Research On The Synthesis And Electrochemical Properties Of Rechargeable Magnesium Battery Materials

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.