Lithium Storage Properties Of Silica-Based Electrode Materials

SiO₂ is being considered as the new candidate to the negative electrode materials of lithium ion batteries?LIBs?due to its large theoretical lithium-storage capacity,low discharge voltage and abundant sources.However,the low conductivity of SiO₂ and the large volume expansion of SiO₂ during the charge/discharge process deteriorates seriously its electrochemical performance.Therefore,in this thesis,several strategies such as physical mixtures of SiO₂ with conductive agent,SiO₂ nanocrystallization,fabrication of nanocomposites of SiO₂ with some conductive materials?TiO₂,rGO,polyaniline?were made attempts to overcome the above shortcomings of SiO₂ as the negative material of LIBs.The results indicate that SiO₂@rGO nanocomposites could effectively advance the specific capacity of SiO₂ electrode and accommodate the volume variation of SiO₂ electrode during charge/discharge process,thus resulting in the excellent cyclic performance and the good rate capability.The investigated contents are as following in detail:?1?The commercially available SiO₂ with different content of the conductive agent BP-2000 and the nano-SiO₂ were investigated electrochemically as the negative electrode materials of LIBs.The results show that the commercially available SiO2 electrode with 30 wt%,20 wt%and 10 wt%BP-2000 deliver the discharge specific capacity of 238.7 mAhg^-1,112.4 mAhg^-11 and 48.4 mAhg^-1,respectively,after 100 discharge/charge cycles at a current density of 100 mA g^-1,indicating the improved conductivity of SiO₂ and thus the enhanced specific capacity of SiO₂ electrode when mixed physically with the conductive agent.On the other hand,the discharge specific capacity of nano-SiO₂ for the first cycle is as high as 1002.4 mAhg^-1 while that of the commercially available SiO₂ is only 512.2 mAhg^-1,which might be attributed to the quantum effect of nano-SiO₂,indicating that SiO₂ nanocrytallization could improve the conductivity of SiO₂ and thus enhance the specific capacity of SiO₂ electrode.?2?The SiO₂@TiO₂ nanocomposites were prepared by tetrabutyl titanate?TBOT?-hydrolyzed TiO₂ deposition on the surface of the obtained monodispersed SiO₂ nanoparticles via the classical St?ber method.Thus,the SiO₂@TiO₂ cavity nanocomposites were synthesized by the NaOH-etching SiO₂ core of the SiO₂@TiO₂ nanocomposites and investigated electrochemically as the negative electrode materials of LIBs.The results show that the SiO₂@TiO₂ cavity nanocomposites with 50 nm SiO₂ present a discharge specific capacity of 527 mAhg^-1 with a approximate coulombic efficiency of 100%at a current density of 100 mA g^-1 after 100 cycles while those with 300 nm SiO₂ present a discharge specific capacity of 405 mAhg^-1 with a approximate coulombic efficiency of 98%after 78 cycles,indicating that the electrochemical performance of the SiO₂@TiO₂ cavity nanocomposites is improved remarkably compared to that of nano-SiO₂ and improved more greatly with decreasing the size of the core SiO₂.This might be attributed to the improved conductivity due to the coated TiO₂ with some conductivity and SiO₂ nanocrystallization.Furthermore,the cavity structure of the SiO₂@TiO₂ nanocomposites could buffer the volume variation of SiO₂ during the charge/discharge process.?3?The SiO₂@rGO nanocomposites were synthesized by the electrostatic interaction of the surface-negative graphene oxide?GO?and the surface-positive monodispersed nano-SiO₂ modified by aminopropyl trimethoxysilane?APS?and the followed reduction of hydrazine hydrate,and investigated electrochemically as the negative electrode materials of LIBs.The results show the SiO₂@rGO nanocomposites electrode present a discharge specific capacity of 486.4 mAhg^-1 with a coulombic efficiency of 97.2%after 99 cycles at a current density of 100 mA g^-1.This might be attributed to the improved conductivity and the buffered volume variation of SiO₂ during the charge/discharge process due to the structural flexibility and good conductivity of rGO.?4?The SiO₂@PANI nanocomposites were synthesized by online surface deposition of polyaniline on the surface of nano-SiO₂ and investigated electrochemically as the negative electrode materials of LIBs.The results show that the SiO₂@PANI nanocomposites present a discharge specific capacity of 415.6 mAhg^-1 with the coulombic efficiency of almost 100%after 100 cycles at a current density of 100 mA g^-1.This might be attributed to the greatly improved conductivity of SiO₂ due to the coated conductive polymer PANI.