Structure Design,Controllable Preparation And Lithium Storage Performance Of Porous Silicon Based Anode Materials

With the depletion of traditional energy and the emergence of environmental pollution and other problems,the market share of new green energy shows a rising trend.Currently,most electronic products and electric vehicles are powered by LIBs.As high-power and long-life electric devices gradually become the mainstream,the development of LIBs with high safety and energy density has become an urgent task.Silicon-based anode possess high specific capacity,and thus it can be used as one of the mainstream anode materials in the future.However,the volume expansion caused by the Insertion-deinsertion process of lithium ions will cause the structural damage of the electrode material,which will produceing serious security risks.Therefore,alleviating or even solving the volume effect of silicon-based anode materials has very important theoretical research significance and industrial application value.According to the research,the structure of nanosilicon,silicon-carbon and silicon-metal composite can release the stress caused by"expansion effect"and improve the electrochemical performance.In order to prepare high performance silicon matrix composites,this thesis adopts chemical etching method to remove aluminum in Al-Si alloy(Al-Si),and obtains porous silicon with unique porous structure.Secondly,in order to solve the volume expansion problem of silicon to a greater extent,silicon-carbon composites were prepared by using the respective advantages of silicon and carbon.Then,the relationship between structure and performance was clarified by physical and chemical characterization,and the influence of the negative component and ratio of silicon on the electrochemical performance of the LIB was revealed.Finally,the variations of the anode structure,morphology and thickness after multiple cycles are analyzed to clarify the mechanism of the structure composition of the silicon-based anode on its macroscopic electrochemical performance,and to reveal the factors affecting the stability of the structure of silicon-based anode.The specific research contents of this thesis are as follows:(1)The main work of this part is the preparation of micron porous silicon(PSi)by aluminum silicon alloy,which can reduce the volume expansion effect to a certain extent.In order to prepare porous silicon anode material,Al-Si alloy was added into hydrochloric acid with certain solubility.After adjusting thereaction conditions,microporous silicon with a specific surface area of 91.05 m² g^-1 and a large number of mesoporous structures were prepared.This characteristic is beneficial to improving the electrochemical performance of silicon anode.Because the porous structure can provide buffer space,the reversible capacity remains 78%of the initial rate after charging and discharging tests at different rates,and the coulomb efficiency can reach 67.7%.Under the condition of high current density(1 A g^-1)and long cycle(500 cycles),the reversible capacity can still reach 322 m Ah g^-1.Compared with commercial silicon,porous silicon anode shows excellent rate performance and cycling performance.Finally,by observing the changes before and after the cycles,the surface of porous silicon appears a certain degree of crack,and the section appears a large volume expansion.Therefore,it is necessary to make use of the synergistic effect of the micron porous silicon and carbon materials by further modification to optimize the electrochemical performance.(2)The work of this part is using carbon nanosheets(CNS)coated on micron porous silicon to limit the volume expansion and shorten the diffusion distance of lithium ions.By using glucose,ethyl orthosilicate and microporous silicon as raw materials,a unique carbon nanosheet cage coated microporous silicon composite(named PSi@CNS)was synthesized by sol-gel method.The coating layer of the material is composed of multi-carbon nanosheets,which form pore channels between the carbon nanosheets to provide an diffusion channel for lithium ions.Moreover,the rigid carbon skeleton can limit the volume expansion of porous silicon cores,thus showing excellent electrochemical performance.Analyses of electrochemical data showed that at high current density(1 A g^-1)and long cycles(500cycles),compared with PSi(322 m Ah g^-1)and PSi@CNS/Si O₂(645 m Ah g^-1),the reversible capacity of PSi@CNSwas up to 1272 m Ah g^-1.The reversible capacity increased by 950 and627 m Ah g^-1,respectively.According to the analysis of electrode materials after multiple cycles,compared with PSi and PSi@CNS/Si O₂,fewer cracks were observed on the surface of the electrodes,and the cracks between the active material and the fluid collector were smaller,thus showing excellent cycle stability and rate performance.(3)This part of the work is using the woven short carbon nanotubes-short reduced graphite oxide(SCNT-SRGO)coating on micron porous silicon,to deeply alleviate the volume expansion,and thus effectively improve the rate performance and poor conductivity.When the porous silicon is coated with hard carbon,it can only limit the outward expansion of the volume of the silicon core.However,the external rigid structure cannot withstand the large volume change of silicon,which will cause serious structure damage of the material.Therefore,a simple and efficient method to prepare silicon-carbon anode materials was designed and developed in this study:porous silicon/short-wall carbon nanotubes-short reduced graphene oxide composite anode materials(optimized PSi/SCNT-SRGO)were prepared by using short-wall carbon nanotubes,GO and porous silicon as raw materials.After acidification,SCNT-SRGO presents strong electronegative properties,and Al-Si is positively charged.Due to the electrostatic effect,the three-dimensional network structure of SCNT-SRGO is tightly coated on the surface of porous silicon.The?-?bond exists between SCNT and SRGO,which enhances the interaction force of composites.SCNT-SRGO is a soft carbon layer,which can effectively buffer the volume effect of silicon core.Therefore,Optimized PSi/SCNT-SRGO showed excellent electrochemical properties,and its reversible capacity was still up to 2173 m Ah g^-1 after charge-discharge tests at different rates of 0.2,0.5,1,2,5 and 0.2 A g^-1.Under the condition of large current density(1 A g^-1)and long cycle(600 cycles),the reversible specific capacity is still as high as 1008 m Ah g^-1.