What are the types of layered compounds for cathode materials?

1.LiCoO₂
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⁺ (t_2g⁶e_g¹), Co³⁺ (t_2g⁶) and Co⁴⁺ (t_2g⁵) are 18D_q+P, -24D_q+2P and -20D_q+2P. If the electron pairing energy P is relatively small, then Co²⁺ (t_2g⁶e_g¹) is the most stable. But in order for Co²⁺ to exist, non-metered Li_1-xCo_1+xO₂ must be formed to meet the requirements of electrical neutrality. Considering the ion radius, r(Co²⁺)/r(O^2-)=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 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. Co^3+/4+, the direct interaction of Co-Co formed by the filled t_2g ^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 Li_1-xCoO^₂, 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.

When charging, the phase change process of Li_1-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.

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.

2.LiNi_1-xCo_xO₂
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 102.69ų. 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 Li_1-yNi_1+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⁺(t_2g⁶), Ni³⁺(t_2g⁶e_g¹) and Ni²⁺(t_2g⁶e_g²) 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(O^2-)=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 Li_1-xNi_1+xO₂.

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 Li_1-xNi_1+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 t_2g and e_g orbitals with different energies, increasing or shortening the bond length of the metal-oxygen bond in the z-axis direction. The t_2g that obtains the extra stable energy is divided into d_xy, d_yz and d_zx, and the e_g is divided into d_z2 and d_x2-y2. Here, the repeated expansion and contraction in the z-axis direction lowers the electrical conductivity and deteriorates the electrode performance.

Figure 5 is the cyclic voltammetry curve of LiNiO₂. When Li_1-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 Li_1-xNi_1+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. LNi_1-xCo_xO₂ and LiNi_1-x-yCo_xAl_yO₂ 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 LiNi_1-xCo_xO₂, 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 LiNi_0.85Co_0.15O₂. The theoretical capacity of LiNi_0.85Co_0.15O₂ 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.

During the charging process, the O3 layered structure of Li_1-xNi_1-yCo_yO₂ 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 Ni_1-yCo_yO₂ is formed. Figure 7 shows the change in the XRD pattern of the material in Li_1-xNi_0.85Co_0.15O₂ as the lithium content changes.

Li_1-xCoO₂ transforms into O3 and P3 coexistence when 0.50<x<1, but for Li_1-xNi_1-yCo_yO₂, the same situation occurs at x=0.70. When charging, because only Ni³⁺ is oxidized, and Co³⁺ is not oxidized, the theoretical capacity of Li_1-x.Ni_0.85Co_0.15O₂ 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 Li_1-xMn_1+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.

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 Li_1-xMnO₂ formed by the extraction of lithium has a very unstable structure, it is easy to transform into a spinel Li_1-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 r_B/r_A<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⁺ (e_g: 3d⁵σ*), and very small from Fe³⁺/Fe²⁺ (e_g: 3d⁵π*).

4.Ni-Co-Mn ternary system
The ternary material Li[Ni_xCo_1-2xMn_x]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 LiNi_1/3Co_1/3gMn_1/3O₂ 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 LiNi_1/3Co_1/3Mn_1/3O₂ 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.

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 LNi_1-xCo_xO₂, when preparing LiNi_1/3Co_1/3Mn_1/3O₂; 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 LiNi_1/3Co_1/3Mn_1/3O₂, 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.

Li_1-xNi_1/3Co_1/3Mn_1/3O₂ 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. Li_1-xCoO₂ has a P3 structure when x=1, and Li_1-xNi_1/3Co_1/3Mn_1/3O₂ 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 Li_1-xNi_1/3Co_1/3Mn_1/3O₂ 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 Li_1-xNi_1/3Co_1/3Mn_1/3O₂ 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.

5.Ni-Mn system
The lattice constant of LiNi_1/2Mn_1/2O₂ is α=2.889Å, c=14.208Å, c/a=4.918, and the unit cell volume is 102.697Å3. Similar to Li_1-xNi_1/3Co_1/3Mn_1/3O₂, the ratio of nickel to manganese in LiNi_1/2Mn_1/2O₂ 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 LiNi_1/2Mn_1/2O₂; Figure 13 is the change of XRD spectrum caused by the lithium content in Li_1-xNi_1/2Mn_1/2O₂. Li_1-xNi_1/2Mn_1/2O₂ maintains a uniform O3 structure within the range of 0≤x≤1, and can actually release a capacity of 260mAh/g at 60°C.

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[Ni_1/2Mn_1/2]O₂ corresponds to the redox process of Ni^2+/4+, and the reversible capacity and voltage plateau exhibited are similar to the Ni^2+/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[Ni_1/2Mn_1/2]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 LixCryMn_2-yO_4+z, the research on (1-x)Li₂MnO_3-x.LiMO₂ (M=Ni, Co, Cr) solid solution (or nanocomposite) has been very active). Li[Ni_xLi_1/3-2x/3Mn_2/3-x/3]O₂ can be regarded as a solid solution containing electrochemically inactive Mn⁴⁺ oxidation state Li₂MnO₃ and Li[Ni_1/2Mn_1/2]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 Mn^3+/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[Cr_xLi_(1-x/3)Mn_2(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 LiNi_0.20Li_0.20Mn_0.6O₂ as an example, charging first oxidizes Ni²⁺ to NI⁴⁺, and continues charging to produce Li₂O, and finally Ni⁴⁺_0.20Li_0.20Mn_0.6O_1.7. During discharge, Mn^4+/3+ is usually reduced, but the reason is not clear.

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.

Recent studies have derived some new cathode materials. For example, adding LiCoO₂ to Li₂MnO₃-Li[Ni_1/2Mn_1/2]O₂ solid solution to form Li₂MnO₃-Li[Ni_1/2Mn_1/2]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[Ni_1/2Mn_1/2]-LiNiO₂-LiCoO₂ can synthesize various forms of active cathode materials

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, LiNi_0.20Li_0.20Mn_0.6O₂, 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.