Study On Lithium Storage Properties Of Modified Silicon Phosphorus Composites As Anode Materials For Li-ion Batteries

Silicon（Si）,with its competitive theoretical capacity(～4200 m Ah g^-1)and low lithium intercalation voltage（～0.4 V）,is a potential candidate as an anode material for Li-ion batteries.The main challenges associated with Si-based anode materials are their structural instability and low electrical conductivity during charging and discharging,which in turn lead to their poor rate and cycling performance,thus seriously hindering the further development of Si-based anode materials.In view of the above problems,this topic focused on the preparation of silicon-phosphorus composite materials and their composite modification as the main research content.Two modified Si-P materials were prepared,namely:Si-P/Ti₃C₂T_x and Si-P@PPy,and their electrochemical properties as anode materials for LIBs were explored.The common advantage of these modified materials were to build the double conductive protective layers,which could improve the conductivity of the material while suppressing the volume expansion of the Si-based material.In addition,the quantitative preparation of the above materials could be achieved by the mechanical ball milling method,which layed a foundation for the practical application of the materials.The main research contents and results of this paper are as follows:1.Si-P and Si-P/Ti₃C₂T_x composites were obtained by mechanical ball milling.The main lithium intercalation potential of Si is～0.4 V（vs.Li/Li⁺）,and the red phosphorus（red P）coated on the outer surface of the nano-Si particles underwent an alloying reaction to form Li₃P during the first cycle of lithium intercalation.Compared with pure red P,Li₃P had higher mechanical strength and improved electrical conductivity,while the deintercalation potential of Li⁺in Li₃P was～1.2 V（vs.Li/Li⁺）.Therefore,by controlling the cut-off voltage of battery charge and discharge at 0.01～1V（vs.Li/Li⁺）,after the first cycle,Li₃P could exist stably,while Si was mainly engaged in the intercalation and deintercalation of lithium ions,resulting in Si（Li_xSi_y）-Li₃P core-shell structure material.Since the Si-P electrode also had a certain volume expansion/contraction during the charging and discharging process,the two-dimensional layered structure material Ti₃C₂T_x was introduced as the matrix,and the surface of the Si-P particles was coated with Ti₃C₂T_x to construct P（Li₃P）and MXene（Ti₃C₂T_x）double conductive protective layers.Among them,the P nanolayer acted as an intermediate layer to form Si-P and P-O-Ti chemical bonds between Si,red P and Ti₃C₂T_x,which effectively promoted the interaction between the protective layer and the Si-based material,thereby strengthening the Si-P composite.The close contact between Ti₃C₂T_x ensured the integrity of the Si-P/Ti₃C₂T_x composite,further suppressed the volume expansion of Si and Si-P electrodes,and stabilized the SEI layer.The electrochemical performance of Si-P/Ti₃C₂T_x electrode was effectively improved.The Si-P/Ti₃C₂T_x electrode could provide a specific capacity of 3486.2 m A g^-1 in the first cycle at a current density of 50 m A g^-1,a capacity of 632.4 m Ah g^-1 at a high rate of 5 A g^-1,and a capacity of 632.4 m Ah g^-1 at a high rate of 50 m A g^-1.After 1 cycle for 500 cycles,there was still a specific discharge capacity of 980.8 m Ah g^-1.2.Si-P@PPy was prepared by in-situ polymerization.As an organic polymer,polypyrrole（PPy）had been widely studied and applied to many materials in recent years due to its good electrical conductivity,reversible lithium storage properties and material characteristics,simple preparation method and less environmental pollution.Therefore,in this paper,PPy was also used to coat Si-P particles.We prepared nano-sized Si-P@PPy composites by a simple in-situ polymerization method and used them as anode materials for lithium-ion batteries.The Si-P particles in the as-prepared Si-P@PPy were effectively coated by PPy,thereby reducing the direct contact of Si-P with the electrolyte and avoiding the continuous growth and destruction of the solid electrolyte interfacial film.Due to the synergistic effect of PPy and Si-P composites,the prepared Si-P@PPy composites exhibited excellent cycling stability and rate capability.