Insights into Polymorphism of Lithium Manganese Oxide, Li_0.95Mn_2.05O₄: A Comprehensive Survey of the High-Pressure Properties

Abstract

Understanding the structural behavior of lithium manganese oxide (LMO) under high pressure, especially under nonhydrostatic pressure conditions, could provide more insights into control of the mechanochemical processes occurring during battery operation and allow predicting new applications for this type of oxides. Here, we report pressure-induced structural changes in Li0.95Mn2.05O4 at two temperatures, 300 and 380 K, investigated in situ through synchrotron X-ray powder diffraction up to 13 GPa in a diamond anvil cell. Compression-induced strain triggers a cascade of local structure deformations, which finally result in the structural phase transition to the high-pressure tetragonal phase. A comparison of bulk moduli at two temperatures reveal that due to the differences in the response of the structure to the strain, Li0.95Mn2.05O4 exhibits the rare property of “warm hardening”. Additionally, a direct correlation between hydrostaticity of the pressure-transmitting medium and structural transformations was found. Our findings demonstrate that high pressure can be a robust tool to tune the structural properties and provide insights into the relationship between the strain and the structure of lithium manganese oxides under extreme conditions.

Structural Stability and Properties of Marokite-Type γ-Mn3O4

“…Recall here that the most common mixed-valence states of Mn ions filling octahedral sites in manganite perovskites are known to be Mn 3+ /Mn 4+ , whose ionic radii have a comparable difference (78.5 pm vs 67 pm for HS-Mn 3+ and Mn 4+ ) . Such mixed-valence Mn 3+ /Mn 4+ states are common for spinels, too, if the tetrahedral position is filled with monovalent cations, e.g., in Li + Mn 2 O 4 . , …”

“…In transition-metal compounds, the electronic states of cations with given oxidation states are primarily dictated by their anion coordinations. Figure a depicts a typical d-orbital splitting for HS-Mn 3+ (3d 4 ) and HS-Mn 2+ (3d 5 ) ions for highly symmetric octahedral and trigonal prismatic oxygen coordinations. − Any deviations in polyhedral symmetry, e.g., Jahn–Teller distortion, should lead to an additional splitting of the energy levels . As shown in Figure d, the Mn1 octahedra in γ-Mn 3 O 4 are subjected to strong Jahn–Teller distortion.…”

“…As shown in Figure d, the Mn1 octahedra in γ-Mn 3 O 4 are subjected to strong Jahn–Teller distortion. Hence, the two upper e g levels of the octahedral ions should be significantly split (Figure a), , thereby restricting an electron transfer between the neighboring Mn 3+ cations. This fact can explain the robust insulating nature of γ-Mn 3 O 4 (Figures and ).…”

“…125 Such mixed-valence Mn 3+ / Mn 4+ states are common for spinels, too, if the tetrahedral position is filled with monovalent cations, e.g., in Li + Mn 2 O 4 . 9,10 In transition-metal compounds, the electronic states of cations with given oxidation states are primarily dictated by their anion coordinations. Figure 8a depicts a typical d-orbital splitting for HS-Mn 3+ (3d 4 ) and HS-Mn 2+ (3d 5 ) ions for highly symmetric octahedral and trigonal prismatic oxygen coordinations.…”

“…Synthetic manganese oxides and manganese oxide minerals play an important role in different technological processes and applications. , For example, some of them are utilized as effective catalysts, − as supercapacitors, as ferroelectrics or multiferroics, − as elements in ion batteries, − as semiconductors, and for many other purposes. − In general, manganese oxides, including Mn-based perovskites, − are inexpensive and highly functional materials, which combine interesting electronic and magnetic properties and, therefore, have remained in the focus of recent investigations.…”