锂离子电池正极材料的制备改性及表征

【作者】 徐化云；

【导师】 陈春华；

【作者基本信息】 中国科学技术大学 ， 材料学， 2007， 博士

【摘要】 现代社会对能源的需求，大大促进了锂离子电池的发展，这也使锂离子电池电极材料的研究成为现在材料研究的热点。锂离子电池的关键材料之一是正极材料，正是因为正极材料的许多问题，限制了锂离子电池的一些应用。为了使锂离子电池的应用范围更广泛、更容易朝大型化发展，正极材料的性能提高十分必要。另外，锂离子电池的安全性能一直是人们关注的焦点，阻燃添加剂的加入可以使易燃有机电解液变成难燃或不可燃的电解液，同时也增加电解液自身的热稳定性，避免电池在过热条件下的燃烧或爆炸。本论文一方面探索了一些高性能锂离子电池的电化学性能及机理，另一方面研究了电解液体系中阻燃剂的加入对锂离子电池的热稳定性的影响。在论文第一章中，作者综述了以下内容：介绍了锂离子电池的发展概况、锂离子电池的工作原理、锂离子电池常用的电极材料、影响正极材料性能的因素、常用的正极材料制备方法以及影响锂离子电池安全性的因素。在第二章中，主要介绍本论文中的实验方法和仪器，详细介绍了实验用的扣式电池的制备过程，以及常用的电化学和结构测试手段。第三章的研究主要针对目前对商品LiCoO₂正极的掺杂改性。作者运用燃烧法成功制得了LiCo_?Zr_（?）2Mg_x/2O₂（0≤x≤0.2）粉末，发现Zr+Mg共掺杂的最佳掺杂量为x=0.06。Zr-Mg掺杂样品的比容量可达138mAh／g和3.6V平台效率都有所提高。由于尖晶石LiNi_0.5Mn_1.5O₄在约4.7V工作电压下有较理想的充放电行为，因此在第四章中作者用辐照凝胶聚合法成功地合成了LiNi_0.5Mn_1.5O₄，研究了粉末材料的电化学性能与烧结温度的关系，950℃烧结的样品显示了139mAh／g的初始容量及在50个循环内96％的容量保持率。采用缓慢降温的措施提高了LiNi_0.5Mn_1.5O₄在4.7V平台的容量。此外，作者还发现“w”型的直流阻抗曲线对LiNi_0.5Mn_1.5O₄电极材料的充放电有着敏感的指示作用。由于安全性问题限制了锂离子电池在大型动力电池例如电动汽车上的应用，所以第五章对锂离子电池的热稳定性进行了探索，研究了TMP（i）在目前商品化的LiCoO₂／Li体系中对电池电化学性能的及热稳定性的影响。作者通过对循环过的处于充电状态的正极表面的非原位FTIR测试，对Li_0.5CoO₂正极表面化学成份进行了了分析。结果表明，5wt.％TMP（i）作为1M LiPF₆+EC／DEC（1:1，重量比）电解液体系的添加剂大大提高了锂离子电池的比容量、循环性能和3.6V平台效率等电化学性能。此外，TMP（i）添加剂的使用明显影响了正极与电解液之间的表面膜的化学成份。发现经过相同循环次数、加TMP（i）添加剂的表面膜有新成份Li₃PO₄生成。同时，其表面膜阻抗和电荷转移阻抗都小于参比样品。用C80测试了满充的Li_0.5CoO₂正极和电解液混合的热稳定性表明，加TMP（i）添加剂的电解液与Li_0.5CoO₂的热稳定性有明显的提高。在第六章作者用辐照聚合凝胶法制得4V正极材料Li[Li_0.16Ni_0.21Mn_0.63]O₂（LNM）和Li[Li_0.2Ni_0.2Mn_0.6]O₂，其中Li／Li[Li_0.16Ni_0.21Mn_0.63]O₂表现出优异的电化学性能，1C放电倍率下比容量达到158 mAhg^-1，相对较低的Ni的价态在增加锂离子的扩散速率、增强电子电导率及抑制表面膜的阻抗等方面起到重要作用。这种锂离子不足的LNM正极材料对于二次锂离子电池的发展有着很大的潜力。最后，第七章对本论文的创新和不足作了简要总结，并对今后可能的研究方向提出了建议。更多还原

【Abstract】 Our ceaseless demands for energy sources have greatly promoted the development of the lithium-ion batteries. This makes the electrode materials become very attractive in the field of materials research. The most important materials for the lithium-ion batteries are the cathode materials. However, some problems of the cathode materials such as safety and cost have hindered the applications of the lithium-ion batteries. Therefore, researches aiming at improving the electrochemical properties of various cathode materials are desirable and very necessary. In addition, safety concerns of lithium ion batteries have been the key problems in their practical applications. One of the effective strategies to address the safety issue is to reduce the electrolyte flammability and in the ideal situation to use a nonflammable electrolyte to avoid explosions of cells.In Chapter 1. a general introduction is given on following aspects: the development and status of the lithium-ion batteries, their working principle, the common cathode materials, the factors that affect the electrochemical properties of cathode materials and some preparation methods.In Chapter 2. we mainly introduce the experimental processes and equipments used in the project of this thesis. A detailed description on the process to making a coin cell is presented. The electrochemical and structural analysis methods are also included.As an example of doped electrode. LiCo_1-xZr_x/2Mg_x/2O₂ （0≤x≤0.2） powders are prepared by a solution-combustion route in Chapter 3. The maximum doping level is between 0.06 and 0.1. The optimal electrochemical performance of this doped UCoO₂ system is obtained when x = 0.06. Both specific capacity and the stability of 3.6V plateau efficiency are improved by the combined Zr-Mg doping.In Chapter 4. a 4.7V class cathode material. LiNi_0.5Mn_1.5O₄ powders have been prepared with radiated polymer gel method. The electrochemical properties of these powders are closely related to the calcination temperature. The sample calcined at 950℃ shows the best electrochemical performance with an initial capacity of 139 mAhg^-1 and 96% capacity retention after 50 cycles. Adopting a slow cooling procedure during the powder calcinations can increase the capacity of LiNi_0.5Mn_1.5O₄ at the 4.7V plateau. In addition, we have found that the approach of a simultaneous DC resistance measurement during the galvanostatic cell cycling can be used as a sensitive probe to the structural change of the electrodes.As safety concerns have limited the full utilization of Li-ion batteries in EV and other high power fields, we also attempt to reduce the flammability of the electrolytes by incorporating a flame-retardant. i.e. trimethyl phosphite （TMP（i））. as the additive in Chapter 5. The results show that 5wt% TMP（i） as an additive to the electrolyte 1M LiPF₆+ EC/DEC （1:1. w/w） has improved the electrochemical performance including cycle performance and the stability of 3.6V plateau efficiency of LiCoO₂ electrodes. In addition, the use of TMP（i） additive can obviously influence the composition of surface layer formed on LiCoO₂ during cycling. In addition, the thermal stability of the electrolyte with 5wt% TMP（i） additive and the thermal stability of the interphase between the cathode and electrolyte have been obviously improved.We also study the 4V-cathode material Li[Li_0.2Ni_0.2Mn_0.6]O₂ （LNM） and Li[Li_0.16Ni_0.21Mn_0.63]O₂ powders prepared with radiated polymer gel method in Chapter 6. Li[Li_0.16Ni_0.21Mn_0.63]O₂ shows excellent electrochemical performance with a high reversible 1C-rate capacity of 158 mAhg^-1 in the voltage range of 2.0 to 4.7V. The presence of lower Ni valence in Li[Li_0.16Ni_0.21Mn_0.63]O₂ is believed to play an important role in facilitating the lithium diffusivity. enhancing the electronic conductivity, and suppressing the surface layer resistance. This lithium-scant LNM is very promising as a potential cathode material for lithium secondary batteries.Finally, in Chapter 7 the author gives an overview on the achievements and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.更多还原