Alloy Anode Materials Of Lithium-ion Battery Theory Of Design And Synthesis

It has been demonstrated that metals and alloys present an attractive altemative to graphite as anode materials for lithium-ion batteries due in particular to the high capacity, an acceptable rate capability,and operating potentials well above the potential of metallic lithium.For example,Sn yields a maximum theoretical capacity of 990 mAh/g or 7200 Ah/L.However,one major problem preventing them from the commercial application is that they undergo large volume changes during cycling,which result in disintegration of the electrodes and subsequent rapid capacity fading.An interesting approach for overcoming these problems is the use of the intermetallic compounds(or called alloys),M'M,which consisting of an "inactive phase M'",referring to the metals that do not react with lithium,and an "active phase M", referring to the metals that react with lithium.The role of"inactive phase M'"is mainly to provide a matrix that will absorb the massive volume change occurred within the electrode upon the lithiation(expansion)/delithiation(contraction) process,thus maintains the mechanical integrity between the particles and also with the current collector.However,many problems occurred during the research of the M'M intermetallic compounds,and one of them is how to choose of the metals of M' and M,as well as how to optimize the composition since there are dozens of active metals M,like Al,Zn,Ag, In,Sn,Sb,and so on,and even more inactive metals M',such as Ti,Mn,Fe,Co,Ni,Cu, etc.There will be a great number of such intermetallic compound of M'M,and it will takes hundreds of man hours of experimental work for the tradition research methods.Since first principles quantum chemistry can now predict physical and chemical properties of molecules and solids,attempts should be made to use this tool for predicting favorable new electrode materials,thereby avoiding needless experimentation and focusing work solely on materials that promise success.The important physicochemical performance parameters of a rechargeable battery are its energy density, specific energy,power density,and cycling stability.The challenge is now to predict some of these properties by first principles quantum mechanics.In this thesis,we want to show that it is possible to predict parameters such as the average voltage,energy density,and specific energy of lithium-ion batteries.After the method of first-principles density-functional theory with pseudopotentials and plane wave basis and the software package of VASP we used were introduced,, some cases of first-principles study in the materials of lithium ion batteries were introduced in Chapter 3.Then,a series of simulations for the most common alloys were carried and their Gibbs energies of the reaction with lithium were calculated.According to this Gibbs energy,the estimation of the alloys' possibility of electrochemical reaction with lithium,hence their potential use for the anode of lithium ion batteries become possible.Base on the above calculation,the CoSn₂ alloy shows great reactivity with lithium ion in a battery environment and was expected to have a high capacity.So,in Chapter 4,the Co-Sn system,including Co₃Sn₂,CoSn and CoSn₂ alloys were systematically studied.A series of Co-Sn intermetallic compounds were prepared by mechanical ball-milling followed by annealing at appropriate temperature.The charge/discharge profile and cycling performance was compared as negative electrodes for lithium ion battery.Moreover,the insertion/extraction mechanism of these alloys were investigated through ex-situ XRD measurement.Further more,the cycle performance of Co-Sn alloy was improved by properly controlling the composition and addition of the graphite in the raw materials during ball-milling.It shows that the electrochemical performance of alloy anodes are closely related with their compositions, crystal structures,and the electronic structures.Firstly,In Chapter 5,a series of Ni doped Ni_xCu_6-xSn₅(χ= 0,0.5,1,2,4) alloys were synthesized.The effects of doped-Ni amount on the structure and lithium intercalation were examined.The relationship between the electrochemical performance and the crystal structure was also studied using the first-principles calculation.The electrochemical tests show that the cycle performance improves while the reversible capacity decreases with the increasing amount of Ni content.A proper amount of Ni doped alloy,NiCu5Sn5 and Ni₂Cu₄Sn₅ show an improved cycling stability with a reversible capacity of 200 mAh/g(1680 mAh/ml).The first-principles calculations suggest that the lithiation depth of Cu₆Sn₅ can reach to Li_4.4Sn,however,the reversible capacity is very small for Ni₃Sn₂ and it may attribute to the high energy barrier required for the structure transformation from the hexagonal structure to the cubic structure.Secondly,in Chapter 6,a series of Co doped Co_xCu_6-xSn5(0≤x≤2) alloys were prepared experimentally.The electrochemical performance,including the average intercalation/alloying voltage,the sharp of charge/discharge curve and the cycling performance,were studied using both the experiment and computational simulation.The results show that the meta-stable intermediate phases of Li₂Co_yCU_1-ySn form during the Li-ion insertion process of Co_xCu_6-xSn₅ and become unstable and even undetectable with increasing amount of Co substituted.A proper amount of Co doped alloy,CoCu₅Sn₅ showed improved cycling stability at the expense of litter capacity,whereas a heavy Co-doped alloy,Co₂Cu₄Sn₅,resulted in poor cycling ability.The crystal and electronic structure,thermodynamic stability of Co_xCu_6-xSn5 and half-lithiated alloy,Li₂Co_yCu_1-ySn, as well as the average voltage of alloying reaction in terms of different discharge depths were investigated using first-principles calculation.In the Chapter 7,the kinetics of Li-ion intercalation into Cu₆Sn₅ electrode was determined by galvanostatic intermittent titration technique(GITT) and electrochemical impedance spectroscopy(EIS) method.Although the Li-ion diffusion coefficient(DLi) values are relatively high(between 10^-11 to10^-10 cm²/s) in the most voltage range,two minimums(below 10^-11 cm²/s) in the D_Li VS.voltage curve were observed at～0.4 V and 0.1 V,coinciding with the voltage plateau in the charge/discharge curves,indicating reversible structural phase transition or order/disorder transition in the compound.The D_Li derived from GITT and EIS were confirmed well both in the magnitudes and in the variation with Li composition(voltage) range.