Morphology Tuning Of Mofs Using PPD As Ligand And The Sodium Storage Properties Of MOFs-derived Carbon Materials

The depletion of non-renewable energy sources（coal,oil and natural gas）on earth is increasing,so it is an urgent need to find alternative renewable energy sources.Secondary energy storage batteries and supercapacitors are gradually developed and commercialized.For lithium-ion batteries（LIBs）,the high cost and shortage of lithium resources limit the future development.Compared with LIBs,the sodium-ion batteries（SIBs）possess the following advantages:abundant sodium resources on the earth,better safety performance,a wide operating temperature range and lower cost.Therefore SIBs have gradually attracted attention in recent years.Metal-organic frameworks（MOFs）derived carbon materials have the advantages of tunable morphology,high specific surface area,and ordered porous structure,and have become a popular choice as anode materials for SIBs in recent years.The coordination compounds formed with p-Phenylenediamine（p PD）as the organic ligand are not very stable,and the structures are easily collapsed during pyrolysis,which is not conducive to subsequent application.In this work,MOFs were synthesized by using p PD as organic ligand and Co²⁺,Ni²⁺and Fe³⁺/Fe²⁺as metal ligands.The framework stability of MOFs were enhanced by preparing bimetallic MOFs and realizing the self-polymerization of organic ligands using MOFs as a template.Finally,the synthesized MOFs were carbonized to obtain carbon materials with different morphologies,which were used as anode materials for SIBs.（1）MOFs（Co-p PD）exhibiting sheet-like morphologies were prepared from Co（NO₃）₂·6H₂O and p PD.Bimetallic MOFs（Co Cu-p PD）and Co-p PD-aniline were synthesized by adding Cu（NO₃）₂·3H₂O and aniline,respectively.Both MOFs exhibit the hollow spherical morphology assembled by the nanosheets.Compared with three kinds of Co-MOFs,Co Cu-p PD has the higher framework stability by thermal stability analysis.When Co-MOFs derived carbon materials with pristine MOFs morphology are used as the anode materials in SIBs,it is found that Co Cu/C exhibites high cycle performance(the capacity is 306 m A h g^-1after 300 cycles at a current density of 1 A g^-1and the cycle retention rate can reach 90%)and rate performance(the capacity is still 240 m A h g^-1at 5 A g^-1).（2）Ni（NO₃）₂·6H₂O and p PD were selected as precursors to prepare Ni-p PD with different solvothermal times.The bimetallic Ni Cu-p PD was also synthesized by adding Cu（NO₃）₂·3H₂O.Ni-p PD and Ni Cu-p PD possess spherical morphologies assembled by sheets.Combined with thermal stability analysis and carbonization results,it is shown that bimetallic Ni Cu-p PD is more stable than Ni-p PD,and the shorter the solvothermal time,the more obvious the advantage of bimetallic MOFs framework stability.When the Ni-MOFs derived carbon materials are used as the anode materials for SIBs,Ni/C exhibits the capacity of 223 m A h g^-1at a current density of 1 A g^-1after300 cycles,with the capacity retention of 93%.When the current density is restored to 0.5 A g^-1after high current cycling,Ni/C exhibits the capacity retention of 90%,showing both the high cycle performance and rate performance.（3）Three kinds of MOFs with different morphologies were synthesized by using p PD as organic ligand and Fe（NO₃）₃·9H₂O,Fe SO₄·7H₂O and Fe Cl₂·4H₂O as iron salts,respectively.The characterization results show that in Fe（NO₃）₃-p PD,p PD is self-polymerized to form poly-phenylenediamine under the catalysis of Fe³⁺.Fe Cl₂-p PD is partially polymerized to form poly-p-phenylenediamine because Fe²⁺is easily oxidized to Fe³⁺during the preparation process.Fe SO₄-p PD is a coordination crystal and does not self-polymerize.Fe（NO₃）₃·9H₂O and Fe Cl₂·4H₂O were simultaneously used as metal salts to coordinate with p PD,obtaining spherical morphology and maintaining the self-polymerization behavior.Thermal stability analysis and carbonization results show that the two kinds of MOFs that can self-polymerize have higher structural stability.Fe-MOFs derived carbon materials with different morphologies are used as the anode materials for SIBs.At the current density of 1 A g^-1,Fe SO₄/C has the highest initial sodium storage capacity(299 m A h g^-1),but poor cycling stability.Compared with Fe SO₄/C,the other three carbon materials have lower initial sodium storage capacity,but higher cycle stability（capacity retention rate at 300 cycles was nearly 90%）.It may be because they contain less N and O doping or more micropores than Fe SO₄/C,which is not conducive to sodium storage.