Effect Of Metal Doping In Carbon Layer On Electrochemical Performance Of Silicon/carbon Composite

With the rapid development of high-tech industries,especially electric vehicles,lithium-ion batteries with higher energy density and longer service life become essential.Silicon,which has high theoretical lithium storage capacity,low lithium intercalation potential,abundant resources and environmental protection,is expected to become the negative electrode material of the next generation of high-performance lithium-ion batteries.However,the huge volume expansion of silicon in the process of deintercalation of lithium results in cracking and pulverization,and the charging and discharging performance decays rapidly.Carbon has good toughness and conductivity,and silica-carbon composites can effectively inhibit the huge volume change caused by silicon deintercalation lithium.Although current researchers have extensively studied the effects of different carbon sources,different preparation methods,and different structural forms on the electrochemical properties of silica-carbon composites,they still not achieve ideal properties.Based on the existing research,this paper proposes the electrochemical properties of silicon carbon composites reinforced with metal in the carbon layer,and focuses on the following research:（1）The effects of different carbon sources on the electrochemical properties of Si-C composites were compared.The silicon-carbon composite materials Si@SC and Si@CC were obtained by mechanical agitation and high temperature calcination using nano-silicon as silicon source,sodium alginate（SA）and sodium carboxymethylcellulose（CMC-Na）as carbon source,respectively.It is found that the carbon layer of the silicon-carbon composite material Si@SC with sodium alginate as carbon source was looser and more porous,with lower oxygen content and higher graphitic degree.The electrochemical performance test results showed that the discharge specific capacity of Si@SC was still 532.9 m Ah g^-1 after 200 cycles,so the carbon source of the metal-incorporated carbon layer was chosen.（2）The effects of Al³⁺,Fe³⁺and Ce³⁺incorporation into carbon layer on the electrochemical properties of silica-carbon composites were investigated.Comparative analysis showed that the surface of the silicon carbon composite with metal in the carbon layer was smoother and denser,but there was a certain degree of sintering agglomeration of the silicon carbon composite with Fe and Al,while the particles of the silicon carbon composite with Ce were dispersed.The incorporated metals Al3+,Fe³⁺and Ce³⁺were transformed into metal oxides and distributed in the carbon layer during calcination.The incorporation of metals reduced the combination of O–C bonds in the carbon layer,and increased the combination of C–C bonds and improves the degree of graphite of the carbon layer.The electrochemical performance test showed that the first discharge specific capacity of Si@C-Al,Si@C-Fe and Si@C-Ce was1887.6,1629.0 and 2374.3 m Ah g^-1,respectively,and maintained 512.5,241.1 and735.7 m Ah g^-1 after 200 cycles,respectively.（3）The effect of Ce incorporated in carbon layer on the electrochemical properties of silica-carbon composites was further studied using micron-scale silicon as silicon source.Analysis showed that carbon layer incorporated with Ce on silicon carbon composite material had three effects:enhance the toughness of carbon layer and improve the cycle performance;reduce the formation of the SEI layer and the coulomb efficiency of the first cycle;improve the lithium-ion conductivity and multiplier performance of silicon carbon composite.The experimental results showed that the microscale Si@C-Ce silicon-carbon composite had an initial specific capacity of1848.6 m Ah g^-1,which remained 502.5 m Ah g^-1 after 180 cycles,and the Initial Coulomb Efficiency（ICE）was higher than that of most reported silicon-carbon composites containing metals.