Synthesis And Composite Preparation Of Polyoxometalate-Resorcin[4]Arene-Based Complexes For Lithium Battery

Lithium-ion batteries（LIBs）have attracted much attention in the field of energy storage due to their advantages such as high energy density and long cycle life.It is crucial for the LIBs to develop electrode materials with high rate capability and highly stable electrolyte materials.Polyoxometalates（POMs）are a class of transition metal oxide nanoclusters with unique physical and chemical properties.Thus,they show great potential in the field of energy storage.Nevertheless,poor conductivity and easy dissolution in electrolytes limit their wide development.Usually,the POM-based complexes constructed from POMs,organic ligands and metal ions are stable in water or organic solvents.At the same time,the POM-based complexes feature the reversible electrochemical redox ability and tunable structure.As a result,the POM-based complexes have attracted extensive attention in the field of electrochemical energy storage.In this thesis,the POM-based composites with good electrical conductivities were prepared via mechanical ball milling of POM-based metal-resorcin[4]arene complexes and carbon materials.Effects of POM type,structural difference and amorphousness on the electrochemical performances of the composites for LIBs anode materials were explored.In addition,the POM-based complexes were used as quasi-solid electrolytes for LIBs due to their high ionic conductivities in electrolytes.1.Three new isomorphic POMs-based complexes were successfully prepared by the self-assembly of TPTR4A,copper（II）ions and different Keggin POMs（phosphomolybdic acid,silicotungstic acid and phosphotungstic acid）under solvothermal conditions,namely,[Cu₄（TPTR4A）₂][PMo₁₂O₄₀]（OH）·2DMA·H₂O（1）,[Cu₄（TPTR4A）₂][Si W₁₂O₄₀]·2.5DMA（2）and[Cu₄（TPTR4A）₂][PW₁₂O₄₀]（OH）·0.5DMA·5H₂O（3）（TPTR4A=4-（pyridin-2-yl）pyrimidine-2-thiolate functionalized resorcin[4]arene and DMA=N,N′-dimethylacetamide）.Crystal structures of 1-3 were determined by single crystal X-ray diffraction.1-3 were isostructural,but their POM types were different.To improve the conductivity of 1-3,they were loaded on the graphene oxide（GO）via ball milling to give1/GO-3/GO composites.Electrochemical properties of the composites as anodes for LIBs were explored.The results showed that the composite 2/GO possessed better electrochemical performance than 1/GO and 3/GO.2/GO featured a reversible capacity of 947 m Ah g^-1at a current density of 100 m A g^-1after 100 cycles.Powder X-ray diffraction（PXRD）and X-ray photoelectron spectroscopy（XPS）demonstrated that the W=O/Mo=O bond energy difference in POMs was probably the main reason that led to the electrochemical performance difference of 1/GO-3/GO.2.To explore the influence of structural difference on the electrochemical performance of POM-based materials for LIBs,the known two-dimensional（2D）and three-dimensional（3D）POM-basedmetal-organicframeworks（POMOFs）[Co₈Cl₁₄（TBR4A）₆]·3[H_3.3Si W₁₂O₄₀]·10DMF·11Et OH·20H₂O（4）and[Co₃Cl₂（TBR4A）₂-（DMF）₄]·[Si W₁₂O₄₀]·2Et OH·3H₂O（5）were studied for LIBs.（TBR4A=benzimidazole groups modified resorcin[4]arene and DMF=N,N′-Dimethylformamide）.Composites4/PG and 5/PG were achieved via ball milling of POMOFs and polypyrrole-reducedgrapheneoxide（PPy-RGO）.Further,nano-sized composites 4/PG-A and 5/PG-A were prepared by calcination of 4@PG and 5@PG.They showed good electrochemical performance as anodes for LIBs due to the rich active sites.4/PG-A exhibited low initial coulomb efficiency at the current density of 100 m A g^-1.Nevertheless,the high reversible capacity was achieved after 100 cycles.The ex-situ PXRD and scanning electron microscope（SEM）determination indicated that 4 experienced pulverization and amorphous process in the first electrochemical cycle.On the contrary,the3D structure of 5 could be recovered after the first electrochemical cycle.Therefore,the initial coulomb efficiency of 4 was lower than 5.However,the unique“amorphization and pulverization”process of 4 produced rich reaction sites,resulting in a high specific capacity after 100 cycles.This work provided a method for the synthesis of POM-based nanomaterials with high cycling performance.3.The above results showed that the amorphousness of POM-based complexes was beneficial to improve their electrochemical performances for LIBs.Thus,the known hydrogen-bonding3DPOM-basedcomplex{[Co₄（TMIR4A）₂（HCOO）₂（OH）₂][Si O₄（W₃O₉）₄]}·6DMF·5H₂O（6）（TMIR4A=methimazole functionalized methyl resorcin[4]arene）was used to further explore the amorphous effect on its electrochemical performance for LIBs.Composite 6/GO with good electrical conductivity was obtained by mechanical ball milling of 6 and GO.PXRD determination showed that the amorphous part of 6 gradually raised with the increased content of GO.At the same time,the composite featured high rate performance compared with other POM-based anode materials.Even at a current density of 5 A g^-1,the 6/GO composite still possessed a specific capacity of 606 m Ah g^-1.It is possible that the amorphous formation of the POM-based complex exposed abundant active sites and further promoted the migration of Li⁺,resulting in high electrochemical performance for LIBs.4.As an important component,electrolyte played a key role in the performance and safety of the battery.In this facet,POMs have high ionic conductivity in the electrolyte.Therefore,aknownPOM-basedcomplex{[Zn₄（TMIR4A）₂（HCOO）₂（OH）₂][Si O₄（W₃O₉）₄]}·6DMF·5H₂O（7）,which was isomorphic with 6,was utilized as a solid electrolyte for LIBs.The sample of 7 was mixed with polymer polyvinylidene fluoride（PVDF）to give the solid electrolyte 7/PVDF,which exhibited high Li⁺conductivity(7.2×10^-3S/cm,25℃)and Li⁺migration number（0.78）.Meanwhile,7/PVDF displayed good cycling stability when applied in both Li|Li and Li|Li Fe PO₄cells.On one hand,the channel of 7.2×7.2（?）in 7 is probably favorable for Li⁺migration.On the other hand,the POMs around the pores of 7 maybe repel entry of the negative charges,which made 7 feature the single-ion conducting characteristics.Therefore,the stable pore structure and single-ion conduction characteristics of 7 provided a stable Li⁺transport channel,thus limiting the formation of lithium dendrites.This work expanded the study of POM-based complexes in the field of energy storage.