The Construction Of Multi Dimensional Germanium Based Composites And Their Lithium Storage Properties

With the accelerating pace of numerous portable electronics and electrical vehicles' continued emergence,it is imperative to develop efficient and environmental benign energy storage devices as alternatives of depletable fossil-fuel resources to cope with the ever-growing energy requirements,among which,rechargeable lithium batteries(LIBs)have attracted great interests on account of the high working voltage,high specific capacity,long cycling life and nonhazardous characteristics.As an indispensable part of LIBs,anode materials with higher energy densities are vital for the fabrication of high performance LIBs,therefore,germanium-based materials are considered as promising candidates for anode materials which have received tremendous attentions due to the extremely outstanding theoretical capacities,besides,it also exhibits a higher lithium diffusivity and a higher electronic conductivity.However,Ge-based material suffers from drastic pulverization upon cycles because of the large volume expansion accompanied by a large mechanical strain,which leads to the cracking and exfoliation of active materials from the current collector,consequently,a rapid capacity degradation appears during lithiation and delithiation process.In this paper,new structured Ge-containing binary alloy and ternary oxide were prepared with the introduction of the second metal.The second metal can act as an lithium active material,simultaneously,it can reduce the cost effectively,the buffer matrix of Li20 and the second metal can be formed during the lithium insertion process,which can effectively buffer the volume variation and enhance the electrical conductivity,additionally,the introduction of transition metal can induce another lithium storage mechanism:conversion mechanism for transition metal,it can offer extra reversible capacity.Graphene(RGO)can function as a supporting buffer matrix to in-situ grow Ge-containing materials,simultaneously,high conductivity graphene can enhance the electrical contact and matain the structure integrity.Through the unique structure design and effective combination with carbon matrix,a much improved electrochemical performance could be achieved.Detailed research contents are as follows:1.A novel layer structured nickel germanate nanosheets were synthesized in combination with reduced graphene oxide by a facile one-pot hydrothermal method(NiGeOx/RGO).The negatively charged GO sheets can tether the nickel metal cations due to the physical interactions,in consequence the NiGeOx nanosheets can be in-situ anchored on RGO networks,meanwhile,the strong interfacial interactions between RGO and NiGeOx nanosheets were formed during the hydrothermal process.The introduction of linkage bridge are beneficial for the good dispersion of nanosheets and largely facilitate the ultrafast electron transfer.Besides,NiGeOx nanosheets with layered structure can provide open channels and shorter lithium-ion diffusion paths;NiGeOx nanosheets are dispersed uniformly on RGO sheets with both flat-lying and vertically-grown nanosheets,the existence of two types of nanosheets guarantees an increased contact efficiency,simultaneously,a stronger interfacial interactions and a higher mass loading of active materials can be achieved.Consequently,the strongly-coupled NiGeOx/RGO nanocomposites exhibit excellent electrochemical performances.After 75 cycles,the reversible capacity of NiGeOx/RGO is as high as 863 mA h·g-1,NiGeOx/RGO also delivers a high rate performance after cycling under different current densities,when it returns to 50 mA·g-1,the specific capacity recovers to 1060 mA h·g-1.2.In this chapter,core-shell architecture of nickel germanide(Ni3.56Ge2)nanoparticles anchored on RGO sheets were fabricated with a simple hydrothermal method,followed by a simple pyrolysis route.the size distribution of Ni3.56Ge2 nanoparticles is 10-50 nm in diameter.Ni3.56Ge2 nanoparticles are uniformly dispersed on the graphene sheets without aggregation,the densely-packed nanoparticles can increase the mass loading of active material on per RGO sheet thus effectively improve the energy density of lithium-ion battery.During Li-alloying and dealloying process,the further volume expansion could be mutually restricted by the adjacent particles in a closely-packed way.The core-shell architecture and RGO sheet can function as a dual-protection layer,preventing the particles from coalescence and acting as a conductive layer and a buffer matrix.In addition,the introduction of transition metal can act as catalyst to induce partial reversible decomposition of SEI layer,consequently,the extra reversible specific capacity could be achieved.Ni3.56Ge2/RGO nanocomposite with the unique structure exhibits excellent cycling and rate performances as the electrode material.At a current density of 100 mA·g-1,Ni3.56Ge2/RGO delivers a specific capacity of 702 mA h·g-1 upon 100 cycles,even at a large current density of 500 mA·g-1,Ni3.56Ge2/RGO composite stabilizes at 410 mA h·g-1 after 500 cycles,besides,the composite also exhibits an excellent rate performance.With the addition of appropriate amount of lithium carbonate,the battery shows a much improved electrochemical performance,it exhibits a specific capacity of 1090 mA h·g-1 after 700 cycles at a current density of 500 mA·g-1.