| The exacerbation of energy crisis and environmental pollution greatly promotes the development of green renewable energy and energy storage technology.However,due to the increasingly depleted lithium resources,high cost and security issues,the commercial lithium ion batteries(LIBs)technology,which dominates the market of portable electronics,cannot meet the needs of large-scale energy storage.Magnesium ion batteries(MIBs)have been regarded as a potential alternative to LIBs,since Mg metal has abundant earth reserves,high theoretical specific capacity(3833 mAh cm-3,2205 mAh g-1)and smooth deposition behavior under normal conditions.But the serious incompatibility between Mg metal and conventional electrolytes has severely limited the progress of MIBs.Using alloy-type anodes with high theoretical specific capacities and low reaction potentials can not only circumvent the passivation problem of Mg metal in conventional electrolytes,but also be favorable to match with high voltage/capacity cathodes,so as to construct high performance MIBs system.Nevertheless,the main alloy-type anode materials(like Bi,Sn,Pb and Sb)still face severe challenges,such as low electrochemical reactivity,sluggish diffusion kinetics and huge volume changes during discharge/charge processes.To cope with these dilemmas,in this dissertation,through magnetron sputtering technology,rolling and painting methods,a series of selfsupporting alloy-type anodes(Pb-Bi,Sn-Bi,Sn-Al and liquid Ga-based electrodes)have been successfully fabricated,and their Mg storage performance and reaction mechanisms have been deeply explored by combining in-situ/ex-situ experiments and simulations.The increased phase and phase boundary in electrodes can effectively alleviate volume change during phase transition and promote ion transport.Based upon phase engineering strategy,Pb-Bi films with different compositions and phase constitutions(single-phase Pb,Bi,Pb0.7Bi0.3 and biphase Pb0.7Bi0.3/Bi)were prepared by the magnetron sputtering method.Compared with the single-phase Pb,Bi and Pb0.7Bi0.3 electrodes,the biphase Pb0.7Bi0.3/Bi electrode delivers significantly enhanced cycling stability and rate performance.The Mg storage mechanisms of the Pbo.7Bi0.3 and Pb0.7Bi0.3/Bi electrodes were revealed by combining operando X-ray diffraction(XRD)experiments and density functional theory(DFT)calculations.Both of them follow a two-step alloying process with the final discharge products of Mg3Bi2 and Mg2Pb,and an intermediate phase Pb0.85Bi0.15 was identified during the discharge/charge processes.Besides,the biphase Pb0.7Bi0.3Bi electrode shows good compatibility with conventional electrolytes.Based on the effect of biphase structure and phase boundary on Pb-Bi electrodes and replacing Pb with Sn possessing low cost and high theoretical specific capacity,the eutecticlike,nanoporpus biphase Bi-Sn films were successfully prepared using the magnetron sputtering method.As benchmarked with the single-phase Bi or Sn electrode,the biphase BiSn electrode displays much better Mg storage performance(specific capacity,cycling stability and rate capability),which could be attributed to the synergetic effect of the interdigitated Bi/Sn phase distribution,increased phase boundaries and nanoporous structure,thus reducing total volume changes and shortening ion diffusion lengths.According to the operando XRD results,the biphase Bi-Sn electrode follows a two-step process of magnesium storage:2Bi+3Mg2++6e-(?)Mg3Bi2,Sn+ 2Mg2++4e-(?)Mg2Sn.In addition,the sputtered pure Sn films are not reactive with Mg even at very low current densities.Based on this,the nanoporous biphase SnBi films with different compositions were obtained by introducing Bi with different contents into Sn via magnetron sputtering.The experimental results revealed that the introduction of the second phase Bi and phase boundary can effectively stimulate the electrochemical reaction of Sn with Mg,and the further increase of Bi content can significantly improve the Mg storage performance of the Sn-Bi electrodes.The activation function of Bi phase and phase boundary on Sn was also verified in the bulk rolled Sn-Bi system.Moreover,the DFT calculations further rationalized the experimental results,considering the introduction of Bi could significantly decrease the defect formation energy of Sn.On the basis of the above study"introducing the second phase Bi and phase boundary",in order to exclude the influence of firstly formed Mg3Bi2 and explore the decisive factors affecting the electrochemical reactivity of Sn,nano-sized biphase Sn-Al,Sn-Pb and Sn-ZnO films were successfully prepared by magnetron sputtering.Using Sn-Al as an example,it was found that the introduction of inactive Al phase and phase boundary can effectively stimulate the electrochemical reaction of Sn with Mg either in nanoscale or bulk size,through combining the experiments and DFT calculations.Especially,the rolled Sn-Al electrode exhibits superior long-term cycling stability over 5000 cycles.In the light of the operando XRD results,only the Sn phase in the Sn-Al electrodes participates in the Mg storage reaction:Sn+2Mg2+ 4e-(?)Mg2Sn,while Al phase exists stably and successfully endows Sn with Mg storage capability.More importantly,the activation effect of second phase and phase boundary to Sn is also applicable to metal phase Pb and oxide phase ZnO,confirming that the introduction of phase and interface to trigger the Mg storage reactivity of Sn is a universal strategy.In order to address rapid specific capacity attenuation of alloy-type anodes caused by huge volume changes during discharge/charge processes,the self-supporting liquid Ga(melting point of 29.8℃)electrodes(ssm-Ga)with self-healing property were simply fabricated by painting onto stainless steel mesh(ssm),but show unsatisfactory cycling stability due to the poor wettability of liquid Ga on the ssm.To address this issue,a CuGa2 layer was constructed on the surface of ssm and the wettability of liquid Ga on current collector was significantly enhanced.As compared with the ssm-Ga electrode,the modified ssm-CuGa2-Ga anode displays much better Mg storage performance at-38℃.On this basis,to achieve high-performance liquid electrodes towards Mg storage near room temperature,the eutectic GaSn(EGaSn)alloy with the melting point(20.5 ℃)below room temperature was obtained by introducing 13.5 wt.%Sn into liquid Ga.Similarly,the wettability between liquid EGaSn alloy and current collector was significantly improved by constructing a CuGa2 layer on the surface of ssm,and the ssmCuGa2-EGaSn electrode delivers further enhanced electrochemical performance(specific capacity,cycling stability and rate performance)either at～38 or 28℃.The Mg storage mechanisms and self-healing feature of the liquid Ga and EGaSn electrodes were unveiled by operando XRD and ex-situ scanning electron microscopy(SEM).The reaction mechanism of the liquid Ga electrode is:5Ga+2Mg2++4e-(?) Mg2Ga5,while the liquid EGaSn electrode follows the Mg storage mechanism of EGaSn (?)Mg2Ga5+Mg2Sn.Noticeably,the solid Mg2Ga5 and Mg2Sn almost simultaneously emerge and disappear during the discharge/charge processes.In addition,both liquid Ga and EGaSn electrodes have good compatibility with conventional electrolytes.In summary,based on alloying strategy,regulation of phase and phase boundary in electrode materials,and interface regulation between active materials and current collectors of liquid electrodes,a series of alloy-type anodes were prepared and their Mg storage performance was significantly improved.Meanwhile,combined with in-situ characterizations and DFT calculations,the Mg storage mechanisms of the alloy-type anodes were deeply investigated.The present results can provide important information for the design and exploration of highperformance anodes,and have practical significance for promoting the development of MIBs. |
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