| 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. |
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