The structure and properties of the lithium iron phosphate

Lithium iron phosphate is introduced

Lithium ion batteries as a kind of high performance green twice The battery Volume, high voltage, high energy density (including energy and mass ratio), low self-discharge rate, use of wide temperature range, long cycle life, environmental protection, no memory effect, and can the advantages of large current charge and discharge. To improve the performance of lithium ion batteries, to a large extent depends on to improve the performance of electrode materials, especially the anode material. The most widely studied nowadays cathode material LiCoO2, LiNiO2, LiMn2O4, but as a result of cobalt toxic and limited resources, lithium nickel acid preparation is difficult, poor circulation performance and high temperature performance of manganese acid lithium factors, restricting their application and development. Therefore, the development of the new type high-energy cheap is very important for the development of the anode materials for lithium ion batteries.

1997, Padhi reported with olivine structure such as lithium iron phosphate (LiFePO4) can be reversibly embedded lithium, and have high specific capacity and good cycle performance, electrochemical performance is stable, the price is low wait for a characteristic, is the first choice of a new generation of green anode materials, especially as power lithium ion battery materials. The discovery of the lithium iron phosphate caused electrochemical academic circles the attention of many researchers both at home and abroad, in recent years, along with The lithium battery More and more wide application, the study of LiFePO4 is increasing.

The structure of the lithium iron phosphate

Lithium iron phosphate (LiFePO4) has olivine structure, for a slightly twisted six-party close packing, the space group is Pmnb type, crystal structure

LiFePO4 by FeO6 octahedral and PO4 tetrahedron form space frame, P tetrahedron position, while Fe and Li fill in octahedral gap, the Fe Angle of octahedral position, Li octahedron of edge position. Lattice a FeO6 octahedron with two FeO6 octahedral and join a PO4 tetrahedral edge, while the PO4 tetrahedron with a FeO6 two LiO6 octahedron octahedron and edge. Because almost six party closely arranged, the oxygen atoms of the accumulation of making lithium ion can only be embedded in the two-dimensional plane to take off, and therefore has a relatively high theoretical density (3.6 g/cm3).In this structure, Fe2 + / Fe3 + relatively metallic lithium voltage of 3.4 V, the theory of material specific capacity is 170 ma · h/g. In the material form strong P - O - M covalent bond, greatly stability of the crystal structure of materials, leading to materials with high thermal stability.

Wang on the electrochemical performance of LiFePO4 made detailed analysis, figure 2.2 is the cycle load voltammo grams LiFePO4, formed in C - V figure two peak, at the anode scan Li + emerge from LixFePO4 structure and oxidation peak at 3.52 V formation; When scanning in 4.0 ~ 3.0 Li + embedded in LixFePO4 structure, the corresponding reduction peak at 3.32 V formation; C - V curves show that the REDOX peak in LiFePO4 electrode embedded with reversible lithium ion reaction.

The performance of lithium iron phosphate

1) high energy density

Its theoretical specific capacity is 170 MAH/g, actual specific capacity of product can be more than 140 MAH/g (0.2 C, 25 ° C).

2) security

Is the safety of lithium ion battery cathode material; Do not contain any harmful heavy metals;

3) long service life

Under the condition of 100% DOD, can charge and discharge more than 2000 times;(reason: lithium iron phosphate lattice stability is good, the lithium ions embedded and emergence of lattice impact is not big, so it has good reversibility. The deficiencies are ionic conductivity electrode is poor, not suitable for large current charge and discharge, stuck in applications. Solution: the electrodes coated on the surface of conductive material, doped with electrode modification.)

Lithium iron phosphate batteries Service life and its temperature is, the use of temperature too low or too high in the charging and discharging process and use process produce greatly bad hidden trouble. Electric cars use especially in northern China, in the fall and winter normal power supply of the lithium iron phosphate batteries or power supply is too low, need to adjust its working environment temperature to maintain its performance. At present, the domestic solve the lithium iron phosphate battery temperature working environment should be considered space limitations, the common solution is to use the aerogel blanket as a thermal insulation layer.

4) charging performance

Lithium iron phosphate cathode material of lithium battery, can use big charge ratio, the fastest will be in 1 hour to the battery is full.

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