Preparation And Research On Sodium Storage Performance Of Composites Of Hard Carbon Coated Carbon Nanotubes

As one of the most promising energy storage technologies in current secondary batteries,sodium-ion batteries（SIBs）have significant advantages over lithium-ion batteries（LIBs）,such as high abundance,low cost,excellent rate performance,outstanding low-temperature performance and safety performance.Given the successful experience of LIBs,SIBs have achieved considerable research achievements in cathode materials,but there are some obstacles in anode materials.Based on sodium storage mechanisms,anode materials for SIBs can be classified into intercalation-type,alloy-type and conversion-type materials.Among them,most of the intercalation-type materials are carbon-based materials,which accomplish sodium storage through the insertion/extraction of Na⁺into the layers of electrode materials.This type of material hardly possesses severe volume expansion as alloy-type and conversion-type materials.However,its limited sodium storage space leads to general low sodium storage performance(～300 m Ah g^-1).Therefore,it is necessary to explore intercalation-type anode materials with appreciable rate performance and reasonable cycling stability.Hard carbon（HC）is one of the most common carbon-based intercalation-type materials.Its large interlayer spacing（0.352 nm）is conducive to Na⁺storage.HC also has superiorities such as low cost,abundant precursors,stable chemical properties and excellent electrochemical performance.As an anode for SIBs,HC has great application potential.With high electrical conductivity and mechanical properties,carbon nanotubes are often used as composites combined with other materials to improve conductivity,thereby optimizing electrochemical performance.In this paper,research has been carried out on the preparation of novel carbon-based anode materials.Especially,the experiment has adopted modification strategies of composite materials and morphology control to form interconnected conductive networks and porous structure to improve the sodium storage performance of carbon-based anodes.The detailed research contents are as follows:（1）The synthesis of hard carbon coated carbon nanotubes（CNTs@C）is carried out via hydrothermal method.The thickness of hard carbon coating layer of composite materials is controlled by changing hydrothermal temperature and feed ratio.And the sodium storage performance of each sample as anodes for SIBs treated at different annealing temperatures is also studied to explore the experimental conditions for achieving optimal electrochemical performance.The interconnected conductive CNTs networks of this material are able to improve electrode conductivity,while the hard carbon coating layer aims to provide excellent cycling performance.The prepared CNTs@C composite materials as anodes for SIBs have excellent electrochemical performance,delivering a reversible capacity of 102.8 m Ah g^-1 at a high current density of 5 A g^-1 with good rate performance.After 1000 cycles at 5 A g^-1,the anode still has a capacity retention and CE of 100%,showing excellent cycling stability.（2）Porous hard carbon coated carbon nanotubes（CNTs@PC）composite materials were prepared via a two-step hydrothermal method by doping Zn into CNTs@C and then conducting annealing to achieve pore-formation.The experiment also explored the impact of feed ratio on structures of hard carbon coating layers and electrochemical performance of composites.Compared with CNTs@C,the porosity and specific surface areas of CNTs@PC are both expanded,increasing active sodium storage sites and shortening ion diffusion paths to further improve sodium storage performance.CNTs@PC can provide a reversible specific capacity of 144.1 m Ah g^-1 at a current density of 5 A g^-1 with splendid rate performance.After 500 cycles at 5 A g^-1,the composites remain a capacity retention and CE of approximately 100%,showing excellent cycling performance.