Metal Organic Framework Derived Zinc Oxide Based Anode Material For Lithium Ion Battery

As an anode material for lithium ion battery,zinc oxide has a lot of advantages such as high specific capacity,low cost,easy to prepare,good stability and nontoxic.ZnO has a lower redox potential and higher Li+ diffusion coefficient compared with other metal oxide.While,similar to other metal oxides,ZnO undergoes volume expansion and poor electrical conductivity during cycling which results in pulverization of the electrodes and limited rate performance.In this work,we modified the zinc oxide based materials by nano-sized,carbon coat and metal dope.Metal Organic Framework derived ZnO possesses many unique advantages.Firstly,the framework and porous struscture were partly retained so that the MOF derived ZnO exists a high specific area.Secondly,a very small particle size can be obtained by calcination of MOF.ZnO quantum dot also can be derived.Thirdly,the ZnO nanoparticles derived by MOF were coated with carbon uniformly.The carbon coated ZnO were embedded in carbon parent,thus the secondary particles formed.In this work,the MOFs were used as precursor to prepare ZnO based anode materials for lithium ion batteries.The details were as follows:1.Cobalt doped MOF-5s?Co-MOF-5s?were firstly synthesized by secondary growth method,and followed by heating treatment for the yielding of Co doped ZnO coated with carbon?CZO@C?.The framework and porous struscture were partly retained and the ZnO nanoparticles were coated by carbon uniformly.Compared with carbon coated ZnO?ZnO@C?,the doping of Co increased the graphitization degree of the carbon on the surface of CZO@C nanoparticles and enhanced the conductivity of the material.The investigation on calcination time indicated that the sample which calcined without heat holding time had the best electrochemical performance.It was found that the as synthesized CZO@C composites enabled a reversible capacity of 725 mA h g^-1 up to 50 th cycle at the current density of 100 mA g^-1,which was higher than that of ZnO@C composites(335 mA h g^-1).2.Co doped ZnO embedded within carbon/carbon nanotube composites?CZO@C/CNT?was in situ prepared during the calcination of Co-MOF-105 at 600 °C.The lower crystallinity demonstrated a weaker binding force in Co-MOF-105,which made it possible that the Co ion breakaway from the crystal and reduced to metal Co during the pyrolysis process.The formation of CNTs was catalyzed by metal Co and the carbon source was terephthalic acid which played as the organic linker in MOF.Moreover,the sp² hybridization during which the carbon atoms were in terephthalic acid decreased the energy barrier during the growth of CNTs.From TEM and SEM observation,the CNTs were interspersed in the material and connected the CZO@C nanoparticles together,which made the electron transfer easier.The other advantages of Co doping were enhancing the conductivity of ZnO and increasing the graphitization degree of the carbon on the surface of CZO@C nanoparticles.When the CZO@C/CNT composite was used as anode material for lithium ion battery,an enhanced electrochemical performance of 758 mA h g^-1 after 100 cycles at the current density of 100 mA g^-1 was obtained.3.Lanthanum doped ZnO coated with carbon composites were derived by the calcination of La doped MOF.The XRD patterns indicated that the La doping was adverse to the growth of ZnO particles.The cyclic voltammetry test stated that the lithium storage process of lanthanum doped ZnO coated with carbon composites was a surface controlled process.The smaller ZnO nanoparticles formed by La doping was beneficial to enhancing the interfacial lithium storage capacity.But the electrochemical impedance spectra proved that the conductivity of ZnO might be reduced with La doped.So when the influence of the two aspects reached a balance point which was 2%,the composites achieved a best electrochemical performance.4.The zinc tin binary MOF was synthesized and calcined under reducibility atmosphere to derive Zn₂SnO₄@C/Sn composites.The nano metal Sn which has a good conductivity and Zn₂SnO₄ nanoparticles were assembled into secondary particles in the as synthesized composites.Metal Sn played as active material and conductive agent at the mean time.The nano sized Zn₂SnO₄@C/Sn composites relieved the volume expansion influence on electrochemical performance.Additionaly,the carbon provided buffer space for volume expansion of active material.The metal Sn growed to larger particle with the extension of calcination time,which induced the transform of lithium storage process from surface controlled to diffusion controlled.When the Zn₂SnO₄@C/Sn composites were used as anode materials for LIBs,a superior electrochemical performance was achieved.A reversible capacity up to 1140 mA h g^-1was obtained at the current density of 100 mA g^-1 after 100 cycles.