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 M₃O_4-δ (M is any cation), which only has vacancies in the oxygen position; the other is Li_1-zMnO_4-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.

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 Mn^{3 +} (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:
LiMn₂O₄=LiMn₂O_4-δ+δ/2O₂

1g of LiMn₂O_4-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 LiMn₂O_4-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 Li_xM_yMn_3-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⁺:
C(mA·h/g)=148[1-3s-(4υ)f]

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 LiM_0.5Mn_1.5O₄ and LiMMnO₄, where M is divalent and trivalent, respectively. In the 5V region, although the expected capacity of LiNi_1/2Mn_1.5O₄ 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 Mn^3+/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.