Lithium iron phosphate battery refers to lithium ion battery using Ferrous lithium phosphate as cathode material. The cathode materials of lithium ion batteries mainly include lithium cobaltate, lithium manganate, lithium nickelate, ternary materials and Ferrous lithium phosphate. Among them, lithium cobaltate is the cathode material used by most lithium ion batteries at present. From the principle of materials, Ferrous lithium phosphate is also a process of intercalation and deintercalation, which is exactly the same as lithium cobaltate and lithium manganate.

  1. Structure and working principle

The internal junction of LiFePO4 battery is olivine structure as the positive electrode of the battery. The P-O bond in Ferrous lithium phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, it will not collapse and heat like lithium cobaltate or form strong oxidizing substances, so it has good safety. It has been reported that a small number of samples were found to be burning in the acupuncture or short-circuit experiment in actual operation, but there was no explosion. In the overcharge experiment, a high voltage which greatly exceeded the self-discharge voltage several times was used to charge, and it was found that there was still an explosion. However, its overcharge safety has been greatly improved compared with that of ordinary liquid electrolyte lithium cobalt oxide batteries.

Two unit cells of LiFePO4 crystal lattice. Green spheres represent Fe atoms; blue spheres, P atoms; yellow spheres, Li atoms; and red spheres, O atoms.(DOI:10.1038/s41598-019-38635-2)

The internal junction of LiFePO4 battery is LiFePO4 with olivine structure as the positive electrode of the battery, which is connected with the positive electrode by aluminum foil, with a polymer membrane in the middle. It separates the positive electrode from the negative electrode, but Li+ can pass through, but e- can't. On the right is the negative electrode of the battery composed of carbon (graphite), which is connected with 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 shell.

Image from SINO VOLTAICS

When the LiFePO4 battery is charged, the lithium ion Li+ in the positive electrode migrates to the negative electrode through the polymer membrane; During the discharge process, lithium ion Li+ in the negative electrode migrates to the positive electrode through the diaphragm. Lithium-ion batteries are named after the lithium ions migrate back and forth during charging and discharging.

    2.Advantages
  • Improvement of life span
    Lithium iron phosphate battery refers to lithium ion battery using Ferrous lithium phosphate as cathode material.

    Long-life lead-acid batteries have a cycle life of about 300 times, with a maximum of 500 times, while Ferrous lithium phosphate Power Battery has a cycle life of more than 2,000 times, which can reach 2,000 times with standard charging (5 hours rate). Lead-acid batteries of the same quality are "new half year, old half year, and maintenance half year", which takes at most 1~1.5 years, while lithium iron phosphate batteries will have a theoretical life of 7~8 years when used under the same conditions. Considering comprehensively, the performance-price ratio is theoretically more than 4 times that of lead-acid batteries. High-current discharge can quickly charge and discharge at a high current of 2C. Under a special charger, the battery can be fully charged within 40 minutes at 1.5C, and the starting current can reach 2C, while the lead-acid battery has no such performance.

    • Good high temperature performance

    The electrothermal peak of lithium iron phosphate can reach 350℃-500℃, while that of lithium manganate and lithium cobaltate is only about 200℃. The working temperature range is wide (-20C-+75C), and the electrothermal peak of lithium iron phosphate with high temperature resistance can reach 350℃-500℃, while that of lithium manganate and lithium cobaltate is only about 200℃.

    • high-capacity

    It has a larger capacity than ordinary batteries (lead acid, etc.). The monomer capacity is 5AH-1000AH.

    • Memory-free effect

    Rechargeable batteries often work under the condition of being full and never discharged, and the capacity will quickly fall below the rated capacity value. This phenomenon is called memory effect. Like Ni-MH and Ni-Cd batteries, there is memory, but lithium iron phosphate batteries do not have this phenomenon. No matter what state the batteries are in, they can be used with charge, without first discharging and then recharging.

    • Light weight

    The volume of lithium iron phosphate battery with the same specification capacity is 2/3 of that of lead-acid battery, and its weight is 1/3 of that of lead-acid battery.

    • Environmental protection

    The battery is generally considered to be free of any heavy metals and rare metals (Ni-MH batteries need rare metals), non-toxic (SGS certified), pollution-free, in line with European RoHS regulations, and an absolute green battery certificate.

    However, some experts said that the environmental pollution caused by lead-acid batteries mainly occurred in the irregular production process and recycling process of enterprises. Similarly, lithium battery belongs to the new energy industry, but it can't avoid the problem of heavy metal pollution. Lead, arsenic, cadmium, mercury and chromium in metal processing may be released into dust and water. The battery itself is a chemical substance, so there may be two kinds of pollution: first, the pollution of process waste in production engineering; The second is battery pollution after scrapping.

      3.Disadvantages

    Whether a material has the potential of application and development, apart from its advantages, the key is whether the material has fundamental defects.

    Lithium iron phosphate battery also has its disadvantages: for example, poor low-temperature performance, low tap density of cathode materials, and the volume of lithium iron phosphate battery with the same capacity is larger than that of lithium ion batteries such as lithium cobaltate, so it has no advantages in miniature batteries. When used in power batteries, lithium iron phosphate batteries, like other batteries, need to face the problem of battery consistency.

    Ferrous lithium phosphate has some performance defects, such as low tap density and compaction density, which leads to low energy density of lithium-ion battery. Low-temperature performance is poor, even if it is nano-sized and carbon coated, this problem has not been solved. Dr. Don Hillebrand, director of the Energy Storage System Center of Argonne National Laboratory, USA, described the low-temperature performance of lithium iron phosphate battery as terrible. Their test results of Ferrous lithium phosphate lithium ion battery showed that lithium iron phosphate battery could not make electric vehicles run at low temperature (below 0℃). Although some manufacturers claim that the capacity retention rate of lithium iron phosphate battery is good at low temperature, it is under the condition of low discharge current and low discharge cut-off voltage. In this situation, the equipment can't start working at all.

    Poor product consistency. From the point of view of material preparation, the synthesis reaction of Ferrous lithium phosphate is a complex multiphase reaction, which includes solid phosphate, iron oxide and lithium salt, carbon precursor and reducing gas phase. In this complicated reaction process, it is difficult to guarantee the consistency of the reaction.

      4.Meaning

    In the metal trading market, cobalt (Co) is the most expensive, and its storage is not much. Nickel (Ni) and manganese (Mn) are cheaper, while iron (Fe) is the cheapest. The price of cathode materials is also consistent with the price of these metals. Therefore, the lithium-ion battery made of LiFePO4 cathode material should be the cheapest. Another feature of it is that it has no pollution to the environment.

    The requirements for rechargeable batteries 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 (no burning or explosion caused by improper operation such as overcharge, overdischarge and short circuit), wide working temperature range, no toxicity or little toxicity, and no pollution to the environment. The lithium iron phosphate battery with LiFePO4 as cathode has good performance requirements, especially in terms of high discharge rate (5 ~ 10c discharge), stable discharge voltage, safety (no combustion, no explosion), long life (number of cycles) and no environmental pollution. It is the best power battery with high current output at present.

      5.Application of Ferrous lithium phosphate Power Battery

    Because Ferrous lithium phosphate Power Battery has the above characteristics and produces various batteries with different capacities, it has been widely used. Its main application fields are:

    Large-scale electric vehicles: buses, electric vehicles, scenic spots tour buses and hybrid vehicles, etc.

    Light electric vehicles: electric bicycles, golf carts, small flat battery carts, forklifts, cleaning vehicles, electric wheelchairs, etc.

    Electric tools: electric drills, chainsaws, lawn mowers, etc.

    Remotely control toys such as cars, boats and airplanes;

    And energy storage equipment for solar power generation;

    UPS emergency lights, warning lights and miner's lamps (the best safety);

    Replace 3V disposable lithium battery and 9V nickel cadmium or nickel hydrogen rechargeable battery (the same size) in camera;

    Small medical instruments and portable instruments, etc.