Design Of Silicon/Carbon Nitride Composite Structure For High Performance Lithium Ion Battery

With the widespread use of portable tools such as mobile phones and laptops and the gradual rise of large-scale vehicles such as electric vehicles,traditional lithium-ion batteries can no longer meet the needs of people.Therefore,the development of high safety,high specific capacity,and excellent cycle performance of lithium-ion batteries has become an urgent goal.Silicon has an ultra-high theoretical capacity of 4200 mAh g^-1 and is regarded as a new generation cathode material for lithium-ion batteries.However,silicon-based anode materials also have problems such as volume expansion,particle comminution and continuous thickening of solid electrolyte interface film?SEI film?,resulting in serious capacity attenuation,so silicon-based anode materials are difficult to be commercialized at present.At present,the solution of silicon modification is mainly focused on nano-scale,alloying,porosity and so on,which are combined with the surface coating technology to modify silicon.As a new inorganic non-metallic material,graphite phase carbon nitride?g-C₃N₄?has been widely used in catalysis sensings and devices because of its advantages of convenient synthesis,soft equality,high thermal stability,acid-base resistance and semiconductor properties.In this thesis,we combined the thermal stability of two-dimensional carbon nitride and the high specific capacity of zero-dimensional silicon nanoparticles and combined them effectively by means of in-situ recombination and in-situ reduction.Through the design of composite structure,there are the superiorities of the improved dispersion of silicon particles,the increased contact area with electrolyte,the provided Li-N active sites,so improve the stability of silicon,and the electrochemical performance of silicon-based anode materials.The main contents and results of this paper are as follows:?1?By exploring the calcination environment?whether the system is open or not?of solid-phase sintering process,the excellent properties of g-C₃N₄ were obtained.The lamellar distribution of the g-C₃N₄ is orderly,which not only facilitates the contact with electrolyte,but also relieves the change of stress caused by volume expansion.Proper nitrogen content contributes to the improvement of electrical conductivity,and higher pyridine nitrogen content provides more N active sites of Li storage.The g-C₃N₄ has an initial discharge specific capacity of 255.1 mAh g^-1 at a current density of 0.1 A g^-1,and a discharge specific capacity of 70.8 mAh g^-1 after a cycle of 120 cycles.?2?A Si@TiO₂@g-C₃N₄ composite anode material was prepared by sol-gel method and in-situ solid-phase sintering recombination method.TiO₂ layer can avoid the direct contact between silicon nanoparticles and electrolyte and inhibit the continuous growth of SEI film.g-C₃N₄ can alleviate volume expansion and provide mesoporous channels and N active sites.At a current density of 0.1 A g^-1,the anode material has an initial discharge specific capacity of 935.9 mAh g^-1,and a discharge specific capacity of 165.9 mAh g^-1 after 50 cycles.?3?A kind of pore Si@g-C₃N_x composite anode material was prepared by in-situ recombination and in-situ magnesium thermal reduction method.The porous silicon and g-C₃N_x obtained by magnesium thermal reduction has a higher specific surface area.The proper N content increases the conductivity and the percentage of pyridine nitrogen,which will increase the N active sites of Li storage.Under the current density of 0.1 A g^-1,pore Si@g-C₃N_x has an initial discharge specific capacity of 540.5 mAh g^-1,and a discharge specific capacity of 95.9 mAh g^-1 after 60 cycles higher than discharge specific capacity of 55.2 mAh g^-1 of pore Si.To sum up,through the superiority combination of two-dimensional carbon nitride and zero-dimensional silicon nanoparticles,we put forward two kinds of composite schemes of silicon and g-C₃N₄,which can improve the cycle performance of silicon to a certain extent.In this study,the composite of inorganic non-metallic materials and g-C₃N₄,and the improvement of electrochemical performance of lithium-ion batteries by g-C₃N₄ can be used for reference.