| With the rapid development of social economy, the serious depletion of traditional energy sources had been drawn great attention in the world. Transportation industries, as a guzzler consumer, had great potential for energy saving and environmental protection. Therefore, electric vehicles which were based on the battery as power source generate accordingly. However, overheating and overcharging took place frequently that could lead to the battery performance severe recession, or even thermal failure of security risks. In this paper, the cylindrical 18650-size Li4Ti5O12/LiMn2O4 battery was used as a study object. The influence of high temperature on the battery performance was studied by overcharged test, and then the composite phase change materials (PCMs) were used for battery thermal management to improve and enhance the comprehensive performance of the battery. The experimental results were as follow:(1) The relationship between performance and temperature of the overcharged battery which the maximum temperature reached 87℃ was investigated. It could be known that electrochemical properties of 18650-size Li4Ti5O12/LiMn2O4 battery declined extremely after charge and discharge test. The internal resistance of the battery increased and the specific capacity decreased obviously. The higher the current density was, the more obviously the capacity decreased. For instance, the overcharged battery had almost no capacity when the current density increased to 5C. At the same time, the overcharged battery presented a much more apparent thermal runaway trend compared to the normal battery. Under the high temperature environment, the highest temperature and the heating rise of overcharged battery reached as high as 57.46℃ and 19.11℃ at 3C, respectively, which has exceeded the reasonable working temperature. These phenomena could be attributed to the following two reasons:1) the decomposition of the electrolyte arisen from the overcharge process resulted in increased internal resistance; 2) the thermal runaway due to the increased internal resistance resulted in the damage to crystal structure/nanostructure and aggregation of the electrode materials, thus leading to the secondary decrease in capacity.(2) According to the latent heat and phase change temperature of the paraffin, the paraffin, graphite and activated carbon in four different proportions (20:5:0,20:4:1, 20:3:2,20:2:3) were prepared into paraffin/graphite/carbon composite phase change material plate. The DSC and the thermal conductivity test results showed that, when the mass fraction of activated carbon was 4%, the latent heat and the thermal conductivity of composite PCM plate was 155.2J·g-1 and 5.591W·m-1·K-1, respectively. Compared with the paraffin/graphite, the latent heat of the paraffin/graphite/carbon was increased by 13.2% and the final cost was decreased about 8.7%.(3) The mass ratio of the paraffin/graphite/carbon was 20:4:1 which was used in 18650-size Li4Ti5O12/LiMn204 battery thermal management. The cycle tests of Li4Ti5O12/LiMn2O4 battery by using PCM and air cooling respectively showed that, the electrochemical performance of air cooling battery declined obviously. With the increased of the number of cycles, this phenomenon was more obvious. The maximum temperature of PCM cooling battery was still less than 37℃ after 30 cycles, and the temperature rise was very small. However, the maximum temperature of air cooling battery had reached 49.38℃ and the temperature risen significantly. The SEM test results showed that, the nanostructure and crystal structure of electrode materials of PCM cooling battery maintained well than the air cooling battery. |
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