Experimental Investigation And Thermodynamic Description Of The Li-Si-Mn Ternary System

Silicon is a great potential anode material in improving energy density of lithium-ion batteries because of the highest theoretical capacity of about 4200 mAh/g.However,it suffers from drastic capacity fading and overall efficiency decreasing,which results from huge volume change of Si anode during the lithiation-delithiation cycle.Research indicates that Si alloys with a Li-inactive element can effectively control the volume change.In the process of lithium intercalation,Si acts as an active central,and reacts with Li to form compounds LixSi.The other inactive alloy phase plays the role of matrix as an inertia phase,which can buffer the volume change.Research indicates that Si-Mn alloys as anode have good conductivity and cycle performance.In order to study the lithiation route of Si-Mn anode materials and to understand the lithium insertion mechanism,it is necessary to study the thermodynamic properties of the Li-Si-Mn ternary system,which can provided a theoretical reference of the insertion/extraction of Li+ in the Si-Mn binary alloy anode materials.The equilibrium alloy method has been used in this work.The samples of equilibrium alloy of the Li-Si-Mn ternary system have been analyzed by means of scanning electron microscopy with energy dispersive spectroscopy(SEM-EDS)and X-ray powder diffraction(XRD),and the isothermal section of the Li-Si-Mn ternary system at 150℃ has been measured.Nine three-phase regions and three two phase regions have been determined,i.e.Li12Si7+Mn11Si19+(Si),Li12Si7+Mn11Si19+MnSi,Li7Si3+Mn5Si3+MnSi,Li7Si3+Mn5Si3+Li13Si4,Li22Si5+Mn3Si+Mn5Si3,Li22Si5+Mn3Si+Mn9Si2,Li22Si5+Mn9Si2+Mn6Si,Li22Si5+Mn6Si+(Li),(Li)+(aMn)+Mn6Si,Li7Si3+Mn5Si3,Li12Si7+MnSi and Li22Si5+Mn6Si.No ternary compound has been found on this isothermal section.Thermodynamic assessment of Li-Mn binary system has been carried out with the CALPHAD approach on the basis of the experimental data reported in literature,and then the thermodynamic parameters of the Si-Mn system from literature have been revised:The compounds MnSi1.75-x and MnSi were modeled by the sublattice models as(Mn)11(Si)19 and(Mn)1(Si)1 respectively to adapt to the multicomponent system.At last,combined the reported thermodynamic parameters of Li-Si system,the Li-Si-Mn ternary system has been directly extrapolated.The calculated 150℃isothermal section can well reproduce the experimental phase relation.Moreover,the liquidus projection of Li-Si-Mn ternary system has been predicted by using the Li-Si-Mn ternary thermodynamic database obtained in this work.