| With the increase of energy consumption for next-generation electric devices and portable electronic devices,more attention has been paid on the development of lithium-ion batteries(LIBs)with high performance.As a host for the insertion/extraction of lithium ions,electrode materials play a crucial role in the performance of batteries.It is difficult to further increase the specific capacities of currently used commercial cathodes.So,the development of new type anode materials with high capacity is an effective way to further improve the performance of LIBs.The commonly used anode material for LIBs is graphite.However,its theoretical specific capacity is low(?372 mAh/g).Silicon and most of the transition metal oxide anode materials have a high theoretical specific capacity.At present,silicon anode has the highest theoretical specific capacity of?4200 mAh/g,and the theoretical specific capacity of most metal oxides is also higher than that of graphite anode,such as Cu2O and Fe2O3.Although silicon,transition metal oxides Cu2O and Fe2O3 anodes possess a high specific capacity,they have the disadvantages of low intrinsic conductivity,large volume expansion/contraction and poor capacity retention.The volume expansion of silicon during the full lithiation process is over 300%,and the volume change of Cu2O and Fe2O3 anodes during the lithiation is also obvious.The huge volume change causes severe pulverization of the active material and the loss of electrical contact,resulting in the continuous rupture and thickening of SEI film on the surface of active material,which increases the charge transport resistance and leads to the decay of battery capacity.This is an obstacle to the practical application of such anode materials.Therefore,it is a challenge in research to mitigate the capacity reduction of silicon,Cu2O and Fe2O3 caused by large volume changes and simultaneously increase their conductivities.In this thesis,the different nanostructures or composite nanostructure are fabricated for silicon,Cu2O and Fe2O3 anode materials,which provide effective space for stress relaxation caused by volume change during charge and discharge process,and achieve lower charge transport resistance,thus obtaining the corresponding anodes with high specific capacity and good capacity retention.We explored the building of nanostructures for silicon,Cu2O and Fe2O3 anode materials.The influence mechanism of nanostructure configuration on charge transport resistance and specific capacity was also discussed:(1)Lithium storage mechanism of mesoporous silicon nanowire anodes:Silicon has a high coefficient of expansion and a band gap of 1.1 eV.Its conductivity can be controlled by doping.In order to solve the capacity retention problem caused during the charge and discharge processes,silicon nanowires were synthesized by etching the silicon wafer in the HF solution,and nanopores were formed in the silicon nanowire by the Ag/Ag+ reaction.Thereby,the specific surface area of silicon could be increased,which is beneficial to the reduction of the current density as well as the pressure on the silicon surface.In this way,its specific capacity can be well maintained;Due to the formation of nanopores in the nano wires,the nanowire structure provides a one-dimensional conductive path way.This special nanostructure provides a channel for fast charge extraction and injection,so its specific capacity and capacity retention could also be increased.The physical mechanism of nano-Ag etching silicon nanowires is systematically studied in this thesis.The equilibrium potential of Ag/Ag+ used is lower than that of silicon valence band.It is easy to capture electrons from silicon and lead to the reduction of Ag+.The preparation of silicon nanowire can be controlled by the etching dynamics of silicon and the formation of Ag particles.The etching rate is then modulated by controlling the size of Ag to obtain mesoporous silicon nanowires with a pore size of?7 nm.Mesoporous silicon nanowires were used as anode materials to evaluate its electrochemical performance and capacity retention properties.It is found that the mesoporous silicon nanowire anode material,which has large specific surface area(323.47 m2/g),high specific capacity and retention rate,could be obtained by optimizing the etching conditions.At a high current density of 1.5 A/g,the reversible specific capacity is as high as 2061.1 mAh/g after 1000 charge and discharge cycles.(2)Study on the mechanism of lithium storage in different configurations of Cu2O anode materials:The energy band gap of Cu2O is 2.1 eV.As an anode material,the particle size has a great influence on the electrode resistance.Since the conductivity of Cu2O is inferior to that of silicon,the branched nanowires are formed on the trunk nanowire instead of building a mesoporous structure,so that the charge transport property is most likely to be ensured.In order to explore the influence mechanism of different configurations on electrode resistance and specific capacity,the Cu2O micro-particles(?5 ?m),nano-sized Cu2O(?15 nm)and branched Cu2O nanowires(trunk>10 ?m,branch:20 nm?60 nm)were fabricated,respectively.Under the testing condition of both low and high current densities,the specific capacity of branched Cu2O nanowire is the highest,about 20%higher than that of Cu2O nanoparticle,and about 100%higher than that of Cu2O microparticle.The charge transport resistance of the branched Cu2O nanowire is only 90%and 70%of its nanoparticles and microparticles counterparts,respectively.(3)Exploring the auxiliary conductive nanomaterial and the flexible electrode to enhance the charge transfer of the ?-Fe2O3 anode material:For the metal oxide anode material with poor conductivity,except for building the branched nanowire structure to improve the charge transfer property,the conductive material can be added to make close contact between the metal oxide and the conductive material.Similar to Cu2O,nano-?-Fe2O3 has poor charge transport capability(Eg=2.2 eV).In this thesis,carbon nanotubes are used as conductive materials to enhance charge transfer,and ?-Fe2O3 is directly synthesized on carbon nanotubes to form close contact and reduce the charge transport resistance of the electrode.The addition of carbon nanotubes reduces the amount of Fe2O3,so the proportion of carbon nanotubes has an effect on the electrode capacity.Besides,we also use flexible carbon nanopaper as an conductive network for the whole electrode,which further reduced the electrode charge transport resistance.It was found that the 50wt.%MWCNT/a-Fe2O3 composite anode maintained a specific capacity of 778.2 mAh/g after 50 cycles at a current density of 200 mA/g.The charge transport resistance of 50wt.%MWCNTs/a-Fe2O3 is just 24%of pure ?-Fe2O3.In addition,carbon nanopaper(CNP)is used as the current collector,which further reduces the charge transport resistance.The resistance value is about 70%of the copper current collector,and its total electrode specific capacity is also greatly increased.The research found that for silicon,Cu2O and Fe2O3 anode materials,the construction of nanostructures can reduce the charge transport resistance,provide space for stress release during the volume change of lithiation process,and increase its specific capacity and capacity retention.The silicon nanowire structure can offer a directional transport channel for charge.For the silicon anode which needs better conductivity,a mesoporous structure can be realized on the nanowire to increase its specific surface area and buffer volume expansion.For metal oxides with poor conductivities,the branched nanowires can be anchored on the trunk nanowire.In addition,by adding highly conductive carbon nanotube,the anode material is in close contact with the carbon nanotube,thereby reducing the charge transport resistance.This thesis provides a direction for the design of anode materials with high capacity retention and good cycle performance.It provides research ideas for these high performance anode materials adopted in the LIBs with high energy density,good cycle life and high power density. |
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