Study On Sodium/Potassium Storage Behavior Of N-Doped Carbon And Its Composites

Clean and efficient energy storage devices play a vital role in combating the issues of global warming and energy crisis.Lithium-ion batteries（LIBs）,among various commercial technologies,have been rapidly developed and dominated in various military and civilian areas.However,the shortage and imbalanced distribution of Li resources further increase the cost of the commercial LIBs,which greatly hinders the application of LIBs in large-scale energy storage.Among various secondary battery systems,owing to the abundant Na and K elements in the earth’s crust and relatively cost-effective,sodium-ion batteries（SIBs）and potassium-ion batteries（PIBs）with similar electrochemical mechanisms to LIBs have become the most promising next-generation energy storage technologies owing to the abundant sodium and potassium resources.As an important component of the battery,the anode material plays a significant factor to promote the developments of SIBs and PIBs.However,compared with Li⁺,Na⁺and K⁺possess larger radii,which makes it difficult for the insertion/extraction of Na⁺/K⁺into/from the electrodes during the charging/discharging processes.Graphite,as the commercial LIBs anode,shows unsatisfied electrochemical performance when used for SIBs and PIBs.Therefore,more alternative anodes with high-performance for SIBs and PIBs should be proposed.In this study,we prepared a series of high-performance anodes for SIBs and PIBs using the carbon materials with excellent stability and carbon-based composites as the research objects.The details are specifically as follows:（1）Sodium storage behavior of the composite of Co_0.85Se/WSe₂ encapsulated in N-doped carbon polyhedronAmong the conversion-type SIBs anodes,compared with metal oxides/sulfides,transition metal selenides have been brought into focus owing to the advantages of their weaker metal-selenium bond,excellent electrical conductivity and higher theoretical capacity.However,the large volume expansion during the cycling results in slower kinetics.Heterostructure engineering and vacancy design are effective strategies to improve reaction dynamics.The composite of Co_0.85Se/WSe₂ heterostructure with Se vacancy encapsulated in N-doped carbon polyhedron（CoWSe/NCP）was prepared by a hydrothermal and selenylation stratergy.The Co_0.85Se/WSe₂ heterostructure can improve electron conductivity and accelerate charge transfer.Se vacancies can enhance the adsorption ability of Na⁺and provide extra active sites for Na⁺adsorption.In addition,the NCP can also play a buffer role during the charging/discharging processes to alleviate the volume change,and enhance the stability of the structure.As a SIBs anode,CoWSe/NCP shows outstanding rate performance(339.6 m Ah g^-1 at 20 A g^-1),exceeding the most of Co/W based selenides,as well as satisfied cycling stability(434.9 m Ah g^-1 after 5000 cycles at 1 A g^-1).（2）Preparation and potassium storage performance of hollow N-doped carbon nanofibersCarbonaceous material is one of the most commonly used anodes for PIBs due to its low cost,high natural abundance,and adjustable interlayer spacing.Here,hollow N-doped carbon nanofibers（HNCNFs）were prepared by carbonizing the polyaniline nanofibers synthesized via a facile self-assembly approach.The interconnected nanofibers can enhance the electron transfer and improve the electrical conductivity.The hollow structure can adapt to the large volume change and provide enough path for electrolyte diffusion.At the same time,the high N doping provides numerous K⁺adsorption active sites and enhances the pseudo-capacitance effect.Thanks to these synergistic effects,the HNCNFs anode in PIBs shows attractive rate performance(139.7 m Ah g^-1 at 30 A g^-1)and cycling stability(188.4 m Ah g^-1 at 1 A g^-1 after4000 cycles).（3）Preparation and potassium storage performance of VN nanoparticle-assembled hollow microspheres/N-doped carbon nanofibersOwing to the high conductivity and theoretical capacity,VN has attracted extensive attention as a PIBs anode material.Similar to other conversion-type materials,large volume expansion will occur during the cycling.Herein,a composite of VN nanoparticle-assembled hollow microspheres/N-doped carbon nanofibers（VN-NPs/N-CNFs）was prepared via an electrospinning process.In the VN-NPs/N-CNFs composite,the ultrasmall VN NPs can shorten the distance of the K⁺insertion/diffusion and the active materials can be fully utilized.N-CNFs connect VN NPs assembled hollow microspheres to prevent their self-agglomeration and improve the structural stability.The hollow structure assembled by the VN NPs is beneficial to electrolyte penetration,buffers the volume change,and increases the contact area between VN NPs and electrolyte.Moreover,the ultra-high N doping provides sufficient active sites for K⁺adsorption and enhances the pseudo-capacitance effect.As a PIBs anode,VN-NPs/N-CNFs exhibits satisfied rate performance(140.2 m Ah g^-1 at 10 A g^-1),and remarkable cycling stability(226.9 m Ah g^-1 after 2000 cycles at 0.5 A g^-1).