Construction Of Silicon/Carbon Nanotube Composites And Its Lithium Storage Performances

With the rapid development of contemporary society,new energy,electric vehicles and portable electronic devices are changing with each passing day,which puts much higher requirement on the electrochemical energy storage devices.Lithium-ion batteries have received extensive attention due to their high energy density,high operating voltage,and long cycle life.As one of the important components of lithium-ion battery anode material,it can determine the capacity and performance of the battery to a large extent.At present,most commercial batteries still use graphite negative electrode.However,the capacity of comercial graphite is close to its theoretical specific capacity(372 mAh/g),which is difficult to further improve.This makes it difficult to meet the needs of electric vehicles and other electronic equipment with high power consumption requirements.Silicon-based materials show high theoretical specific capacity and are currently a hot research topic.However,due to the low conductivity and the huge volume expansion during lithium intercalation.Si anode experience rapid capacity decay and poor rate performance when it is directly used as a negative electrode material for lithium-ion batteries.This dilemma limits the wide application of Si in the lithium batteries.Si/C composite anode materials can not only alleviate the volume effect of silicon nanoparticles during cycling,but also improve their electronic conductivity.Thus,Si/C anode has become the reseach focus nowadays and in the future.In this paper,a series of silicon/carbon nanotube(Si/CNT)composite electrode materials were prepared through different material preparation processes.The as-prepared composite materials were systematically characterized and electrochemically tested.The main research contents are as follows:1)A three-dimensional(3D)network structured Si/CNT/C anode material was fabricated by a hydrogen bonding assisted sol-gel method.In this structure,Si NPs are glued to the CNT sidewall by the amorphous carbon,forming into a unique 3D network structure.The amorphous carbon is coated the surface of Si NPs and CNTs.Besides of acting as a conductive skeleton,CNTs can also form into a porous network,which can buffer the huge volume fluctuation of Si NPs during the lithiation/delithiation process.Therefore,the prepared Si/CNT/C cathode material has excellent lithium storage performance.At a current density of 0.2 A/g,an initial discharge capacity of 2365 mAh/g was obtained and retained as 1 003 mAh/g after 200 cycles.At a higher current density of 1 A/g,the reversible capacity was 718 mAh/g.2)A Si/CNT/C composite material with a "sugar gourd" morphology was prepared through the sol-gel method and the subsequent heat treatment process.In this unique structure,Si NPs are attached to the sidewalls of carbon nanotubes and are encapsulated by amorphous carbon.The carbon nanotubes form into a 3D porous network,which can not only buffer the volume expansion of silicon,but also improve electron/ion conductivity.The Si/CNT/C-0.2 sample,obtained under the optimum addition amount of carbon nanotubes,showed a first discharge specific capacity of 1905 mAh/g at a current density of 0.2 A/g.The capacity is still as high as802 mAh/g after 200 cycles.It still has a reversible specific capacity of 485 mAh/g at a larger current density of 1 A/g.3)The Si/CNT/Ni nanocomposite film with binder-free 3D nanoporous network structure was prepared by one-step electrophoretic deposition method and subsequent heat treatment process.The 3D framework composed of intertwined carbon nanotubes can not only effectively provide sufficient buffer space during Li-ion intercalation/deintercalation,but also a more stable conductive network to increase the ion/electron transfer speed.The nickel shell on the silicon surface further relieves the volume expansion of Si NPs and reduces the degree of pulverization of the material.The Si/CNT/Ni-5,obtained with the optimal addition of nickel nitrate,maintained a capacity of 607 mAh/g after 200 cycles at a current density of 0.2 A/g.876 mAh/g at a current density of 1 A/g reversible specific capacity.4)CNT@Si coaxial nanocable composites were prepared by sol-gel method and subsequent magnesium thermal reduction process.The CNT@Si was coated with carbon to obtain CNT@Si@C.In this unique structure,the Si NPs are coated on the surface of carbon nanotubes.The large contact area endows electrons/ions with fast transmission path.The outer carbon coating layer can buffer volume expansion of silicon,and form into a stable SEI film.The CNT@Si@C-2 sample,obtained under the optimal silicon coating thickness,showed a specific capacity of 700 mAh/g for the first discharge at a current density of 0.2 A/g,and the capacity remained at 538 mAh/g after 200 cycles.The discharge specific capacity was 376 mAh/g at a current density of 1 A/g,showing excellent rate performance.