Preparation And Electrochemistry Performance Of Novel Hollow SnO₂-based Anode Materials For Lithium-ion Batteries

With the growing need for next-generation lithium-ion batteries with high power and energy density,numerous efforts have been made to develop novel anode materials as alternatives to traditional graphite,which has a low theoretical capacity of 372 mAh·g-1.SnO2 is regarded as one of the most promising anode materials due to its high theoretical capacity?782 mAh·g-1?,low operating potential?0.25 V vs.Li+/Li?,abundant reserves and environmental friendliness.However,the practical application of SnO2 has been met with three significant challenges: large volume expansion?³00%?,poor conductivity and low rate capability.In this work,four different kinds of SnO2-based anode materials were designed to circumvent the above-mentioned issues.The blends of PEDOT:PSS and PEO were introduced as the coating layer of hollow SnO2 nanospheres.Such blends can not only alleviate the volume expansion of the nanospheres,but also enhance the conduction of ion and electron of the material system,which are beneficial to the better electrochemical performance of SnO2.In order to achieve a higher ionic conductivity at ambient temperature,PEO was plasticized by PEG-400 before the blends were formed.It was found that above-mentioned blends exhibited superior tensile property and conductivity.After secondary doping with sulfuric acid,the electronic conductivity of blends reaches 5.4×10-2 S·cm-1,which was higher than that of the original.Besides,a better electrochemical performance of the composites was achieved with a reversible capacity of 496 mAh·g-1 after 100 discharge-charge cycles at a current density of 100 mA·g-1 and a discharge capacity of 240 mAh·g-1 at 2000 mA·g-1,which was higher than that of hollow SnO2 nanospheres.As anatase TiO₂ has a relatively small volume expansion ratio?<4%?during lithiation/delithiation,it was selected as a capping layer of hollow SnO2 nanospheres to synthesize hollow SnO2@TiO₂ Yolk-Shell nanospheres.The interlayer void and TiO₂ shell were separately designed to alleviate the volume expansion of hollow SnO2 nanospheres via their accommodation and physical confinement to the expansive volume of the nanospheres and thus improve their cycling stability.According to the experimental results,an optimal cycling stability of the Yolk-Shell nanospheres can be obtained on the condition that the hydrolysis time of TEOS was kept at 1 h and the amount of TBOT was 300 ?L.The reversible capacity of 558 mAh·g-1 after 100 cycles at 100 mA·g-1 and the discharge capacity of 263 mAh·g-1 at 2000 m A·g-1 were reached,respectively,which showed a higher electrochemical performance than that of hollow SnO2 nanospheres.The original structure of such nanospheres was preserved completely after 100 cycles.Self-healing PAH gel cross-linked by phytic acid and MWCNTs with high conductivity were employed to improve the electrochemical performance of hollow SnO2 nanospheres.The self-healing property of PAH is able to ensure the structural integrity when it suffers from volume stress of SnO2 nanospheres.Besides,the conductive network formed by MWCNTs can reduce the polarization of the material system during cycles and help to obtain a higher capacity.The research showed that a better self-healing property can be achieved via increasing the ionic cross-linking density in a PAH gel system,which is helpful to improve the structural integrity of SnO2 nanospheres during cycling.From the experimental results,when the amount of MWCNTs accounts for 4 wt%,the electronic conductivity of resulting composites was up to 9.8×10-2 S·cm-1,which ensured a smaller charge transfer resistance after 60 cycles.The composites showed a better electrochemical performance with a reversible capacity of 574 mAh·g-1 after 100 cycles at 100 mA·g-1,a discharge capacity of 321 mAh·g-1 at 2000 mA·g-1 and a spring-back capacity of 506 mAh·g-1 at 200 mA·g-1.The synergistic effect of TiO₂?B?nanotubes with the wrapping and mechanical supporting functions and three-dimensional interconnected network structure composed of TiO₂?B?nanotubes and MWCNTs was taken to mitigate volume expansion of SnO2 nanocrystals?⁵ nm?and improve the transport of lithium-ion and electron in the network.The experimental results show the amount of SnO2 nanocrystals which TiO₂?B?nanotubes can load has a limit.When reaching the upper limit that the mass ratio of SnO2 nanocrystals and TiO₂ nanoparticles was 16:15,the obtained composites displayed a better cycling performance.The lithium-ion diffusion coefficient of such composites is almost ten times as much as that of SnO2 nanocrystals.Moreover,when the amount of MWCNTs accounts for 6 wt% of the network,the electronic conductivity of resulting composites was up to 9.9×10-2 S·cm-1,which is much higher than that of the composites without MWCNTs.The electrochemical tests indicated that such composites exhibited an excellent cycling stability with a reversible capacity of 462 mAh·g-1 after 100 cycles at 100 mA·g-1,a discharge capacity of 211 mAh·g-1 at 3000 mA·g-1 after two testing processes with alternative current density of 200 mA·g-1 and 3000 m A·g-1 and a spring-back capacity of 338 mAh·g-1 at 200 mA·g-1 after the final testing process.Additionally,the prepared composites were observed to have complete network structure after rate capability test,demonstrating a superior structural stability.