Researches On Preparation Of Composite Zinc Oxide-based Nanostructure And Its Application In Lithium-ion Batteries

As an important wide-band semiconductor material,zinc oxide(ZnO)has favorable characteristics of room temperature luminescence,ultraviolet absorption,biological sterilization,high breakdown voltage and lithium storage capacity,etc.,which is widely used in various fields,such as semiconductor devices,optical glass,photoelectric energy conversion,electrochemical energy storage and photocatalysis,etc.As an energy storage material,ZnO is widely used as an anode material for lithium-ion batteries(LIBs)due to its high lithium storage performance and ion mobility.The lithium-ion batteries with higher energy density and cycle life have become one of the most crucial devices for highly efficient energy storage and conversion.In the internal components of LIBs,anode and separator are two important components affecting the performance of batteries.Currently,numbers of researches have been devoted to developing novel anode materials and separators to improve the electrical performance of batteries.At present,the most common anode materials of LIBs can be divided into carbon-based materials(such as graphite,biomass carbon and hard carbon),metal oxides and silicon-based materials,etc.Among them,graphite has been widely commercialized,but its lower capacity(372 m A h g^-1)has limited its further development.As a green energy source,the reasonable structure design of biomass carbon or combine it with graphite-like materials,can make it as an excellent lithium ion(Li⁺)and even sodium ion energy storage material.Besides electrodes,as an essential component of LIBs,separator provides a channel for the transmission of Li⁺,although it does not participate in electrochemical reactions,preventing the direct contact of positive and negative electrodes to avoid short circuits.A simple and reasonable modification of separator is also of great importance to improve its safety,cycle stabilities and specific capacities.At present,the most universal separator in commerce is polypropylene(PP)membrane because of its excellent electronic insulation and low ion migration resistance,However,some shortcomings of the PP separator still need to be improved,such as unsatisfying electrolytes affinity,poor thermal stability and mechanical properties toward lithium dendrite.To improve the performance of LIBs,we mainly considered ZnO and diamonds.As LIBs anode,ZnO has a high theoretical lithium storage capacity of 978 m A h g^-1 and excellent ion diffusion coefficient,but serious volume expansion during charge/discharge lead to a poor cycle stability.However,ZnO is a structurally controllable intrinsic semiconductor material,a rational three-dimensional structure design or multiphase composite with other materials possessing better electrical conductivity and stability can improve its comprehensive performance as LIBs anode.As important functional materials,nanodiamonds(NDs),especially detonation nanodiamonds with a particle size of 5-10 nm,has unique size effect,chemical inertia,low expansion coefficient and excellent thermal conductivity,etc.And its high Li⁺adsorption capacity extends its application to LIBs and becomes a potential functional material to improve performance of LIBs.In this work,ZnO was selected as basic research material to combine with porous carbon(PC),silicon(Si)and NDs,a series of composite nanomaterials with three-dimensional micronano structures were received to modified anode and separator of LIBs,and an improved capacity and cycle stability was achieved,finally.The following main results have been obtained:1.A porous carbon-coated silicon and zinc oxide(ZnO-Si-PC)three-dimensional composite was synthesized,in which porous carbon(PC)is made by chemical etching of sunflower straw to improve zinc oxide conductivity and protect its structure.During the hydrothermal reaction of mixed ZnO-Si-PC powders with a temperature of 180 ^oC, Si O transformed into nanocrystallite Si and amorphous Si-oxide matrix(Si O₂)due to high-temperature treatment,the Si O₂ were etched in the alkaline reaction system and eventually Si serves as the nucleation point for the growth of ZnO,forming a ZnO-Si-PC three-dimensional structure.The performance of ZnO-Si-PC anode materials in half batteries were studied in detail.The electrochemical tests showed that the special capacity of ZnO-Si-PC anode materials reached 934 m A h g^-1 after 300 cycles at 0.2 C and 547 m A h g^-1 after 300 cycles at 2 C,which significantly higher than the pure ZnO or graphite anode.In addition,the ZnO-Si-PC composites with high specific surface area and ion diffusion rate improved the adsorption and diffusion capacity of Li⁺.Therefore,this three-dimensional structured ZnO-Si-PC composite prepared by hydrothermal method significantly improves the electrochemical performance of lithium-ion batteries.2.A three-dimensional ZnO/ND composite was prepared by hydrothermal reaction and explored its structural evolution process in the atmosphere.Firstly,the surface acid treatment of nanodiamond makes it easier to become a nucleation point of ZnO which finally uniformly embedded in ZnO nanorods.When exposed the ZnO/ND powder to ambient environment from days to weeks with silicon substrate and the assistance of NDs,the ZnO/ND complex reacted with water and carbon dioxide(CO₂)transferring to a gradient structure of ZnO@ZnCO₃ nanowires embedded with well dispersed Zn/ND nanoparticles.When the carbon film is directly selected as the support substrate, the ZnO/ND nanorods are directly decomposed into linear arranged ND@Zn nuclear shell structural nanoparticles,along with the appearance of the phase of C,which is associated with the high concentration of CO₂ content around the carbon film. Experiments further demonstrate that nanodiamond-assisted ZnO structure evolution can reduce CO₂ concentration in the atmosphere,providing a new idea for decomposition of CO₂ in the environment.3 ZnCO₃ nanowires embedded with NDs are used to modify PP separators and improve the electrochemical performance of the LIBs.During preparation,the ZnO/ND complex was deposited on PP separator forming NDs/ZnCO₃ modified PP separator after two weeks growth in air reacting with CO₂ and water,named as DZPP separator. The electrolyte wettability was significantly improved after modification observed by the hydrophilic/hydrophobic electrolyte test.The LIBs based on DZPP separator shows an incremental reversible capacity of 910 m A h g^-1 after 1000 cycles at 2 C,which exhibit significant improvements of capacity and cycle performance compared with PP separator(501 m A h g^-1 at 2 C).The improved stability and capacity can be attributed to the high Li⁺adsorption ability of NDs on separator,and the Li₂CO₃ membrane formed from the reaction of ZnCO₃ and Li⁺,recorded as a buffer layer on separator to mitigate the impact of massive Li⁺at large current density.Meanwhile,numbers of NDs and Zn are embedded in graphite electrodes along with Li⁺,inducing the transition from graphite to few-layered graphene,that explaining the increased capacity.Design novel multifunctional nanostructure materials means a lot to battery energy storage.In this paper,a series of ZnO-based nanocomposites were prepared,including three-dimensional structured ZnO-Si-PC,ZnO/ND nanorods and its derivative of NDs/ZnCO₃nanowires.The above materials were introduced into the anode and separator of LIBs,and an improved specific capacity and cycle stability of LIBs was received compared with the traditional commercial graphite anode and polymer separator.Therefore,this work puts forward new design ideas for the application of ZnO materials in the field of anode and separator of LIBs.