Study On Lithium Bis（Oxalato）-borate And FeC₂O₄

In recent years, with the rapid development of science and technology, we face many problems, such as the energy crisis and environmental pollution. Developing low-carbon economy and seeking clean renewable energy have become most important issues in many countries. Also, high energy density, small size and high safety performance are the main directions in the future research of energy storage devices. Lithium-ion battery exhibits much more advantages and causes widespread concerns since it was first commercialized by Sony in1990s.Currently, the commercial electrolyte for lithium-ion batteries is composed of lithium hexafluorophosphate (LiPF6) salt which dissolves in a mixture of ethylene carbonate (EC) and linear esters. However, LiPF6is thermally unstable and it decomposes into undesired products such as LiF and PF5, which can trigger detrimental reactions on the electrode surfaces. In order to improve this situation, new electrolyte systems should be found. LiBOB is a promising lithium salt for substituting LiPF6due to its many advantages, such as superior thermal stability, safety and a more stable SEI. However, its use in commercial is limited by lower conductivity and difficult to purify.The first part of this thesis is to explore a new method to synthetize and purify LiBOB. This method combines the conventional solid-state method and liquid-state method together. It chooses propylene carbonate (PC) as a solvent because of its good thermal stability, less toxicity and low cost. Raw materials H2C2O4·H2O, Li2CO3and H3BO3were first mixed together to carry out reaction in vacuum oven for12h at120℃, then transferred the material into PC for liquid phase reaction. Later, organic solvent dichloromethane was used to extract PC and the target product was obtained. Also further purification was explored. The following step was to prepare electrolytes by using the as-prepared lithium salt. In this thesis, four groups of organic solvent systems were found. The concentration of the solution was0.8mol L-1. The structure of soluble organic components in the liquid samples was characterized by NMR spectroscopy and MS. The electrochemical properties of electrolytes were measured by linear sweep voltammetry, ionic conductivity, charge/discharge and cycling performance.1H,13C and11B NMR spectra were obtained and the data were in accordance with the structure of LiBOB. In addition, the MS data showed that the molecular ion peak was located at187, indicating that the as-prepared, LiBOB, is very pure. Four electrolyte systems were:(a) γ-GBL:DEC=1:1;(b) γ-GBL:DEC:EC=1:1:1;(c) γ-GBL:DEC:EC:DMC=1:1:1:1;(d) y-GBL:DEC:EC:DMC:DMAC=1:1:1:1:1. The results of electrochemical tests showed that the electrochemical windows of the four electrolyte systems were more than4.7V, the ionic conductivity can be up to8.41mS cm-1at room temperature, which was higher than those reported. Above all, charge/discharge and cycling performance of the battery were carried out. Among these groups, the fourth one exhibited the highest ionic conductivity and best electrochemical performance.LiFePO4is one of the most promising cathode materials for lithium-ion battery due to the low cost of the starting materials, the relative low toxicity and so on. However, the problems of this materials are low electronic conductivity and low lithium diffusivity, which have limited its applications in high power device. Recent research has suggested that these problems of conductivity can be overcome by particle optimizing and adding a conductive material. Solid-state reaction is still the main method to synthesis LiFePO4in industry. The morphology and performance of FeC2O4, which can be used as one of the iron sources to prepare LiFePO4, have a big influence on the synthesis of LiFePO4.The second part focuses on the preparation of nano-structured FeC2O4. By controlling the morphology of the precursor, the performance of LiFePO4is anticipated to be improved. In this thesis, the morphology of FeC2O4was analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that solvent used in the synthesis process determined the morphology of FeC2O4and compared to the aqueous system, organic solvent system can be used to obtain product with higher crystallinity and smaller particle size. Also during the reaction, the temperature should be controlled below0℃to decrease the reaction rate and avoid the oxidation of ferrous ions.