Heteroatom-doped Titanium Dioxide Microspheres:Preparation And Their Electrochemical Performance As Anode Materials For Sodium Ion Batteries

Titanium dioxide（TiO₂）has been generally considered as a promising anode for sodium ion batteries（SIBs）due to its low cost,eco-friendliness,safety insurance,and structural stability.As an intercalation-type anode material,TiO₂ has a high theoretical capacity of 335 m A h g^-1.Nevertheless,the poor rate capability and low actual capacity resulting from its huge band gap and inferior electronic conductivity hinder its practical application in SIBs.By fabricating TiO₂-based hybrids with the conductive matrix,or constructing nanostructures,or doping with heteroatoms,the sodium storage performance of TiO₂ can be effectively improved.Based on these considerations,this paper aims to prepare phosphorus-doped TiO₂ microspheres with uniform N-doped carbon coating layer（（P-An TSS）@NC）and nitrogen and sulfur co-doped TiO₂ microspheres（NS-TiO₂）by using amorphous TiO₂ microspheres（Am TSS）as the heteroatom-doped precursor.Meanwhile,their electrochemical performances have been tested and analyzed in detail.The main research contents are as follows:（1）The（P-An TSS）@NC microspheres are prepared by a two-step route of first P-doping and then carbon-coating.Also,the Am TSS,P-doped titanium dioxide microspheres（P-An TSS）,and carbon-coated titanium dioxide microspheres（An TSS@NC）were used as comparative materials to investigate the potential reasons for the improvement of electrochemical performance of the（P-An TSS）@NC sample.In addition,the mechanism of the storage of Na⁺,the proportion of pseudocapacity contribution and the change of charge transfer impedance during the cycling were also investigated.（2）The influence of the sequence of P-doping treatment and carbon-coating treatment on the electrochemical performance of the achieved TiO₂-based materials was studied.（3）The influence of Am TSS and An TSS as the P-doping precursors on the final P-doping concentration,and the subsequent effect of P-doping amount on their electrochemical performances of the as-prepared materials were investigated and discussed.（4）The electrochemical performances of the NS-TiO₂,N-TiO₂,S-TiO₂,and TiO₂ were explored.（5）The sodium ions storage mechanism,pseudocapacitance contribution and cycle stability of the NS-TiO₂ sample as an anode material for SIBs were studied.Based on the above-mentioned,the following results and/or conclusions are followed:（1）It is proved that the（P-An TSS）@NC sample was successfully synthesized by physical characterization results.The sample displays a uniform carbon coating layer and a high P-doping content（13.48 wt%）.（2）Due to the synergistic effect of phosphorus-doping and carbon coating,the（P-An TSS）@NC sample exhibits higher specific capacity and better cycle stability than P-An TSS and An TSS@NC samples,which were only undergo P-doping or carbon-coating treatment,respectivly.After 150 cycles at 0.1 C and after 2000 cycles at 1 A g^-1,the capacity retention capability was up to 94%and 88%,respectively.In addition,the（P-An TSS）@NC anode shows low charge transfer resistance and a high proportion of pseudocapacitance.（3）It is indicated that the electrochemical properties of the final TiO₂-based materials can be better improved by first P-doping and then carbon-coating treatments.（4）By comparing the P-doping amounts in the P-An TSS and P-（An TSS）samples achieved from the P-doping treatment of the Am TSS and An TSS as the precursors,respectively,it is found that the content of phosphorus in the P-（An TSS）sample is only 4.64 wt%,which is much lower than that in the P-An TSS sample of 11.11 wt%.Furthermore,the P-An TSS with higher P-doping content exhibits a higher specific capacity.（5）The electrochemical performances of the co-doped NS-TiO₂ sample are much better than that of the N-TiO₂ and S-TiO₂ samples.Also,it shows a lower charge transfer resistance.Specifically,the NS-TiO₂ displays excellent cycle stability,with 96%capacity retention after2000 cycles at 1 A g^-1.（6）After analyzing Na⁺storage mechanism and corresponding kinetic characteristics,it is found that the Na⁺storage process is dominated by pseudocapacitance behavior in the NS-TiO₂material.