Design Ultra High Rate And Long Stability Anode Of Lithium Ion Battery Based On 3D Cu-Si Nanostructure Architecture

With the rapid development of intelligent mobile devices and electric vehicle industry,developing high specific capacity,high charging rate,long cycle stability,fast charging and discharging,safe and reliable lithium ion battery has become an inevitable trend.Silicon?Si?is best known for the highest specific capacity,up to?4200mAh/g,as well as its earth-abundant reserves and low manufacturing cost,which make it an ideal competitor to replace the commercial carbon-based lithium-ion battery?LIB?,and thus inspiring a worldwide intense research.The biggest challenge facing Si-based negative electrode is its poor conductivity and a large volume expansion?-400%?during lithiation process,which causes a short cycle life time and poor rate performance that hinder immediate commercial use.To address this issue,low-dimensional nano Si anode structure with a huge surface ratio provides exciting opportunities to increase the effective mass-load and achieve a higher energy density during lithiation.More importnatly,nanostructured Si anode allows far more space among them to accommodate the volume expansion,which is a key point to guarantee a long lifetime of LIB.By constructing highly conductive and stable 3D nano-framework,a continuous and robust conductive pathway is formed that strengthens the mechanical stability and the conductivity of the Si-based anode,while stablizing the SEI film and enhancing the high rate cycling performance.In this thesis,in order to resolve the troubles of Si-based anode structures,a novel design is proposed by constructing highly interconnected three-dimensional network structure,and a highly cross-linked Cu/a-Si core shell nanowires.Furthermore,combining the technology of in-situ hollow nanotube structure and Cu₃Si nanoparticle doping,we demonstrated Cu-Si alloy composite nanotube anode structure,which can help to accomplish a high-speed charge and exellent long term cycling stability in Si-based LIBs.The major results and innovative contributions of this Thesis are listed as follows:1.Two methods of air thermal oxidation and?NH4?2S2O8 NaOH solution chemical reaction were adopted to fabricate mutual crossing CuO NWs.Through a series of synthesis parametric control,we grow vertical CuO NWs with length of?10 um,diameter?40 nm or bending nanobelt with length of-30 um,diameter-60 nm,respectively.A new growth dynamic model has been proposed to explain the CuO NW growth behaviors.2.Based on the bending CuO NWs,a high quality comformal coating of amorphous Si?a-Si?thin film is done in PECVD system to form a crossing matrix of CuO NWs with redundant conductive pathways.After low temperature H2 annealing in H2,the CuO NWs can be reduced into Cu cores.The resultant Cu/a-Si core-shell anodes demonstrated an excellent cycling stability that last more than 700 charge/discharge cycles at a current density of 3.6 A/g,with a high capacity retention rate of 80%.Remarkably,at extremely high charging current density of 64 A/g the LIB can still survive after 25 runs cycling with 75%initial capacity with a reasonably high 80%retention rate and a final capacity of 1026 mAh/g.3.Based on the upright CuO NWs,amorphous silicon was deposited upon the cross-linked CuO NWs,and reduced upon H2 annealing into Cu atoms that further diffuse to alloy with the a-Si shell and produce Cu₃Si alloy nanoparticle,leaving in-situ highly interconnected but hollow Cu-Si alloy nanotubes.In this nanotube anode structure,a high specific capacity of 1010mAh/g has been achieved after 1000 cycles at 3.4 A/g,with a capacity retention rate of?84%,without the use of any binder or conductive agent.In addition,Cu₃Si doping and interconnected hollow structure composite,as so-called "double composite",demonstrated a remarkable high charging rate at high current density of 20 A/g?3 min for one charge cycle?for 1000 cycles with a retention of 88%and a relatively high specific capacity of?780 mAh/g,demonstrating an impressive potential for fast charging and very stable Si-based LIBs application.4.Developing an alloy-embeddment strategy of Cu₃Si nanoparticles into the a-Si shell to largely enhance the conductivity of a-Si shell.Compared to traditional B and P dopant,the Cu₃Si nanoparticle embeddment are non-active lithiated material,and thus avoid the chemical bond breaking during charge and discharge cycling.It is shown that the stability and conductivity of Cu₃Si embedded anodes,as well as its mechanical support,can be greatly improved in developing high performance LIBs.