| The renewable energy has aroused extensive attention due to the fossil fuels depletion and the environmental problems caused by combustion.Simultaneously,the inherent instability of renewable energy has accelerated the development of grid-scale energy storage systems(ESSs)technologies.However,mature lithium-ion batteries(LIBs)are not suitable for ESSs owing to the scarcity and high cost of lithium resources,which cannot meet the cost factors highlighted by large-scale applications.On the contrary,sodium ion batteries(SIBs)have shown extensive application prospects owing to the low cost and easy accessibility of sodium.However,the research of anode materials has fallen into bottleneck because the graphite anode widely used in LIBs cannot effectively store sodium.Therefore,the main effort is devoted to design the anode materials with high specific capacity,excellent rate performance and long stability.Alloy-based anodes have attracted much attention due to their high theoretical specific capacity(such as Bi:385 mA h g-1;Sn:847 mA h g-1;Sb:660 mA h g-1)and low voltage plateaus(less than 1 V(vs.Na+/Na)).However,the serious agglomeration and pulverization caused by the dramatic volumetric variation limit their cycle stability.Constructing the nanoporous structure and introducing the alloying strategy can effectively alleviate volume expansion and promote the ion/charge transfer.In this dissertation,by utilizing rapid solidification-dealloying method and using magnetron sputtering technology,nanoporous Bi-based(Bi,Bi-Sb,Bi-Ni)alloys and sputtering Sb films with nano-sized column-channel structures have been successfully fabricated and their sodium storage properties and mechanisms have been further studied by combining the experiment and DFT calculations.Firstly,the(de)sodiation mechanism of Bi is controversial.The results conducted by operando XRD suggested that the(de)sodiation of Bi follows a two-step(de)alloying reaction.however,the results obtained by ex-situ XRD reported that it performs an(de)intercalation reaction.By using the rapid solidification-dealloying technology and magnetron sputtering technology,nanoporous Bi,sputtered Bi films and commercial Bi with different morphologies and dimensions were synthesized.It was found that all three Bi anodes inherently follow a two-step(de)alloying process(Bi(?)<NaBi(?)Na3Bi)by utilizing the operando XRD.Notably,compared with nanoporous Bi and sputtered Bi films at the nanoscale,the micro size of commercial Bi particles will significantly affect their(de)sodiation process during initial cycle.High polarization induced by slow dynamics results in the unstability of the intermediate sodiation product NaBi and the instantaneous conversion of NaBi→Na3Bi,exhibiting the "one-step" alloying process.By exploring the stability of the Bi discharge product under ex-situ XRD conditions,the contradiction between the results of ex-situ XRD and operando XRD was interpreted,while the problems of ex-situ XRD was emphasized.Secondly,by dealloying the rapidly solidified Mg92BixSby(x,y＝2,6;4,4;6,2)precursor alloys,three nanoporous(np)-BixSby alloy powders were fabricated,exhibiting the three-dimensional bi-continuous nanoporous-ligament structure.Morphological analysis elucidates that the ligament size of the np-Bi-Sb alloy decreases as the Sb content increasing,which can be attributed to the lower diffusion coefficient of Sb compared to Bi in the corrosion solution.Hence,the composition design can effectively adjust the characteristic size of dealloyed nanoporous alloys.Compared with single metal(np-Bi and np-Sb)and two other np-Bi-Sb alloy anodes,np-Bi2Sb6 delivers the highest specific capacity and ultra-long cycle stability,stemming from the nanoporous structure,alloying strategy and appropriate Bi/Sb ratio.The phase evolution of np-Bi-Sb alloy was probed by utilizing the operando XRD.Subsequently,the synchronous(de)alloying sodium storage mechanism((Bi,Sb)(?)Na(Bi,Sb)(?)Na3(Bi,Sb))was proposed and verified by DFT calculation.Thirdly,after studying the "Bi+active material Sb" type anode,the nanoporous(np)-BixNiy(x,y＝50,50;75,25)alloy anodes("Bi+inactive material Ni" type)were fabricated by the rapid solidification-dealloying method.According to the relationship between element diffusion rate and ligament size,the ligament size of np-Bi-Ni alloys were regulated by adjusting the Bi/Ni ratio.It is worth mentioned that,based on the similar XRD results between np-BisoNiso and np-Bi75Ni25(both corresponding to the Bi3Ni phase)and DFT calculations,the unique intra-lattice channel of np-BisoNiso was found and further analyzed according to the sodium ion transfer and structural stability.Compared with np-Bi75Ni25,the performance improvement achieved by np-Bi50Ni50 can be assigned to its unique intra-lattice ion channels and smaller ligament size.From the operando XRD results,np-Bi-Ni alloy anodes followed a two-step(de)alloying reaction(Bi3Ni(or BiNi)+Bi(?)NaBi+Ni(?)Na3Bi+Ni).Finally,by increasing the sputtering working pressure to proper degree during the magnetron sputtering,the internal structure of the sputtered Sb film is refined and exhibits a nano-sized column-channel structure,which can be attributed to the lower kinetic energy of the sputtered atoms,the high diffusion resistance on the substrate and the shadowing effect.Compared to the Sb@0.2Pa and Sb@1Pa electrodes fabricated at lower working pressure,a significant improvement of the electrochemical performance can be observed on the Sb@3Pa electrodes prepared at high working pressure,which stems from their nano-sized column-channel structure,not only mitigating volume expansion but also promoting ion/electron transfer.By using operando XRD,we explored the electrochemical-driven gradually in-situ Cu-Sb alloying between the sputtered Sb layer and the Cu substrate with cycle forwarding,and found that the formed Cu-Sb layer follows the two-step(de)alloying sodiation mechanism of CuxSb1-x(?)+NayCuxSb1-x(?)Na3CuxSbi-x.By utilizing the ex-situ SEM&TEM characterization and DFT calculation,the distribution of Cu(formation of Cu-Sb alloy in the nanochannel or the different concentration of Cu solubilizing in Sb nanocolumn)was clarified,and the Cu content roughly ranges from 0 to 37.5%.In summary,preparation of porous metals during the dealloying process can be controllable by introducing alloying elements,while the magnetron sputtering technology is efficient and has been widely used in the industrial field.Nanoporous Bi-based alloys prepared based on alloying strategy and self-supporting sputtered Sb films with nano-sized column-channel structure obtained based on the principle of working pressure control exhibit the excellent sodiation storage performance,which provides the design route for the alloy-based anodes with high capacity and stability in SIBs.The(de)sodiation mechanisms unveiled by the operando XRD technology provide theoretical foundations and experimental guidance for developing high-performance anode materials for SIBs,as well as for exploring the failure mechanisms for the electrodes.The present results will push forward the practical applications of SIBs. |
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