Study Of Structural Design And Lithium Storage Properties Of Germanium-Based Composites

Lithium-ion batteries（LIBs）have been widely used in portable electronic devices and electric vehicles because of their high energy densities and long cycle lifes.However,limited by the low capacity of graphite,current LIBs with graphite anode cannot meet the growing demand for higher energy density.Benefiting from high theoretical capacity,low working potential and high ionic mobility,alloying-type Ge-based materials become an ideal anode candidate for the next generation LIBs.However,the large volume change during the alloying reaction will lead to the pulverization and degradation of Ge-based anodes,resulting in rapid capacity decay.In this study,through morphology and structure design,carbon-based material compositing and heteroatom doping,we have successfully fabricated GeO_x and Na₄Ge₉O₂₀ anode materials with excellent electrochemical properties.We further investigated their lithium storage behaviors.The main contents and results are as follows:（1）With NaCl as a sacrifice template,porous interconnected GeO_x@C has been fabricated through freeze drying and high-temperature carbonization.The morphology and structure of composites can be optimized by controlling the adding amount of GeO₂ in the precursor,which can consequently affect electrochemical performance.The porous interconnected carbon matrix can alleviate the volume expansion of GeO_xand promote the structural stability of electrodes.Besides,the carbon matrix can also improve the electric conductivity of electrode composites.The GeO_x@C electrode delivers a high reversible capacity of 752.5 mAh g^-1at 0.1 A g^-1 after 100 cycles with capacity retention rate of 92.1%.And after cycled at a high current density of 0.5 A g^-1 for 700 cycles,high capacity of 530.6 mAh g^-1 can still maintain.Moreover,the full cell of GeO_x@C//LiFePO₄demonstrates its prospect of great potential for practical applications.（2）Through ion-exchanging between NaCl and（NH₄）₂GeO₃,and coping with N doping,a three-dimensional porous Na₄Ge₉O₂₀ composite with N-doped carbon coating has been prepared,namely 3D Na₄Ge₉O₂₀@N-C.Confining Na₄Ge₉O₂₀ nanoparticles within the porous N-doped carbon matrix can not only alleviate volume expansion during cycling,but also prevent the agglomeration of nanoparticles effectively,and promoting the structural stability of composite in charge and discharge processes.Moreover,N-doping can lead to the formation of defects and active sites to improve electronic conductivity and facilitate electrode reaction kinetics.As anode material for LIBs,the 3D Na₄Ge₉O₂₀@N-C electrode exhibits a high reversible capacity of 896.2 mAh g^-1 at 0.1 A g^-1after 100 cycles.Even at a high current density of 2.0 A g^-1,the electrode can still achieve a specific capacity of 636.0 mAh g^-1,demonstrating excellent electrochemical performance.