Synthesis Of Germanium Based Anode Materials And Their Electrochemical Properties

Graphite has a relatively low reversible capacity(372 mAh g-1)during lithiation and delithiation.Compared with graphite,germanium(Ge)has higher theoretical specific capacity(1600 mAh g-1)and lower operating potential(<0.5V,vs.Li/Li+),which makes it considered as a promising anode material for next generation lithium ion batteries(LIBs).However,similar to silicon,germanium also has a large volume change during the lithium intercalation/delithiation process,resulting in faster decay of capacity.We have developed a synthesis method of mesoporous germanium nanoparticles from magnesium germanide(Mg2Ge)as raw materials at low temperature in molten zinc chloride,and prepared germanium nanoparticles with a diameter of less than 200 nm from germanium dioxide as raw material at low temperature in molten aluminum chloride.We also prepared a Ge/Cu3Ge@C-based material formed by cross-linking in-situ catalyzed carbon nanotubes;and oxidation of magnesium germanide with glucose,and using a carbonized PVP to dope nitrogen in order to prepare a three-dimensional porous Ge/NC composite.All of the prepared nanostructured germanium and germanium based composite electrodes greatly increased the material capacity and improved their cycle performance.The main research content includes the following aspects:1.We prepared mesoporous germanium nanoparticles(mp-Ge NPs)from magnesium germanide(Mg2Ge)via a novel low-temperature molten salt method.Zinc chloride was applied as the molten salt and reactant;and the reaction was taken place in a high pressure autoclave under 300 ?.When applied as the anode of LIBs,mp-Ge NPs showed a capacity about 1048 mAh g-1 after 1000 cycles at the rate of 1 C;and kept the capacity of 727.1 mAh g-1 after 100 cycles even at the rate of 10 C.Meanwhile,when applied with LiCoO2 as a full-cell,the discharge voltage was 3.4 V,and the capacity retention rate was 85%after 100 cycles.(Dalton Trans.,2018,47,7402)2.Cu3Ge/Ge@C nanocomposites were synthesized by in-situ synthesis of germanium carbon composites by in-situ formation of carbon nanotubes.In this method,Cu*is produced by a reaction between CuCl and Ge,and the newly formed Cu*can in-situ catalyze to form cross-linked carbon nanotubes under an acetylene atmosphere at 500 ?,which greatly alleviates the volume stress during charging and discharging and improve the electronic conductivity of the material.When CNTs-Cu3Ge/Ge@C is used as the anode material for lithium ion batteries,the specific capacity remains 1013.4 mAh g-1 after 1000 cycles at a current density of 0.5 C;and still has a high specific capacity of 931.7 mAh g-1 at 10 C.(Chem.Eng.J.,2018,352,206)3.We developed a method for preparing a three-dimensional porous Ge/NC composite material by using glucose oxidized magnesium germanide while PVP is used as a nitrogen source for doping.After ball milling,magnesium germanide and glucose are reacted at 700? to form porous germanium spheres with an atmosphere of N2,and then form a 3D porous structure with NC formed by carbonized PVP,which not only shortens the diffusion path of lithium ions,but also slows down the volumetric strain during charge and discharge.Volumetric strain also increases the electrical conductivity of the material.The half-cell test found that at a current density of 1 C,the specific capacity stabilized at 917 mAh g-1 after 800 cycles;and the specific capacity remained at 673 mAh g-1 at 5 C.When assembled into a Ge/NC-LiFePO4 full cell,the specific capacity was stabilized at 102 mAh g-1 for 100 cycles at a current density of 1C,and the cycle specific capacity was maintained at 72 mAh g-1 at a high current density of 5 C.4.We present new synthetic approach for synthesis of germanium nanoparticles via Aluminum thermal reduction of GeO2 in low temperature molten salt.Under the low temperature environment of 250 ?,AlCl3 was used as the molten salt.The Ge nanoparticles with diameter less than 200nm were synthesized by thermal reduction of aluminum,which reduced the high energy consumption in the traditional magnesium thermal preparation process.When using Ge nanoparticles as anode of LIBs,we found that it still has a high capacity retention rate after 1000 cycles,and the specific capacity reached 1138 mAh g-1.The specific capacity still remained 898.9 mAh g-1 at 10C,which indicates the excellent rate performance.When applied with a sulfur positive electrode,the assembled Ge-S full cell was maintained a specific capacity of 933 mAh g-1 after cycled 200 times at a current density of 1 C.