Preparation And Electrochemical Performance Of Silicon/Germanium-Based Anode Materials For High Specific Energy Lithium Ion Batteries

In recent years,with the rapid development of consumer electronics,electric vehicles and energy storage technology,the market demand for lithium ion batteries（LIBs）is also increasing.However,the low energy density of conventional LIBs cannot meet the requirements of high energy density energy storage equipment in the future.Therefore,it is imperative to develop LIBs with high specific energy.Using silicon/germanium-based anodes with high specific capacity instead of traditional low capacity graphite anode(372 mAh g^-1)to reduce the mass-loading of anode is one of the effective methods to prepare high specific energy LIBs.Si and Ge alloy anodes are based on allying reaction for lithium storage,with theoretical specific capacities of 4200and 1625 mAh g^-1,respectively.In addition,the appropriate lithiation/delithiation platforms of Si/Ge-based anodes can avoid the formation of lithium dendrites and ensure the high safety of LIBs.Nevertheless,the large volume change of Si/Ge-based anodes during charging and discharging will lead to the pulverization and crushing of active materials,resulting in the decrease of utilization of active materials,and ultimately lead to the continuous decline of capacity and performance deterioration.Thus,it is of great significance to optimize the micro-structure and composition of Si/Ge-based anodes to improve their electrochemical performance for the preparation of LIBs with high specific energy.In this dissertation,the micro-structures of Si/Ge-based composites were mainly designed by combining the strategies of nanocrystallization,porosization and compounding.In addition,the compositions of Si/Ge-based composites were optimized by exploring the preparation technology.We also studied the performance of the above anode materials for LIBs and obtain the following main innovations.（1）Preparation and optimization of carbon-coated mesoporous silicon submicrocube composites（denoted as CM-Si@C）.By designing the morphology and preparation process of Si,the synthesized CM-Si@C anode still has a reversible specific capacity of 740 mAh g^-1 at a current density of 30 A g^-1 after 200 cycles.In addition,the average energy density of the CM-Si@C//LiFePO4 full cell is up to 336Wh kg^-1.Nano-Si（5¹0 nm）particles composed of CM-Si@C composites can withstand large stress,and the pore volume in the porous structure can also weaken the volume effect of Si and improve its structural stability.The carbon coating in CM-Si@C composites not only improves the electronic conductivity of Si,but also avoids the direct contact between Si and electrolyte,which can promote the stable formation of solid electrolyte interphase（SEI）film on the electrode surface and improve the integrity of electrode structure.（2）Design and fabrication of core-shell germanium/carbon composites（denoted as Ge@NC）.The Ge@NC anode synthesized in situ by spray pyrolysis has a high specific capacity of 917 mAh g^-1 at 1 C rate with a capacity retention of 82%.In addition,the average energy density of the Ge@NC//NCM523 full cell is up to 340 Wh kg^-1.The conductive carbon shell in Ge@NC composites can improve the electronic conductivity of Ge,avoid the direct contact between Ge and electrolyte,and reduce the occurrence of surface side reactions and agglomeration.（3）Optimization of germanium/carbon composites（denoted as Ge@C/rGO）prepared by assist with dopamine.In Ge@C/rGO composites,the highly conductive rGO layer can provide a two-dimensional（2D）conductive matrix for Ge@C,thus significantly improving the electronic conductivity of Ge.In addition,the Ge@C particles were anchored to the rGO to enhance the bonding strength between Ge and the conductive substrate,thus ensuring the high stability of the electrode structure.The Ge@C/rGO anode has a specific capacity of 1074 mAh g^-1 after 600 cycles at 2 C rate,and the capacity retention rate is as high as 96.5%.（4）Design and fabrication of sandwich-like germanium/graphene composites in which Ge was encapsulated in interlayer rGO（denoted as rGO/Ge/rGO）.The sandwich conductive network constructed by rGO can completely encapsulate Ge to improve its electrochemical properties.The rGO/Ge/rGO-10 anode still has a reversible specific capacity of 1085 mAh g^-1 after 500 cycles at 1 C rate.In addition,the rGO/Ge/rGO-10//NCM523 full cell can maintain a specific capacity of 940 mAh g^-1 after 100 cycles at1 C rate with a Coulombic efficiency of 99.95%.The sandwich rGO can not only improve the electronic conductivity of Ge,but also inhibit the side reactions between Ge and electrolyte,so as to improve its stability.Furthermore,small amount of voids in the sandwich structure can also accommodate the volume change of Ge,thus improving the structural stability of rGO/Ge/rGO composites.（5）Construction of spherical germanium/graphene composites（denoted as Ge/rGO）with Ge encapsulated in a three-dimensional（3D）rGO conductive framework by means of spray pyrolysis method.The interwoven rGO layer inside the three-dimensional（3D）conductive framework effectively can block the direct contact between Ge particles and avoid the agglomeration phenomenon in the repeated cycles,thus enhancing the utilization of active materials.In addition,the 3D rGO conductive framework can also improve the electronic conductivity and structural stability of Ge.The Ge/rGO-2 anode also has a specific capacity of 811 mAh g^-1 after 1000 cycles at the rate of 1 C,and the average energy density of the Ge/rGO-2//LiFePO₄ full cell is as high as 338 Wh kg^-1.（6）Optimization of silicon-germanium alloy（denoted as Si_xGe_y）anodes prepared by the one-step solid-state method.Si_xGe_y alloy anodes can make full use of the synergistic effect of Si and Ge,and ensure both high capacity and excellent cycling stability.Ge component in the SiGe anode can accelerate the transmission rate of ions and electrons.SiGe anode has a reversible specific capacity of 418 mAh g^-1 after 900cycles at the current density of 2 A g^-1.In addition,Si and Ge in the SiGe alloy with different lithiation/delithiation potentials,which can effectively release stress and enhance its structural stability.