Generally, the electrolyte of lithium-ion batteries is liquid, but later the development of solid and gel-type polymer electrolyte, lithium-ion batteries are called lithium polymer batteries, whose performance is better than that of the lithium-ion batteries with liquid electrolyte.
The full name of the lithium iron phosphate battery should be lithium iron phosphate lithium-ion batteries, the name is too long, referred to as lithium iron phosphate batteries. Because of its performance is particularly suitable for power applications, it is added to the name of the "power" two words, that is, lithium iron phosphate power battery. Some people also call it "lithium iron (LiFe) power battery".
The significance of using LiFePO4 material as anode
Currently, the anode materials used in lithium-ion batteries include LiCoO2, LiMn2O4, LiNiO2 and LiFePO4. Among the metal elements that make up the anode materials of the batteries, cobalt is the most expensive and has little storage, while nickel and manganese are the cheaper ones and iron is the cheapest. Fe) is the cheapest. The price of cathode materials is also in line with the price quotations of these metals. Therefore, a lithium-ion battery made with LiFePO4 cathode material should be the cheapest. Another feature is that it is non-polluting to the environment.
The requirements for rechargeable batteries are: high capacity, high output voltage, good charging and discharging cycle performance, stable output voltage, high-current charging and discharging, electrochemical stability, safety in use (will not cause combustion or explosion due to improper operation of overcharging, over-discharging, and short-circuiting, etc.), a wide range of operating temperatures, non-toxic or less toxic, and non-polluting to the environment. LiFePO4 as anode lithium iron phosphate battery is good in these performance requirements, especially in the large discharge rate discharge (5 ~ 10C discharge), discharge voltage smooth, safety (no combustion, no explosion), life (cycle times), no pollution of the environment, it is the best, is the best high-current output of power batteries.LiFePO4 Battery Structure and Working Principle
The internal structure of LiFePO4 battery is shown in Figure 1. On the left is the olivine structure of LiFePO4 as the positive electrode of the battery, which is connected to the positive electrode of the battery by an aluminum foil, and in the middle is a polymer diaphragm, which separates the positive electrode from the negative electrode but the lithium ion Li+ can pass through it while the electrons e- can't pass through it, and on the right side is the negative electrode of the battery, which is composed of carbon (graphite), which is connected to the negative electrode of the battery by a copper foil. Between the upper and lower ends of the battery is the electrolyte of the battery, which is hermetically sealed by a metal shell. Figure 1 Internal structure of LiFePO4 battery
When charging a LiFePO4 battery, the lithium ion Li+ in the positive electrode migrates to the negative electrode through the polymer diaphragm; during discharge, the lithium ion Li+ in the negative electrode migrates to the positive electrode through the diaphragm. The lithium-ion battery is named for the lithium ion migration back and forth during charging and discharging.LiFePO4 Battery Main Performance
The nominal voltage of LiFePO4 battery is 3.2 V, the termination charging voltage is 3.6 V, and the termination discharging voltage is 2.0 V. Due to the difference in the quality of the positive and negative electrode materials, electrolyte materials, and the process adopted by each manufacturer, there are some differences in their performance. For example, the capacity of the same model (standard battery in the same package) has a big difference (10% to 20%).
The main performance of lithium iron phosphate power battery is listed in Table 1, in order to compare with other rechargeable batteries, other types of rechargeable battery performance is also listed in the table. It should be noted that different factories produce lithium iron phosphate power battery in the performance parameters will have some differences; in addition, there are some battery performance is not included, such as battery internal resistance, self-discharge rate, charging and discharging temperature.
The capacity of lithium iron phosphate power batteries varies greatly, and can be divided into three categories: small ones with zero to a few milliampere hours, medium ones with tens of milliampere hours, and large ones with hundreds of milliampere hours. There are also some differences in the similar parameters of different types of batteries. Here is another introduction to the parameters of a small standard cylindrical package of lithium iron phosphate power battery, which is widely used at present. Its outer dimensions: diameter of 18mm, height of 650mm (model 18650), and its parameter performance is shown in Table 2. Typical Discharge Characteristics and Lifetime
The discharge characteristics of a lithium iron phosphate power battery model STL18650 (capacity 1100mAh) at different discharge rates are shown in Figure 2. The minimum discharge rate is 0.5C, the maximum discharge rate is 10C, and five different discharge rates form a set of discharge curves. As can be seen in Figure 2, regardless of which discharge rate, the voltage is very flat during the discharge process (i.e., the discharge voltage is smooth and basically constant), and the curve bends downward only when it reaches the termination of the discharge voltage (downward bending occurs only after the discharge volume reaches 800mAh). In the discharge rate range of 0.5 to 10C, the output voltage varies mostly in the range of 2.7 to 3.2V. This indicates that the battery has good discharge characteristics. Figure 2 Discharge characteristics of STL18650
The discharge curve of STL18650 with a capacity of 1000mAh under different temperature conditions (from -20 to +40C) is shown in Figure 3. If the discharge capacity is 100% at 23℃, it drops to 78% at 0℃ and 65% at -20℃, and its discharge capacity is slightly more than 100% when discharged at +40℃.
From Figure 3, it can be seen that STL18650 lithium iron phosphate battery can work at -20 ℃, but the output energy to reduce about 35%. Figure 3 Discharge curve of STL18650 under multi-temperature conditions
The charge/discharge cycle life curve of STL18650 is shown in Figure 4. Its charging and discharging cycle conditions are: charging at 1C charging rate, discharging at 2C discharging rate, after 570 charging and discharging cycles. From the characteristic curve in Fig. 4, it can be seen that after 570 charge/discharge cycles, its discharge capacity remains unchanged, indicating that the battery has a high life span. Figure 4 Charge/discharge cycle life curve of STL18650 Over-discharge to zero voltage test
The STL18650 (1100mAh) lithium iron phosphate power battery was used to do the discharge to zero voltage test. Test conditions: 0.5C charging rate will be 1100mAh STL18650 battery full, and then 1.0C discharge rate discharge to the battery voltage of 0 C. And then will be put into the 0 V battery in two groups: a group of storage 7 days, the other group stored for 30 days; storage of the expiration of the 0.5C charging rate full, and then discharged with 1.0 C. Finally compare the two zero-voltage storage period of lithium iron phosphate power battery, and then discharged to zero voltage test. Finally compare the difference between the two zero-voltage storage period is different.
The results of the test were that after 7 days of zero-voltage storage, the batteries had no leakage and good performance, with 100% capacity; after 30 days of storage, there was no leakage and good performance, with 98% capacity; and the batteries after 30 days of storage did three more charge/discharge cycles, and their capacity returned to 100%.
This test shows that even if the battery is over-discharged (even to 0V) and stored for a certain period of time, the battery is not leaking or damaged. This is a characteristic that other types of lithium-ion batteries do not have. Characteristics of lithium iron phosphate batteries
Through the above introduction, LiFePO4 batteries can be summarized in the following characteristics.
1 High-efficiency output: standard discharge of 2-5C, continuous high-current discharge up to 10C, instantaneous pulse discharge (10S) up to 20C;
2 Good performance at high temperatures: the external temperature of 65 ℃ when the internal temperature is as high as 95 ℃, the battery discharges the end of the temperature of up to 160 ℃, the structure of the battery is safe, intact;
3 Even if the battery is harmed internally or externally, the battery does not burn.
3 Even if the battery is harmed internally or externally, the battery does not burn, does not explode, and has the best safety;
4 Excellent cycle life, after 500 cycles, its discharge capacity is still greater than 95%;
5 No damage even when it is over-discharged to zero volts;
6 Rapid rechargeable;
7 Low cost;
8 No pollution to the environment. Application of lithium iron phosphate power battery
Because of the above characteristics of lithium iron phosphate power battery, and the production of a variety of different capacity of the battery, it will soon be widely used. Its main application areas are:
1 Large electric vehicles: buses, electric cars, attractions tour buses and hybrid vehicles, etc.;
2 Light electric vehicles: electric bicycles, golf carts, small flat battery trucks, forklifts, cleaning trucks, electric wheelchairs, etc.;
3 Electric tools: electric drills, electric saws, lawnmowers, etc.
4 Remote control cars, boats, airplanes and other toys;
5 Solar and wind power energy storage devices;
6 UPS and emergency lights, warning lights and mining lights (the best safety);
7 Replacement of 3V disposable lithium batteries and 9V nickel-cadmium or nickel-metal hydride rechargeable batteries in cameras (identical size);
8 Small medical instruments and equipment, and portable apparatus and equipment, and so on.
Here is an application example of replacing lead-acid batteries with lithium iron phosphate power batteries. Using 36V/10Ah (360Wh) lead-acid batteries, its weight 12kg, charging a charge can walk about 50km, charging times of about 100 times, the use of about 1 year. If the use of lithium iron phosphate power battery, using the same 360Wh energy (12 10Ah battery series composition), its weight is about 4kg, charging once can walk about 80km, charging times up to 1,000 times, the service life of up to 3 ~ 5 years. Although it is said that the price of lithium iron phosphate power battery is much higher than lead-acid batteries, but the total economic effect is still using lithium iron phosphate power battery is better, and in the use of more lightweight. Small lithium iron phosphate power battery
Small lithium iron phosphate power battery is standard, cylindrical and rectangular. Such as cylindrical models are 18650, 26650 and so on. The first two in the model number is to indicate the diameter, the last two or three to indicate the height (in mm), that is, 18650 size of the diameter of 18, the height of 65. rectangular models are 103450R, 183665R, etc.. Its first two are the thickness of the battery, the middle two are the width of the battery, and the last two are the length of the battery (in mm). Battery manufacturers often add three letters of the alphabet in front of the model for the factory label, for example, the model number is × × × 18650.Conclusion
Lithium iron phosphate power battery is a new type of power battery, due to its excellent performance, by all aspects of the importance of the battery. China now has a number of factories to produce lithium iron phosphate battery cathode materials and the production of a variety of different capacities of lithium iron phosphate power battery. Due to the production time is not long, the scale is not large, resulting in the situation of demand exceeding supply. However, this situation is expected to change in 2 to 3 years, lithium iron phosphate power battery will be cheaper, and its application will be more common.