Study On Preparation Of Ni_aX_b(X=O,S,P)/C Composites And Their Lithium Storage Properties

The nickel-based compounds Ni_a X_b?X=O,S,P?are considered to be one of the lithium ion anode materials,which are expected to replace the low-theoretic graphite anode due to their features of high theoretical capacity and abundant reserves.However,the practical application of nickel-based compounds still faces problems such as low inherent conductivity,severe volume effects and sluggish electrochemical reaction kinetics.These problems can lead to the attenuated capacity and poor rate performance.In this regard,the methods that can improve the performance of nickel-based compounds can be summarized into the following two points:the first is to improve the electrical conductivity of the electrodes by compounding highly conductive materials,and the second is to maintain the stability of the electrode structure through the rational design of the nano-composite structure.This paper combines the two above-mentioned methods to modify the nickel-based compounds Ni_a X_b?X=O,S,P?with ionic liquid-derived carbon,thereby improving their electrical properties.The main research contents and results are as follows:?1?The nitrogen-doped carbon-coated Ni-NiO is synthesized by high-temperature pyrolysis-oxidation method to obtain the Ni-NiO/NC composite.When the Ni-NiO/NC composite is used as the anode material of lithium ion battery?LIBs?,the reversible discharge specific capacity of 335.9 mAh g^-1 is maintained after1000 cycles at a current density of 5 A g^-1.In rate performance,when the current density returns from 5 A g^-1 to 0.25 A g^-1,its discharge specific capacity rises from348.6 mAh g^-1 to 731.7 mAh g^-1.Its excellent lithium storage properties are attributed to the following two aspects:on the one hand,the conductive metallic Ni significantly improves the electrode conductivity,promotes the electrode reaction kinetics,and reduces electrochemical polarization.On the other hand,the nitrogen-doped carbon matrix can alleviate the volume change of NiO during lithiation and delithiation,and enhance the conductivity of electrode materials in the meantime.?2?Ni-NiS₂/C composite is prepared by directly sulfiding Ni/C precursor with sublimed sulfur.The TEM images show that the conductive metallic Ni and NiS₂ in the composite are dispersed in the ionic liquid-derived carbon.Electrochemical performance tests demonstrate that Ni-NiS₂/C shows good reversibility and stability.During a cycle performance with a current density of 0.2 A g^-1,its reversible specific capacity is maintained at 735.7/727.7 mAh g^-1 after 60 cycles.When the current density rises to 1 A g^-1,its reversible specific capacity remains at 348.7/343.6 mAh g^-1after 350 cycles.This is due to the fact that metallic Ni can improve the electrical conductivity of Ni-NiS₂/C composite,while the carbon matrix is beneficial to alleviate the volume strain of Ni-NiS₂/C composite.?3?A simple pyrolysis method is used to synthesize Ni₃S₂ nanoparticles encapsulated with nitrogen,sulfur dual-doped carbon nanotubes hybrid?Ni₃S₂/NSCNTs?by using thiourea as a sulfur source.When it is used as an anode of LIBs,the outer NSCNTs can not only provide fast channels for charge,but also improve the conductivity and electrochemical activity of the electrode.In addition,the TEM images after rate cycling also show that the NSCNTs outside Ni3S2/NSCNTs can keep structural integrity well.Therefore,its discharge specific capacity can maintain smoothly at 486.9 mAh g^-1 after 300 cycles during a long cycle performance at 0.5 A g^-1,and maintain at 84%of the second cycle.In the rate performance,the discharge specific capacity of Ni₃S₂/NSCNTs is 374.1 mAh g^-1 under 3 A g^-1.When the current density returns from 3 A g^-1 to 0.1 A g^-1,the discharge specific capacity is maintained at 628.0 mAh g^-1.?4?Ni₂P/C and Ni₂P/NPC composites are synthesized by using sodium hypophosphite as the phosphorus source and ionic liquid[EMIm]N?CN?₂ as the carbon source.Through SEM and TEM tests,it is found that fine and uniform Ni₂P nanoparticles are dispersed on the nitrogen,phosphorus dual-doped carbon skeleton in the Ni₂P/NPC composite.The Ni₂P/NPC composite has excellent electrochemical performance,showing a discharge specific capacity of 381.0 mAh g^-1 after 50 cycles at 0.1 A g^-1.But for Ni₂P/C,its discharge specific capacity has decayed to 336.4 mAh g^-1 after 34 cycles at 0.1 A g^-1.The better electrochemical performance of Ni₂P/NPC composite is mainly due to the following two aspects:on the one hand,the N,P dual-doped carbon skeletons can not only keep the active material fully contact with electrolyte,and provide sufficient charge transfer channels,but also alleviate the volume fluctuation of Ni₂P during lithiation/delithiation,and maintain the structural stability.On the other hand,the nano-scaled Ni₂P particles themselves facilitate the transport of Li⁺.