Preparation And Electrochemical Properties Of Porous Silicon-based Composites Anode Materials For Lithium-ion Batteries

Nowadays,lithium-ion batteries are in an irreplaceable position in the field of electrochemical energy storage and power batteries.The energy density of a lithium ion battery depends mainly on the specific capacity of the positive and negative materials of the battery.The theoretical capacity of commercially graphite anode materials currently used is only 372 mAh g^-1,and the performance of commercial products is close to the theoretical capacity.With the further increase in the energy density requirements of lithium batteries,the development of positive and negative materials with higher capacity density is imminent.The silicon-based anode has a theoretical capacity of up to 4200 mAh g^-1 and a low operating voltage?<0.5V vs.Li/Li+?,making it the next generation anode material most likely to replace graphite.However,silicon is accompanied by?300%volume change during the process of delithiation/lithiation,and as a semiconductor material,its conductivity is poor.These two intrinsic properties of silicon lead to the formation of unstable SEI film and pulverization,resulting in attenuation of battery capacity,greatly reducing battery life,and restricting the wide application of silicon-based anode materials.Based on the commercial aluminum-silicon alloy,this paper constructs a stable three-dimensional network skeleton inside the silicon.This hollow structure effectively buffers the volume expansion of silicon at the process of charge and discharge to a certain degree,and improves the conductivity in various ways.These innovative design methods of micro structure effectively improve the electrochemical performance of composite materials.?1?The silver particles were successfully synthesized in-situ in the porous silicon by the modified silver mirror reaction,and the carbon layer was uniformly coated on the silicon surface by CVD.Finally,the core-shell structure p-Si-Ag/C with double-layer conductivity was obtained.This well-designed structure greatly enhances the electrical conductivity inside and outside the structure by the introduction of silver particles and the coating of the carbon layer,providing a channel for the transport of lithium ions and electrons,while the inherent skeleton and outer carbon layer of the porous silicon provide a guarantee for the stability of the overall structure,alleviating the damage caused by volume expansion during charging and discharging.The battery prepared using this negative electrode material can maintain a specific capacity of up to 1000 mAh g^-1 after circulating 200 times at a current density of 0.2 A g^-1,and still exhibit 765 mA g^-1 at a high current density of 2 Ag^-1.which indicate the strong cycle performance and rate performance.?2?Using melamine as a nitrogen source and asphalt as a carbon source,a porous silicon-carbon composite p-Si/NC with a nitrogen-doped carbon layer was prepared by a simple one-step carbonization method,which is simple and cost-effective for large-scale production applications.The nitrogen-doped carbon layer greatly improves the electronic conductivity of the composite electrode.As the carbon source,asphalt makes the outer carbon layer of the porous silicon uniform and dense,and avoids the excessive loss of lithium ions caused by the direct reaction between the internal silicon and the external electrolyte.At a current density of 0.2 A g^-1,the p-Si/NC electrode remain hold a capacity of 720 mAh g^-1 after 200 cycles,and a capacity of 550 mAh g^-1 at a high current density of 2 A g^-1.