Studies On The Manufacture And Performance Of C-LiFePO₄ Power Battery For Electric Vehicles

Lithium-ion power battery has a broad application prospect in electric vehicles where battery life, thermal behavior and safety are key factors. The lithium iron phosphate power battery with long life, high thermal stability, good safety and low cost, is considered one of the most promising lithium-ion power batteries for electric vehicles. This dissertation is targeted at the preparation of the LiFePO4 battery with long life and high safety for electric vehicles. Firstly, the LiFePO4 batteries with different structures and capacities were prepared, and their performances of charge-discharge and safety were tested to study the influence of battery structure on battery performance. Secondly, the cycle performance of battery were analyzed and their capacity fading reasons were tested by the analysis methods of electrochemical, XRD, SEM and EDS. Thirdly, the battery thermal behaviors were simulated using the principle of energy conservation. And finally, the influence of charge-discharge methods on the properties of the battery module has been studied.Using lithium iron phosphate as cathode material, graphite as anode material, the five types of high specific energy Lithium-ion power battery for electric vehicles with cylindrical winding structure for 13Ah,100Ah and 120Ah batteries, square winding structure for 9Ah battery, square laminated structure for 300Ah battery were developed. In the five batteries, the 9Ah and 120Ah battery has been produced in mass production and small quantities, respectively. The performances of initial charge-discharge and large current discharge were studied. The results showed that:the battery capacity depends on the characteristics of the positive and negative materials and the ratio of the design capacity of anode/cathode material when the discharge current is low. The change of discharge plateau voltage was related to the structure of battery, and the difference of charge plateau and discharge plateau of the cylindrical battery is less than that of the square battery. With the discharge rate increasing, the battery discharge capacity and discharge voltage are related to its marked capacity and structure. When the marked capacity of battery was close, the discharge capacities with the square 9Ah battery and the cylindrical 13 Ah battery were not different significantly, but the performance of rate discharge with 13 Ah battery was better than that with 9Ah battery. In the case of same cell structure and different marked capacity, the rate of capacity fading and the decrease of discharge plateau voltage of 120Ah battery are higher than those of 13 Ah battery. For the large capacity battery, the influence of ohmic voltage drop on the discharge plateau voltage is more obvious. The battery's specific energy, high and low temperature performance, storage performance and safety were tested. The result showed that:the specific energy was between 100-120Wh/kg; for the 13Ah battery, the discharge capacities in 55℃and-20℃were 98.9% and 47.9% of that in room temperature, respectively. The charge retention rate was 71% at room temperature for one year and the capacity recovery rate was 92%. The 120Ah and 13Ah batteries neither explode nor catch fire in the safety test. The Lithium-ion power battery with lithium iron phosphate as cathode material, graphite as anode material has a good combination performance.The cycle performances of five types of lithium iron phosphate power battery were studied. The test results showed that:the fading of battery characteristics (capacity, discharge voltage, etc.) have the same trends which can be divided into two stages. In the first stage, the performances fade slowly, showing a characteristic of the passive film growth in the negative electrode. In the second stage, the performances fade fast, showing the characteristic of lithium deposition in the surface of negative electrode. The study on structures and performances of 13Ah batteries with no cycling and the 1785th cycle showed that:the rate of capacity retention of positive electrode is 85% after cycling which is less than that of battery. The positive electrode after cycling also maintain the integrity of the olivine structure, and its surface morphology are not significantly changed with before cycling; the rate of capacity fade of graphite anode was faster than that of LiFePO4 cathode. In the end of cycling, the capacity of negative electrode lost seriously, and in some place, the lithium was desposited, its surface morphology is significantly changed. The changes of capacity and voltage in the second stage have a great relationship with negative electrode.The analysis on the surface temperature of five categories of lithium iron phosphate battery during discharge showed that:the surface temperature changes faster and the temperature gradually increased when the discharge current increases. The relationship of the surface temperature rising and the discharge current is parabolic at the same discharge time, but the surface temperature rising at the end of discharge is a liner relationship with the discharge current. The analysis on the thermal behavior of 13Ah battery showed that: the total heat generation at 0.3C,1C,2C and 3C rate is about 0.47W,2.04W,6.04W and 11.85W and the average value of irreversible resistive heating at 0.3C,1C,2C and 3C rate is about 0.4W,2.0W,6.0W and 11.8W, respectively. Inside the battery, the irreversible resistive heating is major heat generation source, and the proportion of reversible entropic heat is very small. At low discharge current, the surface temperature drop was dominated by reversible entropic heat. The temperature simulation results of 300Ah,120Ah,13Ah and 9Ah lithium iron phosphate power battery show that:the specific heat capacity of 300Ah,120Ah,13Ah and 9Ah battery were 2.7 JK-1g-1,1.8 JK-1g-1,1.8JK-1g-1 and 1.5 JK-1g-1, respectively. Battery components, battery case material and other will exert some influences for the specific heat capacity of the battery. The total heat capacity of the battery, the battery and the external heat exchange coefficient has a greater impact on the trend of the surface temperature of battery.Eventually, the influences of the charge and discharge mode on the performances of 13 Ah battery module (four series) were analyzed. If the upper and the lower voltage of the series battery module equal the single battery charge and discharge cut-off voltage multiply number the battery number, respectively, the over-charge and over-discharge could easily occur in the end of charge and discharge, The low capacity battery reached firstly cut-off voltage during the charge process. The chance that the large capacity battery reached cut-off voltage during the discharge process is greater. When the current is large enough, the battery of larger resistance may be first to reach cut-off voltage. In the voltage range of 11.6-14.2V, the capacity of battery pack is 85% of marked capacity with constant current charge and discharge. After 900 cycles with 1C at room temperature, the capacity retention rate of 13Ah battery module capacity is 85%. During cycling, the over-charge and over-discharge didn't occur. Before the 900 cycles, the difference of battery voltage in the end of charge unchanged. In the end of discharge, the difference of battery voltage increased with cycles. The smaller capacity battery voltage increased, and the larger capacity battery voltage is decreased. In short-circuit and compression test, the battery pack is not burning, no explosion, the battery pack has a good anti-abuse properties.