What is non-stoichiometric manganese spinel?
Twenty years ago, researchers discovered that manganese spinel-type compounds can be oxidized electrochemically. Early research on manganese spinel focused on the performance of 3V cathode materials. Studies have shown that spinel compounds can be divided into cation-deficient type (Li2Mn4O9) and lithium-rich type (Li4Mn5O12). In addition, anoxic spinel compounds are synthesized at high temperatures. Lithium manganese spinel is a complex non-stoichiometric compound in which lithium and manganese are distributed in the cubic close-packed oxidation of oxygen anions.
Regardless of whether the close-packed structure of oxygen ions remains stable, spinel compounds can be divided into oxidation stoichiometric spinel and anoxic spinel, and the cycle performance of the two spinels is significantly different.
Oxidation stoichiometric compound as a battery cathode material has excellent cycle performance. It is composed of three oxidation stoichiometric spinels: LiMn2O4, Li4Mn5O12 (χ represents the molar ratio of Li4/3Mn5/3O4), Li2Mn4O9 (y represents The molar ratio of Li8/9Mn16/9O4), these compounds can be represented by the general formula Li1+χ/3-y/9Mn2-χ/3-2y/9O4. If y>0, then its oxygen is excessive, because the oxidation stoichiometric spinel is composed of oxygen-rich Li2Mn4O9 (cation absent).
The composition of Li1+χ/3-y/9Mn2-χ/3-2y/9O4 is as in the ABC triangle of Li-Mn-0 phase diagram (see Figure 1), three oxidation stoichiometric spinels LiMn2O4, Li4/3Mn5/3O4 and Li8/9Mn16/9O4 are located at the apex of the triangle. The empty cation lattice (8a and 16d) and the empty anion lattice (32e) are represented by the same symbol “口”. The phase diagram is expressed by two parameters: the average oxidation number of manganese (m) and the atomic ratio of lithium to manganese in the spinel (n). The parameters can be directly determined by chemical analysis. The spinel parallel to AC has the same n value, and the spinel parallel to BC has the same m value. If n<0.5, the oxidation stoichiometric spinel is outside the AGC triangle. Spinel can be divided into two categories according to its structure. One type is lithium-rich spinel where cations are only vacant at position 8a (triangular ABD area), and the other type is oxygen-rich spinel where cations are vacant at position 8a and/or 16d (polygonal ADCG area).
The polygonal ABEFG area contains hypoxic spinel, and E, F, and G are compounds in the critical state. The relational equation at the straight line AG is: m+n=4, the relational equation at the straight line AE is: 3m +n = 11; point F is a point on the AB extension line when n=0.45. The lithium content at position 8a is lower than the content of Mn3+ in the ABE triangle area, and the relationship is reversed in the AEFG area. The preparation of spinel at a higher temperature will lead to hypoxia. As the temperature increases or the time increases, its composition will move up to the AB line in parallel, and the value of n will not change. The spinel compound located in the polygonal AGFH region is vacant at the position of cation (8a) and oxygen (32e); the spinel located in the triangular AEH region has only oxygen vacancies. Pure hypoxic spinel with high crystallinity is distributed near point A.
Yoshio and Xia took the lead in expounding the relationship between spinel’s cyclic performance and hypoxia in 1997. The research on the capacity of the hypoxic spinel at a voltage of 3.2~3.3V verifies that the hypoxic spinel is a LiMn2O4-& type crystal structure, which will be described in detail below. However, there are several different opinions on the structure of hypoxic spinel: it has no substantial hypoxia, only manganese ions move to position 8a; manganese ions move to position 16c; there are undetectable impurities of Mn2O3. From the quadrangular spinel structure of Mn3O4, it can be inferred that manganese ions occupy the 8a position. When the lithium content is reduced, the average oxidation number of manganese is close to 3. Due to the Jahn-Teller effect of Mn3+, the crystal structure is transformed from a cube to a tetragon, and manganese ions are transferred to the 8a position. Kanno et al. used neutron diffraction to have high sensitivity to oxygen and lithium, and obtained highly reliable data, confirming that the cubic spinel is anoxic. They also confirmed that the 8a position of the hypoxic spinel was not mixed with cations.
After the first systematic study of the relationship between the oxygen content in spinel and battery performance and the changes in crystal structure, it was found that the battery characteristics of spinel compounds largely depend on the oxygen content. Studies have shown that the electrochemical performance of spinel is poor, especially the electrochemical performance of hypoxic spinel is worse. At room temperature, the capacity of the spinel compound will decrease during the cycle, which is caused by the Jahn-Teller effect. The above-mentioned spinel compound is anoxic spinel, which will be described in detail below.
The charge-discharge curves of stoichiometric spinel and hypoxic spinel are shown in Figure 2. In the figure (a) and (b) are the charge-discharge curves of the oxidation stoichiometric spinel LiMn2O4.02 and the oxygen-deficient spinel Li1.002Mn1.998O3.9812O4.02, respectively. The charge-discharge curves of LiMn2O4.02 There are two flat voltage platforms near 4V: 4.0V (low voltage platform) and 4.15V (high voltage platform). Here, the charge and discharge products of LiMn2O4 are represented as Li1-χMn2O4. The low-voltage plateau (χ<0.5) is a single-phase region (only the cubic II phase exists), and the a-axis of the spinel Li1-χMn2O4 shrinks continuously with the increase of χ. The high voltage plateau (χ>0.5) is a two-phase region, two cubic phases with different lattice parameters, Li0.5Mn2O4 (cubic II phase) and λ-MnO2 (cubic III phase) coexist.
On the other hand, in the discharge curve of anoxic spinel such as Li1.002Mn1.998O3.981, in addition to the high voltage plateau and the low voltage plateau, there are also two voltage plateaus at 3.2V and 45V. Its electrochemical reaction is different from the oxidation stoichiometric spinel under the low-voltage platform, and it is the two-phase coexistence mechanism of the cube phase I and II. In the two-phase region, since the charge and discharge process is accompanied by a phase transition, the capacity is prone to attenuation during the cycle.
Figure 3 shows the relationship between spinel compound hypoxia and cycle performance at room temperature. It can be seen from the figure that the relationship between hypoxia (δ) and capacity retention is linear, and the cycle performance at room temperature is determined by the hypoxia. In addition, when δ = 0, the extrapolated value of the capacity retention rate is 100%, which indicates that the room temperature cycling capacity will not decrease in the absence of oxygen deficiency. Based on the above discussion, the capacity decline of hypoxic spinel during the room temperature cycle is caused by the two-phase reaction on the high and low voltage platform.
In addition, the capacity decay of the stoichiometric spinel during the cycle is also caused by the two-phase reaction of the high voltage plateau, but the capacity decay is slower than that of the hypoxic spinel. In other words, the decay of the remaining capacity of the stoichiometric spinel is due to the formation of the λ-MnO2 phase (cube III) during deep delithiation. If the deep delithiation of spinel is suppressed, all electrochemical reactions in the 4V range are single-phase, and the cycle performance will also be improved. Using spinel Li1+χMn2O4, which is not oxygen-deficient and rich in lithium, can overcome deep delithiation.
However, when a lithium-rich compound is used, it is easy to form an oxygen-deficient spinel. This problem can be easily solved by doping metal ions to promote the formation of oxygen-rich spinel. In other words, using the above-mentioned doping technology to process lithium-rich compounds can easily form an oxidation stoichiometric spinel. The amount of cation absent of metal-doped spinel can be calculated from the relationship between the number of doped metal ions and the capacity (although it depends on the synthesis temperature). These spinels doped with 1% of chromium, 1% to 1.5% of cobalt and aluminum, and 1.5% to 2% of nickel proved that doping with metal ions can promote the formation of stoichiometric spinel. Even in the case of metal doping, the lack of oxygen when preparing spinel at about 800°C cannot be ignored. Therefore, this synthesis method needs to meet two conditions, namely, ensuring the stoichiometry of oxygen and preventing the formation of a λ-MnO2 voltage plateau at 3.2V in the charged state, which confirms that the spinel compound is hypoxic.
At the same time, battery manufacturers recommend using high-temperature synthesis to prepare spinel with low specific surface area because they believe that the capacity decay of spinel compounds described in some papers under high-temperature cycles depends on the dissolution of manganese. So some companies used hypoxic spinel and tested its physical and chemical properties and electrochemical properties. However, these early reports on the properties of hypoxic spinel caused people to misunderstand the characteristics of spinel as follows:
① Spinel deteriorates during the cycle;
② Experiments on the relationship between storage time and capacity decay at different depths of discharge show that performance degradation is the most serious when the depth of discharge is 60% to 100%;
③When the temperature is lower than room temperature, the structure of the spinel compound will change.
The above is the description of hypoxic spinel, which does not apply to oxidation stoichiometric spinel. It should be particularly emphasized that the stoichiometric spinel does not have these three characteristics. As mentioned earlier, the oxygen content data of spinel compounds can be reliably measured by chemical analysis methods.
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