Study On Preparation Of Novel Porous Silicon From Hypoeutectic Al-Si Alloy And Its Electric Storage Properties

As a traditional commercial anode material,graphite’s actual capacity is close to the theoretical specific capacity(372 m Ah/g).Therefore,it is urgent to develop new high-capacity and low-cost anode materials to replace traditional graphite to meet people’s demand for highenergy density lithium-ion batteries.Silicon is considered to be one of the most promising anode materials for lithium-ion batteries because of its highest theoretical specific capacity.However,in the process of inserting/extracting a large amount of lithium ions from silicon-based anode materials,huge volume changes will result in rapid capacity attenuation.The problem has not been completely solved,which also hinders the large-scale practical application of silicon-based anode materials.Therefore,this thesis aims to control the morphology and size of eutectic silicon phase of the cast hypoeutectic Al-Si alloy through melt processing such as modification and refinement.Then the new porous silicon materials were prepared by dealloying process.In order to obtain a new method for the preparation of new porous silicon,the related morphology characterization and electric storage performance of the prepared porous silicon materials were tested.In this thesis,the hypoeutectic Al-10 Si binary alloy is taken as the research object.The target alloy melt is treated through modification and refinement processes,and the effects of modifier added,holding temperature,holding time,cooling rate,and compound deterioration are systematically studied.A new type of porous silicon material with 3D dendritic structure with high purity and no other impurities was prepared by acid etching and dealloying method after the microstructure of the alloy was controlled by the appropriate melt treatment parameters.The morphology,structure and other characteristics of the new porous silicon materials were characterized by XRD and SEM.The prepared porous silicon is made into button type half-cell according to the lithium-ion battery assembly process,and the battery storage performance of the half-cell was tested with the battery testing system.The electric storage properties of porous silicon anode materials prepared by different melt treatment processes were systematically studied,and the changes of electrode morphology before and after the electric storage performance test were characterized,so as to reveal the electric storage mechanism of porous silicon prepared by this method.Through experiments and analysis,the following conclusions are finally reached:(1)After the Al-10 Si alloy melt is modified,the eutectic silicon microstructure with smaller size can be prepared by direct water quenching.Compared with the alloy melt cooled at room temperature,the target alloy melt treated with 0.03 wt.% Sr at 760°C for 10-15 minutes,and then the alloy melt is quenched with water quenching can prepare finer size eutectic silicon,that is,the average size of the eutectic silicon structure is refined from 30 μm before water quenching to about 0.8μm.The iron-rich phase appears in the eutectic silicon structure prepared by this method,and with the extension of the holding time,the iron-rich phase will converge in the eutectic silicon structure.The eutectic silicon structure is sparse in the segregated area of the iron-rich phase,The eutectic silicon phase has a tendency to become larger.(2)A novel porous silicon material with high purity was successfully prepared by acid etching dealloying the precursor of Al-Si alloy prepared by water quenching process after modification.After SEM and XRD characterization,it can be inferred that the powder products prepared by this process have the characteristics of three-dimensional dendritic and porous structure,and are in good agreement with the diffraction peak of silicon material,and contain no other impurity elements.(3)The new porous silicon prepared as anode material of lithium-ion battery has good electric storage performance.It was found that the new porous silicon material was mixed evenly according to the active material: acetylene black: PVDF=1:1:1(mass ratio),and then evenly coated and made into buckle type half battery.At the current density of 200 m A/g(activated by the first three 100 m A/g cycles),the first specific discharge capacity of the new porous silicon prepared by metamorphism and water quenching is as high as 3844.2m Ah/g,the first coulomb efficiency is 68.85%,and the charge-discharge specific capacity is stable at 200 m Ah/g after 60 cycles.At this point,the charging and discharging efficiency is 100.16%.The charge-discharge cycle curve shows that the new porous silicon begins to form SEI film at about 0.7 V during the first discharge process,and the charging and discharging voltage platforms are 0.44 V and 0.01 V,respectively.The voltage platform gradually decreases with the increase of charging and discharging current density,and the charging platform gradually decreases with the increase of charging times.After the charging and discharging cycle,the dendritic porous silicon structure changes into fragmentary single dendrite.(4)Based on the combination of modification,water quenching process and T6 heat treatment,a spherical particle-like silicon powder with an average diameter of about 1.75 μm can be prepared.The silicon material prepared by this process also exhibits good electrical storage performance.The study found that at a current density of 200 m A/g(the first 3 times of activation was 100 m A/g),the first discharge specific capacity of the button half-cell made of this kind of spherical silicon particles was about 3250 m Ah/g,and the first coulombic efficiency was lower than dendritic porous silicon is 54%,which is lower than the new porous silicon.According to the charge-discharge cycle test,the capacity loss in the first 10 cycles is relatively slow,and the cycle performance after activation is relatively unstable.From the morphology of the spheroidal silicon electrode before and after 80 cycles,it can be seen that the spherical particle-like silicon changed into an amorphous structure wrapped by SEI film,and the surface was rough.The original spheroidal structure was irregular and cracked into flower shape,and the volume increased.