The design and synthesis of the theory of alloy anode materials in lithium-ion battery
|Course||Physical and chemical|
|Keywords||Lithium ion battery Quantum chemical calculations First-principles VASP Alloy Intermetallic compound Diffusion coefficient|
Graphite materials as the lithium-ion battery anode material, its utilization has reached its limit (372mAh / g), and the development of a new generation of lithium-ion battery has become the hot spot of the current research in the field of lithium-ion batteries with higher capacity. Metal and alloy anode material for lithium-ion batteries have the potential, because they generally have higher capacity and appropriate charge and discharge potential. Such as the metal Sn, it has up to 990 mAh / g (7200 mAh / L) of the theoretical capacity. Quality than the capacity is 2.7 times that of graphite, and the volume ratio of capacity was actually 8.8 times that of graphite. A single mass of metal, however, it can not directly be used to do the negative electrode material of lithium ion battery, the main reason is that they are in the process of reaction of the lithium metal alloy with lithium ions occur there is a larger volume of expansion and contraction, which causes the material by the action of the internal stress cracking, and thus lose activity, eventually leading to the decline in the cycling performance. Improve cycling performance of metal and alloy materials become the key issues of the development of the alloy lithium-ion battery materials. In recent years, how to suppress the volume expansion and contraction of the metal material to improve the alloy material cycling performance as anode material in lithium-ion battery research attracted much attention. One of the more effective method is to use an intermetallic compound (often referred to as alloy) MM ', which contains M is \non-reactive metal \Which the of Cu 6 Sn 5 , FeSn 2 , of Cu , 2 Sb belong to this type of alloy. The reason why this system is possible to suppress the volume expansion is \The expansion generated cracking, to prevent the detachment of the active material and the current collector. However, the development of such MM 'type intermetallic compounds as lithium-ion battery anode material encountered many difficulties, the most important of which is: how to choose the composition of the alloy of the metal M and M', as well as how to adjust them ratio. Is well known, in the periodic table the nearly ten can be alloyed with lithium and a metal (M), such as Al, Si, Zn, Ag, In, Sn, Sb, etc., there can not be alloyed with lithium metal ( M '), such as of Ti, Mn, Fe,, Co,, Ni,, Cu, etc.. The binding between them, according to the different types of elements and the ratio of the composition do not, will be an infinite number of compounds. So far, the alloy materials research and development, mainly built on the basis of experimental data and personal experience in a large number of suitable lithium-ion high-performance alloy materials development, and will spend a lot of funding for research and human. First-principles quantum chemical calculations based on density functional theory only provides a crystal structure starting from the compound of the element type, an effective means to predict its physical and chemical properties. By quantum chemical calculations, faster and more convenient to come to a material suitable for battery materials, many can not through conventional experimental synthesis or difficult to synthetic materials, calculated to detect whether it has the performance of electrochemical cells. To guide experimental development, materials development this will bring great convenience. The subject by means of quantum chemical calculations, the design of alloy material, select the appropriate preparation, combined electrochemical site and a variety of physical and chemical characterization methods. Research and development of stable structure, swelling, good cycle performance lithium ion battery with a new type of high-energy alloy material, clarify the lithium ion intercalation / deintercalation process of crystal structure, the relationship between the powder structure and the expansion ratio, the cycle performance in quantum computing based on the development of high-performance lithium-ion battery with high capacity alloy material, will open up a new field of study. The first chapter of this thesis, will brief the lithium-ion battery as well as common anode material. The second chapter describes the package VASP calculation method based on first-principles density functional theory calculations used in this paper. Then the third chapter first introduces the application of quantum chemical calculations in the study of the lithium-ion battery materials. Focuses on how to use the results of quantum chemical calculations to predict the electrochemical properties of the material. And common alloy material of the periodic table of elements, design structure and theoretical models, the Gibbs free energy calculations. Its purpose is to see if they can and lithium electrochemical reaction occurs, to determine which alloy is suitable for use in lithium-ion battery anode material. Found through the calculation of the system, including Co-Sn alloys containing Co 3 Sn 2 , CoSn, and CoSn 2 three kinds of compounds, while CoSn 2 is a very material development potential of the alloy. Chapter synthesis and study of systems they would. Electrochemical performance tests show that these three alloys, as the Sn content increases, the reaction with lithium deintercalation easier capacity also increases. Further, in the use of synthetic samples of the high-energy milling method, a small amount of graphite is added in the raw material, and can effectively improve the cycle performance of the material. By more than theoretical, experimental development and research, we found that the electrochemical properties of the alloy anode materials of lithium-ion batteries have a close relationship with the nature of the composition of the alloy element type and composition, crystal structure, electronic structure, and charge distribution. In the fifth chapter, in order to find the relationship alloy lithium intercalation capacity of the crystal structure and elemental composition, specifically selected two crystal structure is very similar to the alloy material Cu 6 Sn 5 sub the> and Nickel 3 Sn 2 research. To design experimental synthesis of the papers with a single-phase Ni 2 the In type hexagonal structure of the alloy series the Ni x Cu 6-x Sn < / sub> (x = 0.0,0.5,1.0,2.0,4.0), method of combining the use of quantum chemical calculations and experimental studies, found that the alloy capacity gradually decreased with Ni doping content increases, but the cycling performance gradually better. Further, in the lithium ion with the alloy occurs lithium deintercalation reaction process, the lithium ions are embedded first in the internal space of the alloy grain package, which will lead to increase the energy of the crystal package only occur after a certain amount of lithium ion intercalation of lithium and Cu or Ni is substituted. In this process, the rise of the energy equivalent to the reaction to be crossed to an energy barrier, if the energy barrier is too high, such as NI 3 Sn 2 , the reaction would not be able to proceed, this many alloy materials can not be the cause of lithium deintercalation reaction occurs, the reasons for the low capacity. In the sixth chapter, the synthesis of design experiments alloy series of Co. x Cu 6-x Sn 5 (0 ≤ x ≤ 2) and test their electrochemical properties. Study found that the right amount of Co doping and CoCu 5 Sn 5 alloy has the best electrochemical performance. Calculated from the theoretical point of view, the papers from the crystal structure of the alloy material, thermal stability, and lithium-ion deintercalation reaction of the intermediate product in the electronic structure, the density of the charge state distribution angle depth discussions, a good explanation the electrochemical properties and the structure of the relationship. The results show that, Alloy Co. x Cu 6-x Sn 5 in lithium deintercalation reaction process generates the intermediate phase compounds of Li 2 < / sub> Co y Cu 1-y Sn, and this intermediate phase compound as the Co content increases, gradually becomes unstable, resulting in a change in the electrochemical performance occurred . Chapter VII of the paper, in order to in-depth study dynamical properties of the lithium-ion diffusion in the alloy internal classic electrochemical test methods (PITT and EIS) Cu 6 Sn 5 sub > the diffusion coefficient of the lithium ion migration in the alloy. Not imagine slow lithium ion diffusion in the alloy material, most of the potential range of the charging and discharging, the diffusion coefficient in 10 -11 sup> ~ 10 -10 sup > cm 2 sup> / s between. However, the charging and discharging platform potential, lithium ion diffusion is very slow, the diffusion coefficient is lower than 10 -11 sup> cm 2 sup> / s. And the minimum value of the diffusion coefficient of discharge potential of mapping curve, it is obvious there are two diffusion coefficients, and Cu 6 Sn 5 discharge curve places the two platforms anastomosis. The experimental data obtained by the two methods PITT and EIS also very consistent. This is likely to hinder the diffusion of lithium ions due to phase transition in the alloy.