Design And Preparation Of Si/Ge Based Anode Materials For High-Capacity Lithium-Ion Batteries

Lithium-ion batteries?LIBs?with high capacity density,power density and long-term cycling have been used in all kinds of electric devices in our life.However,the commercial LIBs we used now is consist of graphite and LiCoO₂ electrodes,the energy density is already reach the bottleneck cannot improve much more base on these materials.Higher energy density and long-term cycling stability rechargeable Li-ion batteries are being urgently needed in the large-scale electrochemical energy storage systems,especially the electric vehicles and advanced power grid.As a key component of LIBs,all kinds of anode materials with improved storage capacity and thermal stability have been proposed for LIBs.Silicon and Germanium based anode material has attracted the most extensive researches in recent years due to its unparalleled theoretical capacity?4200 mAh/g for Si and 1600mAh/g for Ge?,relatively low discharge potential?<0.5 V vs Li/Li⁺?,abundance reserves and low cost?Si?.However,Si and Ge based anodes are notoriously plagued by poor capacity retention resulting from large volume changes?300%?during Li-ion insertion and extraction processes.This volume change induces severe pulverization and electrical disconnection between active materials and the electrode framework,and also causes continual formation of unstable solid electrolyte interphase?SEI?on the silicon surfaces newly exposed to the electrolyte.All those undesirable factors lead to rapid capacity fading.The thesis mainly focuses on the design and assembly of high energy density and long-life electrochemical energy storage devices.The nanostructure design and preparation of silicon and germanium based composite,surface modify and charge storage mechanism as well as devices construction and optimization have been carried out systematically in our thesis,summarized as follows:?1?Ge/grapheme/carbon nanotube composite anode for high performance lithium-ion batteries:A Ge/grapheme/carbon nanotube composite electrode was constructed by germanium?Ge?nanoparticles anchored on reduced graphene oxide?Ge/RGO?intertwined with carbon nanotubes?CNT?.In this unique structure,the graphene sheets improve the electrical conductivity and buffer severe volume changes.Additionally,the CNT mechanically binds together with Ge/RGO to maintain the integrity of the electrodes and stabilize the electric conductive network for the active Ge nanoparticles,leading to better cycling performance.As a result,the designed anode exhibits an outstanding energy capacity up to 863.8 mAh/g at a current density of 0.1 A/g after 100 cycles?capacity reservation 70%?and good rate performances of 767.6,and 644.8 mAh/g at current densities of 1600 and 3200 mA/g,respectively.Our results indicate that the hybrids exhibit considerably improved lithium storage performance.?2?Titanium Dioxide/Germanium Core–Shell Nanorod Arrays Grown on Carbon Textiles as Flexible Electrodes for High Density Lithium-Ion Batteries:Three-dimensional?3D?titanium dioxide@germanium?TiO₂@Ge?core–shell nanorod arrays on carbon textiles are fabricated by a facile two-step method and further investigated as flexible electrode for Li-ion batteries?LIBs?.The synthesis of TiO₂@Ge composite involves the hydrothermal growth of TiO₂ nanorod arrays on carbon textiles and a subsequent coat with a thin layer of germanium with radio frequency?RF?magnetron sputtering.The TiO₂ nanorod arrays can effectively not only increase the unit mass loading as a role of skeleton but also remarkably enhance the electrical conductivity via control the lithiation/delithiation voltage in the range of 0.01–1.0 V,where TiO₂ can be in situ lithiated to LixTiO₂after the first discharge cycle.Moreover,each TiO₂@Ge nanorod has enough space to accommodate the large volume expansion of Ge during charge and discharge cycles.Benefiting from unique electrode architectures,this additive free,self-supported electrode exhibits the high reversible capacity,outstanding rate capability,and the extremely long cycling stability even at a high rate?700.3 mAh/g is still retained at 5.0 A/g after 600 cycles,capacity reservation is 91%?.?3?Rational Design of Void-Involved Si@TiO₂ Nanospheres as High Performance Anode Material for Lithium-Ion Batteries:A unique core-shell structure of silicon@titania?Si@TiO₂?composite with silicon nanoparticles encapsulated in TiO₂ hollow spheres is synthesized by a simple hydrolysis method combined with magnesiothermic reduction method.It is found that the TiO₂ shell is effective for improving the electrical conductivity and structural stability.More importantly,the well-designed nanostructure with enough empty space would accommodate the volume change of silicon during the cycling.Reversible capacities of 1911.1 and 795 mAh/g can be obtained at 0.21 A/g and a high current rate of 4.2 A/g,respectively.After 100 cycles at 0.42 A/g,the composite electrode still maintains a high capacity of 804 mAh/g.This excellent cycling stability and high-rate capability can be ascribed to the unique core-shell nanostructure and the synergistic effect between Si and TiO₂.?4?Si nanoparticles encapsulated in elastic hollow carbon fibres for Li-ion battery anodes with high structural stability:The core–shell structure in the 1D fibres of Si@C was fabricated by electro-spinning method,surface polymerisation,and magnesiothermic reduction.The Si NP@C nanofibres exhibited high reversible capacity,excellent rate capability,and good cycling performance.The enhanced electrochemical properties could be attributed to the interpenetrating network of carbon nanofibres and their provision of high conductivity,good mechanical properties,facile ion transport,and the in situ formation of sufficient empty space,thus leading to facile electrode kinetics and high strain tolerance during volumetric changes.The capacity of Si NP@C nanofibres retained a specific discharge capacity of 466.8 mAh/g at current density to 12.8 A/g,a high discharge capacity of 908.5mAh/g could achieve after 50 cycles at 0.1A/g and capacity reservation is 80%and 546.9 mAh/g could still be obtained after 200 cycles at 1.0 A/g.?5?Raspberry-like Nanostructured Silicon Composite Anode for HighPerformance Lithium-Ion Batteries:Based on the relationship between shell thickness and the radius,we precisely design and synthesize raspberry-like hollow nanospheres which can meet the 3 times volume expansion of the Si hollow spheres by the molten salt magnesiothermic reduction of mesoporous hollow silica nanospheres?HSiO₂?and polymerization of dopamine on the surface of HSi,followed with carbonization and without any other template.This distinguished material design brings about multiple fascinating merits:?a?the large stress and volume changes of Si during cycling can be effectively alleviated by the plentiful porous structure of the hollow nanospheres;?b?the transportation distance of the ionic and electron can be extremely reduced benefit from the hollow nanospheres with a thin shell;?c?thermal decomposition of carbon precursors as coatings provide high electrical conductivity of the electrode as well as a buffer layer to remission the stress of volume changes.Outstanding cycling stability and distinguished rate capability are obtained when such novel HSi@C are evaluated as anode materials in LIBs.Therefore,these specially designed hollow structures exhibit a stable and high capacity of 886.2 mAh/g for 200 cycles with a high Coulombic efficiency?CE?up to 99.4%;a high capacity of 516.7 mAh/g can be maintained at 2.0 A/g even after500 cycles when used as anode materials for LIBs.?6?High Graphitization Carbon Coating of SiO as Anode Material for High Performance Lithium-ion Batteries:A novel of conductive and electrochemically stable carbon coating is rationally designed to modify the silicon monoxide?SiO?anode materials by controlling sintering a conductive polymer,pyrene-based homopolymer poly?1-pyrenemethyl methacrylate??PPy?,which achieved high graphitization carbon layers at a low temperature and avoids silicon carbide formation.When evaluated as the anode of Lithium-ion batteries,the composite delivered a high discharge capacity of 2058.6 mAh/g at 0.05 A/g of the first formation cycle with an initial Columbic efficiency?ICE?of 62.2%.After 50 repeated cycles at 0.1 A/g this electrode was still able to store an impressive capacity of 1090.2 mAh/g?83.5%capacity retention,relative to the capacity of the second cycle?and a high specific capacity of 514.7 mAh/g was attained at 0.3 A/g after 500 cycles.Furthermore,the coin-type full cell composed of the pre-lithiated SiO composite anode and the LiNi_0.5Co_0.2Mn_0.3O₂cathode attained excellent cycling performance,which shows the prospects of application for next-generation high energy density LIBs.