Preparation Of Ge-based Nanomaterials And Their Application In Li Ion Battery

Lithium ion batteries?LIBs?are widely used in portable electronic devices,electric vehicles,and hybrid electric vehicles because of their long cycle life,high energy density,and high output power.However,commercial graphite materials used as the anode materials of LIBs show a low theoretical capacity of 372 m A h g-1,which can not meet demand for electrodes with high performance.Ge-based nanomaterials are the most promissing materials,which can deliver a high capacity.However,the volume expansion ratio of Ge is 370%,which limits the application of Ge-based nanomaterials.Here,we prepare Ge-based nanomaterials with different morphologies and structures in order to accommodate the volume change and improve the conductance of electrochemical active materials.Furthermore,we study the electrochemical reaction mechanisim and relationship between the electrochemical performance and the structure of materials.Ge nanotubes?NTs?and nanowires?NWs?are prepared by template assisted electrodeposition from ionic liquid [EMIM]Tf₂N.The result shows ion liquid [EMIM]Tf₂N is fit for electrodeposition as a electrolyte by tesing the viscosity,contanct angle,and conductivity.Ge NTs and NWs with different aspect ratios are obtained by adusting the polycarbonate?PC?template,electrolyte concentration,and deposition time.At a high current density,Ge grows by the wall-surface growth pattern for the inner wall is a preferential site for depostion,and thus Ge NTs are formed.At a low current density,Ge grows by bottom-up pattern that produces Ge NWs.The stability of Ge NT electrodes is remarkble?dropping by only 0.01% per cycle?.The Ge NT electrodes also show an improved rate capability with a specific capacity of 825 m A h·g-1 at 2 C.The improved performance of the Ge NTs is attributed to the formation of an amorphous phase and a hollow tubular structure.The Ca₂Ge₇O₁₆ nanorode?NR?and NW electrodes are fabricated by a facile hydrothermal method on Cu foam substrates.The orthorhombic phase Ca₂Ge₇O₁₆ NRs and NWs are obtained when the hydrothermal time is 1 h and 25 h,respectively.The formation of 1 D Ca₂Ge₇O₁₆ undergoes nucleation and ostwald ripening process.The Ca₂Ge₇O₁₆ NR and NW electrodes decompose into Ca,Ge,Li2 O at ¹ V during the first cycle and the lithium diffusion coefficient of NRs is 3.92×10-13 cm2·s-1 measured by cyclic voltammograms?CV?and electrochemical impedance spectroscopy?EIS?.When cycled at a high current density of 2 A·g-1,capacity retention is 143 % with initial reversible capacity of 699.5 m A h·g-1 after 1000 cycles.The superior electrochemical performance is attributed to the synergetic effect of 1 D structure of NR arrays and good conductivity of Cu foam.The Ca₂Ge₇O₁₆ porous,weed and urchin like electrodes are synthesised by hydrothermal methods assisted with CTAB and urea due to surface tension,self-assemble,and recrystalization.CV results show the lithium storage reversible of smaller pore and urchin like electrode is better.The electrodes with smaller pores cycled at a high current density of 0.2 A·g-1,capacity retention is 123 % with initial discharge capacity of 1746 m A h·g-1 after 400 cycles.The urchin like electrodes cycled at a high current density of 0.2 A·g-1,capacity retention is 56 % with initial discharge capacity of 1812.7 m A h·g-1 after 500 cycles.The improved electrochemical performance is attributed to the porous and urchin like structure,which can provide space to buffer volume change of electrodes and shorten the ionic diffusion length.Ca₂Ge₇O₁₆ NS/CNT composites are prepared by a simple hydrothermal method on Ni foam insitu.The synthetic process of the Ca₂Ge₇O₁₆ NS/CNT nanocomposite goes through three processes which are nucleation,self-assemble and adsorption of CNT.The capacity retention of electrodes is 87% of its 2nd cycle capacity after 1000 cycles.After the rate performance test,the coin cell is cycled for additional 100 cycles,maintaining a capacity of 1001.6 m A h g-1.This superior cycling performance is attributed to the synergetic effect of structure of NS and CNT.Ca₂Ge₇O₁₆ /C core-shell nanowires?NWs?are fabricated on Ni foam insitu by annealing at 800 ? after mixing with polyamide acid?PAA?.The characteristic D band and G band of C are observed at 1342.7 cm-1 and 1583.1 cm-1,respectively.The intensity ratio of D band and G band is less than 1,which suggests a good crystallinity of C.The Ca₂Ge₇O₁₆ /C core-shell NW electrodes cycled at a current density of 0.5 A·g-1,capacity retention is 204% with initial reversible capacity of 459.9 m A h·g-1 after 500 cycles.The exellent electrochemical performance is attributed to the coating C which can increase the conductivity of electrodes.