Microstructures and Properties of Al Alloy for Lithium Ion Battery Shell

Author:Zhang Jun Chao

Supervisor:ding dong yan


Degree Year:2016





Lithium-ion battery has been a preferred choice to be applied to electric vehicles and hybrid electric-vehicles due to many advantages such as high potential,high aspect of rate capability,no memory effect,no pollution,flexible design and long using life,especially with its cost down and improvement in safety performance.However,shell materials of power battery based on traditional Al alloys usually have low strength,undesirable corrosion property or poor thermal stability,which cannot meet the increasing demands on the shell material.Both multi-alloying and rare earth micro-alloying are efficient ways to improve comprehensive properties of the Al alloy.In this thesis,new type Al-Cu-Mn-Mg alloys with rare earth elements were designed and fabricated.The effects of Cu,Mg,Si and Ce element on the microstructures,mechanical properties and electrochemical performances of as-rolled alloys and as-annealed alloys were investigated through optical mircroscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),transmission electron microscopy(TEM),tensiling and electrochemical test.Main conclusions are as follows:(1)For Al-Cu-Mn-Mg alloys,the addition of Ce element could refine the grains of alloys and induce the formation of phases.And the effect of Ce was weakened with increase of the Ce content to 0.5wt%.The addition of Si could also decrease the grain size.The grain size of the alloy with low Cu content was bigger than that of high Cu-containing alloy due to less precipitation of secondary phases.The main phases in all of the alloys areα-Al,Al(Mn,Fe)Si phase and AlCuMnFe phase.And Al8Cu4Ce phase and AlCuSiCe phase were formed in the Ce-containing alloy.Al2Cu phase existed in the alloys with higher Cu content,and Al6(Mn,Fe)phase tend to form in the alloy with low Cu content.Besides the Al2Cu phase and Al6(Mn,Fe)phase,there was also Al2CuMg(S)phase in the high Mg alloys.Al12(Mn,Fe)3Si phase with spherical shape and Al20Cu2Mn3(T)phase with rod-like shape were found in the Al-Cu-Mn-Mg alloys.Addition of Ce element promoted precipitation of the T phase,and the density of T phase decreased with increase of the elements such as Mn and Si.It is worthy that there was noθ’strengthening precipitate in the alloys with low Mg content during annealing.The size of pre-exist phase grew up slightly with increase of the annealing temperatures and times,and the addition of 0.25wt%Ce could suppress the growth of phases.During annealing,a new plate-like S phase precipitated from the Ce-containg high Mg alloys,and no plate-like S phase formed in the Ce-free high Mg alloys.(2)Addition of Ce to Al-Cu-Mn-Mg alloys and increasing the content of Cu and Mg could increase micro-hardnesses and strengths remarkably.With increase of Ce content,the strengthening effect was weakened.The effect of Si on the strength of the alloys was negligible.The strengths and hardnesses of the alloys increased first and then decrease slightly during annealing process,but the elongation of the alloys increased monotonously.Addition of Ce could improve thermal stability of the strength of the Al-Cu-Mn-Mg alloys.(3)The corrosion properties of Al-Cu-Mn-Mg alloys were enhanced with addition of Ce,but droped quicklywhen the content of Ce reached 0.50t%.Increasing the content of Cu or Mg could damage the corrosion properties of the alloys,and the effect of Si on the corrosion properties of the alloys was negligible.The corrosion resistance of the alloys decreased with increase of Cl ion concentration of the testing solution.The corrosion type of the alloys was local corrosion,and there occurred main corrosion rings and corrosion domes on the alloys surface of the with high Cu and low Mg content or the Ce-contianing alloys with high Mg content.Corrosion products and pitting holes occurred on surface of the alloys with low Cu and low Mg content or the Ce-free alloy with high Mg content.After annealing process,the corrosion potentials of the alloys shifted negatively,and the corrosion current density increased.The variation of corrosion properties of the 0.25wt%Ce-containing alloys was least.Increasing the Mg content of the alloys will remarkably decrease the corrosion properties and the alloy stability.After annealing,the corrosion patterns of the alloys did not change,and the corrosion morphologies of the alloy with low Mg content varied little.However,the quantity of the corrosion rings of the alloys with high Mg content decreased considerably.