The working principle and structure of lithium iron phosphate battery for solar energy
MANLY Battery Technical Support: The full name of lithium iron phosphate battery is lithium iron phosphate lithium-ion battery, this name is too long, and it is called lithium iron phosphate battery for short. Because its performance is particularly suitable for power applications, the word "power" is added to the name, that is, lithium iron phosphate power battery. Some people call it a "Lithium Iron (LiFe) Power Battery".
working principle
Lithium iron phosphate battery refers to a lithium-ion battery that uses lithium iron phosphate as a positive electrode material. The cathode materials of lithium-ion batteries mainly include lithium cobaltate, lithium manganate, lithium nickelate, ternary materials, lithium iron phosphate, and so on. Among them, lithium cobalt oxide is currently the cathode material used in most lithium-ion batteries.
significance
In the metal trading market, cobalt (Co) is the most expensive and has a small storage capacity. Nickel (Ni) and manganese (Mn) are cheaper, while the iron (Fe) has more storage capacity. The price of the cathode material is also consistent with the price of these metals. Therefore, the lithium-ion battery made of LiFePO4 cathode material should be quite cheap. Another feature of it is that it has no pollution to the environment.
Like a rechargeable battery, the requirements are high capacity, high output voltage, good charge, and discharge cycle performance, stable output voltage, high current charge and discharge, electrochemical stability, safety in use (not due to overcharging, over-discharge, and short circuit Burning or explosion caused by improper operation), wide operating temperature range, non-toxic or less toxic, and non-polluting to the environment. Lithium iron phosphate batteries using LiFePO4 as the positive electrode are good in these performance requirements, especially in high discharge rate discharge (5～10C discharge), the discharge voltage is stable, safety (non-burning, non-explosive), and life (cycle times) ). It is the best in terms of no pollution to the environment and is currently the best high-current output power battery.
Structure and working principle
LiFePO4 is used as the positive electrode of the battery. It is connected with the positive electrode of the battery by aluminum foil. In the middle is a polymer separator, which separates the positive electrode from the negative electrode, but lithium-ion Li can pass through but electron e- cannot pass. The right side is made of carbon (graphite). The negative electrode of the battery 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, and the battery is hermetically sealed by a metal casing.
When the LiFePO4 battery is charged, the lithium-ion Li in the positive electrode migrates to the negative electrode through the polymer separator; during the discharge process, the lithium-ion Li in the negative electrode migrates to the positive electrode through the separator. Lithium-ion batteries are named after lithium ions move back and forth during charging and discharging.
Main performance
The nominal voltage of the LiFePO4 battery is 3.2V, the final charging voltage is 3.6V, and the final discharge voltage is 2.0V. Due to the different quality and technology of the positive and negative materials and electrolyte materials used by various manufacturers, there will be some differences in their performance. For example, the battery capacity of the same model (standard battery in the same package) is quite different (10%-20%).
What I want to explain here is that lithium iron phosphate power batteries produced by different factories have some differences in various performance parameters; in addition, there are some battery performances that are not listed, such as battery internal resistance, self-discharge rate, charge and discharge temperature, etc.
There are big differences in the capacity of lithium iron phosphate power batteries, which can be divided into three categories: small ones from a few tenths to several milliamp-hours, medium-sized tens of milliamp-hours, and large-scale several hundred milliamp-hours. Similar parameters of different types of batteries also have some differences.
Over-discharge to zero voltage test:
STL18650 (1100mAh) lithium iron phosphate power battery was used for over-discharge to zero voltage test. Test conditions: Fully charge a 1100mAh STL18650 battery with a 0.5C charge rate, and then discharge it with a 1.0C discharge rate until the battery voltage is 0C. The batteries placed at 0V are divided into two groups: one group is stored for 7 days, and the other group is stored for 30 days; after the storage expires, it is fully charged with a 0.5C charging rate, and then discharged with 1.0C. Finally, compare the difference between the two zero-voltage storage periods.
The result of the test is that the battery has no leakage after 7 days of storage at zero voltage, with good performance, and the capacity is 100%; after 30 days of storage, there is no leakage, with good performance, and the capacity is 98%; the battery after 30 days of storage is subjected to 3 charge-discharge cycles. The capacity has returned to 100%.
This test shows that even if the lithium iron phosphate battery is over-discharged (even to 0V) and stored for a certain period of time, the battery will not leak or be damaged. This is a characteristic that other types of lithium-ion batteries do not have.
