【作者】 马玉林；

【导师】 高云智； 尹鸽平；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2010， 博士

【摘要】 与金属锂电池相比,锂离子电池有较高的安全性,但是在过充、过热、挤压等极端条件下锂离子电池仍然会出现爆炸、起火等现象。目前,车用动力电池的发展对锂离子电池提出了更高的安全要求。由于锂离子电池使用易燃的有机电解液,电解液的热稳定性对电池的安全性至关重要。本文采用自熄时间法（SET）、差示扫描量热法（DSC）、线性扫描（SLV）、循环伏安（CV）及充放电测试等研究了磷酸酯类阻燃剂对电解液性能的影响及其与正负极材料的兼容性,并采用电化学阻抗谱（EIS）对电极的界面性质进行了研究,X射线光电子能谱（XPS）对电极的表面成分进行了分析。研究了磷酸三甲酯（TMP）、二甲基甲基磷酸酯（DMMP）及亚甲基二磷酸四异丙酯（TPPP）三种烷基磷酸酯对锂离子电池常用电解液热稳定性的影响,发现DMMP具有最高的阻燃能力。研究了含有DMMP的电解液与正极材料（LiFePO₄和LiCoO₂）、石墨负极材料的兼容性,结果表明,相对于LiCoO₂,DMMP与LiFePO₄材料具有更好的兼容性;DMMP电解液与石墨负极的兼容性较差,在1mol·L-1 LiPF6 / EC:DEC:EMC （1:1:1,vol）+ 10%DMMP电解液中,石墨电极首次库仑效率仅为41.1%,而在不含DMMP的同种电解液中,石墨的首次库仑效率为86.1%。XPS测试结果表明,DMMP在石墨表面的还原分解是石墨首次库仑效率降低的主要原因。采用密度泛函理论研究了阻燃剂、溶剂、成膜添加剂双乙二酸硼酸锂（LiBOB）和氟代碳酸乙烯酯（FEC）的最低未占据分子轨道（LUMO）及最高占据分子轨道（HOMO）,结果显示,LiBOB和FEC的还原能力比DMMP强,可以在石墨电极表面优先成膜。进而研究了LiBOB、FEC对石墨在1mol·L-1 LiPF6 / EC:DEC:EMC （1:1:1,vol） +10%DMMP电解液中的电化学性能的影响,发现LiBOB和FEC都能够大大提高石墨与该电解液的兼容性,首次不可逆容量损失明显减小,石墨脱锂容量得到明显提高。XPS分析表明,FEC和LiBOB的使用抑制了DMMP在石墨表面的还原分解。研究了亚磷酸三苯酯（TPPi）和磷酸三苯酯（TPP）两种苯基磷酸酯阻燃剂对电解液基本物理性质的影响,结果发现,TPPi的加入能够提高电解液的耐热能力,但是会降低电解液的电导率,含有TPPi的电解液具有较低的氧化电位,只能用于以LiFePO₄为正极材料的电池。进而研究了含TPPi的电解液与正负极材料的兼容性,发现TPPi与LiFePO₄及石墨材料均有较好的兼容性。含有10%TPPi的电解液不仅对LiFePO₄/石墨电池300次循环的循环性能影响较小,而且对LiFePO₄电极的过充电有一定的钳制作用。XPS及SEM分析表明,过充会在电极表面形成较厚的钝化膜。TPP对电解液的电化学窗口影响较小,能够与LiCoO₂材料兼容,6%以下TPP的使用对LiCoO₂/石墨电池250次循环性能的影响较小,而且能够提高LiCoO₂/石墨电池的安全性。采用醇盐水解法制备了Sb₂O₃包覆LiMn2O₄材料, XRD测试结果表明,Sb₂O₃包覆并不影响LiMn2O₄材料本体的结构。对Sb₂O₃包覆LiMn2O₄材料的电化学性能测试表明,Sb₂O₃能够提高LiMn2O₄材料的循环稳定性,但是初始容量有所降低。热稳定性测试表明,Sb₂O₃与TPP同时在LiMn2O₄材料中使用时会有协同阻燃作用,能够进一步提高LiMn2O₄材料的热稳定性。更多还原

【Abstract】 Lithium ion batteries have higher safety than lithium battery. However, lithium ion batteries will ignite or explode under extreme conditions, such as overcharge, excessive heating, crushing and so on. At present, with the development of power battery for hybrid electric vehicle or electric vehicle, the research of high-safety lithium ion batteries is becoming more urgent. Because the frequently used electrolyte of lithium ion battery is flammable, the thermal stability of electrolyte is very important to the safety of batteries. In this dissertation, the safety mechanism and methods of improving battery safety were reviewed. Effect of phosphate flame-retardant on performance of electrolyte was studied by self-extinguishing time method （SET）, differential scanning calorimetry （DSC）, and linear sweep voltammetry （LSV）. The compatibility between phosphate and cathodes and anodes was investigated by cyclic voltammetry （CV）, electrochemical impedance spectroscopy （EIS）, charge-discharge test and X-ray photoelectron spectroscopy （XPS）.The effects of trimethyl phosphate （TMP）, dimethyl methyl phosphate （DMMP） and methylene diphosphate isopropoxide （TPPP） on the thermal stability of electrolyte were investigated and the results showed that DMMP have the highest flame-retardant capability. The compatibility of DMMP-contained electrolyte with cathodes （LiFePO₄, LiCoO₂） and anode （graphite） materials was investigated. The results showed that the LiFePO₄ had better compatibility with DMMP than that of LiCoO₂. On the other hand, DMMP had poor compatibility with graphite. The first coloumbic efficiency was 86.1% in the electrolyte without DMMP, but the efficiency was decreased to 41.1% with addition of 10% DMMP due to the decomposition of DMMP on the graphite surface.The highest unoccupied molecular orbit （HOMO） and lowest occupied molecular orbit （LUMO） of flame-retardants, LiBOB and FEC were investigated by the density functional theory. The results showed that LiBOB and FEC have higher reduction ability than DMMP and can form passivation film preferentially. Furthermore, electrochemical properties of graphite electrode were studied in 1mol·L-1 LiPF6 / EC: DEC: EMC （1:1:1, vol） +10%DMMP with LiBOB or FEC. The compatibility of graphite and DMMP was improved by LiBOB or FEC. The first irreversible capacity loss decreased significantly and the discharge capacity increased obviously. Analysis of the electrodes after passivation film formation revealed that FEC and LiBOB inhibited the reduction of DMMP on the graphite surface.The effect of triphenyl phosphite （TPPi） and triphenyl phosphate（TPP） on the performance of electrolyte was investigated. At the same time, the compatibility of electrolyte containing TPPi or TPP with cathodes and anode materials was researched. These results showed that the electrolyte with TPPi have high thermal stability and low conductivity. TPPi can only be used for battery employed LiFePO₄ as cathode due to the decreased anodic potential. TPPi has excellent compatibility with graphite and LiFePO₄. The cyclic performance of LiFePO₄/graphite cells was not affected by 10%TPPi and the potential of LiFePO₄ electrode was restrained during overcharged. The passivation film was formed on the surface of LiFePO₄ electrode during overcharged. The electrochemical window was not affected by TPP. Charge-discharge performance of LiCoO₂/graphite was still excellent in the electrolyte containing 6% TPP during 250 cycles, while the safety of LiCoO₂/graphite cells was improved by TPP. Sb₂O₃ coated LiMn2O₄ was prepared by alkoxide hydrolysis method. X-ray diffraction exhibited that Sb₂O₃ coating did not affect the structure of LiMn2O₄. The electrochemical test showed that the cyclic performance of LiMn2O₄ was enhanced by Sb₂O₃ but the first discharge capacity decreased. The thermal stability of LiMn2O₄ with Sb₂O₃ and TPP was investigated by means of differential scanning calorimetry. The results showed that the thermal stability of LiMn2O₄ can be elevated by synergistic effect of Sb₂O₃ and TPP.更多还原