Synthesis And Performance Characterization Of Tin-based Anode Materials For Lithium-ion Batteries

Recently, graphite and modified graphite are mostly used as anode material in commercial lithium-ion batteries. However, the graphite materials have low theoretical capacity (372 mAh/g), and the current used graphite anode is close to its capacity limitation. Exploring new generation anode materials with higher capacity has become the focus of the research on lithium-ion batteries. It has been demonstrated that metals and alloys, present higher capacity and lower lithium intercalation potential. 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. How to improve the cycleability of tin anode is the key point to develop high performance anode.This dissertation is on the foundation of existing research, and creatively introduces a new method that combines two existing ways. A core-shell carbon-coated nano-scale alloy anode has been prepared in this dissertation, and shows excellent electrochemistry performance. Moreover, in the preparation, we used in-situ polymerization technique and surface modification to avoid the low-melting point alloy pouring out from shell in heating process. This method can be applied extensively to other nano-alloy preparation work. In addition, this dissertation explores the matrix materials for tin-based anode, and synthesize a LiSn2(PO4)3 compound in which the Li3PO4 is used as matrix. The lithiuation mechanism of this material has also been investigated. The ordered three-dimension structure anode is also included in this dissertation.In Chapter 3, a core-shell-structure carbon-coated nanoscale Cu6Sn5 is prepared by using an in situ polymerization method integrated with a surface modification technology. The composite combines the merits of intermetallic compounds and nano-sized anode materials, and exhibits an excellent cycling stability with a reversible capacity of 437 mAh/g and no obvious fading after 50 cycles, which is the best cycle performance among alloy anodes reported up to date. The improvement in the cycling stability could be attributed to the fact that the well-coated carbon layer can effectively prevent the encapsulated low melting point alloy from out-flowing in a high temperature treatment process, as well as preventing aggregation and pulverization of nano-sized Cu6Sn5 alloy particles during charge/discharge cycling.It is convenient and has practical meaning to use oxides which is cheap and easily got as precursors to prepare nano-alloy anodes. In Chapter 4, the thermodynamic characters of carbothermal reduction of various oxides have been studied. And differential scanning calorimetry (DSC) is applied to identify the reduction temperatures. The nano CuO and SnO2 are used as precursors, and hexadecyltrimethoxysilane (HTMS) is used as surface modifier. A "Si-O-metal" bond which is detected by Fourier infrared spectrometry (IR) technique is formed during the modification to keep the oxide particles dispersing well in the organic phase. This is a key point to guarantee that polymerization occurs on a single particle surface and ensure that the polymer layer firmly coats the grains. The prepared Cu6Sn5 delivers a reversible capacity of 420mAh/g with capacity retention of 80% after 50 cycles.In Chapter 5, CoSnC system has been investigated. It is reported that although the amorphous CoSnC has large capacity, the aggregation of nano particles deteriorate the electrochemistry performance. To solve this problem, for the first time, stannate, CoSnO3, which mixes the elements Co and Sn evenly at the atomic level, is used as precursor to prepare CoSnC by a modified carbothermal reduction method. The alloy formation is easier due to the existence of liquid tin. However, the composition of the product consists of CoSn and CoSn2 due to inhomogeneous quenching process and the chemical formula can be expressed as CoSnC8 identified by Thermogravimetric (TG) measurements. The synthesized CoSnC delivers a reversible capacity of 450mAh/g with a capacity retention of 72% after 50 cycles.A systematic research of LiSn2(PO4)3 is presented in Chapter 6. Through the optimization of the synthesis condition, a NASICON structure LiSn(PO4) is prepared in air at 900?with nano-SnO2 as precursor. This material delivers a reversible capacity of 320mAh/g with a capacity retention of 85% after 50 cycles. The lithiuation mechanism of this material is studied by ex-situ XRD and SEM, which confirms that a Li4.4Sn phase is eventually formed accompanying with the structure collapse. The formed Li3PO4 matrix restricts the volume expansion of the tin particles and greatly improves the cycleability of tin-based anode. This work analyzes the irreversible capacity in the first cycle and deepens the understanding of tin/matrix anode.The three-dimensional structure materials have some unique advantages and show great promising for enhancing the performance of rechargeable lithium-ion batteries. In Chapter 7, highly ordered three-dimensional macroporous 3DOM FePO4 material was prepared by using a colloidal crystal template. The effects of the annealing temperature on the morphology changes and the electrochemical properties of the composite were investigated. The 3DOM FePO4 prepared at 400?shows the excellent cycling stability and good rate capability. This material delivers a reversible capacity of 120mAh/g at current density of 0.05C (2.5V-4.0V v.s Li+/Li). These improvements are mainly due to the following facts:first, the macropores can make the electrolyte solution infiltrate the electrode better. Second, the 3DOM structure with the wall thickness only several tens of nanometers, can reduce the diffusion distances of lithium ions markedly. Third, the continuous network of 3 DOM materials should have better electrical conductivity.