Synthesis,Characterization And Electrochemical Properties Of Zn-based Anode Materials By Atomic Layer Deposition

Lithium-ion batteries(LIBs)have been widely used due to its good safety performance,large energy density,long cycle life and high output voltage.In recent years,with the development of microelectronics and electric vehicles,people put forward higher requirements for LIBs.It is particularly imperative to develop new electrode materials with higher specific capacity and better cycling performance.Zinc-based anode materials have drawn much attention because of their high theoretical capacity and environmental friendliness.More importantly,the zinc resource reserves in China rank second in the world.There are abundant varieties of Zn-based compounds with diverse preparation methods,leading to low cost.Research and development of high-performance zinc-based anode materials have important strategic significance for China’s energy industry.Among zinc-based anode materials,ZnO and ZnS with good electrochemical performance have aroused some researchers’ interests.Unfortunately,the dramatic volume expansion/shrinkage caused by Li+ insertion/extraction during the discharge/charge process and low intrinsic electric conductivity issue limit their development.A large number of experiments have proved that the design and preparation of distict composite nanostructures can effectively alleviate the volume change during charge/discharge process and enhance their electrical conductivity,thus the electrochemical performance will be improved.Atomic layer deposition(ALD)is a kind of developing thin film preparation technology with the advantages of three-dimensional conformality,precise sub-monolayer thickness control,and large area uniformity.ALD is being widely applied in the field of lithium ion batteries as a reaserach hotspot.It not only can modify the surface of the electrode materials so as to improve the battery performance,but also can be used to synthesize anodes,cathodes,and solid state electrolytes of the LIBs.Diverse electrode materials with excellent performance can be prepared by ALD by means of the combination of different materials and unique structural design.In this thesis,several Zn-based nanostructures have been designed and fabricated by ALD,including ZnO/TiO2 nanolaminates,Co doped ZnO thin films and Carbon nanotube(CNTs)@ZnS core-shell structures,in order to solve the technical problems of ZnO and ZnS anode materials,such as sharp capacity fading and low rate capacity due to the dramatic volume change during the lithiation/delithiation process and poor electrical conductivity.The composition,microstructure and morphology of the samples of ZnO/ZnS-based anode materials were characterized and their electrochemical properties were studied.The main achievements are summarized as follows:1.ZnO/TiO2 nanolaminates with thickness ratio of ZnO and Ti02 of about 5:1 were designed and fabricated by ALD.The total thickness of ZnO/TiO2 nanolaminates is about 140 nm with the grain size of about 100 nm.Compared to pure ZnO thin films,ZnO/TiO2 nanolaminates,as the LIB anode material,exhibit excellent cyclability with a capacity of 634 mAh/g after 1200 cycles at a current density of 500 mA/g and a voltage range of 0.05-2.5 V without declining tendency.This is mainly due to the structural support of TiO2 in ZnO/TiO2 nanolaminates,which alleviates the stress and strain and avoids the pulverization effect of the material caused by the volume change.In comparison with the bulk materials,the nanostructured thin films shorten the transport pathway of Li+ and electrons and improve the migration rate of electrons and Li+.2.In order to reduce the initial irreversible capacity of pure ZnO film,Co doped ZnO films were designed and prepared by combination of thermal ALD and plasma-enhanced ALD.Although the Co dopant amount in ZnO films is lower and only～3.0 wt%of the total mass,Co doped ZnO films,as the lithium ion battery anode material,show a higher initial coulombic efficiency of 63%at a current density of 500 mA/g and a voltage of 0-2 V than pure ZnO ones of 45%.After 100 cycles,the discharge capacity of Co doped ZnO films still maintain at 302 mAh/g,which is much higher than pure ZnO films of 108 mAh/g.Obviously,compared to pure ZnO film,the Co dopant of ZnO films significantly improves the electrochemical performance.This may be related to the Co metal nanoparticles formed in the first charge/discharge cycles,which is beneficial to the reduction reaction from Li2O to Li and the improvement of reversible capacity.3.The CNTs@ZnS core-shell structures were successfully prepared by depositing 100 cycles of ZnS on the hydroxylated multi-walled carbon nanotubes by ALD.The ZnS shell of CNTs@ZnS core-shell structure shows nanostructured polycrystalline with thickness of about 6 nm and mass percentage of 52.1%.The CNTs@ZnS core-shell structures exhibit excellent cycling stability as a LIB anode material.The capacity remains at 474 mAh/g after 140 cycles at a current density of 100 mA/g.Moreover,the capacity shows futher increasing trend due to the activation role of anode material.This is mainly due to the fact that the conductive CNTs not only improve the conductivity of the material but also provides the structural support.At the same time,nano-sized ZnS can alleviate the stress and strain caused by Li+insertion/extraction and shorten the transport pathway of Li+ and electrons.