磷酸铁锂电池用高低温电解液的研究

【作者】 付茂华；

【导师】 黄可龙；

【作者基本信息】 中南大学 ， 应用化学， 2010， 硕士

【摘要】 磷酸铁锂(LiFePO4)是动力电池最具发展前景的正极材料之一,研究其配套电解液也具有现实的意义。本文采用恒电流充放电、循环伏安、交流阻抗、等离子发射光谱(ICP)、扫描电镜(SEM)和能量散射(EDS)等方法,研究了电解液的锂盐、溶剂和添加剂对电极界面及电池性能方面的影响。首先研究了新型锂盐二氟二草酸硼酸锂(LiODFB)对磷酸铁锂／石墨电池高低温性能的影响。结果表明,使用LiODFB作为锂盐加入到电解液中对LiFePO4/Li电池没有负面影响,LiODFB基电解液能有效地抑制LiFePO4在高温条件下析出铁离子。LiODFB基电解液中在石墨负极表面形成的SEI膜更致密、更稳定。根据EIS结果可知,一方面,在LiODFB基电解液中形成的SEI膜热稳定性更好；另一方面,LiODFB基电解液能抑制铁离子在负极上还原,有利于降低SEI膜阻抗,因此能显著提高LiFePO4/石墨电池的高温循环性能。另外可能是由于二氟二草酸硼酸锂具有较低的电荷转移阻抗,将其作为添加剂加入到电解液中,电池具有良好的低温性能,其反应原理有待进一步研究。其次研究了二元体系和新型溶剂丙酸乙酯(EP)对磷酸铁锂／石墨电池性能的影响。结果表明,在三种二元体系(EC+DMC、EC+DEC、EC+EMC)中,EC+DEC体系的高温性能较对较好,EC+EMC体系的低温性能相对较好。在EC+EMC二元溶剂中,丙酸乙酯的加入能显著提高电池的低温性能,但加入太多会对电池的高温性能产生一定的负面影响。最后还分别考察了添加剂亚硫酸丙烯酯(PS)、氟代碳酸乙烯酯(FEC)对磷酸铁锂／石墨电池性能的影响。结果表明,在电解液中加入PS或FEC后,石墨负极表面能形成平滑致密的SEI膜,提高电池的高温性能。加入FEC后,能提高磷酸铁锂电池的低温放电平台,提高低温下的电池放电容量。更多还原

【Abstract】 LiFePO4 is one of the most promising cathode materials for dynamical battery, and the study of supporting electrolyte for LiFePO4 battery also has practical significance. In this paper, galvanostatic charge-discharge, cyclic voltammetry (CV), electrochemical impedance spectros-copy (EIS), inductively coupled plasma (ICP), energy dispersive spectroscopy (EDS) were used to study the performance of batteries with different lithium salt, solvents, and additives.First of all, we have studied the effect of a new lithium salt, which was called lithium difluoro(oxalato)borate (LiODFB), on the performance of LiFePO4/graphite batteries at high and low temperature. Results showed that, LiODFB used in electrolyte as a lithium salt had no negative effect in LiFePO4/Li cells, and the LiODFB-based electrolyte could restrain iron dissolution from LiFePO4. According to the EIS results, on the one hand, the SEI film formed in LiODFB-based electrolyte had better thermal stability; on the other hand, LiODFB-based electrolyte could restrain iron dissolution from LiFePO4 and prevent the reduction of dissolved iron ions’reducing at anode’s surface which decreased the impedance effectively, thereby improve the cycling performance of LiFePO4/graphite batteries at high temperature. LiODFB has a lower charge-transfer resistance, which may be the reason for the good property of battery at low-temperature when LiODFB is added, and the mechanism remains to be studied further.Secondly, the effects of bi-solvent、a new solvent (ethyl propionate) on the performance of LiFePO4/graphite were tested. Compared with other bi-solvent systems, the battery with ethylene carbonate(EC) and diethyl carbonate(DEC) solvent system has the best cycling performance at high temperature, and the battery with EC and ethyl methyl carbonate(EMC) solvent system has the better low temperature discharge property. In the EC+EMC bi-solvent electrolyte, adding some ethyl propionate could increase the battery’s low temperature performance, but adding too much ethyl propionate would have some negative impact on the battery’s high temperature performance.Finally, the performances of LiFePO4/graphite batteries with two different additives, propane soltone (PS) and fluoroethylene carbonate (FEC), were investigated respectively. The results showed that a smoother and compacter SEI film structure generated by the introduction of the additive PS or FEC to the electrolyte, which could improve the battery’s high temperature performance. The results revealed that the flat of battery increased after adding FEC to the electrolyte, and the presence of FEC improved the low temperature discharge property of the battery.更多还原