(1) Technical performance parameters
The main technical performance parameters of a typical lithium iron phosphate (LiFePO4) battery for EV and PHEV made by a company are shown in Figure 1.
(2) Charging and discharging characteristics of lithium-ion batteries
1) Discharge characteristics under different discharge rates. The discharge characteristics of a 55Ah lithium iron phosphate (LiFePO4) battery at different discharge rates are shown in Figure 2. The minimum discharge rate is 0.5C, the maximum discharge rate is 10C, and 5 different discharge rates form a set of discharge curves. It can be seen from Figure 2 that no matter what kind of discharge rate, the voltage during the discharge process is very flat (that is, the discharge voltage is stable and basically remains unchanged), and only when the discharge voltage is almost terminated, the curve bends downward ( The downward bending occurs only after the discharge volume reaches 40Ah). In the discharge rate range of 0.5~10C, the output voltage mostly changes in the range of 2.7~3.2V. This shows that the battery has good discharge characteristics.
2) Discharge characteristics at different temperatures. The discharge characteristics of 55Ah lithium iron phosphate (LiFePO4) batteries at different temperatures are shown in Figure 3, and the discharge curves under different temperature conditions (from -20 to 40°C) are shown in Figure 3. If the discharge capacity is 100% at 23°C, the discharge capacity at 0°C will drop to 78%, and it will drop to 65% at 20°C, and the discharge capacity will be slightly greater than 100% when discharged at 40°C. It can be seen from Figure 3 that the lithium iron phosphate battery can work at -20°C, but the output energy should be reduced by about 35%.
3) Charging and discharging cycle life characteristics. The 55Ah lithium iron phosphate (LiFePO4) battery charge-discharge cycle life curve is shown in Figure 4. The conditions of the charge-discharge cycle are: charge at 1C charge rate, discharge at 2C discharge rate, and go through 570 charge-discharge cycles. It can be seen from the characteristic curve of Fig. 4 that after 2800 charge-discharge cycles, the discharge capacity has not changed, indicating that the battery has a very long life.
(3) Features of lithium iron phosphate battery
Lithium iron phosphate batteries have the following advantages of lithium-ion batteries:
1) Large specific capacity. If lithium-intercalated carbon materials are used as the negative electrode instead of lithium metal, the mass specific capacity of the negative electrode material will decrease. In the metal lithium secondary battery, in order to ensure a certain cycle life of the battery, the metal lithium usually needs to exceed 3 times of excess. Therefore, the actual decrease in mass specific capacity is not very large, but from the perspective of volume specific capacity, there is almost no decrease. Therefore, the lithium ion battery still maintains the characteristic of large specific capacity of the metal lithium secondary battery. Its energy density is 120Wh/kg, which is 6 times that of lead-acid batteries, 2.5 times that of Ni-Cd batteries and 1.8 times that of Ni-MH batteries.
2) The battery working voltage is high. Using lithium intercalation compound instead of lithium metal as the battery negative electrode will reduce the battery voltage, but the secondary lithium ion battery uses graphite or petroleum coke as the battery negative electrode, because their lithium intercalation potential is low (0.01~1.5 V vsLi/Li+), the voltage loss can be reduced to a minimum. Choosing a suitable lithium intercalation compound as the positive electrode of the battery can make the secondary lithium ion battery have a higher working voltage. At present, the working voltage of lithium-ion secondary batteries is generally 3.0~4.0V, while lead-acid batteries are 2.0V, Ni-Cd batteries are 1.2V, and Ni-MH batteries are 1.26V.
3) Good safety performance and long cycle life. Lithium ion secondary batteries do not contain metal lithium, only lithium intercalation compounds exist, especially from the perspective of kinetics, lithium compounds are much more stable than metal lithium; on the other hand, lithium ion secondary batteries are The ions are inserted into the lattice of the negative electrode intercalation compound material, instead of being deposited on the surface of the metal lithium negative electrode like the metal lithium secondary battery, so that the formation of lithium dendrites can be avoided, so that the safety of the lithium ion secondary battery is significantly improved. The cycle life is also greatly improved.
4) The self-discharge rate is small. Lithium ion secondary batteries will form a solid electrolyte mesophase (SEI) film on the surface of the carbon negative electrode during the first charging process, which allows ions to pass but not electrons, so it can better prevent self-discharge and make the battery self-discharge. The discharge rate is greatly reduced, much smaller than the self-discharge rate of Ni-MH batteries, and much smaller than that of lead-acid batteries.
5) Clean and pollution-free. Lithium-ion secondary batteries do not contain toxic substances such as lead, cadmium, mercury, etc. At the same time, the battery is well sealed, and very little gas is released during use. It is a non-toxic and non-polluting battery system.