| Due to the flexible frame structure,large specific surface area and adjustable pore structure,organometallic compounds are considered to be promising anode materials for potassium ion batteries.This paper takes bismuth-titanium-ethylene glycol binary metal organic compound（Bi-Ti-EG）as research subject to analysis its structural composition,revealing the potassium storage performance and mechanism of Bi-Ti-EG and its derivatives.In order to improve the conductive performance of Bi-Ti-EG,enhance the electrochemical activity,and further optimize its potassium storage performance,introducing conductive carbon materials to build a three-dimensional conductive network is adopted.The main research results obtained are as follows:（1）The metal-organic compound Bi-Ti-EG was synthesized by solvothermal method.The crystal structure analysis results indicate that the as-prepared Bi-Ti-EG belongs to typical layered compound.Nevertheless,the layered structure and its inferior electronic conductivity would cause K+to migrate inward layer by layer,further lead to the phase transition reaction mechanism during the first discharge process and follow-up poor potassium storage performance of P-Bi-Ti-EG prepared with deionized NMP as solvent and PVDF as binder.Further studies find that the H-Bi-Ti-EG electrode prepared with deionized water as solvent and CMC as binder transformed the dense layered structure into a long-range disordered amorphous structure due to the hydrolysis reaction of Bi-Ti-EG during slurrying.Thus H-Bi-Ti-EG owns a higher porosity and a larger pore volume than P-Bi-Ti-EG,which provides a convenient channel for the rapid transmission of potassium ions.Therefore,H-Bi-Ti-EG shows a better electrochemical potassium storage performance than P-Bi-Ti-EG.HRTEM,ex-situ XRD,FTIR and XPS analysis show that the Bi metal center with hydroxyl and carbonyl groups in H-Bi-Ti-EG serve as blanket active site for potassium storage.Thanks to the synergistic effect of the nanosize effect of the distributed ultrafine metal Bi quantum dots formed in situ and the elastic properties of the electrochemically active organic matrix,H-Bi-Ti-EG exhibits a specific capacity above the theoretical capacity of metal Bi and a ultra-long cycle performance.To further improve the rate performance,Bi-Ti-EG was grown in situ on the activated multi-walled carbon nanotubes（MWCNTs）by solvothermal method.With different MWCNTs contents,Bi-Ti-EG/CNT composites were successfully prepared,in which Bi-Ti-EG nanoparticles were uniformly dispersed on the cross-linked MWCNTs,and the 3D conductive network effectively shortened the migration distance of electrons and ions.Therefore,Bi-Ti-EG/CNT（8 wt.%）anode shows the most excellent potassium storage performance of 371m Ah g-1 after 400 cycles at a current density of 0.1 A g-1.At the current density of 0.5 A g-1,it could deliver a specific capacity of 278 m Ah g-1 after 800 cycles with capacity retention rate of 99%.Even at a high current density of 1 A g-1,the capacity could still be maintained at 207 m Ah g-1 after1550 cycles.（2）Taking Bi-Ti-EG as precursor,oxygen-doped carbon coated nano Ti O2/micron Bi sphere composite（denoted as ODC@Bi/Ti O2）were fabricated through pyrolysis of precursor by self-sacrificial template method under argon atmosphere.To promote the electrochemical performance of ODC@Bi/Ti O2,dopamine（PDA）coating was adopted to modify precursor.Thus nitrogen/oxygen co-doped carbon coated nano Ti O2/micron Bi sphere composite（denoted as NOCDC@Bi/Ti O2）were fabricated through the same experimental process without other changes.The research results indicate that on the one hand,doping of nitrogen atom is beneficial to conductivity and electrochemical activity of electrode.On the other hand,the nitrogen/oxygen co-doped porous carbon layer on the surface of the Bi micron particles can both enhance the electronic conductivity of the Bi active material and alleviate the huge bulk expansion of the metal Bi during the charge and discharge process.The electrochemical results show that550-NOCDC@Bi/Ti O2 presents the best electrochemical performance among derivatives.At the current density of 0.1 A g-1,550-NOCDC@Bi/Ti O2 shows a superb discharge capacity of 401 m Ah g-1.At the current density of 0.5A g-1,the specific capacity could remain 312 m Ah g-1 after 800cycles with a stable coulomb efficiency above 99%,the average rate of fade capacity per cycle is0.010%.Besides,superior rate performance has been proven by delivering a capacity of 252 m Ah g-1 at 1 A g-1 after 450 cycle. |
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