| Sodium superionic conductor?NASICON?compounds derived from polyanionic systems have the advantages of open structural framework,large gap channels and high ion mobility,which have been widely used in energy storage devices such as sodium-ion batteries?SIBs?as active materials.Among them,NASICON sodium titanium phosphate material?NTP?is considered as a promising anode material for sodium-ion batteries due to its high theoretical capacity,good thermal stability,low cost and environmental friendliness.However,NTP has the inherent defects of low electronic conductivity leading to the poor electronic transmission,which seriously affect the full play of its rate performance and cycling performance.Thus,its further development is limited in the field of SIBs.It is important to develop effective synthesis methods to improve the electron transport properties of NTP materials.In order to improve the poor electronic conductivity of NTP,anode composites with carbon materials and NTP were designed and prepared successfully.The preparation conditions,structure,morphology and electrochemical performance of materials were optimized,and the evolution of the structure and morphology were further explored during the charge/discharge process by in-situ TEM.In addition,we studied the phase transformation during the sodiation process to investigate the reaction mechanism in depth.Thus,the reaserch provides a novel technical means and theoretical basis for the development of NTP.The carbon coated NTP composite electrodes were prepared with hexadecylamine?HDA?as the structure guide agent by one-step solventhermal method.The amorphous carbon layer was formed by the sintering treatment of the long-chain alkyl groups belong to HDA.By regulating the additive amount of HDA,the impurity phase of titanium pyrophosphate was reduced sighnificantly and the NTP materials with the high purity were obtained.The specific capacity of the electrode materials could reach 117.9 mAh g-1 at 0.5 C.Besides,the specific capacity is 104.5 mAh g-1 at 1 C after 1500 cycles and the capacity retention is 88.6%,indicating that the NTP anode achieved the better electrochemical properties.Through TEM and a series of electrochemical analysis,the negative influence of titanium pyrophosphate was discussed for the electrochemical performance of NTP.It was found that the close contact resisted the diffusion of sodium ion between the NTP and titanium pyrophosphate,leading to the increased charge transfer resistance and the decreased diffusion coefficient of sodium ion.NTP/carbon nanotubes composites?NTP/C-CNTs?were prepared by the solvothermal-hydrothermal method.The average particle sizes of the prepared NTP/C materials are less than 100 nm,and the carbon nanotubes formed an interlaced network,which wrapped the NTP/C material and provided the fast electronic transfer path.NTP/C-CNTs showed excellent electrochemical performance and the initial specific capacity was 112.3 mAh g-1 at 1C.After 200cycles,the specific capacity was 108.5 mAh g-1 and the capacity retention reached 97%.At the low temperature of-20 oC,the discharge specific capacity was 62.2 mAh g-1 at 10 C,showing good low temperature adaptability.NTP/graphene composite?NTP-rGO?was prepared using Ti-based Metal Organic Frameworks?MIL-125?as raw material and graphene oxide as additive.The porous structure of MIL-125 not only serves as the titanium source,but also acts as an in-situ etching template.As the result,the prepared active material exhibits the multistage structure assembled by NTP nanosheets.During the hydrothermal process,graphene oxide?GO?self-assembled into the precursor of NTP through the tight bond.This structural design combines the characteristics of rapid ion diffusion of NTP with the high conductivity of graphene to enhance the electrochemical properties of electrode materials.The discharge specific capacity of NTP-rGO material could reach 129.2 mAh g-1 at 0.1 C,and the specific capacity retention was 76.9%under the 500-fold increase in current density.In addition,the specific capacity retention was 92.0 mAh g-1 at 10 C after1000 cycles.By means of XRD and in-situ TEM,this work analyzed the evolution of structure and morphology during the charge/discharge process.Further,the phase transition mechanism of NTP was studied to explain the electrochemical reaction mechanism during the sodiation.The DFT results showed that more electron states appeared near the Fermi level,indicating that the composite interface has the stronger electrochemical activity. |
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