ISSN 1674-8484CN 11-5904/U 汽车安全与节能学报, 2011年, 第2卷第1期J Automotive Safety and Energy, 2011, Vol. 2 No. 1Manufacture and Performance Tests of Lithium Iron PhosphateBatteries Used as Electric Vehicle PowerZHANG Guoqing, ZHANG Lei, RAO Zhonghao, LI Yong(Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, ChinaAbstract: Owing to the outstanding electrochemical performance, the LiFePO 4 power batteries could be used on electric vehicles and hybrid electric vehicles. A kind of LiFePO 4 power batteries, Cylindrical 26650, was manufactured fromcommercialized LiFePO 4, graphite and electrolyte. To get batteries with good high-current performance, the optimal content of conductive agent was studied and determined at 8% of mass fraction. The electrochemical properties of the batteries were investigated. The batteries had high discharging voltage platform and capacity even at high discharge current. When discharged at 30 C current, they could give out 91.1% of rated capacity. Moreover, they could be fast charged to 80% of rated capacity in ten minutes. The capacity retention rate after 2 000 cycles at 1 C current was 79.9%. Discharge tests at -20 ℃ and 45 ℃ also showed impressive performance. The battery voltage, resistance and capaci ty varied little after vibration test. Through the safety tests of nail, no in fl ammation or explosion occurred.Key words: hybrid and electric vehicles; power batteries; lithium iron phosphate; lithium ion batteries;电动汽车用磷酸铁锂动力电池的制作及性能测试张国庆、张磊、饶忠浩、李雍( 广东工业大学材料与能源学院,广州 510006, 中国摘要: 磷酸铁锂电池的优异性能使其可以应用在电动汽车和混合动力汽车上。
用市售磷酸铁锂、石墨和电解液制作了圆柱型26650磷酸铁锂动力电池。
为改善电池的大电流性能,研究了正极导电剂的最佳质量分数为8%。
研究了所制备的动力电池的充放电性能。
电池在高倍率下放电仍有较高的电压平台和放电容量。
30 C (96 A放电时,可放出额定容量的91.1%。
电池大电流充电性能较好,5C (16 A充电 10 min 左右,可充入额定容量的80%。
1 C 充放电循环 2 000次,仍能保持额定容量的79.9%。
高低温下电池放电性能良好。
电池经过振动测试,内阻、电压和容量变化很小。
针刺实验中没有发生起火和爆炸,电池温度峰值为 94.7 ℃。
关键词: 混合动力汽车/电动汽车;动力电池;磷酸铁锂; 锂离子电池中图分类号: TQ 152收稿日期/ Received : 2010-12-13基金项目/ Supported by : The Research Cooperation Project of Guangdong Province and the Ministry of Education / 广东省教育部产学研结合项目 (2008B090500013第一作者/ First author : 张国庆(1963-,男(汉,河北,教授。
E-mail: pdzgq008@ 第二作者/ Second author : 张磊,E-mail :rockyzhang2010@IntroductionWith the demand for more power to satisfy the rapidly growingautomotive markets, focus is being directed at the lithium ion batteries, which have energy densities exceeding 130 Wh ·kg -1and cycle life of more than 1 000 cycles. However, compared with traditional markets like laptops and cellular phones, new applications have much higher energy and power requirements. In these applications, where safety is of paramount importance,10/1368 — 7169 ZHANG Guoqing, et al:Manufacture and performance tests of lithium iron phosphate batteries used as electric vehicle powerthe use of LiCoO2 and its derivatives raises serious concerns for developers because of inherent thermal instability. These inherent safety limitations have until now prevented lithium ion batteries from entering the large applications such as electric and hybrid electric vehicles.Comparatively, iron-based olivine phosphate has been the focus of research[1]. LiFePO4 has high theoretical capacity of 170 mAh·g-1 and an average voltage of about 3.5 V vs. Li+/Li. Due to the low cost, environmental benignity, excellent structural stability, long cycling life and high reversible capacity, lithium iron phosphate has been recognized as a promising candidate material for cathode of lithium ion batteries[2]. However,the poor conductivity, resulting from the low electronic conductivity of the LiFePO4, has posed a bottleneck for commercial applications[3]. Therefore, researches of LiFePO4 materials and batteries mainly focus on enhancing their high-current performance[4-5]. In this paper, effect of conductive agent content was studied to get batteries with good high-current performance as well as acceptable capacity sacrifice, and their charge-discharge performance was investigated.1 ExperimentsCylindrical 26650 LiFePO4 power batteries were manufactured. Lithium iron phosphate, or graphite, was mixed togetherwith super P, Polyvinylidene Fluoride (PVDF and N-Methyl Pyrrolidone (NMP in proportion, and then stirred to obtain homogeneous slurry. The slurry was then coated on aluminum or copper foil. After fully dried, the electrode sheet was rolled to appropriate thickness, and then sliced to adequate small size. Positive, negative electrode sheet andseparator were stacked and coiled into battery core. The battery core was put into the battery shell and the positive, negative electrodes were weld with the battery cap and the shell respectively. Electrolyte (1 mol/L LiPF6, EC+DEC+DMC, 1:1:1 was then infused into the battery shell. The battery was then mounted by the battery cap and sealed. At last, the batteries were activated with particular charging-discharging method.To optimize their properties, batteries with different weight ratio of the conductive agent (super P in cathode were manufactured. After the optimization, battery properties such as high-current charging-discharging performance, high and low temperature performance, cycle life, vibration endurability and security, were tested.2 Results2.1 Effect of Conductive Agent ContentTo get batteries with good high-current performance, the optimal content of conductive agent in cathode was studied[6]. Batteries were fabricated in which Super P contents (mass fraction, w were 4%, 6%, 8% and 10% in cathode respectively. (Binder contents were the same as the conductive agent Resistances and capacities of these batteries were shownin Figure 1. It indicated that both the resistances and the capacities of the batteries decreased as the increase of the Super P content. Low resistance could result in good high-current performance, but the capacity is also important. When the mass fraction of Super P is above 8%, the resistance decline is not obvious any more, but the capacity decrease didn’t slow down. To get batteries with good high-current performance as well as acceptable capacity, the mass fraction of the conductive agent was determined at 8%.2.2 High-Current Discharge PerformanceOne cell was charged at a current of 1 C (3.2 A, then discharged at different rates of 0.5, 1, 2, 4, 10, 30 C (1.6, 3.2, 6.4, 12.8, 32, 96 A. The discharge capacities were 3.243,3.168, 3.157., 3.130, 3.115, 2.955 Ah, respectively. Capacities at 1, 2, 4, 10, and 30 C reached 97.6%, 97.2%, 96.4%, 95.9%, and 91.1% of the capacity at 0.5 C. Voltage-capacity curves were shown in Figure 2. Every curve had quite flat platform, and only when approaching the end-voltage of discharge, these curves began to decline. Voltage platform varied from 3.23 V to 2.65 V when discharge rate changed from 0.5 C to 30 C. Both capacity andvoltage performed excellently.Fig. 1 Resistances and Capacities of the Batteries Fig. 2 Voltage-Capacity Curves of Discharge at DifferentCurrents70J Automotive Safety and Energy 2011, Vol. 2 No. 12.3 High-Current Charge PerformanceWhen using fuel vehicles, people are used to the convenience of fast refueling. When electric vehicles took the place, they need to be charged quickly sometimes. This requires electric vehicle batteries could be fast charged at high currents. One fully-discharged cell was charged to 3.65 V with a constant current of 5 C. The voltage-capacity curve was shown in Figure 3. The charge capacity was 2.676 Ah, that ’s 82.0% of the battery ’s 1 C discharge capacity. The process only took 10 min. That means the cells had high-current and fast charge capability.2.4 Discharge Performance at High & LowTemperatureElectric vehicles are used outdoors; the ambient temperature varies from summer to winter. That demands the batteries can work both at high and low temperature. One battery was charged at room temperature, and then discharged at 25, 45 and -20 ℃respectively. When discharged at 45 ℃ and -20 ℃, the battery was placed at that temperature for not less than 6 h. The voltage-capacity curves were shown in Figure 4. Discharge capacities at 25, 45 and -20 ℃ were 3.223, 3.231 and 2.773 Ah, respectively. The discharge capacity at 45 ℃ was a little higher than that at room temperature. The batteries could work at -20 ℃, and discharge capacities only declined by 14.0%.2.5 Cycle LifeLong operational life of electric vehicle batteries is important, because it means less maintenance costs and more competitiveness against fuel vehicles. The cycle life of batteries we made was tested. The charging and discharging currents were both 1 C. As shown in Figure 5, after 2 000 cycles, the battery capacity dropped from 3.257 Ah to2.601 Ah, and capacity fading rate was 20.1%. Average fading rate per cycle was only 0.01%. Hence the batteries had excellent cycle performance and long operational life.2.6 Vibration EndurabilityWhen travelling on road, electric vehicles were in the status of irregular vibration. As the power source for electric vehicles, the batteries must have sufficient vibration endurance. 50batteries were investigated in a simulation vibration test. In the vibration parameters, the constant acceleration is 30 m/s 2; the scan frequency range is 30-35 Hz; the vibration time is 2 h. The resistances, voltages and capacities of the batteries were tested both before and after the vibration. Changes of these properties were shown in Figure 6.As figured in the graphs, the resistance-risings did not exceed 0.4 m Ω; the voltage-droppings were no more than 20 mV; and the capacity retention rates were above 96.8%. After one cycle of discharge and charge, capacities of all batteries recovered to above 98%. Changes of these properties were all in acceptableranges.Fig. 3 Voltage-Capacity Curve of Charge at 5 C CurrentFig. 5 Cycling Curve at 1 C CurrentFig. 4 Voltage-Capacity Curves of Discharge at DifferentTemperature2.7 SecurityConsidering the application on electric vehicles, security of the batteries was of paramount importance [7-8]. Extreme damage to the batteries was simulated by piercing a nail through the battery horizontally. The voltage and temperature were inspected through the process and shown in Figure 7. The voltage of the battery dropped to zero immediately whenthe battery was nailed. Meanwhile, the surface temperature71ZHANG Guoqing, et al: Manufacture and performance tests of lithium iron phosphate batteries used as electric vehicle power Figure 6 Properties Change Through Vibration TestFig. 7Voltage and Temperature Change Curve of Nail Test(a ResistanceChangeof the battery rose to the peak of 94.7 ℃ in a few seconds. Then the flame retardant in electrodes worked to enlarge the resistance of the battery, so the temperature started to decrease. No inflammation or explosion occurred through the whole process, so the security of the batteries is satisfying.3 ConclusionLiFePO4 power batteries are considered to be the most competitive candidate for electric vehicles ’ power source. Increasing content of conductive agent can improve the high-current performance of the batteries but lower the capacity. In our manufacture procedure, mass fraction of 8% of super P brought good high-current performance with acceptable capacity sacrifice. The cylindrical 26650 LiFePO 4 powerbatteries we manufactured could output 91.1% of rated capacity at highest 30 C discharge current, simultaneously had a high voltage platform of 2.65 V, and thereforecould supplied strong power for electric vehicles. They could be fast charged to 80% of rated capacity in ten minutes at 5 C charging current, which saved charging time by far. After 2 000 cycles at discharging current of 1 C, the capacity retention rate was 79.9%; the working life was gratifying. High and low temperature, vibration conditions were common to vehicles, and the simulating tests performed impressively. Even damaged extremely, the batteries did not explode or burn. Due to their extraordinary electrochemical and safety performance, the LiFePO4 power batteries could be used on electric vehicles and hybrid electric vehicles.References[1] Padhi A K, Nanivndaswamy K S, Goodenough J B. Phospho-olivines as positive-electrode materials for rechargeable lithiumbatteries [J]. J Electrochemical Society, 1997, 144 (4: 1188-1194. [2] Padhi A K, Nanivndaswamy K S, Masquelier C, et al. Effect ofstructure on the Fe 3+/Fe 2+ redox couple in iron phosphates [J]. J Electrochemical Society, 1997, 144 (5: 1609-1613.[3] Franger S, Bourbon C, Le Cras F. Optimized lithium ironphosphate for high-rate electrochemical applications [J]. J Electrochemical Society, 2004, 151 (7: A1024-A1027.[4] Amine K, Liu J, Belharouak I. High-temperature storage andcycling of C-LiFePO 4/graphite Li-ion cells [J]. Electrochemistry Communications, 2005, 7 (7: 669-673.[5] Liao X Z, Ma Z F, He, Y S, et al. Electrochemical behavior ofLiFePO 4/C cathode material for rechargeable lithium batteries [J]. J Electrochemical Society, 2005, 152 (10: A1969-A1973.[6] Toprakci O, Toprakci H A K, Ji L W, et al. Fabrication andelectrochemical characteristics of LiFePO 4 powders for lithium-ion batteries [J]. Kona Powder and Particle J, 2010, 28: 50-73.[7] HUA Ning, SUO Liuming, WANG Chenyun, et al. Effectsof different carbon sources on properties of LiFePO 4/C by acarbothermal reduction method [J]. Functional Materials Letters,2010, 3 (3: 155-160.[8] YANG Kai, AN Jinjing, CHEN Shi. Temperature characterizationanalysis of LiFePO 4/C power battery during charging anddischarging. [J]. J Thermal Analysis and Calorimetry, 2010, 99(2: 515-521.(b Voltage Change(b Capacity Retention & Recovery Rate。
