles,indicating that EuB₆ material has excellent cycle stability.After 1000 cycles at 0.5 A g^-1,the specific capacity of EuB₆ was increased from 54.6 mA h g^-1 to 131.6 mA h g^-1.Electrochemical impedance spectroscopy(EIS)results show that the diffusion coefficient of Li⁺after 1000cycles is 8.62×10^-14 cm s^-1,which is 2 orders of magnitude higher than the diffusion coefficient of 4.2×10^-16 cm s^-1 before cycling.As the number of cycles increases,the EuB₆ is continuously pulverized,which shortens the diffusion distance of lithium ions and thus increases the diffusion coefficient of Li⁺.This is also one of the main reasons for the increase in specific capacity after long cycles.In addition,we also tested the magnetic properties of EuB₆and the electron transport properties of a single EuB₆nanowire.The fitting of temperature-dependent magnetic susceptibility curve shows that the effective magnetic moment of each Euion in EuB₆ is 6.0?_B,which is close to the theoretical magnetic moment of Eu³⁺of 6.9?_B.The Arrott plots analysis indicates that the Curie temperature T_C of ferromagnetic ordering is about 10.1 K.Yttrium hexaboride(YB₆)were successfully prepared by our designed HPSS method at200-240?.XRD patterns showed that the powder was pure phase YB₆.From the growth morphology at different time stages and EDS evidence,we proposed that YB₆nanowires were nucleated and grown from the submicron particle clusters.As the anode materials for LIBs,YB₆ nanostructures showed good charge-discharge capacity and cycle stability.At the current density of 10 mA g^-1,the discharge specific capacities in the first three cycles were 373.4,193.0 and 175.1 mA h g^-1,respectively.The reversible specific capacity under 0.2 A g^-1was increased to 295.34 mA h g^-1 after 2000 cycles.The test results of electrode materials at different rates after 2000 cycles show that the specific capacity and rate performance are greatly improved compared with those before long cycles.Compared with the electrodes before cycling and after 2000 cycles,we found that its specific capacity and the rate performance were greatly improved after long cycles.After 2000 cycles,the reversible specific capacity reached 493.3 mA h g^-1 at 10 mA g^-1,and its Coulombic Efficiency was 97.8%.Combined with XRD and XPS,we explored the lithium storage mechanism of YB₆,which is an embedded and extracted lithium mechanism.It is speculated that Li⁺accumulated near B atoms in YB₆ framework during the process of lithium intercalation,and bonded with B atoms to form LixB(x=1-5)alloy.In addition,the temperature-dependent magnetization results indicate that YB₆ nanostructures undergo a superconducting transition with T_c=7.8 K.In a summary,SnO₂ composites are prepared by template method,which have high reversible specificcapacity and cycle stability when used as anode materials for lithium-ion batteries.Three 1D RB₆(CeB₆?EuB₆?YB₆)nanostructured materials are also synthesized by our designed high pressure solid-state method,and as lithium anode materials they display fully reversible lithium-ion storage and extraction performance and cycling stability.