【作者】 宋刘斌；

【导师】 李新海；

【作者基本信息】 中南大学 ， 冶金工程， 2013， 博士

【摘要】 锂离子电池以其高能量密度、高电压、无记忆效应、低自放电率等优点已广泛应用于笔记本电脑、手机、数码相机等小型便携式电器和航空航天领域,并逐步走向电动汽车领域。然而,锂离子电池特别是电动汽车用锂电池开发面临的安全性问题有待进一步解决。为了解决电池安全问题,有必要对电池的热效应进行分析。本文采用电化学-量热联用技术系统地研究以LiFePO4和LiMn2O4为正极材料的锂离子电池在不同温度和倍率下充放电过程中的热电化学行为,为电池热管理提供了基础数据,为全面评价电池材料的热、电性能提供了一种新的手段。同时,建立了锂离子电池的电-热耦合模型,应用有限元法预测了电池内部的温度分布。构建了锂离子电池体系中电极材料的晶体结构模型,应用第一性原理预测了电池的平均电压及正、负极材料的热力学性质,对于电池结构设计的优化及安全性能的提高具有非常重要的意义。本文运用热电化学方法和计算机模拟技术分别从宏观和微观角度对锂离子电池及其电极材料的结构和性能等若干问题进行了研究,获得了以下三个方面的研究结果：1.采用八通道等温微量量热仪与蓝电电池测试系统联用技术,测量分别以LiFePO4和LiMn2O4为正极材料的锂离子电池的电学特性、热学特性与温度的关系,进一步开展了正极材料的电、热性能评价。LiFePO4研究结果表明：温度和充放电倍率是影响电池比容量和发热量的重要因素,随着充放电倍率和温度的增加,比容量减小而发热量增大。在低倍率(O.1C.0.2C)下,电池极化较小,可逆性较好,电池的循环产热来自于可逆热和不可逆热共同作用。而在高倍率(0.5C、1.0C)下,不可逆热远远大于可逆热而处于主导地位,且随着温度的升高,放热效应更显著。通过热电化学研究,获得了电池充放电过程中的一系列热力学参数(化学反应焓变△rHm、化学反应熵变△rSm、化学反应吉布斯自由能变△rGm),该热力学参数在低倍率(O.1C和0.2C)下受温度影响较小；而在高倍率(0.5C和1.0C)下,随着温度的升高,△rHm显著增加。在低倍率(0.1C和0.2C)下,与正极材料LiFePO4相比,LiMn2O4的△rSm更小,其可逆性更好,循环性能更优。2.基于热传导理论建立了锂离子电池电-热耦合模型,采用有限元ANSYS模拟了LiFePO4锂离子电池在不同环境温度和充放电倍率下的稳态温度场。同时采用热电偶监测电池内部温度变化,对电池模型进行验证。结果表明：锂离子电池充放电过程中,电池内部的最高温度均出现在负极层与隔膜层之间,即电池内部偏中心位置。在相同充放电倍率条件下,环境温度越高,电池内部最高温度和表面温度之间的温差越大,电池内部温度场分布均匀性越差。在相同环境温度下,充放电倍率越大,电池内部温度场分布的均匀性越差。采用热电偶测量到的电池内部温度值与模型计算结果基本吻合,验证了本电-热耦合模型的可靠性。3.采用第一性原理的超软赝势平面波法,结合广义梯度近似(GGA)的PW91算法,计算了锂离子电池电极材料(LiFePO4. Li)的电子结构、热力学性质及LiFePO4体系的平均电压。结果表明：锂离子电池LiFePO4/Li的平均电压为3.22V,和实验值(3.40V)基本一致。正极材料LiFePO4和负极材料Li的熵S和焓H均随温度升高而增大,而吉布斯自由能G随温度升高而减小,这与热力学规律相符合。本研究获得了锂离子电池正极材料LiFePO4和负极材料Li的微观结构及热力学性质,可为锂离子电池的实际应用提供理论指导。更多还原

【Abstract】 Lithium ion batteries have been widely used in portable appliances such as laptops, cell phones and digital cameras because of their high-energy storage density, high voltage, memoryless effect, low self-discharge rate, etc. When lithium ion batteries are developed from small size used in the portable electronic devices to up-scaling system investigated for the potential applications of aerospace fields and electrified vehicles, safety concerns have come to the public attention. It is necessary to analysis the thermal effects of lithium ion batteries in order to resolve their safety problem. In order to disclose the thermo-electrochemical behaviors of LiFePO4and LiMn2O4lithium ion batteries during charge-discharge process at various ambient temperatures and rates, electrochemical-calorimetric measurements were employed in this study. These experimental results provided basic data for battery thermal management and a new technique for comprehensive evaluation of thermal and electric performance of battery materials. In addition, the electric-heat coupling model of lithium ion battery was established with finite element method. Temperature distribution inside the battery was predicted with the aid of this model. Based on established crystal structure model of electrode materials, average voltage of lithium ion battery and thermodynamic properties of the cathode and anode materials were predicted by using the first principles. It is significant to optimize the battery structure and improve safety performance of battery based on the above results.Here three main aspects of the dissertation have been achieved by thermo-electrochemical method and computer simulation techniques at both macro and micro levels:electronic structures, electrochemical and thermodynamic properties of lithium ion battery and their electrode materials:1. With LiFePO4and LiMn2O4as cathode materials, the relationships among electrical characteristics, thermal characteristics, and temperature of lithium ion batteries was investigated by using an eight-channel micro-calorimeter combined with battery test system. The performances of the cathode materials like thermal and electrical were evaluated further. The research results of LiFePO4show that the specific capacity and the amount of heat were strongly affected by ambient temperature and charge-discharge rate. With the increase of rate and temperature, the specific capacity decreased and the amount of heat increased. At low rate (0.1C,0.2C), the battery had a smaller polarization and a better reversibility, both reversible and irreversible heat contributed to the overall heat production, while irreversible heat dominated the overall heat production when the battery cycled at high rate (0.5C,1.0C). From the comparison of thermal behavior at target temperature (30℃,40℃,50℃), a pronounced exothermic thermal behavior was observed during charge-discharge process in the high current region (0.5C,1.0C) at elevated temperature. Through the study of thermo-electrochemistry, a series of thermodynamic parameters of lithium ion batteries during charge-discharge process, such as△rHm,△rSm and△rGm, were achieved. These thermodynamic parameters were weakly affected by ambient temperature when the battery cycled at low rate (0.1C,0.2C), but at high rate (0.5C,1.0C), enthalpy change of chemical reaction (△rHm) increased significantly with the increase of temperature. At low rate (0.1C,0.2C), compared with LiFePO4cathode material, LiMn2O4had a smaller entropy change of chemical reaction (△rSm), a better reversibility and a better cycle performance.2. The electric-heat coupling model of LiFePO4lithium ion battery was established with theory of thermal conduction. The steady temperature field of lithium ion battery during charge-discharge process at different ambient temperatures and charge-discharge rates was simulated with ANSYS software. Moreover, temperature change inside the battery was monitored by using the thermocouple in order to validate the battery model. The results show that the highest temperature inside the battery appeared between the anode layer and the separator layer. That is, it appeared at partial center position inside the battery. Temperature distribution of lithium ion battery was strongly affected by charge-discharge rate and ambient temperature. With the ambient temperature increasing temperature difference between the highest temperature and surface temperature inside the battery increased. Under the same rate, and uniform temperature distribution decreased. When the ambient temperature was the same, with the rate increasing uniform temperature distribution decreased. Experimental values, which had been achieved by using the thermocouple measurement, were basically anastomosed with the theoretical calculations. The result showed the reliability of this model.3. By using the ultrasoft pseudopotential plane wave method based on the first principles, combining the generalized gradient approximation (GGA) and PW91algorithms, the electronic structures and thermodynamic properties of the electrode materials of lithium ion batteries (LiFePO4and Li) as well as average voltage of battery were calculated. The results show that the calculated average voltage of LiFePO4/Li battery3.22V, is basically agreement with3.40V that was experimentally observed. Entropy S, enthalpy H and Gibbs free energy G of the electrode materials (LiFePO4and Li) of lithium ion batteries were calculated by the phonon spectra state density. With the ambient temperature increasing entropy S and enthalpy H increased, but Gibbs free energy G decreased. This result complied with the thermodynamic law. In a word, the micro calculation provided the theoretical guidance about the practical application of lithium ion batteries.更多还原