The Study On Microstructure And Discharge Behavior Of Mg-Sn Based Alloys As Anode Materials For Mg-air Batteries

Mg-air batteries have great potential for development in the fields of electrochemical energy storage and conversion because of their high theoretical discharge voltage and specific energy density,rich source of material,no environmental pollution.Nevertheless,the high anodic polarization and low anodic efficiency significantly reduce the inherently excellent discharge performance of Mg anode material,which limits the wide-spread application and commercialization of Mg-air batteries to some extent.Generally speaking,tailoring the microstructure through alloying and processing technology is an effective way to improve the discharge property of Mg anode.Environment-friendly Mg-Sn based alloys are potential and attractive anode materials for Mg-air batteries.Based on the Mg-Sn alloy system,the microstructure characteristics of alloy are regulated by the Zn,Ag,Mn,Ca alloying,pre-treatment and extrusion deformation.The dependence between microstructure characteristics and discharge behavior is revealed by the analysis of the microstructure,electrochemical behavior,discharge performance and surface morphology after discharge.The specific research contents and results are shown as follows:(1)The influence of Sn content on the microstructure evolution and discharge behavior of the extruded Mg-x Sn-Zn-Ag(x=2,4 and 8 wt.%)was investigated.The results indicate that the grain size gradually decreases and dispersed second phases(Mg₂Sn)gradually increases with the increase of Sn content.Furthermore,the Mg-4Sn-1Zn-1Ag(TZQ411)alloy has a high volume fraction of dynamic recrystallized grains and non-basal orientation grains compared with the counterpart alloys.During the half-cell and full-cell test,the TZQ411 alloy exhibits more excellent discharge performance,which is primarily related to the enhanced discharge activity and loose discharge products.(2)In order to further optimize the alloy microstructure characteristics and reduce the negative effect of the second phases relative discharge property,a dilute Mg-0.5Sn-0.5Mn-0.5Ca alloy was designed and the effect of homogenization prior to extrusion(HPE)treatment on the microstructure characteristics and resultant discharge behavior of the alloy was studied.The results signify that HPE treatment exerts a prominent effect on the crystallographic structure and orientation,distribution of the second phase particles as well as the dislocation density of the extruded alloy.The HPE alloy provides a stable discharge process,high cell voltage and specific energy,which are mainly attributed to the uniform grain structure,excellent electrochemical activity and weak“chunk effect”.(3)The influence of sampling direction on microstructure and discharge property of the dilute Mg-0.5Sn-0.5Zn-0.5Ca alloy was investigated.The results imply that the sampling direction has little effect on the grain size,morphology and distribution of the second phase particles as well as the dislocation density of the extruded alloy.Nevertheless,the longitude surface(LS)sample is mainly composed of(0001)orientated grains,while the transversal surface(TS)one is consisted of(10-10)/(11-20)orientated grains.The TS sample exhibits a remarkably high cell voltage,anodic efficiency and specific energy,which can be ascribed to the enhanced discharge activity,loose discharge products and uniform dissolution of the matrix.As a consequence,the TS sample of dilute extruded alloy is an ideal anode material for superior performance Mg-air battery.The above results indicate that the uniform microstructure,high volume fraction of non-basal orientated grains,dispersed nanoscale second phase particles,weak“chunk effect”and loose discharge products can be beneficial to obtain the outstanding discharge performance of Mg anode.The above research results provide the necessary experimental data and theoretical basis for boosting the comprehensive performance of the anode material of the Mg-air battery by tailoring the microstructure characteristics.