As a rechargeable battery 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 overcharge, over discharge and short circuit and other improper operation caused by combustion or explosion), wide operating temperature range, non-toxic or less toxic, no pollution to the environment.
Lithium iron phosphate ion battery using LiFepO4 as the positive electrode in these performance requirements are good, especially in the large discharge rate discharge (5 ~ 10C discharge), smooth discharge voltage, safety (no combustion, no explosion), life (number of cycles), no pollution to the environment, it is the best, is the best high-current output power lithium battery.
Structure and working principle
LiFepO4 is used as the positive electrode of the battery, which is connected by aluminum foil and the positive electrode of the battery. In the middle is the polymer diaphragm, which separates the positive electrode from the negative electrode, but lithium ion Li can pass through but electron e- cannot, and on the right is the negative electrode of the battery, which is composed of carbon (graphite) and connected by copper foil and the negative electrode of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal shell.
LiFepO4 battery during charging, the lithium ion Li in the positive electrode migrates to the negative electrode through the polymer diaphragm; during discharging, the lithium ion Li in the negative electrode migrates to the positive electrode through the diaphragm. Li-ion batteries are named after the lithium ions that migrate back and forth during charging and discharging.
LiFepO4 batteries have a nominal voltage of 3.2 V, an end charge voltage of 3.6 V, and an end discharge voltage of 2.0 V. There are some differences in performance due to the quality and process of the positive and negative electrode materials and electrolyte materials used by each manufacturer. For example, the same model (standard battery in the same package) has a large difference (10% to 20%) in the capacity of the battery.
It should be noted here that different factories produce lithium iron phosphate power lithium battery in the various performance parameters will have some differences; in addition, there are some battery performance is not included, such as battery resistance, self-discharge rate, charging and discharging temperature, etc..
Lithium iron phosphate power lithium battery capacity has a large difference, can be divided into three categories: small zero point a few to a few mAh, medium-sized tens of mAh, large hundreds of mAh. Similar parameters of different types of batteries also have some differences.
1, the safety performance of the improvement of lithium iron phosphate crystal in the p-O bond solid, difficult to decompose, even at high temperatures or overcharge will not be the same as lithium cobalt structure collapse heat or the formation of strong oxidizing substances, and therefore has a good safety.
2, life improvement lithium-iron phosphate ion battery is the lithium-ion battery with lithium iron phosphate as the cathode material. Long-life lead-acid battery cycle life of about 300 times, up to 500 times, and lithium iron phosphate power lithium batteries, cycle life of more than 2000 times, the standard charge (5-hour rate) use, can reach 2000 times.
3, high-temperature performance of lithium iron phosphate electric peak of 350 ℃ -500 ℃ and lithium manganate and lithium cobalt acid only in about 200 ℃.
4, high-capacity ∩ rechargeable batteries in often in the full not put out under the conditions of work, the capacity will quickly fall below the rated capacity value, this phenomenon is called the memory effect. Like nickel-metal hydride, nickel-cadmium batteries have memory, while lithium iron phosphate ion batteries without this phenomenon, no matter what state the battery is in, can be used as it is charged, no need to first put out and then charged.
5, light weight, the same capacity of lithium iron phosphate battery volume is 2/3 of the volume of lead-acid batteries, the weight of lead-acid batteries 1 / 3.
6, environmental protection
Lithium iron phosphate ion batteries are generally considered to be free of any heavy metals and rare metals (NiMH batteries require rare metals), non-toxic (SGS certification through), non-polluting, in line with European RoHS regulations, for the absolute green battery certificate.
Lithium iron phosphate also has fundamental defects that cannot be ignored, which boils down to the following important points:
1, in the preparation of lithium iron phosphate sintering process, iron oxide in a high-temperature reducing atmosphere there is the possibility of being reduced to singlet iron. Monolithic iron will cause a micro-short circuit in the battery, is the most taboo substance in the battery.
2, lithium iron phosphate has some performance defects, such as vibration density and compaction density is very low, resulting in a low energy density of lithium-ion batteries. Poor low-temperature performance, even if it is nanosized and carbon cladding does not solve this problem.
3, the material preparation cost and the manufacturing cost of the battery is high, the battery yield is low, and the consistency is poor. Nanosized lithium iron phosphate and carbon cladding, despite the improved electrochemical performance of the material, but also brings other problems, such as the reduction of energy density, increased synthesis costs, poor electrode processing performance and harsh environmental requirements.
Although the chemical elements Li, Fe and p in lithium iron phosphate is abundant and low cost, but the preparation of lithium iron phosphate product cost is not low, even if you remove the preliminary research and development costs, the material process cost plus the higher cost of preparing the battery, will make the final unit of energy storage power cost is higher.
In view of the above-mentioned problems of lithium iron phosphate, it is difficult to be widely used as the cathode material for power-type lithium-ion batteries in new energy vehicles and other fields. If the problems of high temperature cycling and poor storage performance of lithium manganate can be solved, there will be great potential for its application in power-type lithium-ion batteries with its advantages of low cost and high multiplier performance.