The full name of lithium iron phosphate battery is lithium iron phosphate lithium ion battery, which is too long and is referred to as 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. It is also called "lithium iron (LiFe) power battery".
working principle
Lithium iron phosphate battery refers to a lithium ion battery using lithium iron phosphate as a positive electrode material. The cathode materials of lithium-ion batteries mainly include lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Among them, lithium cobalt oxide is the cathode material used in the vast majority of lithium-ion batteries.
significance
In the metal trading market, cobalt (Co) is expensive and there is not much storage, nickel (Ni) and manganese (Mn) are cheaper, and iron (Fe) is more stored. The prices of cathode materials are also in line with those of these metals. Therefore, lithium-ion batteries made of LiFePO4 cathode materials should be quite cheap. Another feature of it is that it is environmentally friendly and non-polluting.
As a rechargeable battery, the requirements are: high capacity, high output voltage, good charge-discharge cycle performance, stable output voltage, high-current charge-discharge, electrochemical stability, and safety in use (not due to overcharge, overdischarge and short circuit). It can cause combustion or explosion due to improper operation), wide operating temperature range, non-toxic or less toxic, and no pollution to the environment. LiFePO4 batteries using LiFePO4 as the positive electrode have good performance requirements, especially in terms of large discharge rate discharge (5 ~ 10C discharge), stable discharge voltage, safety (non-burning, non-exploding), life (cycle times) ), no pollution to the environment, it is good, 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 to the positive electrode of the battery by an aluminum foil. In the middle is a polymer separator, which separates the positive electrode from the negative electrode, but the lithium ion Li can pass through but the electron e- cannot pass through. The right side is composed of carbon (graphite). The negative electrode of the battery is connected to the negative electrode of the battery by 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 LiFePO4 batteries are charged, the lithium ions Li in the positive electrode migrate to the negative electrode through the polymer separator; during the discharge process, the lithium ions Li in the negative electrode migrate to the positive electrode through the separator. Lithium-ion batteries are named after lithium ions migrate back and forth during charging and discharging.
Main performance
The nominal voltage of the LiFePO4 battery is 3.2V, the final charge voltage is 3.6V, and the final discharge voltage is 2.0V. Due to the different quality and process of positive and negative electrode materials and electrolyte materials used by various manufacturers, there will be some differences in their performance. For example, the capacity of the battery of the same type (standard battery in the same package) is quite different (10% to 20%).
It should be noted here that lithium iron phosphate power batteries produced by different factories will have some differences in various performance parameters; in addition, some battery performance is not included, such as battery internal resistance, self-discharge rate, charge and discharge temperature, etc.
The capacity of lithium iron phosphate power batteries is quite different, and can be divided into three categories: small tenths to a few milliamp hours, medium tens of milliamp hours, and large hundreds of milliamp hours. There are also some differences in the same parameters of different types of batteries.
Overdischarge to zero voltage test: STL18650 (1100mAh) lithium iron phosphate power battery was used for discharge to zero voltage test.
Test conditions: 1100mAh STL18650 battery is fully charged with a 0.5C charge rate, and then discharged to a battery voltage of 0C with a 1.0C discharge rate. Then divide the batteries placed at 0V 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. Compare the difference between the two zero-voltage storage periods.
The result of the test is that after 7 days of zero voltage storage, the battery has no leakage, good performance, and the capacity is 100%; after 30 days of storage, there is no leakage, good performance, and the capacity is 98%; after 30 days of storage, the battery is subjected to 3 charge-discharge cycles, The capacity is back to 100%. This test shows that even if the lithium iron phosphate battery is overdischarged (even to 0V) and stored for a certain period of time, the battery will not leak or be damaged. This is a feature that other types of lithium-ion batteries do not have.
Advantage
- Improvement of safety performance The P-O bond in the lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, it will not collapse and generate heat like lithium cobalt oxide or form strong oxidizing substances, so it has good safety. sex. A report pointed out that in the actual operation, a small number of samples were found to be burning in the acupuncture or short-circuit experiments, but there was no explosion. explosion phenomenon. Even so, its overcharge safety has been greatly improved compared with ordinary liquid electrolyte lithium cobalt oxide batteries.
- Lifetime improvement Lithium iron phosphate battery refers to a lithium ion battery that uses lithium iron phosphate as the positive electrode material. The cycle life of long-life lead-acid batteries is about 300 times, and the highest is 500 times, while the cycle life of lithium iron phosphate power batteries can reach more than 2,000 times, and the standard charging (5-hour rate) can reach 2,000 times. The lead-acid battery of the same quality is "new half year, old half year, and maintenance and maintenance for half a year", which is usually 1 to 1.5 years, while lithium iron phosphate battery is used under the same conditions, and the theoretical life will reach 7 to 8 years. Comprehensive consideration, the performance-price ratio is theoretically more than 4 times that of lead-acid batteries. High-current discharge can quickly charge and discharge high-current 2C. Under the special charger, the battery can be fully charged within 40 minutes of 1.5C charging, and the starting current can reach 2C, but lead-acid batteries do not have this performance.
- Good high temperature performance, the electric heating peak of lithium iron phosphate can reach 350℃-500℃, while lithium manganate and lithium cobaltate are only around 200℃. Wide operating temperature range (-20C--75C), high temperature resistance, lithium iron phosphate electrothermal peak can reach 350℃-500℃, while lithium manganate and lithium cobaltate are only around 200℃.
- Large-capacity ∩ rechargeable batteries often work under the condition of being fully charged, and the capacity will quickly drop below the rated capacity. This phenomenon is called the memory effect. Like nickel-metal hydride and nickel-cadmium batteries, there is memory, but lithium iron phosphate batteries do not have this phenomenon. No matter what state the battery is in, it can be used at any time without having to discharge it before charging.
- Light weight
The volume of the lithium iron phosphate battery with the same specification and capacity is 2/3 of the volume of the lead-acid battery, and the weight is 1/3 of the lead-acid battery.
- Environmental protection
Lithium iron phosphate batteries are generally considered to be free of any heavy metals and rare metals (nickel-metal hydride batteries require rare metals), non-toxic (SGS certified), non-polluting, comply with European RoHS regulations, and are an absolute green battery certificate. Therefore, the reason why lithium batteries are favored by the industry is mainly environmental protection.