Construction Of Silicon And Titanium Dioxide/Graphene Anode Material And Their Energy Storage Properties

In this rapidly developing information age,the vigorous development of electronic equipment and electric vehicles has put forward higher requirements for the performance of lithium ion batteries?LIBs?.It is urgent to develop a new generation of LIBs with high energy density and power.Compared with LIBs,sodium ion batteries?SIBs?have a cost advantage despite their lower energy density.Silicon material is the potential new generation of high energy density LIBs anode with the advantages of high theoretical capacity and low voltage platform.However,it has the shortcomings of serious volume expansion and low lithium ion conductivity.Numerous researchs have shown that the electrochemical performance of silicon materials can be greatly improved by constructing nano or porous structures.TiO₂/RGO composites are also potential new generation of high-power anode materials.Due to that the capacitive effect dominates energy storage mechanism,the composites exhibite the excellent cycling stability and rate performance as the anodes for lithium/sodium ion batteries.However,challenges remain in the preparation of nano-silicon,porous silicon and TiO₂/RGO composites,such as high cost,complicated process and uncontrollable reaction,which have seriously hindered their commercialization.In this dissertation,the low temperature reaction system of Mg H₂-Al Cl₃-SiO₂ for preparing nano-silicon,a method of using metal hydride as reducing agent to convert purified sand into porous silicon and a supercritical CO₂fluid-assisted strategy to prepare TiO₂/RGO composites are proposed respectively to solve the above problems.The efficient and controllable preparation of the above materials was realized,and the relationship between their structure and electrochemical properties was systematically studied.The main research results are as follows:?1?A new method has been developed to convert purified sand to morphology-controllable porous silicon with metal hydride as reducing agent,and the relationship between the structure and lithium storage performance of the products was studied.By systematically studying the reaction of Li H,Na H,Mg H₂ and Mg with SiO₂ respectively,it was found that Mg H₂ is best the reducing agent to convert SiO₂into Si,and the sand was successfully reduced to porous silicon at the reaction temperature of 300?.And the purified sand was successfully converted into porous Si with Mg H₂ as reducing agent at low temperature?300 ^oC?.When the reaction temperature was raised to 500 ^oC,the yield reached as high as 94.5%.Through further study,it was found that the heating rate had a great influence on the morphology of the product,and the porous silicon product with a specific surface area of as high as331.2 m²·g^-1 were obtained.The product also exhibits excellent electrochemical performance(1663 m A h g^-1 after 500 cycles at 0.2 A h g^-1).?2?A novel low temperature Mg H₂-Al Cl₃-SiO₂ reaction system was developed to prepare nano-Si.The system uses Al Cl₃ as molten salt medium and Mg H₂ as reducing agent to convert SiO₂ to nano-Si at low temperature.It is an efficient liquid-solid reaction system with no by-products.Therefore,the nano-Si product prepared by this method has a high purity without involving HF purification treatment.The reaction can be triggered at a temperature as low as 150 ^oC?the lowest temperature reported for the preparation of nano-Si by reduction method?.When the reaction temperature increased to 200 ^oC,the yield of silicon reached as high as 97.6%.The average particle size of the product was 22.4 nm,70.6 nm and 162.6 nm corresponding to the reaction temperature at 200 ^oC,300 ^oC and 400 ^oC,respectively.As the anode for LIBs,the Si-product with an average particle size of 22.4 nm exhibits excellent electrochemical properties(1185 m A h g^-1 after 300 cycles at 0.2 A h g^-1 and 854 m A h g^-1 after 500 cycles at 2 A h g^-1)and a low volume expansion effect(14.2%after 500cycles at 2 A h g^-1).?3?An innovative and efficient supercritical CO₂ fluid-assisted strategy?SCFAS?was developed to synthesize 3D TiO₂ nanowires/reduced graphene oxide?3D TiO₂NWs/RGO?composite.With the aids of high permeability and superior solvability of supercritical CO₂ fluid,highly uniform ultrafine nanowires were successfully obtained.Even if the TiO₂ loading is increased to about 80 wt.%,the average diameter of TiO₂nanowires can be well controlled within 5 nm.As the anode for LIBs,the composite has ultra-high reversible capacity(666 m A h g^-1 at 0.1 A g^-1 after 300 cycles),excellent cycling stability and rate performance(460 m A h g^-1 after 2000 cycles at 2 A g^-1).The lithium storage mechanism of the material is mainly attributed to capacitive effect.When used as anode for SIBs,it still has a reversible capacity of 113 m A h g^-1.?4?TiO₂ quantum dots/reduced graphene oxide?TiO₂ QDs/RGO?composites were prepared by SCFAS.The particle size of TiO₂ in the TiO₂ QDs/RGO composites prepared by this method is 3.7 nm,reaching the quantum level?<5 nm?.As anode material for SIBs,it shows remarkable cycling stability(241 m A h g^-1 at 0.05 A h g^-1over 300 cycles)and excellent rate performance(108 m A h g^-1 at 5 A h g^-1 over 5000cycles).The energy storage mechanism of the material is mainly attributed to capacitive effect(76%non-intercalation capacity at a scanning rate of 3.2 m V s^-1).It is worth noting that SiO₂ QDs/RGO composites was also successfully prepared by the above methods.