What are the types of layered compounds for cathode materials?

LiCoO2 has an R3m rhombic hexahedron structure. Although it is expensive, it is suitable for large-scale production and is therefore widely used as a cathode material for lithium-ion batteries. Depending on the heat treatment temperature, LiCoO2 can form two different structures. Layered LiCoO2 is obtained by solid-phase reaction synthesis above 800°C, while spinel Li2Co2O4 is synthesized at about 400°C. When the synthesis temperature is low, there are many defects in the crystal lattice and the crystallinity is low, resulting in poor electrochemical performance. Therefore, layered LiCoO2 synthesized at high temperature is generally used as a positive electrode active material.
Considering the crystal field stabilization energy (CFSE) of Co ion, the relative energy levels of Co2+ (t2g6eg1), Co3+ (t2g6) and Co4+ (t2g5) are 18Dq+P, -24Dq+2P and -20Dq+2P. If the electron pairing energy P is relatively small, then Co2+ (t2g6eg1) is the most stable. But in order for Co2+ to exist, non-metered Li1-xCo1+xO2 must be formed to meet the requirements of electrical neutrality. Considering the ion radius, r(Co2+)/r(O2-)=0.627, which falls within the range of 0.414~0.732 of the octahedron. But r(Co2+)/r(Li+)=0.878, which is 12.2%, which is close to the 15% limit of the Hme-Rothery rule. Therefore, the synthetic amount of LiCoO2 is more stable.

Figure 1 Charge and discharge curve of O3-LiCoO2 synthesized at high temperature

Figure 1 shows the charge/discharge curve of LiCoO2 cathode material when metallic lithium is used as the anode. A platform of about 4V can be seen, and the discharge voltage does not change significantly over time. Co3+/4+, the direct interaction of Co-Co formed by the filled t2g 6-x track in the middle part makes the material have high conductivity. Due to the low spin and stable Co4+ ions in the octahedral position, irreversible phase transitions are unlikely to occur during charging and discharging.
But for Li1-xCoO2, when x>0.5, the layered O3 structure and the monoclinic P3 structure are mixed, and the phase change may be irreversible. Less than 50% of the lithium in LiCoO2 can be reversibly inserted and extracted. The CoO2 obtained by complete delithiation has a hexagonal close-packed O1 layered structure formed by irreversible phase transformation. The theoretical specific capacity of LiCoO2 is 274mAh/g, but the actual specific capacity is only 145mAh/g, which is slightly higher than half of the theoretical specific capacity of 137mAh/g.

Figure 2 shows the XRD spectrum of Li1-xCoO2 structure changes during charging/discharging. During the charging process, LiCoO2 transforms into a completely different structure, which can be seen from the gradual increase of the small peak on the right side of the (003) peak.

Figure 2 X-ray diffraction analysis of Li1-xCoO2 with different lithium content

When charging, the phase change process of Li1-xCoO2 is O3→[O3+P3]→P3(01). As the oxygen in the P3 (01) structure is arranged along the c axis as AABBCC and ABABAB, the Co-O bond is maintained, and the CoO2 layer slips to form a new structure, as shown in Figure 3.

Figure 3 Phase transition of Li1-xCoO2 caused by changes in lithium content

When overcharged to 4.5V, the increase of Co oxidation number will cause the O-Co-O bond length to shorten, and the lattice constant a of the x-y plane shrinks by 0.3%. However, due to the repulsion of the CoO2 layer, the C axis increased by more than 2%, and the total unit cell volume expanded. Note that when x approaches 1, the lattice constant c drops rapidly.
When x>0.72, the charge compensation is not through the oxidation of Co but the release of lattice oxygen, which will lead to a decrease in the reversible capacity. When the temperature is higher than 50°C, the massive release of oxygen causes the crystal lattice to collapse. At the same time, the side reaction of the positive electrode material and the electrolyte causes the electrolyte to decompose and release gas, which makes the battery swell and makes the battery unsafe.

O3-LiNiO2 has an R-3m rhombohedral structure, a=2.887Å, c=14.227Å, c/a=4.928, and the unit cell volume (V=√3/2a2c) is 102.69Å3. Due to its actual specific capacity 20% more than LiCoO2, layered mountain LiNiO2 has been studied as an alternative material for LiCoO2. Due to the existence of stable Ni2+ in LiNiO2, the synthesized product is often non-metered Li1-yNi1+yO2. This is because part of the octahedral positions of Ni3+ and the positions of lithium ions can be replaced by Ni2+ under the condition of maintaining electrical neutrality. The unpaired electrons formed by the low-spin Ni3+ octahedral coordination are unstable and easy to transform into Ni2+. Therefore, the synthesis of LiNiO2 usually needs to be carried out under oxidizing conditions.
Comparing the crystal field stabilization energy of Ni ions, the relative energy levels of Ni4+(t2g6), Ni3+(t2g6eg1) and Ni2+(t2g6eg2) are -24Dq+2P, -18Dq+P and -12Dq, respectively. Assuming that the electron pairing energy P is relatively small, the electronic configuration of Ni2+ is the most stable. Similar to Co, considering the ionic radius, r(Ni2+)/r(O2-)=0.659 is within the octahedral configuration range of 0.414~0.732, r(Ni2+)/r(Li+)=0.922, that is, in the Hume-Rothery rule 7%~8% of Compared with Co2+, the radius of Ni2+ is closer to that of Li+. Therefore, Ni2+ is prone to appear in the lithium ion layer, generating non-quantitative Li1-xNi1+xO2.

Figure 4 Simulation diagram of cation migration path

Figure 4 shows the ion migration path when Ni2+ replaces lithium ions. In the cubic close-packed structure of oxygen atoms, transition metals and lithium occupy 3b and 3a octahedral positions, respectively, and all tetrahedra are empty. The transition metal in the 3b octahedron position can be moved to the nearest T1 tetrahedron position, and then to the farther 3a octahedral position. The other path is to move to the T2 tetrahedron position first, and then move to the octahedral position of the chamber 3a. The total distance of the two paths is equal.

In the non-metered Li1-xNi1+xO2, since lithium is replaced by Ni2+, the NiO2 layer forms a local three-dimensional structure. This hinders the diffusion of lithium ions and reduces the charge and discharge efficiency. The irreversible phase change during charging and discharging also reduces the capacity of the battery. Metered LiNiO2 can be prepared by hydrothermal method or redox ion exchange method, but the repeatability is poor. In addition, the low-spin Ni3+(d7) electronic configuration has the Jahn-Teller effect, which increases the bond length in the z-axis direction. The Jahn-Teller effect is caused by the CFSE of the MO6 octahedron that causes the d orbital to split into t2g and eg orbitals with different energies, increasing or shortening the bond length of the metal-oxygen bond in the z-axis direction. The t2g that obtains the extra stable energy is divided into dxy, dyz and dzx, and the eg is divided into dz2 and dx2-y2. Here, the repeated expansion and contraction in the z-axis direction lowers the electrical conductivity and deteriorates the electrode performance.

Figure 5 Cyclic Voltammetry of LiNiO2

Figure 5 is the cyclic voltammetry curve of LiNiO2. When Li1-xNiO2 is in the range of 0≤x≤0.75, the battery performance is hardly affected by the three irreversible phase transitions. When charged above 4.2V, a capacity of 160mAh/g can be obtained, which is higher than that of LiCoO2. The three phases involved in the phase transition are the rhombohedral phase in the range of 00.75, the battery performance is reduced, this is because lithium ions cannot be inserted back into the NiO2 that has been generated.

Although LiNiO₂ has a higher actual capacity than LiCoO2, it has not been used as a cathode material. The first reason is that in the non-metered Li1-xNi1+xO2, the diffusion of lithium ions is hindered by the Ni ions in the lithium layer, resulting in a decrease in capacity. Secondly, as the charging progresses and the lithium content decreases, the structural instability increases, causing the decomposition of oxidized substances to increase the oxygen pressure, and will react with the organic electrolyte to make the battery unsafe. LNi1-xCoxO2 and LiNi1-x-yCoxAlyO2 obtained by partially replacing nickel ions with other transition metal elements can be used instead of LiNiO2. When other M3+ with a fixed oxidation number is incorporated, it is difficult for N2+ to replace lithium ions and maintain electrical neutrality. The introduction of Co ions can effectively prevent Ni2+ from substituting lithium ions; while the emotional Al will affect the insertion/extraction of lithium ions. In high-temperature synthesis, an excessive amount of lithium salt is usually used to supplement the volatilization of lithium, which also helps prevent the production of Ni2+.

Because transition metal ions have different oxidation-reduction potentials, if the chemical reaction is not uniform, the performance of the electrode material will decrease during the charging and discharging process. In order to make the reaction uniform, other synthesis methods are often used instead of solid-phase reactions, such as co-precipitation sintering method, co-precipitation molten salt method and spray pyrolysis method.

In LiNi1-xCoxO2, the increase of Co substitution reduces the material lattice parameter, so the volume specific capacity can be increased. Figure 6 shows the charge and discharge curves of LiNi0.85Co0.15O2. The theoretical capacity of LiNi0.85Co0.15O2 is 274mAh/g, and the actual capacity is about 174mAh/g. Due to the substitution of Co, the Jahn-Teller effect is weakened, and less Ni2+ exchange (or the formation of NiO) occurs, and the phase change during charging and discharging is suppressed, so that the electrochemical performance is improved.

Figure 6 Charge and discharge curve of LiNi0.85Co0.15O2

During the charging process, the O3 layered structure of Li1-xNi1-yCoyO2 remains stable in the range of 0≤x≤0.70, and when 0.70<x<1, it transforms into a layered structure of O3 and P3 to coexist. When the lithium ions are completely extracted, a new O3 layered structure phase Ni1-yCoyO2 is formed. Figure 7 shows the change in the XRD pattern of the material in Li1-xNi0.85Co0.15O2 as the lithium content changes.

Figure 7 X-ray diffraction analysis of Li1-xNi0.85Co0.15O2 with different lithium content

Li1-xCoO2 transforms into O3 and P3 coexistence when 0.50<x<1, but for Li1-xNi1-yCoyO2, the same situation occurs at x=0.70. When charging, because only Ni3+ is oxidized, and Co3+ is not oxidized, the theoretical capacity of Li1-x.Ni0.85Co0.15O2 is about 233mAh/g. If phase change is not involved, the actual capacity of the material is 163mAh/g. However, if the irreversible phase transition of O3 and P3 coexistence is also taken into consideration, the capacity value can be increased to 174mAh/g. During charging, the low-spin Ni3+(d7) is oxidized to Ni4+, and the NiO6 octahedron twisted by the Jahn-Teller effect turns back into a symmetrical form.

3.LiMO2 (M=Mn, Fe)
Due to the thermodynamic instability, the layered structure of LiMnO2 is difficult to synthesize. But it can be prepared by first preparing the α-NaMnO2 precursor, and then performing the exchange reaction of lithium ions and sodium ions. Generally, 3%~10% of manganese occupies the non-metered oxide of lithium, rather than the layered LiMnO2 with well-developed crystal structure and conforming to the stoichiometric ratio. In Li1-xMn1+xO2, when 3% of lithium is replaced by manganese, the layered structure will be transformed into a spinel-like structure, and 4V and 3V voltage platforms will appear on the corresponding discharge curve, as shown in Figure 8 Shown. For layered LiMnO2, although lithium ions can be extracted 100%, they will irreversibly transform into a spinel structure during charging and discharging. Despite this transition, during the cycle, the capacity rises at the beginning and then gradually decreases.

Figure 8 A) 50th discharge curve (25 mA/g) and b) cycle characteristics of Na0.09Li0.59Mn0.94O2 obtained by ion exchange of Na0.51 Mn0.90O2 synthesized at 600°C at 80°C

If the orthorhombic LiMnO2 with a salt structure is charged at 2.0~4.5V, the initial capacity is as high as 200mAh/g. Since Li1-xMnO2 formed by the extraction of lithium has a very unstable structure, it is easy to transform into a spinel Li1-xMn2O4 with a relatively stable capacity (see Figure 8). Some studies have shown that the electrochemical performance of LiMnO2 can be improved by surface modification and improved synthesis process.

Although there are a lot of researches on the preparation of layered LiFeO2 by α-NiFeO2 ion exchange, its unstable layered structure will be transformed into spinel in the initial charging stage. The reason is that, from the general relationship between structure and relative size of cations, LiCoO2 and LiNiO2 satisfy the stable conditions of rB/rA<0.8, which are 0.76 and 0.78, respectively. However, LiFcO2 exceeds the limit value, which is rFe3+/rLi+=0.87. Secondly, there is no crystal field stabilization energy for high-spin Fe3+, and it is easy to move to the tetrahedral position. Corrugated, needle-shaped and tunnel-shaped iron compounds, such as FePS3, FeOCL and FeOOH, all show poor electrochemical performance. Compared with LiCoO2, the average voltage of iron-containing materials is not too high (Fe4+/Fe3+ ) Is too low (Fe3+/Fe2+) (see Figure 9). This is because in iron compounds in which oxygen is an anion, the interaction between iron and electrons is relatively large. At the same time, the Fermi potential of lithium is very different from Fe4+/Fe3+ (eg: 3d5σ*), and very small from Fe3+/Fe2+ (eg: 3d5π*).

Figure 9 Li/LiFeO2 charge and discharge curve

4.Ni-Co-Mn ternary system
The ternary material Li[NixCo1-2xMnx]O2 is a composite of high-capacity LiNiO2, LiMnO2 with good thermal stability and low price, and LiCoO2 with stable electrochemical performance. Demonstrates excellent electrochemical performance. According to first-principle calculations, the ratio of LiNi1/3Co1/3gMn1/3O2 with P3112 space group symmetry composed of low-spin Co3+, Ni2+ and Mn4+ to LiNi3+O2, LiCo3+ O2 and LiMn3+O2 mixed in a ratio of 1:1:1 are more stable. This means that LiNi1/3Co1/3Mn1/3O2 can be synthesized by an appropriate method. In this low-spin mixture, the lattice constant α is 4904Å, which is √3 times that of the LiMO2 material, which is caused by the uniform and regular arrangement of the three transition metal elements, as shown in Figure10. At the same time, manganese increases the lattice constant c to 13.88Å.

Figure 10 A simulation diagram of Li Ni1/3Co1/3Mn1/3O2 with [√3 x√3]R30° form superlattice [Ni1/3Co1/3Mn1/3O2] layer

Generally, LiCoO2 can form a solid solution with LiNiO2, but cannot form a solid solution with LiMnO2, while LiNiO2 and LiMnO2 can form a solid solution. Compared with the preparation of LNi1-xCoxO2, when preparing LiNi1/3Co1/3Mn1/3O2; uniform mixing of transition metals is more important. Therefore, the co-precipitation method is commonly used. When the hydroxide precursor is obtained by co-precipitation, Mn(OH)2 will be oxidized to produce MnOOH or MnO2. The manganese in the precipitated precursor can be maintained as Mn2+, but it is not easy to be completely eliminated in the subsequent sintering process. Impurities such as NiO and Li2MnO3 make the battery performance unsatisfactory.

If the electrical neutrality of the compound is to be maintained, lithium ions and manganese ions tend to adopt the electronic configuration of Ni2+ and Mn4+ instead of Ni3+ and Mn3+. In LiNi1/3Co1/3Mn1/3O2, the valence of nickel is 2+, the valence of cobalt is 3+, and the valence of manganese is 4+. It is generally believed that Ni2+ participates in the charge and discharge process, Co3+ is activated at the end of the charge, and the surface Mn4+ does not participate in the charge and discharge process, but the stability of the entire crystal structure is provided by the crystal field stabilization energy at the octahedral position.

Li1-xNi1/3Co1/3Mn1/3O2 will undergo structural changes during charging and discharging. The main manifestation is that the lattice parameter c increases with the decrease of lithium content, but when (1-x)<0.35, it will It is reduced due to the production of oxygen; the change of the lattice parameter α is just the opposite. In general, this material has a small volume change and is suitable as a positive electrode active material. Li1-xCoO2 has a P3 structure when x=1, and Li1-xNi1/3Co1/3Mn1/3O2 has an O1 structure when x=1. Considering that the two materials have an O3 structure in the range of 0≤x≤0.8, so The stable structure of Li1-xNi1/3Co1/3Mn1/3O2 material can be well maintained during the entire charging process. The excellent charge and discharge characteristics of this material make the corresponding battery exhibit long life and high safety. Figure11 is the discharge curve of Li1-xNi1/3Co1/3Mn1/3O2 cathode material under different discharge currents.
As the capacity is similar to LiCoO2, considering the performance, safety and cost, this material is a good substitute for LiCoO2. Recently, a variety of material composite research has appeared, the purpose of which is to increase the capacity by increasing the Ni content while retaining its advantages.

Figure 11 Rate performance of LiNi1/3Co1/3Mn1/3O2 charged to 4.6 V.
a) 2400 mA/g, b) 1600mA/g, c) 800mA/g, d) 400mA/g, e) 200mA/g,
f) 100mA/g and g) 50mA/g. Active material 88%; Acetylene black 6%; PVdF6%

5.Ni-Mn system
The lattice constant of LiNi1/2Mn1/2O2 is α=2.889Å, c=14.208Å, c/a=4.918, and the unit cell volume is 102.697Å3. Similar to Li1-xNi1/3Co1/3Mn1/3O2, the ratio of nickel to manganese in LiNi1/2Mn1/2O2 is 1:1, and the electrochemical performance is reduced due to the presence of NiO and Li2MnO3 impurities. Its theoretical capacity is 280mAh/g. Figure 12 is the charge and discharge curve of LiNi1/2Mn1/2O2; Figure 13 is the change of XRD spectrum caused by the lithium content in Li1-xNi1/2Mn1/2O2. Li1-xNi1/2Mn1/2O2 maintains a uniform O3 structure within the range of 0≤x≤1, and can actually release a capacity of 260mAh/g at 60°C.

Figure 12 Charge and discharge curve of LiNi1/2Mn1/2O2
Figure 13 X-ray diffraction analysis of different lithium content in Li1-xNi1/2Mn1/2O2

LiNiO2-LiMnO2The solid solution is essentially different from the LiCoO2-LiMnO2 and LiCrO2-LiMnO2 solid solutions. Ni2+ and Mn4+ are formed in the LiNiO2-LiMnO2 solid solution, while Co2+ and M4+ do not exist in LiCoO2-LiMnO2. In LiCrO2-LiMnO2, when the content of LiMn0₂ exceeds 30%, due to the stability of Cr3+, manganese exists in the form of Mn3+, and the material is monoclinic.

In the LiNiO2-LiMnO2 solid solution, because Mn4+ does not participate in the redox reaction in the charge and discharge process, there is no phenomenon of a layered structure changing to a spinel structure. The charging and discharging of Li[Ni1/2Mn1/2]O2 corresponds to the redox process of Ni2+/4+, and the reversible capacity and voltage plateau exhibited are similar to the Ni2+/4+ redox process of LiNiO2. According to theoretical calculations, when Mn-Ni changes from 2+/4+ to 4+/4+, the change in the interaction between them can explain the high working voltage. However, the synthesis of Li[Ni1/2Mn1/2]O2 is more complicated because of the impure spinel phase and Ni2+ appearing in the lithium layer. To overcome these problems, excess lithium is used in the reaction.

6.Lithium-rich materials
Since the discovery of LixCryMn2-yO4+z, the research on (1-x)Li2MnO3-x.LiMO2 (M=Ni, Co, Cr) solid solution (or nanocomposite) has been very active). Li[NixLi1/3-2x/3Mn2/3-x/3]O2 can be regarded as a solid solution containing electrochemically inactive Mn4+ oxidation state Li2MnO3 and Li[Ni1/2Mn1/2]O2, and its capacity can be as high as 250mAh /g. LiMO2-LiMnO2-Li2MnO3 solid solution can be regarded as a lithium metering phase, LiMO2-Li2MnO3 can be regarded as a lithium saturated phase, and LiMO2-LiMnO2-Li2MnO3 can be regarded as a lithium-rich phase. The cation at position 3b The occupancy number is 1, and its total oxidation number is +3. The redox reaction of manganese oxide is not Mn3+/4+. Because the valence state of manganese in these cathode materials remains Mn4+, the Jahn-Teller effect of Mn3+ is not Obviously, it has no effect on electrode performance.
Although the above-mentioned oxides are generally considered to be solid solutions, the TEM analysis of Li[CrxLi(1-x/3)Mn2(1-x/3)]O2 shows that when a certain composition is reached, this type of material will transform from a solid solution to a composite Objects (see Figure 14). When charged to 4.6~4.8V, the corresponding charge and discharge capacities of the material are 352mAh/g and 287mAh/g, respectively, and there is a voltage plateau at 4.5V (see Figure 15). The irreversible capacity loss originates from Li2O being released from Li2MnO3 during charging. Taking LiNi0.20Li0.20Mn0.6O2 as an example, charging first oxidizes Ni2+ to NI4+, and continues charging to produce Li2O, and finally Ni4+0.20Li0.20Mn0.6O1.7. During discharge, Mn4+/3+ is usually reduced, but the reason is not clear.

Figure 14 Electron diffraction pattern
Figure 5 First charge and discharge curve of Li/0.3Li2MnO3·0.7LiMn0.5Ni0.5O2 (5.0~2.0V)

According to recent studies, xLi2MnO3·(1-x)LiMO2 is charged to 4.4V, and after Li of LiMO2 is completely released, xLi2MnO3·(1-x)MO2 is formed; above 4.4V, as Li2O is released, (x- δ) Li2MnO3·δMnO2·(1-x)MO2 (see Figure16). When the voltage is higher than 4.4V, due to the extraction of lithium in Li2MnO3, Li2O and MnO2 are generated and oxygen is released. This process can be expressed as Li2MnO3→xLi2O+xMnO₂+(1-x)Li2MnO3. Since the amount of Li2MnO3 margin is determined by the cut-off voltage, increasing the cut-off voltage can reduce its proportion. It is necessary to set a suitable cut-off voltage to control the electrochemical reaction of Li2MnO3, so as to achieve the purpose of maintaining the stability of the material structure. In addition, during the discharge process after the first charge is completed, (x-δ)Li2MnO3·δLiMnO2·(1-x)LiMO2 is generated.

Figure 16 Composition changes and electrochemical reaction path based on the three-component phase diagram of xLi2MnO3·(1-x) LiMO2

Recent studies have derived some new cathode materials. For example, adding LiCoO2 to Li2MnO3-Li[Ni1/2Mn1/2]O2 solid solution to form Li2MnO3-Li[Ni1/2Mn1/2]O2-LiCoO2 can improve conductivity.
Li2iMO3 is added to the LiCoO2-LiNiO2-LiMnO2 oxide whose oxidation state is M3+, and the solid solution (Li2iMO3-LiCoO2-LiNiO2-LiMnO2) formed contains different valences, such as N2+ and Mn4+. As shown in Figure 17, adding Li2MnO3 to Li[Ni1/2Mn1/2]-LiNiO2-LiCoO2 can synthesize various forms of active cathode materials

Fig.17 Phase diagram of quaternary Li2MnO3-Li [Ni1/2Mn1/2 ]O2-LiNiO2-LiCoO2

The above oxide is more stable than LiCoO2, but when used as a positive electrode material, the electrolyte is required to be stable at 4.6V or more. In Figure 18, LiNi0.20Li0.20Mn0.6O2, which contains excessive lithium, has a much lower true density than LiCoO2, and its capacity as high as 220mAh/g fails to fully demonstrate its advantages. However, these oxides are good choices for batteries that require high safety and high energy. The replacement of Li2TiO3 with the same +4 compound as manganese is the focus of recent research, which is the result of comprehensive consideration of various factors such as manufacturing cost, ease of synthesis, and toxicity.

Figure 18 Discharge capacity of LiNi0.20Li0.20Mn0.6O2 and LiCo0.20Li0.27Mn0.53O2

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