Preparation Of Polyoxometalates-based Composites And Their Researches For Lithium Ion Batteries

Polyoxometalates(POMs),are a class of single molecule clusters with structural diversities and rich functions,which show excellent properties and broad application prospects in the fields of catalysis,electrochemistry,energy storage,photochemistry,ionic conductor,magnetand so on.Therefore,they have become more and more important to utilize the unique features and structural characteristics of POMs,in order to acquire the novel POMs-based materials and device.Although POMs have high lithium storage capacity meanwhile they also have some defects,such as poor electrical conductivity and high solubility in the LiPF6 electrolyte.All these shortcomings restrict the application of POM as cathode materials in lithium ion batteries.So how to solve these shortcomings and improve eletrochemical performance of POM-based Li ion-batteries becomes hot topic in the research of POMs based lithiumion-batteries.(1)Keggin-type POM H3PW12O40(PW12)can reversibly gain or loss multi electron atom meanwhile crystal structure still can keep unchanged.With high-performance lithium storage properties,it can be used as cathode materials for lithium-ion batteries.However,due to its poor conductivity(almost an insulator),we have combined PW12 with conductive polymer polyaniline(PANI)by electrostatic force compound.The composite material[(C6H5N)6(PW12O4))3-]n is formed by the combination of the cationic group of polyaniline and the anion group of PW12.We found this composite material can still be partially dissolved in the LiPF6 electrolyte while bare PW12 can be easily dissolved into the electrolyte.We have prepared three dimensional rGO?PANI/PW12 composite materials through the hydrothermal reaction with large specific surface area and excellent electrical and mechanical properties of graphene,and then we carried out a series of morphological and structural characterizations,such as infrared,XRD,Raman,TGA and so on.At last,the composite materials was assembled into 2032 type lithium ion batteries,and the electrochemical properties were also measured by the battery test system:At different current densities of 50mA/g?100mA/g?200mA/g?500mA/g?1A/g?2A/g?50mA/g,the discharge specific capacities are 280mAh/g?270mAh/g?230mAh/g?190mAh/g?155mAh/g?127mAh/g?280mAh/g?275mAh/g.(2)We use alkali metal cation(Cs+)as cation to access modified polyoxometalates.Cs3PW12O40 polyoxometalate is difficult to dissolve in the LiPF6 electrolyte,and Cs+,alkali metal cations combined with polyoxometalates show nanosphere morphology.However,the prepared polyoxometalates salt is still poor in the electric performance,so we still use hydrothermal method to coat graphene and to prepare 3D graphene/heteropoly acid salt composite,which is excellent in its conductive,cycle stability,discharge specific capacity.At the same time,we also carried out such as SEM,TEM,FTIR,XRD,Raman,TGA characterizations,and assembled lithium ion batteries to test the electrochemical performance.It is found that at the current density of 50mA/g the discharge capacity reached 240mAh/g,the discharge specific capacity is still 100mAh/g after 700 cycles at the current density of lA/g(retention 83.3%).(3)Based on the above two parts,we use the hydrothermal reaction,in the process of making composite material cycle is long,energy consumption is big.In the third part,we use the strong redox properties of phosphomolybdic acid to synthesis rGO@PANI/PMo12 composite by one-step in situ reaction Three dimensional sandwich structure not only prevent?-? effect of graphene and can space the layers of graphene,but also increase the specific surface area of 3D sponge to provide a location for more lithium ions intercalation/embedding cathode material,thus improving the discharge specific capacity of the rGO@PANI/PMo12 composite.At 0.1C rate,the discharge capacity reached 305mAh/g.At 1C rate,the initial discharge capacity was 120mAh/g,and after 500 cycles it can still achieve 60mAh/g,(rention 50%).