What are the types of spinel compounds for cathode materials?

Spinel compound: LiMn₂O₄ / LiM_xMn_2-xO₄ (M=transition metal)

1.LiMn₂O₄
In the cubic spinel structure LiM₂O₄ (M=Ti, V, Mn), oxygen is closely packed according to the cubic ABCABC (see Figure 1), located at position 32c. LiMn2O4 has a lattice parameter α=8.245A, which is a representative spinel active material with high capacity. At the same time, manganese is easily available, cheap, and environmentally friendly.

The valence of manganese is affected by the composition of the reactants, heat treatment temperature and other synthesis conditions, and can vary from 2+ to 4+. Because Mn can occupy the tetrahedral or octahedral position in the spinel structure, the synthesized materials may have different compositions and the corresponding electrochemical properties are also different. Therefore, the synthesis of LiMn₂O₄ is more complicated than the synthesis of other cathode materials, and the reaction involves a variety of phase transitions. Figure 2 shows various manganese oxides with different compositions and valences.

Transition metals and lithium ions occupy vacancies created by oxygen accumulation in spinel LiM₂O₄, and their arrangement is determined by electrostatic attraction, repulsion and ion radius. 3d transition metals with a valence of 3+ or 4+, when the ratio of the ion radius to the oxygen ion radius is 0.476≤r(M³⁺)/r(O2﹣)≤0.702 or 0.492≤r(M⁴⁺)/r(O^2﹣) When ≤0.591, it can be located in the octahedral position. In fact, in layered materials and spinel materials, all transition metal elements occupy octahedral positions. One MO₆ is arranged around six adjacent MO₆ to form a two-dimensional layered structure. In spinel, the same arrangement is three-dimensional (see Figure 3). This leads to the formation of transition metal ions with different valences. Because of the difference in charge distribution and the bond length of M³⁺O₆ and M⁴⁺O₆, the three-dimensional octahedral arrangement is more uniform than the two-dimensional.

In the three-dimensional structure of (M₂O₄)^1-, in order to maintain electrical neutrality, lithium ions are the farthest away from M³⁺ and M⁴⁺, at the 8a position where the electrostatic repulsion is the smallest. Lithium ion and M³⁺ and M⁴⁺ occupy positions 8a and 16d respectively, so they can be represented by (Li)_8a[M₂]_16d[O₄]_32e. Spinel compounds provide a way for lithium ion migration during charge and discharge through three-dimensionally connected coplanar octahedrons.

Generally, the electrochemical characteristics of Mn spinel are manifested as two voltage platforms. In Li_1-xMn₂O₄, when 0≤1-x≤1, lithium ions are inserted/extracted at about 4V, maintaining a cubic structure. In Li_1+xMn₂O_4, when 1≤1+x≤2, the lithium ion in the 16e position is inserted/extracted at about 3V, accompanied by the phase change between the cubic phase LiMn2O4 and the tetragonal phase Li₂Mn₂O₄ (see Figure 4 and Figure 5). That is to say, the same Mn^3+/4+ oxidation-reduction reaction has a 1V potential difference, which is mainly caused by the energy gap between the lithium at the 8a position in the cubic phase LiMn₂O₄ and the lithium at the 16c position in the tetragonal phase Li₂Mn₂O₄.

In Li_1-xMn₂O₄, when 0≤x≤0.73, the insertion/extraction of lithium ions is reversible. The electrode potential of Li_1-xMn₂O₄ is mainly affected by the average valence of Mn until 50% of the bond ions at the 8a position are removed. However, when x>0.5, it is also affected by the change in the energy of lithium ion extraction, which is caused by the rearrangement of the remaining lithium ions. This change at Li_0.5Mn₂O₄ makes the material appear two voltage plateaus at 4.0~4.2V.

The lattice constants of LiMn₂O₄ and Li_0.5Mn₂O₄ are 8.245Å and 8.029Å, respectively. When the average valence of Mn ions is greater than 3.5, the cycle performance is improved due to the inhibition of the dissolution of Mn and the Jahn-Teller effect of Mn³⁺. When excessive lithium ions are inserted into Li_1+xMn₂O₄ (0.03≤x≤0.05), lithium enters the 16d position, which increases the valence of Mn, thus making the material display a more stable reversible cycle capacity. At this time, the lattice constant can be calculated from a=8.4560-0.2176x, and the average valence of manganese is greater than 3.58. This means that the change of lattice parameters during charging and discharging is very important because it directly affects the average valence of Mn. However, due to the reduction of Mn³⁺ involved in the redox reaction, the theoretical capacity of Li_1.05Mn_1.95O₄ at 4V is only 128mAh/g. In the similar Li_1.06Mn_1.95Al_0.05O₄, while increasing the valence of Mn, it replaces Mn with Al and further increases the stability.

Initially, the average valence of Mn in [Li⁺]_8a[Mn³⁺Mn⁴⁺]_16dO₄ was 3.5. Since the position of the 8a tetrahedron is close to the position 16c, which accounts for 50% of the octahedron, lithium ions can reversibly migrate along the path of 8a→16c-→8a-→l6c→8a, while the [M₂]O₄ spinel structure remains unchanged. The three-dimensional spinel structure provides a short diffusion path for lithium ions, that is, high ion conductivity, and this structure has good thermal stability during charging. When Li_1+xMn₂O₄ is discharged, the excessively inserted lithium ions occupy the empty 16c octahedral positions, and have strong electrostatic repulsion with the lithium ions at the 8a tetrahedral position. This is because the 8a tetrahedron and the 16c octahedron are coplanar. At this time, the lithium ion at the position of the 8a tetrahedron will move to the position of the 16c octahedron, thus forming a rock salt structure (Li₂) _16c[M₂]_16d[O₄]_32e° As the excessively inserted lithium x in Li1+xMn2O4 increases, 16d Most of the Mn at the octahedral position becomes Mn3+(d4). The cubic-tetragonal structure change caused by the Jahn-Teller effect increases c/a by 16%, the unit cell volume increases by 6.5%, and the capacity drops rapidly.

In the initial stage of excessive lithium ion insertion, the surface of the positive electrode material is in an overdischarged state, and its thermodynamic equilibrium state is destroyed, resulting in an irreversible phase transition of cubic to tetragonal. The 4V platform using only LiMn₂O₄ can obtain an actual capacity of 120mAh/g.

During the discharge process, the disproportionation reaction of Mn ions on the electrode surface (2Mn³⁺=Mn²⁺+Mn⁴⁺) produces M²⁺. The dissolution of M²⁺ in the acid electrolyte reduces the amount of LiMn₂O₄ active material. At the same time, the dissolved manganese destroys the electrodeposition of the healthy ions on the negative electrode or becomes a catalyst for the decomposition of the electrolyte, thereby reducing the capacity. At high temperatures, this catalytic reaction is strengthened, making the capacity drop more significant.

2. LiM_xMn_2-xO₄ (M=transition metal)
   In order to reduce the Jahn-Teller distortion caused by Mn³⁺ ions and maintain a stable structure, transition metal ions with valence lower than 3+ (M=Co²⁺, Ni²⁺, M^g2+, Cu²⁺, Zn²⁺, A1³⁺, Cr³⁺) or healthy ions are often used to replace M³⁺. When a transition metal of 2+ or 3+ valence replaces Mn, the average valence of Mn increases, thereby increasing stability and improving cycle life. Figure 6 shows how the capacity of LiMn_2-yM_yO₄ varies with charge and discharge cycles. Comparing the two figures, it can be seen that Mn substitution improves the cycle life. By coating the surface of LiMn₂O₄ with transition metal oxides, such as Al₂O₃, LiCoO₂, MgO and ZrO₂, to reduce the interface reaction with the electrolyte, the cycle life can be further improved. It is more effective because Mn substitution to increase its valence will bring about a decrease in capacity, and modification of the surface of the material in contact with the electrolyte can minimize the decrease in capacity, so it is more effective.

The spinel stabilized by the above substitution or surface modification can be used in high-power lithium ion secondary batteries for hybrid vehicles. With the expansion of the application of batteries in the transportation field, their capacity continues to increase, and safety has become the first issue to be considered. Mn spinel material is more stable than the existing layered structure material. Although it still has problems such as high temperature attenuation and self-discharge caused by Mn dissolution, it is still considered to be the best cathode material for batteries used in the transportation field.

When in Li_1+xMn_2-xO₄, x=0.33, Li₄Mn₅O₁₂ is obtained. It can also be seen that 1/6 of the manganese ions at the 16d position in LMn₂O₄ are replaced by lithium ions to form Li[Li_1/3Mn_5/3]O₄. Since the valence of Mn in Li₄Mn₅O₁₂ is 4+, lithium ions can only enter electrochemical intercalation. The intercalation of lithium causes an increase in Mn³⁺ with the Jahn-Teller effect, but the cubic-tetragonal irreversible phase transition does not affect Li_6.5Mn₅O₁₂, because the average valence of Mn is 3.5+. Therefore, Li₄Mn₅O₁₂ can be used as a 3V cathode material with a theoretical capacity of 163mAh/g and an actual capacity of 130-140mAb/g. The relatively small capacity of LiMn₂O₄ makes it unable to replace current cathode materials.