Magnetic characterization of orthorhombic LiMnO2 and electrochemically transformed spinel LixMnO2 (x<1)

Orthorhombic LiMnO₂ exhibits complex magnetic behavior. In addition to short- and long-range antiferromagnetic ordering, we observed spin-glass behavior in the reported temperature regime of long-range antiferromagnetic ordering. Lithium extraction from LiMnO₂ further complicates its magnetic behavior. A broad maximum of susceptibility at ≈360 K, characteristic of well-ordered LiMnO₂, disappears upon electrochemical delithiation to Li_0.39MnO₂, indicating that two-dimensional ordering on the folded triangular Mn lattice in LiMnO₂ is destroyed as the cation sublattice begins to transform to a spinel. Spin-glass behavior is, however, observed in Li_0.39MnO₂ as well. Compared to conventionally prepared spinel LiMn₂O₄, a lower degree of frustration is deduced, which is attributed to incomplete spinel ordering in the early stages of the cycling-induced transformation. In addition, the fraction of Mn ions occupying tetrahedral sites during the spinel transformation has been quantitatively determined for the first time, using magnetic susceptibility data. The results, surprisingly, support the existence of low-spin Mn ions on tetrahedral sites in the electrochemically transformed spinel.