Preparation And Electrochemical Energy Storage Performances Of SnO₂ Hollow Nanospheres And Composites

In recent years,compared with the slow electron and ion transport of lithium-ion batteries,supercapacitors have been favored by researchers because of their fast charging and discharging speed and high power density.Although supercapacitors are currently commercialized,they still face problems such as high cost and low specific capacity.Therefore,the preparation of electrode materials with high specific capacity and low cost has become the focus of current research.As we all know,electrode material as the core of supercapacitors is also an important factor that determines their performance.The transition metal oxide SnO₂ is the best choice for supercapacitor electrode material because of its low price,easy availability,high theoretical specific capacity,non-toxic and harmless.However,SnO₂ is easy to agglomerate,and its low ion migration rate affects its electrochemical activity.By adjusting the morphological structure,the micro-nano hollow materials can effectively increase the contact area of electrode material and electrolyte with its huge"cavity",provide more active sites,and reduce the ion transmission distance and accelerate the ion transmission rate at the same time to increase the specific capacity of the material.In this paper,hard template method is used to prepare SnO₂-based hollow nanomaterials,which effectively solves the problems of tin-based materials easy to agglomerate and low ion migration rate,and greatly improves the specific capacity and cycle stability.The main research work is as follows:（1）We have discussed the optimal preparation conditions of hard template carbon spheres prepared by hydrothermal method.The preparation process of carbon spheres is optimized by adjusting the carbon source,reaction time,and reaction temperature.The results showed that when the sucrose was used as the carbon source and reacted at 180℃for 2 h,the prepared carbon spheres had a uniform spherical shape,good dispersion,and a size of about 300 nm to 600 nm.The surface was rough and porous,which facilitated the adsorption of metal cations.（2）The controllable preparation and electrochemical energy storage properties of SnO₂ nano hollow spheres in aqueous phase were studied.Using carbon spheres as template,the tin salt concentration is adjusted under aqueous phase to obtain a Sn⁴⁺/carbon spheres precursor,and after being calcined in air atmosphere,SnO₂ nano hollow spheres are obtained.The result showed that due to the hydrolysis of Sn⁴⁺,aqueous solution systems are not conducive to the preparation of structurally stable SnO₂ nano hollow spheres.Through strict control of the experimental conditions,it was found that when the tin salt concentration was adjusted to 5 mmol/L,SnO₂ nano hollow spheres（denoted as TO-5）with uniform morphology and relatively stable structure were prepared.The electrochemically active sites and high specific surface area provided by the micro-nano hollow structure made TO-5 exhibit good electrochemical performance,and its specific capacity could reach 467 F/g at a current density of 1 A/g.In addition,TO-5 also showed good cycle stability.The capacitance retention reached 77.8%After 500 cycles at 2 A/g.（3）The preparation of SnO₂ nano hollow spheres in organic phase and its electrochemical energy storage performance were studied in detail,and the synthesis mechanism of SnO₂ nano hollow spheres in organic phase was preliminarily discussed.In order to suppress the hydrolysis of Sn⁴⁺,ethanol and acetone were used as solvents in the synthesis system,and by adjusting the ratio of them,SnO₂ nano hollow spheres were prepared.The results showed that when the adsorption and stirring temperature is 60℃,ethanol:acetone=2:1,the prepared SnO₂ nano hollow spheres（denoted as TOHS-2）have the best electrochemical energy storage performance.TOHS-2 had a specific capacitance of 783 F/g at a current density of 1 A/g,which is nearly 1.7 times that of SnO₂ nano hollow spheres in aqueous phase.The capacitance retention of TOHS-2 reached 87%after 2,500cycles at 2 A/g,showing excellent cycle stability.Assemble the TOHS-2//AC button-type asymmetric capacitor with activated carbon,the specific capacitance of it was 40 F/g at 1A/g.The energy density can reach 14.23 Wh/kg and the power density is 839 W/kg.（4）The preparation and electrochemical energy storage properties of heterostructure MnO₂@SnO₂ nano composite hollow spheres were studied.Using the carbon spheres prepared by hydrothermal method as template to adjust the ratio of tin salt and manganese salt,a heterostructure MnO₂@SnO₂ nano composite hollow spheres with MnO₂ as the"skeleton"and SnO₂ nanoparticles as the"muscle"was successfully prepared.The results showed that when the addition ratio of tin salt and manganese salt is 1:6,the prepared heterostructure MnO₂@SnO₂ nano composite hollow spheres had the best electrochemical performance.Depending on the unique hollow structure and the synergy of MnO₂ and SnO₂,the material had a relatively high electrochemically active area（RF=～383）;its specific capacitance can reach 540 F/g at a current density of 1 A/g.The specific capacitance of it can still maintain 498 F/g after 1500 cycles at 2 A/g,indicating that it had good cycle stability.Assemble a button-type asymmetric supercapacitor with activated carbon at the mass ratio of 1:3.At a current density of 0.5 A/g,the specific capacitance of the device can reach 50.76 F/g,the energy density is 18.05 Wh/kg,and the power density is 403.6 W/kg.