Applications Of Crystal Structure Affiliation In Preparation And Performance Improvement Of Lithium-ion Battery Electrode Materials

The advent of the era of4G and the development of electric vehicles (EVs) have given rise to the urgent need of new lithium-ion batteries (LIBs) electrode materials with high energy density and set off a wave of new materials research. Quick, convenient and large-scale preparation of new materials is the key. We developed a fast, convenient and effective route to the preparation of new high-voltage and high-capacity LIBs electrode materials, which were too difficult to be synthesized by traditional methods, based on the structural affiliation between the precursor and the finally formed product by selecting the suitable precursor and the suitable reaction system, and optimizing the reaction conditions. The process of phase changes of crystal structure and morphologies, and the related electrochemical performance and were studied systematically. Based on the structural affiliation between the precursor and the finally formed product, the probable synthetic mechanisms were proposed. At the same time, the synthetic route based on the crystal structure affiliation was extended to the preparation of other compounds.The main contents of the dissertation can be summarized as follows:1. In the previous reports, the synthesis of the new high-voltage materials AFeSO4F (A=Li, Na) was always difficult and complex, including the following points:(1) the preparation of the precursor FeSO4·H2O was unavoidable;(2) the reaction medium used in the reaction system was ion liquid, which is very expensive;(3) the reaction time was very long, such as30h. Based on the analysis of the crystal structure affiliation between the precursor FeSO4·7H2O and the final product NaFeSO4F, a novel benzene-water azeotrope route was designed to selectively prepare the NaFeSO4F and NaFeSO4F·2H2O using the FeSO4·7H2O and NaF as the raw materials and the benzene as the reaction medium. The reaction time was very short (1min) and the reaction temperature was200℃. The product in the autoclave with the iron-cap was NaFeSO4F; if the coppor-cap, the product was NaFeSO4F·2H2O. With the further studies, the NaFeSO4F·2H2O can be converted to be the dehydrate phase NaFeSO4F by prolonging the reaction time from1min to40h in the autoclave with the coppor-cap. An ideal reaction model was proposed to explain the reaction mechanism based on series of control experiments. Lastly, the electrochemical performances of the NaFeSO4F和NaFeSO4F·2H2O were valuated and the3.5V (vs Li+/Li) of the NaFeSO4F was detected. In our reaction system, the Fe2+can be avoided to oxide to Fe3+. At the same time, the benzene-water azeotrope route can be extended to NaMSO4F·2H2O (M=Co, Ni). This work has been published on the RSC journal CrystEngComm,2012,14,4251-4254. The comment of the reviewers is "......a major breakthrough in the targeted synthesis of these compounds......"2. One-dimensional nanostructure is of importance for the improvement of the electrochemical performance of the metal oxides. However, some layer structured metal oxides, such as MoO3, V2O5and WO3, are difficult to be one-dimensional nanostructure, such as nanowire et al because of the limitation of the crystal structure properties. We propose a new concept that preparing the MoO3, V2O5, and WO3nanowires by inhibiting and/or destroying the connecting of1-D MO6octahedra chains existed in their hydrated states in the radial direction based on the dehydration of their hydrated metal oxides, and for the first time, we report a vacuum topotactic transformation route to mesoporous orthorhombic MoO3highly textured nanowire bundles from single-crystal α-MoO3·H2O nanorods. The topotactic conversation is based on the1-D double chains of MO6octahedra sharing two common edges for the α-MoO3·H2O and MoO3and the structural matching of [001]α-MoO3·H2O//[100]MoO3. During the chemical and structural change from α-MoO3·H2O to MoO3, the nanorod morphology can be well maintained. Investigation of the transformation progress indicates MoO3nanowires constructing a porous nanobundle prefer textured and epitaxial growth from {001} plane of a single-crystal α-MoO3·H2O nanorod. The topotactic transition growth in vacuum based on the relationship of the crystal structures provides an effective and practical approach to synthesize highly organized porous nanomaterials, such as V2O5, and WO3nanowires, etc. The result of galvanostatic electrochemical testing in the voltage range of0.001-3.0V versus Li+/Li at200mA g-1give reversible capacities of954.8mA h g-1for the mesoporous orthorhombic MoO3nanowire bundles. The capacity can be comparable to the best recently reported data. The sample consisting of mesoporous MoO3nanowire bundles with an average pore size of13nm and high BET surface-area shows more stable reversibility and higher capacity than others, indicative of micro/nano-structure-enhanced performance, which can also be used in other application fields, such as supercapacitor and catalysis, etc. This work has been published on the ACS journal J. Phys. Chem. C,2014,118,5091-5101. The comment of the reviewers is "......The work is of creative and well done......This paper successfully shows the novelty with its effective and versatile synthesis approach......"3. We have designed a special nanocomposite SnO2-(MoO3), nanocrystalline SnO2(below3nm) distributing in amorphous MoO3matrix, to simultaneously solve the two problems to obtain a high-capacity and high cycle stability anode material. Because the nanocomposites have two advantages:Firstly, the nanocrystalline SnO2with critical size can insure good cycling stability. Secondly, the surrounding MoO3with small size not only sufficiently make the wasted Li2O yielded in irreversible process convert to Li+, but also prevent the nanocrystalline SnO2aggregating and forming larger particles during the cycling process to avoid the capacity fading. A facile two-phase method has been introduced to successfully prepare the designed nanocomposites in one-pot. In our water-in-oil system, we ingeniously combine the reactions of preparing SnO2and MoO3by intermediate product HC1. The assembly structure of the nanocomposites is affected by the reaction subsequence of the formation of SnO2and MoO3. As anode materials, the nanocomposites reveal much higher reversible capacity2356mAh g-1(approximate3times as high as the theoretical capacity of SnO2790mAh g-1) at200mA g-1. We also give the detailed analysis. A perhaps obvious appeal of our approach is that it can be extended to generate nanocomposites of other metal oxides, such as TiO2-(MoO3), for obtaining improved physical-chemical properties.