The Preparation Of Layered Double Hydroxides Based Electrode Materials And Their Sodium Storage Performance

With the increasing reliance on lithium ion batteries(LIBs)of digital products and transition vehicles,lithium source is facing serious challenge of limited reserve.The development of sodium ion batteries(SIBs)alleviates the situation limited by the shortage of lithium reserves to a great extent.Compared with LIBs,the advantages of SIBs are as follows.Firstly,the sodium resource with uniform distribution is abundant and the cost of sodium resource is relative low.Secondly,the standard electrochemical potential of Na is higher than that of Li,which leads to a broad choice range of electrolyte.Thirdly,the SIBs are more safety than LIBs due to its relative stable electrochemical abilities.However,the larger diameter of Na+than Li+impedes electrochemical reaction kinetics,resulting in difficulties in finding applicable anode materials with reversible and facile shuttling of Na+.Layered double hydroxides(LDH)as a classical two-dimensional structure is promising host materials.The structure of LDHs can be maintained during the intercalation/de-intercalation of sodium ions due to the topotactic nature of such transformation.In this paper,the expanded LDHs as anode materials for sodium ion batteries were obtained with large capacities and long cycling performance by controlling the layered space of LDH.(1)CoFe-NO3--LDH nanoplatelets with a particle size of 80-120 nm and a thickness of～10 nm was prepared by a modified coprecipitation method.The nitrate-pillared CoFe-LDH nanoplatelets are used as anode material for SIBs for the first time.A capacity of 209 mAh g-1 is delivered after 200 cycles at a rate of 1 A g-1.The nitrates as pillaring anions expand the layered space of CoFe-LDH to facilitate the diffusion of Na+.By adjusting the voltage window in the range of 0.4-2.3 V,an exceptional intercalation-type mechanism is taking place with enhanced stability of LDH,rather than commonly-believed conversion reaction mechanism in LIBs.By the means of ex-situ XRD,XAFS and XPS,this work not only reveals the intercalation/de-intercalation sodium storage mechanism of CoFe-LDH,but also discover the inferior reversible capacities and cycling performance of CoFe-CO32--LDH(d=7.5 A)and anion-free layered Co2+/Fe2+hydroxide.In addition,theoretical calculation reveals a low Na+diffusion barrier of 0.147 eV in the interlayer space of CoFe-NO3--LDH,which manifests LDH as a good Na+ ion conductor.(2)C@NiCo-LDH particles with size of 350-550 nm were synthesized by in situ transformation of carbonized ZIF-67 crystals into NiCo-LDH through hydrothermal method.The structure feature and electrochemical performance of materials were studied by adjusting the calcination temperature.It was found that under calcination temperature of 350 0C,well-defined C@NiCo-LDH@Co3O4 was obtained with hollow interior and stable chemical property.A capacity of 300 mAh g-1 is delivered after 200 cycles at a rate of 1 A g-1.When the current density is increased to 2 A g-1,the reversible capacity still can remain around 200 mAh g-1.The hollow core/shell structure can not only improve the specific surface area,but also alleviate the stress produced by the intercalation/de-intercalation of sodium,leading to an enhanced capacity and improved structure ability.