Preparation And Lithium Storage Properties Of Germanium/Carbon Composites Anode

Lithium-ion batteries are characterized by high specific energy and environmental protection,which is an important research direction to solve the current resource shortage and environmental pollution problems.It plays an increasingly important role in the production and daily life.The conventional graphite anode is limited by the lower theoretical capacity,which cannot meet the requirements of high energy density and power density.The theoretical specific capacity of germanium metal is up to 1600 mAh·g-1,and its electrical conductivity is excellent,which is one of the representatives of new anode materials.However,there is a serious volume expansion phenomenon during the lithium insertion/extraction process,resulting in its poor electrochemical performance.In this thesis,the relatively inexpensive GeO2 is used as raw material to synthesize modified germanium-based materials by means of restriction,coating and dispersion to alleviate the issues caused by volume expansion and aggregation effects.The main research contents are as follows:(1)S-OMC/Ge nanocomposite with Ge nanoparticles embedded in spherical ordered mesoporous carbon was synthesized by nanocasting method.The effects of Ge contents on the performance of S-OMC/Ge electrodes were also investigated.The Ge nanoparticles are uniformly anchored in the pores of ordered mesoporous carbon with parallel perforating channels.The existence of the pores effectively accommodates its volume expansion,and the agglomeration phenomenon is also well suppressed.The spherical structure owns good mechanical stability,and the connected channels facilitate the ion transmission.When the mass ratio of S-OMC to GeO2 is 1:1.5,the S-OMC/Ge sample shows the highest capacity retention in different rate,which yields 996 mAh·g-1 with very good cycle stability over 160 cycles at a specific current of 100 mA·g-1,and 530 mAh·g-1 after 200 cycles even at a high current of 1 A·g-1.(2)The Ge@C-rGO composite in which the cubic hollow Ge@C blocks are uniformly dispersed on reduced graphene oxide nanosheets was synthesized by dopamine-coated precursors and subsequent carbothermal reduction process.The carbon shell formed by the degradation of polydopamine serves as a support structure to leave space for the volume expansion of Ge.Graphene further disperses and confines the Ge@C cubes,and forms a double carbon layer protection structure together with the carbon shell.Both N-doped carbon shell and highly conductive graphene networks contribute to the improved conductivity of Ge@C-rGO electrode.The reversible capacities of the Ge@C-rGO electrode can be obtained by charge/discharge process for 200 cycles,and it owns 1183 mAh·g-1 at 100 mA·g-1 and 707 mAh·g-1 at 1 A·g-1.In addition to the high capacity retention,the Ge@C-rGO electrode has a stable cycle tendency,good rate recovery performance and excellent electrical conductivity.(3)CuGeO3/rGO composite with copper germanate nanorods uniformly dispersed on reduced graphene oxide nanosheets was prepared by a simple one-step hydrothermal method.CuGeO3/rGO with different reduced graphene oxide contents were synthesized by adjusting the amount of GO added.The graphene nanosheets in the CuGeO3/rGO-30 sample containing 30%GO addition are not stacked,and the CuGeO3 nanorods can be dispersed in a large area to alleviate the agglomeration.At the meantime,the presence of rGO promotes the overall conductivity of the material.CuGeO3/rGO-30 delivers the capacity retention of 909 mAh·g-1 in 200 cycles at the specific current of 100 mA·g-1,and its rate performance and conductivity are superior to those of the CuGeO3 electrode.