【作者】 冯真真；

【导师】 杨军；

【作者基本信息】 上海交通大学 ， 应用化学， 2008， 博士

【摘要】 镁二次电池是一种有良好前景的化学电源。镁的离子半径、化学性质和锂有许多相似之处,且具有价格便宜（约为锂的1/24）、安全性高及环境友好等优点;镁的理论比容量较大(2205 mAh g^-1),能提供比铅酸电池和镍-镉电池系统高得多的能量密度,虽然镁二次电池在小型装置方面的应用难以与锂电池相竞争,但在大负荷用途方面有潜在优势,被认为是很有望适用于电动车的一种绿色蓄电池。对镁二次电池的研究,目前还处于初级阶段,主要集中于可实现镁可逆沉积的非水电解液以及可嵌入Mg²⁺的正极材料。本论文研究了有机镁铝络合物的四氢呋喃电解液中金属镁的电化学可逆沉积-溶出的金属基质效应,并尝试制备可实现镁可逆沉积的新型电解液,在此基础上设计并制备了两种可用于镁二次电池的新型正极材料,通过改进制备方法、优化工艺及表面改性等手段,获得高的放电平台及电化学放电容量。主要研究内容如下:1.研究了金属镁的电化学可逆沉积-溶出的金属基质效应。以二丁基镁（MgBu₂）和二氯乙基铝（AlCl₂Et）为原料,自制高纯四氢呋喃（THF）为溶剂,制备得到了可用于电化学镁沉积-溶出的Mg（AlCl₂EtBu）₂/THF（0.25M）电解质溶液。在该溶液中,我们研究了镁在银、铜、镍等金属基质上的电化学沉积-溶出行为。发现在该体系中,镁在几种不同的金属基质上都可以进行沉积与溶出,但是沉积-溶出性能却存在较大差别。其中,镁在银上的沉积过程中因为可以形成银-镁合金,对镁的进一步沉积起到了很大的促进作用,从而使得连续沉积-溶出效率、过电位以及循环效率都明显好于其他几种金属基质。2.制备了二丁基镁（MgBu₂）、二丁基镁-三氟化铝（MgBu₂-AlF₃）的四氢呋喃（THF）有机电解质溶液,及三氟甲基磺酸镁的丁基甲基咪唑四氟硼离子液体电解质溶液(Mg（CF₃SO₃）₂/BMImBF₄（0.3 M）,研究了两种不同类型新型电解液中镁沉积-溶出的电化学性能。结果表明这些新型电解液体系中都可以较好的进行镁的沉积-溶出电化学反应,其中有机电解液中,以MgBu₂-AlF₃/THF体系中镁沉积过电位较小;不同电解液中均以基质银上镁沉积-溶出的效果最佳,过电位小,循环效率最高。在Mg（CF₃SO₃）₂/BMImBF₄ （0.3M）离子液体电解液体系中,最初的几个循环过程中镁沉积过电位很高,并且还存在电位不稳定、波动的现象,但经过初始的几个循环后,镁可逆的沉积-溶出会达到一个相对稳定的状态,此时的过电位很低。两种不同类型电解液中不同的镁沉积-溶出性能表现,对应着不同的镁沉积-溶出过程机理。3.使用价格便宜、环境友好的导电含硫材料（CSM）及其与聚苯胺的复合物（CSM/PAn）作为镁二次电池的新型正极材料。与单纯的导电含硫材料相比,其与聚苯胺的复合物表现出更高的电极容量和较好的充放电循环性能,电池放电容量可达117.3 mAh g^-1,22次循环之后容量仍能维持在第二次容量的78%。4.采用多种制备工艺及不同的碳包覆手段,制备了含有较大聚阴离子的硅酸锰镁正极材料,并将其应用于镁二次电池,研究了其在电解液0.25 M Mg（AlEtCl₂）₂/THF中的充放电电化学性能。结果表明,该硅酸盐嵌、脱镁的电化学性能与材料的导电性能及其颗粒尺寸密切相关。其中改性溶胶-凝胶法制备的碳包覆硅酸锰镁材料(MSG-Mg_1.03Mn_0.97SiO₄/C)在C/100倍率下放电容量可以达到244 mAh g^-1。碳包覆提高了材料的导电性,同时抑制了纳米颗粒间的团聚现象,因此增大了材料颗粒之间、电极材料和电解液之间的电子接触。而且,材料颗粒尺寸的减小缩短了颗粒内部镁离子的扩散路径。因此,活性材料的利用率大大提高,电化学极化现象明显减弱。同时,我们根据Mg_1.03Mn_0.97SiO₄的晶体结构特征对其充放电过程进行了一定的解释。高的电化学储镁容量以及好的嵌、脱镁循环性能使得该材料成为可用于镁二次电池的很有前景的正极材料。更多还原

【Abstract】 Rechargeable magnesium battery is a promising electrochemical power source. Magnesium has similarity with lithium in many respects, such as ionic radius and chemical properties. In addition, magnesium is relatively inexpensive （1/24 of lithium）, environmentally friendly and safe to handle. Magnesium has large enough theoretical specific capacity (2205 mAh g^-1), it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems. Although rechargeable magnesium battery can not compete with lithium battery in terms of energy density for small-scale uses, it has potential advantages in heavy load applications. Rechargeable magnesium battery is a hopeful green storage battery for application in an electric vehicle.The research on rechargeable magnesium battery is under elementary development, and concentrated on non-aqueous electrolyte systems, from which magnesium can be reversibly deposited and dissolved, and cathode materials that can reversibly intercalate magnesium ions. In this dissertation, we studied the effect of metal substrates in the tetrahydrofuran electrolyte solution based on magnesium organohaloaluminate salts on magnesium electrochemical deposition-dissolution behavior, and tried to synthesize novel electrolytes for magnesium deposition and dissolution. On this basis, we designed and prepared two kinds of new cathode materials for rechargeable magnesium battery, trying to obtain a high discharge voltage plateau and a large electrochemical discharge capacity by promising preparation methods, optimizing the technical condition, and applying some surface modifications. The major research content is as follows: 1. The effect of different metal substrates on magnesium deposition and dissolution has been studied. The electrolyte of Mg（AlCl₂EtBu）₂/THF （0.25 M） was prepared from Bu₂Mg, AlCl₂Et and home-made high purity tetrahydrofuran. The magnesium deposition and dissolution on various metal substrates （Ag, Cu, and Ni） were surveyed from this electrolyte. In this electrolyte, magnesium can realize deposition and dissolution on all of these substrates, but with different performances. Better results were obtained using silver substrate with the formation of silver-magnesium alloy, which decreased the over-potential of electrochemical magnesium deposition-dissolution and promoted the cycling efficiency.2. Two kinds of electrolytes containing organic solutions of MgBu₂/THF （0.25 M）, MgBu₂-AlF₃/THF （0.25 M）, together with ionic liquid solution of Mg（CF₃SO₃）₂/BMImBF₄ （0.3 M）, were prepared. The magnesium electrochemical deposition-dissolution performances were studied in these two kinds of electrolytes. Among organic solutions, the lower overpotential was obtained in the MgBu₂-AlF₃/THF electrolyte, while better results were obtained using silver substrate in all of the electrolyte solutions. In the Mg（CF₃SO₃）₂/BMImBF₄ （0.3M） ionic liquid solution, the high overpotential with unstable fluctuation appeared in the initial several cycles. The process with considerably low overpotential could be obtained after several cycles, then the reversible cycles were maintained and residual magnesium was electrochemically dissolved. That is to say, the electrolyte can exert great influence on magnesium deposition performance, which may imply different electrochemical mechanisms in the two types of electrolytes.3. The composite consisting of low cost and environmentally friendly conductive sulfur-containing material and polyaniline （CSM/PAn） demonstrates electrochemical reversibility as cathode material for rechargeable magnesium battery. Comparing with single CSM, the CSM/PAn composite exhibits higher discharge capacity and better charge-discharge cycle ability. The specific discharge capacity can reach to 117.3 mAh g^-1 and the capacity retention calculated on the basis of the 2nd cycle is about 78% after 22 cycles.4. Magnesium manganese silicate (Mg_1.03Mn_0.97SiO₄) containing large polyanion was prepared with diverse processes and different carbon coating technologies as a novel high energy density cathode material for rechargeable magnesium batteries. The electrochemical intercalation/de-intercalation of magnesium from/into Mg_1.03Mn_0.97SiO₄ in 0.25 M Mg（AlCl₂EtBu）₂/THF electrolyte was investigated. It is found that the available Mg-storage capacity of the silicate is strongly dependent on the particle size and the electric conduction of the material. A large capacity of ca. 244 mAh g^-1 at a rate of C/100 is obtained in carbon-coated nano-Mg_1.03Mn_0.97SiO₄ prepared by a modified sol-gel process. Carbon coating enhances the material’s electrical conductivity, suppresses the agglomeration of nano-particles, and thus increases the electric contact between the particles and between the electrode and the electrolyte. Moreover, decrease of the particle size shortens the diffusion path of the Mg²⁺ ions within a particle. Thus, the utilization of the active material is enhanced and the electrochemical polarization is minimized. Combined with the crystal structural characteristic of Mg_1.03Mn_0.97SiO₄, we gave a certain interpretation about the material’s charge and discharge process. The high magnesium storage capacity and the good cyclability of the material indicate that Mg_1.03Mn_0.97SiO₄ is a promising cathode material for rechargeable magnesium batteries.更多还原