A structure of composition AB₂O₄ in which cations occupy tetrahedral and octahedral sites in cubic close-packed oxygen. The cathode LiMn₂O₄ has three-dimensional Li⁺ pathways.
Japanese name: スピネル型構造
Spinel structure appears in 1 practice question across 1 topic, including Magnetic materials.
Field: Battery and energy materials
The spinel structure is named after the mineral spinel, MgAl₂O₄. Its unit cell contains 8 AB₂O₄ units, with 32 oxygens forming cubic close packing. Of the 64 tetrahedral interstices among them, 8 are occupied by A, and of the 32 octahedral interstices, 16 by B. The arrangement with A in tetrahedral and B in octahedral sites is called normal spinel, and the one in which half of B occupies tetrahedral sites and A octahedral sites is called inverse spinel; magnetite, Fe₃O₄, is the classic inverse spinel. Which arrangement forms depends on ion size and on differences in crystal field stabilization.
The cathode LiMn₂O₄ is a normal spinel with Li⁺ in tetrahedral sites (8a) and Mn in octahedral sites (16d). Li⁺ moves to a neighboring 8a site via an empty octahedral site (16c), and because these pathways connect in 3 directions, the material can be charged and discharged quickly, suiting high-power uses. Its average voltage is about 4.0 V and its theoretical capacity about 148 mAh/g; it uses no Co, is inexpensive, and is comparatively thermally stable. The Mn₂O₄ framework hardly collapses when Li⁺ is removed, and the fully delithiated form is called λ-MnO₂.
The weakness of LiMn₂O₄ is capacity fade, especially at high temperature. Through the action of hydrogen fluoride and similar species in the electrolyte, Mn³⁺ disproportionates into Mn²⁺ and Mn⁴⁺; the Mn²⁺ dissolves into the electrolyte, deposits on the anode, and damages the SEI, accelerating degradation. In addition, discharging to around 3 V inserts excess Li⁺ and raises the fraction of Mn³⁺, and the Jahn–Teller effect distorts the octahedra, causing a large phase change from cubic to tetragonal. Replacing part of the Mn with Al, Ni or similar elements, and coating the surface, are used to suppress this degradation.
In LiNi₀.₅Mn₁.₅O₄, in which a quarter of the Mn is replaced by Ni, the Ni²⁺/Ni⁴⁺ reaction is used and a high voltage of about 4.7 V is obtained. At this voltage, however, ordinary electrolytes decompose easily, so developing suitable electrolytes is a challenge. On the anode side, spinel Li₄Ti₅O₁₂ reacts at about 1.55 V, and its lattice size hardly changes as Li is inserted and removed (zero strain), so it has a very long life, forms little SEI and is safe. Its higher voltage lowers the energy density, but it is used for fast charging and applications such as buses.
Answer: B. 4 µB
The B-site moment is 5 + 4 = 9 µB and the A-site moment is 5 µB, and since they are antiparallel, 9 − 5 = 4 µB remains. You can also think of the Fe³⁺ ions on the A and B sites as exactly canceling each other, leaving only the contribution of Fe²⁺. The value obtained from the measured saturation magnetization is about 4.1 µB, in good agreement with this estimate. 14 µB assumes all the moments are parallel (the mistake of treating it as ferromagnetic), and 0 µB assumes everything cancels (the mistake of treating it as antiferromagnetic). Replacing Fe²⁺ with another divalent ion (such as Ni²⁺ or Mn²⁺) changes the saturation magnetization according to that ion's moment.
In normal spinel, A occupies tetrahedral sites and B octahedral sites. In inverse spinel, half of B occupies tetrahedral sites and the remaining B and all of A occupy octahedral sites. Fe₃O₄ and nickel ferrite, NiFe₂O₄, are inverse spinels.
Assuming 1 Li⁺ is removed per mol of LiMn₂O₄, with a formula weight of about 180.8 g/mol, the capacity is 96485/(3.6 × 180.8) ≈ 148 mAh/g.
Because the Li⁺ pathways are connected in three dimensions, so the ions can move in any direction. Li⁺ enters and leaves the particles easily regardless of their orientation, making charge and discharge at high current easier.
Saturation magnetization Layered rock salt structure Olivine structure Polymer electrolyte fuel cell Shockley–Queisser limit Solid electrolyte Theoretical capacity Thermoelectric figure of merit