| Rechargeable magnesium batteries may be a potential candidate for heavy load application due to high energy density, low cost, safe to handle and environmentally friendly nature of metallic magnesium. It's essential for rechargeable magnesium batteries to develop cathode materials with larger specific capacity and higher discharge plateau. In this dissertation, based on our previous research findings, we designed and prepared MgCoSiO4 as a new kind of cathode material for rechargeable magnesium batteries and researched its electrochemical behaviors during intercalation and de-intercalation process. The main efforts of this thesis were focused on solving two key issues in the charge-discharge process-Mg2+ diffusion and charge transfer.(1) Pure MgCoSiO4 materials were synthesized by high-temperature solid-state reaction, molten salt method and mixed solvothermal approach, which were named HSS-MgCoSiO4, MS-MgCoSiO4 and ST-MgCoSiO4, respectively. It was found that the preparation process had a profound effect on the morphology, particle size and specific surface area of the materials, and mesoporous MgCoSiO4 was obtained for the first time without templates by a non-surfactant mixed solvothermal approach. Mesoporous MgCoSiO4 exhibited the best electrochemical performance with a discharge capacity of 167.0 mAh·g-1 (ca. 54.6% of the theoretical capacity) and a high voltage plateau of 1.65 V vs. Mg, which were generated from the unique advantages of mesoporous structure. With the existing pores, a relatively large surface increased the contact area at the electrode/electrolyte interface and was beneficial for facile charge-transfer reactions at the interface to improve specific capacity. Moreover, the well-defined channels could reduce ion and electron diffusion distances within particles hence increasing rate capability. Furthermore, the buffer effect of mesoporous structure diminished the structural changes of material during charge-discharge process then provided outstanding cyclical stability.(2) Modification experiments were conducted to improve the electrical conductivity of the cathode materials. Various means such as ex-situ carbon blending, in-situ carbon coating and CNTs decoration by CVD method were attempted using different kinds of carbon sources. In-situ carbon coating approaches using sucrose and phenolic resin were not successful for preparing desired products. The mixture of MS-MgCoSiO4 and super-P with a high carbon mass ratio of 20% displayed smaller discharge capacity or lower voltage plateau compared to the untreated MS-MgCoSiO4. However, the ex-situ carbon blending MS-MgCoSiO4 with MCNTs markedly increased the discharge plateau to 1.70 V vs. Mg. It was considered that MCNTs constructed a well-connected conductive network among activated particles after heat treatment, which was favorable to reduce polarization. The trace amount of CoO impurity in N-ST-MgCoSiO4 (using nano-sized SiO2 as silicon source) was reduced into Co using CVD method, which could catalyze the growth of CNTs to decorate MgCoSiO4 particles. The special net-like structure formed by direct growth of CNTs made the infiltration of electrolyte into active materials more easily and speeded up the diffusion of ions and electrons, and hence shortened the activation period. The obtained N-ST-MgCoSiO4/CNTs showed a stable discharge capacity of 214.8 mAh·g-1 (ca. 70.3% of the theoretical capacity) with the increase of capacity in lower discharge plateau. |
