Studies On The Synthesis And Electrochemical Performance Of Cobalt Terephthalate

Transition metal terephthalate TMTP(TM = Mn,Fe,Co or Ni)has the advantages of easy preparation and high lithium storage capacity,it is a promising new lithium-ion battery anode material.As for the divalent transition metal of cobalt,changing the solvent or pH value of precipitation system could also modify the structural type of terephthalates:the general dihydrate cobalt terephthalate(CoTP-2H2O),the solvent(i.e.,N,N-dimethylformamide,DMF)-containing form of CoTP·DMF and the anhydrous basic salt of Co2(OH)2TP.However,as LIB anode active substances,the function of presented constitute DMF or.OH-in chemical structure CoTP is still ambiguous presently,and so does the presence of crystal water.Therefore,.the controlling syntheses and electrochemical performance of CoTP-2H2O and CoTP withπ-π stacked self-assembled structure and Co2(OH)2TP are chosen as main topics in this thesis,and the structure formation mechanism and the lithiation-delithiation mechanism are revealed.This dissertation mainly deals with two aspects,which is shown as the following:(1)The π-π stacking self-assembled structure of crystal water-containing cobalt terephthalate CoTP-2H2O and the high lithium storage property of dehydrated product CoTP.CoCl2 aqueous solution and CoCl2·6H2O particles are comparatively used as starting materials to react with aqueous lithium terephthalate(Li2TP),resulting in the hierarchical architectures of dihydrate cobalt terephthalate(CoTP-2H2O No.1,No.2)by a solution-based π-π stacking self-assembly,respectively.Then,dehydrated CoTP No.1 and CoTP No.2 were obtained through a simple dehydration reaction.By analyzing the morphology of the product obtained at different reaction times,we found that the aggregate structure of CoTP-2H2O No.2 π-π stacking nanosheets is due to the sacrificial template effectiveness of CoCl2·6H2O particles.Each of the four samples has an extremely low vaIlue of specific surface area,but possesses a reversible specific capacity much higher than the theoretical capacity of commercial graphite(i.e.,372 mAh g-1).No matter cycling at a low or high C-rate of 200 or 1000 mA g-1,there exists an increasing order of specific discharge capacity in each cycle(i.e.,CoTP No.2>CoTP No.l>CoTP-2H20 No.2>CoTP-2H20 No.1),being associated with the R,t decreasing order of CoTP No.2(263.3 Ω)<CoTP No.1(284.52)<CoTP·2H20 No.2(474.0 Ω)<CoTP·2H20 No.1(588.8 Ω)that indicates an adverse effect of crystal water.The ultrahigh lithium-storage capability(e.g.,～1234.2 mAh g-1,at 100 mA g-1,in the 350th)of structure CoTP No.2 could be explained by a three-stage lithiation mechanism:Co2+-iorn reduction(2Li,Stage 1),group O=C-O-enolization(2Li,Stage Ⅱ)and benzene ring lithiation and its interlayer lithium intercalation(8Li,Stage Ⅲ).(2).Synthesis and lithium storage’performance.of Co2(OH)2TP.By changing the reactant concentration,pH and other conditions of the precipitation reaction system of cobalt ions and terephthalic acid ions,two kinds of Co2(OH)2TP(Co2(OH)2TP-No.1 and.Co2(OH)ZTP-No.2)were prepared in the presence of anionic surfactant sodium dodecyl sulfate(SDS)and inorganic salt sodium chloride(NaCl),respectively.Structural characterization and electrochemical measurements display that,no matter cycling at a low or high C-rate of 200 or 1000 mA g-1,in each cycle the specific discharge capacity of Co2(OH)2TP-No.2 is always higher than that of Co2(OH)2TP-No.l,By further considering the Rct value of Co2(OH)2TP-1(214.7 Ω)and Co2(OH)2TP-2(146.1 Ω),this may be mainly due to the fact that SDS molecules adsorbed onto Co2(OH)2TP-No.1 nanosheets could cut down the conductivity and content of the active substance.When cycling at a current density of 200 mA g-1,initial discharge capacity of Co2(OH)2TP-No.2 electrode could reach a high value of 2156.9 mAh g-1 with an initial columbic efficiency of 80.15%.Even after a long-term cycling,the reversible capacity of Co2(OH)2TP-No.2 is still as high as 1291 mAh g-1 at 100 mA g-1in the 230th cycle.Combining voltage profile with its differential curve of Co2(OH)2TP-No.2 electrode reveals a 19-electron transfer lithiation-delithiation mechanism,which deserves to be further conducted in future.