What is the capacity of spinel compounds?

Studies have confirmed that the capacity of the oxygen stoichiometric spinel compound is consistent with the capacity calculated using chemical analysis to test Mn3+, but there are certain analytical errors. The calculation formula for the capacity of spinel compounds has been established using the lithium-manganese ratio (n) and the average oxidation number (m) of manganese obtained in the experiment. Classification of spinel compounds, spinel structure formula, and calculation of theoretical capacity The formula is shown in Figure 1. Based on different cation vacancies, oxygen stoichiometric spinels can be divided into lithium-rich spinels and oxygen-rich spinels. However, the ratio of Mn3+ to total Mn in the above two spinels is expressed by (4-m)/(m + n), and the theoretical capacity formula of the two spinels is the same even if n<0.5 This capacity formula is also applicable to the oxygen-depleted spinel with 4≤m+n. Two kinds of oxygen-depleted spinels with different chemical formulas can be formed, one is M3O4-δ (M is any cation), which only has vacancies in the oxygen position; the other is Li1-zMnO4-δ (z>0), It has a cation vacancy at the 8a site because manganese occupies only the 16d site in the oxygen-poor spinel. During the charging process, if the Mn3+ in the spinel compound is oxidized to Mn4+ and accompanied by the release of Li+, the capacity of the spinel is determined by the content of Mn3+ or Li+, and it can be considered that the Li+ at the 8a position is electrochemically active.

What is the capacity of spinel compounds?
Figure 1 Classification, structural formula and capacity of spinel Li-Mn-O compounds

The oxygen-depleted spinel can be classified according to the capacity. The capacity of one type of oxygen-depleted spinel is determined by the content of Mn3+, and its capacity calculation formula is the same as that of the oxygen-stoichiometric spinel mentioned above. Although not yet experimentally verified, this spinel capacity can be calculated to be 296n if the lithium content in the spinel limits its capacity.

At voltages of 3.2V and 4.5V, the oxygen-depleted spinel additionally has two identical discharge plateaus. The experiment confirmed the relationship between the oxygen deficit δ and the capacity at 3.2V, C3.2V (mA • h/g) = 444δ (see Figure 2). This capacity can be explained from the crystallographic point of view by the introduction of MnO5, as shown in Figure 3, when one oxygen ion disappears to form a hole, three manganese ions and one lithium ion surround one oxygen ion to form three Mn() 5. Three octahedral MnO6 share edges with three MnO5. The redox potentials of the 12 Mns change and generate new voltage plateaus at 3.2V and 4.5V. Here, the capacity of oxygen-depleted spinel is discussed. The capacity of 6 Mn3 + (half of 12 Mn) corresponding to each oxygen vacancy can be reflected by the total capacity at 3.2V and 4.5V. The formation of oxygen vacancies in LiMn2O4 can be expressed as:

What is the capacity of spinel compounds?
Figure 2 Relationship between Ca.3.2V and δ in LiMn2O4-δ
What is the capacity of spinel compounds?
Figure 3 Local structure of oxygen vacancies

1g of LiMn2O4-δ sample (relative molecular mass is Fw), the number of moles of oxygen missing is δ/FW. Therefore, the number of moles of Mn3+ changed from 6δ/FW due to the amount of oxygen deficiency. If the value of δ is small, the value of Fw in LiMn2O4-δ can be approximated to the molecular weight of LiMn2O4. In the 3.2V and 4.5V regions, the total capacity is 6δX148mA·h/g=888δmA·h/g, and the 3.2V capacity corresponds to half of the total capacity, 444δmA·h/g.

For the positive spinel material, the cycle performance can be improved by suppressing the formation of the λ-MnO2 phase, and by introducing Li+ and other metal ions to the 16d site, the electrode reaction under the high voltage platform becomes a single-phase reaction. However, the control of the 16d site composition reduces the content of Mn3+, resulting in a decrease in capacity. Therefore, this explains the capacity of spinel at Li/Other Metal = 0.5. In this case, the chemical formula of an oxygen-stoichiometric spinel containing metal (M) can be expressed as LixMyMn3-x-yO4 When the molecular weight of the spinel is close to that of LiMn2O4, the atomic ratio M/(Mn+M) is defined For, ƒ[=y/2], the specific capacity C (mA h/g) of metal-doped spinel can be calculated by the molar amount of Mn3+ contained in 1g spinel, and its expression is shown in Equation , the charge of the doped metal ion (M) is represented by υ+:

That is to say, the smaller the charge amount of the doped metal ion, the larger its ƒ value, and the lower the capacity. The maximum capacity can be obtained by designing the spinel composition where the cationic vacancy s is equal to zero.

In an ideal 5V spinel positive electrode material, manganese ions are tetravalent, and the doped metal ions are redox types. Well-known doped metals include nickel, copper, iron, cobalt, and chromium. The highest 5V capacity can be obtained by the combination of LiM0.5Mn1.5O4 and LiMMnO4, where M is divalent and trivalent, respectively. In the 5V region, although the expected capacity of LiNi1/2Mn1.5O4 can reach 145~147 mA h/g after doping with divalent metal compounds (Cu and Ni) under the double electron migration mechanism, its actual discharge capacity is only 140mA • About h/g. Oxygen deficiency in this spinel leads to the emergence of a 4V voltage plateau based on Mn3+/4+, which can be repaired using oxygen absorption techniques (annealing in an oxygen atmosphere) to improve 5V capacity, which is very important. Iron, cobalt, and chromium doped spinel (LiMMnO4) is the ideal compound with the highest capacity because the three metals are trivalent. Since there is no oxygen loss, LiCrMnO4 has no 4V plateau, so the capacity will not exceed 100mA • h/g in the 5V region.

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