Olivine type compound: LiFePO4 /LiMPO4(M=Mn, Co, Ni),Vanadium compounds
Olivine type compound
Iron is a rich metal, which is cheaper and more environmentally friendly than Co. Through the research on iron-containing cathode materials, it was found that olivine-type LiFePO4 has the most promising development prospects. LiFcPO4 is derived from LiFeO2, but the latter cannot be applied due to poor electrochemical performance because its working voltage is only 3.2V. This is because the Fermi energy level of Fe3+/Fe2+ is similar to that of lithium. But by replacing the oxygen in LiFeO2 with polyanion XO4y- (X=S, P, As; y=2, 3), the voltage can be increased to 3.4V, because for the Fe-O-X bond, XO4y- has a strong XO bond The Fe-O bond is weakened, and the ionization tendency of Fe3+/Fe2+ is increased, thus increasing the voltage.
LiFePO4 has the characteristics of structural stability and chemical stability. The disadvantages are low electronic conductivity and slow lithium ion diffusion. The conductivity of LiFePO4 can be improved by coating a conductive agent such as carbon or nano silver.
In the usual M2XO4 olivine structure, in the voids formed by hexagonal close-packed oxygen, 50% of the octahedral positions are M, and 1/8 of the tetrahedral positions are occupied by X, which is similar to cubic spinel. The hexahedron structure in Li[Mn2]O4 is the same. If the X ion radius is small, such as Be2+, B3+, Si4+ or P5+, the olivine structure is better than the spinel structure formed first. Table 1 compares the spinel structure and the olivine structure. In the inverse spinel structure, the migration of lithium ions is disrupted by the irregular arrangement of lithium ions and nickel ions in the octahedral position. In the spinel structure, because lithium ions occupy tetrahedral positions, and manganese ions and drill ions occupy octahedral positions, lithium ions can migrate along the 16c-8a-16c path. In the olivine structure, although the two octahedrons have different crystallographic parameters and sizes, they form a uniform structure that allows lithium ions to diffuse in one dimension.
|Lithium ion position||tetrahedron||Octahedron||Octahedron|
In the Pmnb structure of olivine, Fe occupies the M2 tetrahedral position, while lithium ions occupy the M1 octahedral position.
Figure 1 is a schematic diagram of the hexagonal close-packed structure of olivine-type LiFePO4. Lithium ions form a linear chain of co-sided octahedrons in the c-axis direction, while FeO6 octahedrons are arranged in a zigzag shape. Each lithium ion octahedron and two iron ion octahedrons, and two XO4 tetrahedrons share an edge.
The distortion of oxygen in the hexagonal close-packed structure causes electrostatic repulsion between coedge cations. In the coedge octahedral chain, the insertion/extraction of lithium ions is similar to the insertion/extraction of lithium ions in the LiMO2 (M=Co, Ni) layered structure. In LiMO2, the free space of lithium ions between MO2 layers is limited by the Li-O bond, while in LiMPO4 it is limited by the XO4 tetrahedron connected to Fe.
The theoretical density of LiFePO4 is 3.6g/cm3, which is smaller than LiCoO2 (5.lg/cm3), LiNiO2 (4.8g/cm3) and LiMn2O4 (4.2g/cm3), but is lower than LiMnPO4 (3.4g/cm3) and NaSICON (sodium super Ionic conductor) is large. The theoretical capacity of LiFePO4 is 170mAh/g (2.0~4.2V), and the average working voltage is 3.4V, which is not enough to decompose the electrolyte and at the same time provide a certain energy density. Therefore, it is an excellent cathode material among iron compounds. From the charge and discharge curve of LiFePO4 in Figure 2, it can be seen that Li1-xFePO4 has a flat voltage plateau in a wide range of x. The same phenomenon can also be seen in Li4Ti5O12 and Li4Mn5O12, because during the charge and discharge process of these materials, the migration of lithium ions is controlled by diffusion or phase boundary control. According to the phase rule, the insertion/extraction of LiFePO4 and FePO4 will inevitably lead to a two-phase redox reaction, because on the Gibbs free energy curve of the two-phase reaction, the chemical potential of lithium remains unchanged, and therefore, the corresponding voltage is also unchanged.
The lithium insertion/extraction reaction in LifePO4 can be described as follows: LiFePO4 in the fully discharged state and FePO4 in the fully charged state have the same crystal structure, and the rate of the charge-discharge reaction is determined by the rate of phase boundary movement. It is generally believed that during charging and discharging, the movement of lithium in the crystal structure can be divided into two types. The first is the diffusion reaction, that is, the crystal structure remains unchanged, and the diffusion of lithium appears as a gradient change curve on the charge-discharge curve. When charging, due to the interaction between different units in the crystal structure, the lattice constant changes, but the crystal structure remains unchanged. The second is the phase boundary movement of the two-phase reaction. Figure 3 is a schematic diagram of the phase boundary movement.
When charging, the lithium on the surface of the particles is released, and the lithium-free A phase (FePO4) and the stoichiometric B phase (LiFePO4) coexist. As the charging progresses, the A phase keeps increasing and the B phase keeps decreasing until only A phase remains. As the lithium is released, the phase boundary moves continuously. The capacity of Li1+xMn2O4 decreased significantly at 3V, while LiFe PO4 showed a good cycle life. This can be explained by the structural stability of LiFePO4 and the similar crystal structure of the discharged product LiFePO4 and the charged product FePO4.
The FePO4 after delithiation was heated to 350°C without weight loss in a nitrogen atmosphere. X-ray diffraction analysis showed that the heat treatment hardly changed the structure. In addition, FePO4 without impurities is stable to most physicochemical salts and electrolytes at high temperatures. Unlike other materials, charging will reduce the volume of LiFePO4 by about 6.8%, which is an advantage for battery design because it can compensate for the volume expansion of the carbon negative electrode during charging. Because of its structural stability and thermal stability, LiFePO4 has good application prospects in various occasions that require high-capacity, large-volume, and long-life batteries, such as hybrid vehicles, power tools, and power storage.
In LiFePO4, because the diffusion of lithium is carried out through a one-dimensional channel, it will be significantly affected by material defects. When the position on the lithium diffusion channel is occupied by other cations such as iron ions, the diffusion channel will be blocked, restricting the migration of lithium ions. Due to the poor migration ability of iron, this blocked lithium channel remains inactive and cannot undergo electrochemical reactions. Therefore, in the synthesis of LiFePO4, it is important to generate a defect-free crystal structure.
The biggest disadvantage of LiFePO4 is its low conductivity, which is a common feature of polyanions such as PO43-compounds, which will cause significant polarization during charging and discharging. If the conductive agent cannot be uniformly dispersed, its capacity will decrease significantly.
Researchers use various methods to improve the rate performance of LiFePO4. For example, by adjusting the particle size to increase the ionic conductivity and by wrapping carbon on the surface of the particles to increase the ionic conductivity. Another method is to use Nb as a doping element to produce high-conductivity phases such as Fe2P, but this method is difficult to repeat and has unknowable effects on electrochemical performance.
In order to avoid the generation of Fe3+ ions in the high-temperature solid-phase reaction, nitrogen protection is required when synthesizing LiFePO4. At the same time, the uniformity of raw materials must be ensured to avoid impurities such as Fe2O3 and Li3Fe2(PO4)3. When the sample is synthesized at high temperature, the particle size increases, the specific surface area decreases, the lithium ion diffusion channel becomes less, and the battery performance deteriorates. Recent studies have used low-temperature synthesis techniques to inhibit particle growth.
② LiMPO4(M=Mn, Co, Ni)
LiFePO4 and LiMnPO4 solid solution LiFe1-xMnxPO4 can also be charged and discharged. The lattice constant of LiFe1-xMnxPO4 satisfies Vegard’s law, and the phase operation between Fe3+-O-Mn2+ can generate a high voltage of Mn3+ /Mn2+ at 4.IV, as shown in Figure 4 In LiFe1-xMnxPO4, when iron appears near the Mn position, it will activate Mn3+/Mn2+. For the fully charged material (Mny3+Fe1-y3+) PO4, when y>0.75, the Jahn-Teller effect of Mn3+ will cause serious The crystal lattice deforms, thereby affecting electrochemical activity.
Comparing VO4 tetrahedron and PO4 tetrahedron, it was originally thought that the shared feature of PO4 makes it easier to form an olivine structure instead of a spinel structure. This makes the redox energy of Mn3+ /Mn2+ at the octahedral position stable, and its potential It also increased from 3.7 V in V [LiMn]O4 to 4.1 V in LiFe0.5Mn0.5PO4. The 4.1 V region I is a flat two-phase region, and the region II is an S-shaped single-phase region. The overpotential of area I is more obvious than that of area II. This is because lithium ions are blocked from passing through the mixed phase, and it is also because of the Jan-Teller effect of Mn3+ that the effective mass of 3d electrons is greater. The voltage of area Ⅱ is 3.5 V, which is slightly higher than the 3.4 V of Li1-xFePO4. This is because the superexchange effect of Fe3+-O-Mn2+ increases the potential of Fe3+/Fe2+, and reduces the potential of Mn3+/Mn2+. The lattice constants of the two phases in the region I do not change, and the lattice constants of the corresponding phases in the region II change continuously.
LiCoPO4 can be charged and discharged at 5.1/4.8 V. It can be used as a high-voltage cathode material by optimizing its conductivity and particle size. Other high-voltage cathode materials include LiNiVO4 and LiCrxMn2-xO4. When charged to 5.1 V with a current density of 0.2 mA/cm2, the capacity of LiCoPO4 can reach 100 mAh/g, and the corresponding volume change is 4.6%. The energy density is equivalent to LiCoO2, and the power performance is better than LiFePO4. The charging product Li0.4CoPO4 undergoes the first phase transition at 290 C, which is similar to the phase transition temperature of FePO4 (315 T). According to the measurement of the magnetic moment, the change of electrochemical properties changes the ratio of high-spin Co2+(t2g)4(eg)2 and low-spin Co3+(t2g)6 in the material. In LiMnPO4, it was originally thought that the thermal instability of MnPO4 made it impossible for lithium to escape from the crystal lattice, but recently it was discovered that this material can release a capacity of about 140 mAh/g. In addition, studies have reported that LiMnPO4 with a capacity of about 70 mAh/g can be obtained by direct precipitation and carbon ball milling, and MnPO4 also has a thermally stable phase. Through first-principle calculations, the equilibrium voltage of LMnPO4 is 4.1V. Due to its forbidden bandwidth and slow pole movement, its conductivity must be very low. Experiments also proved the low power density of the material.
Research on LiNiPO4 is still relatively small, but the equilibrium potential of 5.1 V predicted by first-principles calculations has recently been confirmed.
Vanadium oxides and their derivatives have various phases and crystal structures, including V2O5, V2O3, VO2 (B), V6O13, V4O9, V3O7, Ag2V4O11, AgVO3, Li3V3O5, δ-MnyV2O5, δ-NH4V4O10, Mn0. 8V7O16, LiV3O8, CuxV2O5 and CrxV6O13.
V205 synthesized by polymer media will have different morphologies: spherical, nanorods and nanowires, with an initial capacity of 250~400mAh/g. The discharge curves of these oxides present a wide voltage range (1.5~4.0V), as shown in Figure 5.
VO2 (B) is an unstable phase and easily transforms into rutile phase VO2 at temperatures above 300°C. The VO2(B) nano-oxide synthesized by low-temperature oxidation-reduction reaction exhibits a slowly decreasing discharge curve, and the capacity can reach 300mAh/g (see Figure 6). The VO2(B) aerosol synthesized by freeze-drying and low-temperature heating methods has a high capacity of 300 ~ 520mAh/g. The heat treatment temperature is crucial to whether there is a flat area on the discharge curve.
In addition, vanadium oxides (Ag2V4O11, AgVO3) synthesized in silver-containing nanowires and nanorods have a capacity higher than 300 mAh/g. Figure 7 shows the discharge curves of various nanowire and nanorod compounds.
The LiV3O8 active material with a size of 100 nm synthesized by the sol-gel method shows a capacity greater than 300 mAh/g and a discharge curve with a slow change level. The chromium-doped compound Cr0.36V6O13 has a capacity of 380 mAh/g and a relatively flat discharge curve near 3 V, but its synthesis process is more complicated and needs to be synthesized by the sol-gel method. After pickling, it is doped with Cr by ion exchange. .
Vanadium can be used as a high-capacity electrode material. The vanadium oxide with micron size and high crystallinity shows a unique plateau voltage for each oxidation-reduction reaction that changes in valence. Recently, researchers have tried to synthesize nano-sized vanadium oxides. This type of material has a metastable phase between the amorphous phase and the highly crystalline phase, corresponding to the slowly changing discharge curve.