What is the spinel lithium manganese oxygen phase diagram?

There are many kinds of spinel lithium manganese oxides. In the research of lithium ion battery cathode materials, the main lithium manganese oxides that have received attention are spinel LiMn₂O₄, Li₂Mn₄O₉ and Li₄Mn₅O₁₂. Selecting different Li/Mn ratios and controlling the atmosphere and process conditions can obtain lithium manganese-oxygen solid solutions with different chemical compositions. Figure 1 is a phase diagram and a partial enlarged view of a manganese compound. It can be seen from Figure 1 that there are many compounds formed by lithium manganese oxide, and they can be transformed into each other under different conditions. This is the complexity of lithium manganese oxide as a cathode material. Figure 1(b) is an enlarged part of the MnO-Li₂MnO₃-λ-MnO₂ phase region in Figure 1(a), and Figure (c) is an enlarged view of the shaded phase region in Figure (b). In the figure, the MnO-Li₂MnO₃ connecting line represents the stoichiometric rock salt component; the Mn₃O₄-Li₄Mn₅O₁₂ connecting line represents the stoichiometric spinel component.

Figure 1 The oxide phase diagram of a lithium tank composed of spinel, defective spinel and rock salt structure

The triangular region composed of Mn₃O₄-Li₄Mn₅O₁₂-λ-MnO₂ is a defective spinel phase region, and lithium manganese oxide composed of any point in the region has a spinel structure.

The quadrilateral region formed by MnO-Li₂MnO₃-Li₄Mn₅O₁₂-Mn₃O₄ is a defective rock salt (layered structure) phase region. The defective lithium manganese oxide component in the triangular area formed by LiMn₂O₄-Li₄Mn₅O₁₂-MnO₂ can be obtained by chemical delithiation of stoichiometric spinel lithium manganese oxide Li_1+xMn_2-xO₄, so along the connecting line λ-MnO₂-Li₄Mn₅O₁₂ It means that Li₂O and λ-MnO₂ can form a completely mutually soluble solid solution. The LiMn₂O₄-λ-MnO₂ connecting line represents the electrochemical delithiation process of LiMn₂O₄. The λ-MnO₂-LiMn₂O₄-LiMnO₂ connection line represents the electrochemical lithium insertion process and phase transition of LiMn₂O₄ .

Figure 1(c) shows the spinel structure of interest, which can be divided into metered spinel Li_1+xMn_2-xO₄(0≤x≤0.33) and non-metered spinel. The latter includes two types: oxygen-rich (such as LiMn₂O_4+δ, 0<δ≤0.5) and hypoxia (such as LiMn₂O_4-δ, 0<δ≤0.14). In Figure 1(c), from A to C, the oxygen concentration increases, and from A to B, from C to B, the lithium ion concentration increases. It can be seen that the oxygen partial pressure and the lithium/manganese ratio have a direct impact on the composition and structure of the non-metered spinel. Oxygen-rich (such as LiMn₂O_4+δ, 0＜δ≤0.5) and hypoxia (such as LiMn₂O_4-δ, 0<δ≤0.14) type spinel lithium manganese oxide have different values ​​of δ, causing the average valence of manganese to change, thereby Make the non-metered spinel have different discharge specific capacity.

Strictly control the pressure and temperature of the atmosphere to make the reaction reach equilibrium, and stoichiometric compounds can be obtained. Therefore, to study non-stoichiometric compounds, such as the δ setting of LiMn₂O_4+δ, a series of stoichiometric compounds can be obtained by controlling a certain oxygen partial pressure and temperature. With the help of TG, XRD and elemental analysis are used to determine the non-stoichiometric δ value in the non-stoichiometric compound structure model. Spinel lithium manganese oxide with different lithium content will show different chemical reaction activity and electrochemical performance due to different chemical positions.

The spinel-type compound LiMn₂O₄ cathode material, in which the anion lattice contains cubic densely packed oxygen ions, is similar in structure to α-NaFeO₂, except for the distribution of cations at the octahedral position and the tetrahedral position. The discharge process is mainly divided into two steps, that is, around 4V, and the other platform is below 3V, as shown in Figure 2.
Charging: LiMn₂O₄→Mn₂O₄+Li (embedded in negative electrode, such as graphite carbon)

Figure 2 Voltage characteristics of lithium-manganese composite oxide materials with different doping levels

The intrinsic defect of spinel lithium manganese oxide mainly refers to the situation where Li and O deviate from the stoichiometric ratio. Because the synthesis conditions have a great influence on the content of Li, Mn, and O in LiMn₂O₄, it is very important to control various processes in the synthesis process. The first discharge capacity of stoichiometric spinel is high, up to 140mA·h/g, but in the high voltage area of ​​x<0.45 during the cycle, the coexistence of unstable two-phase structure causes the capacity attenuation; non-stoichiometric spinel Although the initial capacity of Li_1+yMn₂O_4+δ is low, only 110~120mA·h/g, the cycle stability is very good, and the desorption/intercalation of lithium is a homogeneous reaction. The stoichiometric spinel exhibits capacity loss during the cycle, which causes Mn to enter the 8a position, which turns the metered spinel into a defective spinel, and vacancies appear at the 16d position. Correspondingly, in the high voltage region of x<0.45 , The charge and discharge curve is changed from L type to S type. This shows that both lithium excess and oxygen enrichment are beneficial to improve the structure and stability of the solid solution.

Based on the results of DSC and in-situ XRD, some people analyzed the reason why Li_1+yMn_2-yO_4-δ changed from a cubic structure to a tetragonal structure during cooling to 210K, and believed that the existence of oxygen defects is the only necessary condition for phase transition. The synthesis temperature, heat treatment time, cooling method, etc. can all cause different degrees of oxygen defects, with different oxygen defect concentrations and different phase transition temperatures. The metered spinel LiMn₂O₄ has no oxygen defects, so there is no phase transition. With the increase of oxygen defects, the average valence of Mn decreases, the unit cell parameter a increases, and the phase transition temperature increases. Low-temperature baking of LiMn₂O₄ in an argon atmosphere may produce two types of point defects: oxygen defects or interstitial manganese ions. If the oxygen-deficient samples are fired again in an oxygen atmosphere, the original structure can be restored. This is the low-temperature baking of LiMn₂O₄ in an argon atmosphere. , Produced a direct proof of oxygen deficiency. It can be seen that processes such as synthesis temperature, heat treatment time, and cooling methods can all cause oxygen defects to varying degrees.

The cycle performance of the material is determined by its cubic lattice parameter value, and the cubic lattice parameter value is related to the average valence state of manganese. For example, the cubic lattice parameter value is about 0.823nm, this value is consistent with the lattice parameter value of the lithium-rich material Li_1+xMn_2-xO₄; in the lithium-rich material, the average valence of manganese is about 3.58; the valence is a numerical value The material can reduce the dissolution of manganese and prevent the Jahn-Teller distortion effect caused by Mn³⁺. The related values ​​of the lattice parameter a₀ in the chemical composition Li_1+xMn_2-xO₂ are shown in Figure 3(a). The general formula for calculating the parameters is a₀=8.4560-0.21746x.

Figure 3 Influence of different conditions on the lattice parameters of spinel Li_1+xMn_2-xO₄

The lattice parameters can be used to indirectly calculate the oxidation state of manganese, as shown in Figure 3(b). In addition, Figure 3(c) clearly shows the influence of the lattice parameters on the capacity loss rate after the first 120 cycles. At the same time doping with aluminum ions and fluoride ions can maintain the stability of the cycle capacity at high temperatures, such as Li_1+xMn_1-x-yAl_yO_4-zF_z. Here, the value of y and 2 is 0.1~0.3. In addition, if the voltage on the spinel surface is kept higher than the formation voltage of the Li₂Mn₂O₄ phase, the reaction of Mn³⁺ to Mn²⁺ on the uneven surface will decrease, and the balance of the following equation will shift 2Mn³⁺↔Mn²⁺+Mn⁴⁺ to the left. Since divalent manganese ions are easily dissolved in the acid electrolyte, it is necessary to prevent the formation of divalent manganese ions. Once it is dissolved in the electrolyte, the divalent manganese ions will diffuse to the cathode and be reduced to manganese metal at the cathode , Thereby depleting lithium ions and reducing the electrochemical capacity of the battery.

Spinel-type compounds are the research and development hotspot of high-energy lithium-ion battery cathode materials used in hybrid electric vehicles (HEV), although their capacity can only reach about 90mA·h/g at high rates. This type of material will self-discharge when it is fully charged, especially at high temperatures. LiBOB can be used to replace the LiPF₆ electrolyte, which is easily decomposed to generate HF in a wet state, to solve this technical problem. In the spinel used, lithium replaces part of the manganese to obtain a stable structure, such as Li_1.06Mn_1.95Al_0.05O₄. An alternative method is to coat the surface of spinel particles with certain materials, such as ZrO₂ or AlPO₄, which can also serve to adsorb HF.

Spinel Li₄Ti₅O₁₂ can also be used as a cathode material for high-energy batteries. Its charging voltage (lithium insertion) is about 1.55V. Therefore, there is no danger when lithium metal deposition occurs on graphite carbon at high rates. When the magnification is as high as 12C, the nanostructure and microstructure of the material coexist at 60℃. If the electrode is combined with a high-rate cathode material such as composite layered oxide or spinel LiMn₂O₄, it can be prepared with low cost and safety A good working voltage is a 2.5V battery; it can also be combined with olivine LiFePO₄, the resulting battery has excellent cycling performance at 1.8V, and a battery with no capacity loss after 100 cycles; if combined with high-potential spinel, the battery voltage It can reach 3.5~4V.

In terms of improving the cycle performance of spinel LiMn₂O₄, the use of impurity ions to stabilize the structure of spinel LiMn₂O₄ is considered to be one of the most effective methods to solve cycle capacity degradation. The choice of impurity ions directly affects the doping effect, and there are many metal ions available for doping spinel LiMn₂O₄.