Lithium-rich, oxygen-deficient spinel obtained through low-temperature decomposition of heterometallic molecular precursor
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Abstract
A heterometallic single-source molecular precursor Li2Mn2(tbaoac)6 (1, tbaoac = tert-butyl acetoacetato) has been specifically designed to achieve the lowest decomposition temperature and a clean conversion to mixed-metal oxides. The crystal structure of this tetranuclear molecule was determined by single crystal X-ray diffraction, and the retention of heterometallic structure in solution and in the gas phase was confirmed by nuclear magnetic resonance spectroscopy and mass spectrometry, respectively. Thermal decomposition of this precursor at the temperatures as low as 310 oC resulted in a new metastable oxide phase formulated as lithium-rich,
Keywords
INTRODUCTION
Gaining access to kinetically metastable compounds is an important step toward the discovery of new functional materials having complementary or even superior properties to the known thermodynamically stable phases[1]. The intrinsic instability of such compounds often stems from their structural and compositional flexibilities that are sensitive to the external environment and build up the foundation for new interesting characteristics and reactivities[2]. The keys for those low-temperature/metastable phases are their constituent elements and chemical/coordination versatility[3].
Manganates are among the most promising classes of compounds for exploring new materials due to the unique position that manganese holds within the family of transition metals[4]. Manganates typically feature Mn in oxidation states of +3 or +4, often in varying ratios. The easy transition between Mn+3 and Mn+4 oxidation states can result in oxygen deficiency causing distorted coordination environments[5,6], while the Jahn-Teller effect, originating from the unique electronic configuration of Mn3+, induces significant distortion in the [MnO6] octahedra[7]. These two factors lead to a considerable variation in Mn-O bond lengths in manganates. Those distortions enhance the structural diversity of phases that are otherwise typically constructed with highly symmetric Oh octahedra[7,8]. Manganates are well-known for their diverse arrangements of octahedral building blocks, resulting in unique structures (such as tunnel structures) that are uncommon among other transition metals[9,10].
Incorporating a second metal into the manganate framework can further enhance its structural and compositional diversity, significantly broadening the potential applications of Mn-based materials[11,12]. Li+ ions can occupy both tetrahedral and octahedral lattice sites introducing structural and compositional variations that are unknown for manganese neighbors on the Periodic Table[13,14]. These variations translate into a broad range of the Li:Mn ratios and different oxidation states for manganese from +3 (LiMnO2) to mixed +3/+4 (LiMn2O4) to +4 (Li2MnO3 and Li4Mn5O12). Structurally, these variations result in tertiary oxides with at least three distinct structure types: layered (LiMnO2), spinel (LiMn2O4), and rock-salt
It is not surprising that a number of unusual compounds in the Li-Mn-O system have been reported as metastable phases that exhibit unique properties[18]. For example, Li4Mn2O5 with a rock-salt type structure was obtained through a room-temperature cation exchange approach[17]. This compound decomposes into thermodynamically stable phases such as LiMnO2, Li2O, and Li2MnO3 at elevated temperatures[19]. Li4Mn2O5 has demonstrated a discharge capacity of 355 mAh/g as a positive electrode (cathode) in Li-ion batteries, which is the highest among all known lithium-transition metal oxides[17]. Another metastable phase, commonly referred to in literature as cubic spinel Li1.6Mn1.6O4, was obtained at 350-450 oC by the calcination of orthorhombic LiMnO2[20,21]. Upon further heating, the compound begins to decompose, as evidenced by the appearance of Li2MnO3[20]. This phase is currently under extensive investigation due to its very high Li+ cation intercalation capacity. Some researchers believe that the above-mentioned compounds represent only a small fraction of the still untapped phases in the Li-Mn-O system[21,22].
Since the conventional high-temperature (> 500 °C) solid-state approaches are apparently not suitable for obtaining metastable phases, new synthetic strategies need to be explored. Those should be capable of affording fast, controllable kinetic routes that allow for the immediate formation of the metastable phase, and the whole preparation process should be mild enough in order to keep the operating temperatures below the decomposition point of the target compound. One viable approach is to employ heterometallic molecular precursors[23,24]. Within these molecular assemblies, the Li and Mn atoms are effectively set in a
In this work, we present evidence for a new metastable Li manganate, formulated as lithium-rich,
EXPERIMENTAL
Materials
Anhydrous manganese(II) chloride (MnCl2) and lithium methoxide (LiOMe) were purchased from
Preparation of Li2Mn2(tbaoac)6 (1)
Li(tbaoac) (0.391 g, 2.38 mmol) and MnCl2 (0.100 g, 0.794 mmol) were added to a 100 mL flask under argon atmosphere, followed by the addition of 40 mL of dry ethanol. The resulting colorless solution was stirred at room temperature for one hour. The solvent was then evaporated under vacuum at room temperature and the white solid residue was further dried overnight at 100 oC. The final white product was isolated by dichloromethane extraction followed by evaporation of the solvent at room temperature, yielding approximately 0.400 g (95 %). The purity of the bulk crystalline product was confirmed by X-ray powder diffraction [Supplementary Figure 1 and Supplementary Table 1].
Characterization
Le Bail fit for X-ray powder diffraction patterns was performed using the TOPAS version 4 software package (Bruker AXS, 2006). The attenuated total reflection (ATR) spectra were recorded on a PerkinElmer Spectrum 100FT-IR spectrometer. NMR spectra were collected using a Bruker Avance 400 spectrometer at 400 MHz for 1H and at 155.5 MHz for 7Li. Chemical shifts (δ) are given in ppm relative to the residual solvent peaks for 1H, and to 7Li peak of external standard (0.1 M solution of LiCl in D2O). Mass spectra were acquired using a Direct Analysis in Real Time -Standardized Voltage and Pressure (DART-SVP) ion source (IonSense, Saugus, MA, USA) coupled to a JEOL AccuTOF
RESULTS AND DISCUSSION
Synthesis and characterization of heterometallic precursor 1
The heterometallic precursor complex [Li2Mn2(tbaoac)6] (1) was obtained by a simple solution reaction procedure that employs commercially/readily available starting reagents on a large scale with nearly quantitative yield:
This reaction represents an interaction of anhydrous manganese(II) chloride with an excess of unsolvated Li(tbaoac) salt. The heterometallic complex can be readily separated from LiCl byproduct based on their different solubilities in dichloromethane. The isolated precursor 1 appears as a white powder that is relatively stable in the open air and can be handled outside of the glove box for a reasonable period of time in the course of characterization and decomposition studies. Precursor 1 was found to be quantitatively resublimed at 160 oC under dynamic vacuum conditions and can be further purified by sublimation at temperatures below its decomposition point (ca. 185 oC). Precursor 1 is instantly soluble in a variety of common solvents.
Single crystal X-ray diffraction analysis revealed that precursor 1 consists of tetranuclear heterocyclic molecules [Li2Mn2(tbaoac)6] [Figure 1], similar to those of other transition metals (Fe[23], Co[26], and Ni[25]). All crystals of precursor 1 were found to be allotwins[26] consisting of an intergrowth of two polymorphs (triclinic and monoclinic), regardless of the crystal size or growth conditions. A satisfactory Le Bail fit for the experimental X-ray powder diffraction pattern of bulk product 1 was obtained by comparing it with the one calculated for the mixture of two modifications: the major triclinic and the minor monoclinic phases
Figure 1. Solid state structure of heterometallic molecular precursor [Li2Mn2(tbaoac)6] (1). Bridging M-O (M = Li, Mn) bonds are highlighted in blue. Hydrogen atoms are omitted for clarity. Single crystal growth of precursor 1 is shown in Supplementary Table 2. Detailed views of molecular structure 1, including thermal ellipsoids, are available in Supplementary Figure 2. Unit cell parameters, selected bond distances and angles of precursor 1 are recorded in Supplementary Tables 3 and 4.
The ATR spectrum of precursor 1 was measured and shown in Supplementary Figure 3. The 1H and
Figure 2. Positive-ion DART mass spectrum of solid [Li2Mn2(tbaoac)6] (1). The isotope distribution pattern for the [M-L]+ {M =
Low-temperature decomposition oxide product
Heterometallic precursors similar to 1 have been reported[23,25,26] to yield crystalline lithium-transition metal oxides LiMO2 (M = Fe, Co, Co/Ni) with a Li:M ratio of 1:1 upon thermal decomposition at temperatures as low as 280 oC. Thermogravimetric analysis (TGA) of heterometallic precursor 1 shows that the thermal decomposition process is quite complex and is mostly complete by 310 oC [Figure 3]. Lower decomposition temperature (280 oC) would lead to the appearance of impurities [Supplementary Figure 7]. The isotherm recorded at 310 oC reveals residue weight stabilizes after 24 h. The X-ray powder diffraction pattern indicates that phase-pure material is obtained after the decomposition at 310 oC [Figure 4].
Figure 3. TGA plot (blue) of the heterometallic molecular precursor 1 recorded at a heating rate of 1.0 oC/min under an N2 protection flow of 25 mL/min. The red curve is the isothermal analysis at 310 oC over time. TGA: Thermogravimetric analysis.
Figure 4. Powder X-ray diffraction pattern of the decomposition residue of precursor 1 at 310 oC and the Le Bail fit for the cubic unit cell (Sp. gr. Fd-3m) with a = 8.1170(1) Å. The black and blue curves are the experimental and calculated patterns, respectively. The grey line is the difference curve. Theoretical peak positions are shown as blue bars at the bottom.
The Le Bail fit of the X-ray powder diffraction pattern revealed the cubic crystal system with a unit cell parameter of a = 8.1170(1) Å [Figure 4], which is smaller than those (8.221 - 8.162 Å) for any reported
The low-temperature phase obtained from the thermal decomposition of the molecular precursor was investigated using ED and high-resolution TEM (HRTEM). Overview TEM images are shown in the Supplementary Figure 8. The phase consists of highly agglomerated crystallites ranging in size from a few nanometers up to approximately 60 nm. An ED ring pattern measured from a group of crystallites is shown in Figure 5A. It can be completely indexed in a cubic Fd-3m lattice with unit cell parameter a≈8.0 Å. The intensity profile was measured from this pattern. Figure 5B represents the Le Bail fit of the profiled ED data. The refined cell parameter was found to be a = 7.98(2) Å.
Figure 5. (A) Electron diffraction ring pattern and (B) the corresponding intensity profile measured from the electron diffraction ring pattern, the tick marks represent the Le Bail fit; (C-E) electron nanodiffraction patterns of Li1.5Mn1.5O3.5; (F-H) HRTEM images and corresponding fast Fourier transforms (FFT) taken from different crystallites of Li1.5Mn1.5O3.5. HRTEM: High-resolution transmission electron microscopy.
Electron nanodiffraction patterns taken along the [001], [-110], and [-112] zone axes of the spinel-type
Elemental analysis (large sample) and ICP analysis (small sample) yielded Li/Mn ratios of 1:1.02 and 1:0.998, respectively. Iodometric titration gave the average oxidation state of +3.76 for Mn in the oxide spinel phase. XPS confirmed the presence of mixed Mn3+/Mn4+ oxidation states and Mn:O molar ratio of 1:2.33 [Supplementary Figure 9].
The spinel structure model was applied for the Rietveld refinement using the powder X-ray diffraction (XRD) pattern. The cation and oxygen positions were refined with the same thermal parameter. Occupancy factors for the tetrahedral and octahedral metal sites were refined assuming the full occupancy of the tetrahedral site by Li cations and joint occupancy of the octahedral site by Li and Mn ions in the ~0.25:0.75 ratio. Very good agreement between the experimental and calculated XRD profiles has been achieved after the refinement [Figure 6]. The structure parameters and reliability factors are summarized in Table 1. Based on all the above results, the new oxide phase was formulated as lithium-rich, oxygen-deficient spinel
Figure 6. Experimental, calculated and difference Rietveld profiles after the refinement of the Li1.494Mn1.506O3.5 structure. The red and blue curves are the overlaid experimental and calculated patterns. The green curve is the calculated pattern. The grey line is the difference curve. Theoretical peak positions are shown at the bottom.
Crystallographic data for Li1.494Mn1.506O3.5 obtained by the rietveld refinement
| Formula | Li1.494Mn1.506O3.5 | ||||
| Space group | Fd-3m | ||||
| Z | 8 | ||||
| a, Å | 8.1398(6) | ||||
| Position | Occupancy | x/a | y/b | z/c | Uiso, Å2 |
| Li1 | 1 | 1/8 | 1/8 | 1/8 | 0.0236(3) |
| Mn1 | 0.753(8) | 1/2 | ½ | 1/2 | 0.0236(3) |
| Li2 | 0.247(8) | 1/2 | ½ | 1/2 | 0.0236(3) |
| O1 | 0.875(3) | 0.2567(4) | 0.2567(4) | 0.2567(4) | 0.0236(3) |
High-temperature decomposition products
TGA of the Li1.5Mn1.5O3.5 oxide obtained at 310 oC shows almost no weight loss until 600 oC [Figure 7], but two significant features are observed. First, a strong endothermal signal is present in the Differential thermal analysis (DTA) curve between 475 oC and 600 oC [Figure 7]. Second, X-ray powder diffraction reveals the appearance of additional peaks at around 400 oC [Figure 8]. These observations suggest that a stoichiometric transformation or disproportionation is occurring without the release of any volatiles. Continued temperature increase leads to a new phase, identified as monoclinic Li2MnO3, accompanied by its characteristic red color in the solid samples. Additionally, the unit cell parameter of the spinel phase gradually increases, indicating a transformation of Li1+xMn2-xO4-δ towards a spinel structure with lower lithium/higher oxygen content to compensate for the formation of lithium-rich Li2MnO3.
Figure 7. TGA (blue) and DTA (red) curves of Li1.5Mn1.5O3.5 obtained at 310 oC. ΔT represents the temperature difference between the sample and reference. TGA: Thermogravimetric analysis; DTA: differential thermal analysis.
Figure 8. Powder X-ray diffraction patterns of the residues obtained by decomposition of heterometallic precursor 1 at different temperatures. Theoretical peak positions shown at the bottom represent the spinel phase (green) and Li2MnO3 (blue).
At 600 °C, the X-ray powder diffraction pattern clearly reveals the presence of two distinct phases [Figures 8 and 9A]: Li2MnO3 and LiMn2O4. The main phase can be indexed using a monoclinic C-centered lattice with unit cell parameters a = 4.9298(4) Å, b = 8.5276(6) Å, c = 5.0223(4) Å, β = 109.333(5)° [Supplementary Figure 10 and Supplementary Table 7], consistent with a Li2MnO3-type structure. However, the Li2MnO3 phase alone does not allow adequate fitting of the powder X-ray diffraction pattern [Supplementary Figure 11]. A reasonable fit was achieved only when an F-centered cubic spinel-type phase with a = 8.1524(5) Å was included in the refinement [Supplementary Figure 12].
Figure 9. (A) Fit of the powder X-ray diffraction pattern of the sample obtained by the thermal decomposition of Li1.5Mn1.5O3.5 at 600 °C with a mixture of Li2MnO3 and LiMn2O4 phases; (B) HAADF-STEM images of the nanocrystalline phases in the sample obtained by the thermal decomposition of Li1.5Mn1.5O3.5 at 600 °C; (C) Ring electron diffraction patterns of the small (left) and large crystals (right) in phase (B); (D) Intensity profiles of the ring ED patterns indexed with the spinel F-centered cubic cell (top) and the Li2MnO3C-centered monoclinic unit cell (bottom); (E) Theoretical electron diffraction pattern of the Li2MnO3 phase (top) and the theoretical electron diffraction pattern of the Li2MnO3 phase affected by preferred orientation; (F) HAADF-STEM image, Mn and O EDX signal maps and a mixed colored map; (G) EDX spectra at the O-Kα and Mn-Lα lines indicating that the large Li2MnO3 crystal (area 2 in the panel F) has a higher O-content (red spectrum). The spectrum of the area of small LiMn2O4 crystals (area 1, blue spectrum) demonstrates a clearly lower intensity of the O-K line compared to that of the large Li2MnO3 crystals (red); (H) ED patterns of the Li2MnO3 phase. The reflections due to spinel intergrowth are marked with arrows. HAADF-STEM: High angle annular dark field scanning transmission electron microscopy; ED: Electron diffraction; EDX: Energy dispersive X-ray.
HAADF-STEM images revealed that the sample contains two distinct types of crystals differing in size and shape [Figure 9B]. The larger crystals are octahedral in shape and range in size from 50 nm to 300 nm, while the smaller crystals, measuring 5 nm to 50 nm, do not exhibit well-defined facets. Although the two types of crystals are closely intermixed, it was possible to obtain ring ED patterns from areas with a preferential abundance of either small or large crystals [Figure 9C]. The intensity profiles integrated around the rings are shown in Figure 9D. The ring ED pattern of the small crystals displays an intensity distribution similar to that expected for the spinel LiMn2O4 phase. The shoulder at the 111 spinel reflection is attributed to the simultaneous presence of the Li2MnO3 phase, in which these reflections originate from the Li and Mn “honeycomb” ordering in the LiMn2 layers. The ring ED pattern from the large crystals corresponds to the Li2MnO3 phase (Figure 9E, top) considering a preferred orientation correction (March-Dollase parameter of 1.4 along the [001] direction) (Figure 9E, bottom). This is typical for large, anisotropic crystals deposited onto a flat surface such as the carbon support. It should be noted that the intensities are also affected by the simultaneously present spinel phase.
Although the exact quantification of the Mn to O ratio using EDX spectra is impossible, this ratio can be qualitatively estimated from the relative intensities of the O-Kα (at 525 eV) and Mn-Lα (at 637 eV) lines, which are close in energy and, hence, their ratio is not strongly affected by absorption due to variations in crystal thickness. EDX analysis of the residue revealed a highly homogeneous distribution of manganese and oxygen [Figure 9F]. The oxygen content in the Li2MnO3-like large crystals is clearly higher than that in the small spinel-like crystals [Figure 9G]. Additional HAADF-STEM images of different crystals can be found in the Supplementary Figure 13. Besides existing as separate crystallites, the Li2MnO3 and spinel phases also form intergrowths, as indicated by the extra reflections, marked with arrows in Figure 9H, in the ED patterns of Li2MnO3, which are attributed to the spinel phase[32].
High-temperature treatment (750 °C) of Li1.5Mn1.5O3.5 resulted in a mixture of Li2MnO3 and LiMn2O4 (Equation 2) as confirmed by X-ray powder diffraction [Figure 10]. The unit cell parameters of Li2MnO3 and LiMn2O4 obtained from the Le Bail fit correspond well with the values reported in the literature [Table 2].
Figure 10. Powder X-ray diffraction pattern of the residue obtained by heating Li1.5Mn1.5O3.5 at 750 oC in the air and the Le Bail fit. Black and green curves are experimental and calculated overlaid patterns. Blue and red curves are calculated as single peak patterns for
Comparison of the unit cell parameters for Li2MnO3 and LiMn2O4 oxides obtained by heating of Li1.5Mn1.5O3.5 at 750 oC with the literature data
From a crystal chemistry point of view, the metastable nature of the Li1.5Mn1.5O3.5 spinel phase is not surprising considering mixed Mn valence of +3.67, incomplete coordination environment for both cations due to oxygen vacancies and mixed occupancy of the 16d crystallographic site with the Mn3+, Mn4+ and Li+ cations with quite different ionic radii: r(Mn3+) = 0.645 Å, r(Mn4+) = 0.53 Å, and r(Li+) = 0.76 Å. No wonder that it tends to be thermally decomposed upon exsolving Li2MnO3 with a very strong Mn4+-O bond, complete octahedral oxygen coordination around the Mn4+ cations and robust “honeycomb” Li/Mn cation ordering accommodating the size and charge differences. This can be considered as the main driving force behind the phase transition under discussion.
The mechanism of transformation of the Li-rich spinel structures into the Li2MnO3-type structures has already been discussed in the literature. According to the experimental and theoretical study by Pei et al.[35], the transformation occurs through a migration of the tetrahedrally-coordinated Li at the 8a crystallographic site to the octahedral 16c site that creates a fragment of the Li2MnO3-type layered structure. It should be noted that in the original work of Pei et al.[35], this cation migration is facilitated by oxygen vacancies generated due to O2 release upon heating of their Li-rich spinel. The reported Li1.5Mn1.5O3.5 spinel phase does not need oxygen release to create vacancies as they are already present in this anion-deficient composition. This makes the mechanism discussed above highly plausible in this case.
Historical perspective
We strongly believe that all tertiary oxides, particularly those in well-studied systems such as Li-Mn-O,
CONCLUSIONS
Compounds with low thermal stability that cannot withstand traditional synthetic methods often remain undiscovered due to the limited variety of low-temperature synthetic routes available. Mixed-metal manganates are prime candidates for revealing new metastable phases that can be accessed using
In our previous works with low-temperature synthesis of manganates, we detected a number of new (unknown) phases[38,39]. Many of those compounds appeared to be complex oxyfluorides. However, in every instance, these phases were observed in mixtures, making it nearly impossible to determine their structure and composition, let alone to measure their properties. Perhaps the clearest example was the preparation of a Pb-Mn oxide[39] with a todorokite structure that was still obtained as a mixture, albeit with a known phase.
We report here a new Li-Mn oxide phase and define it through a thorough investigation of its composition, structure, Mn oxidation state, and oxygen content as the Li-rich, oxygen-deficient spinel, Li1.5Mn1.5O3.5. We show that this phase disproportionates stoichiometrically upon heating into the thermodynamically stable phases Li2MnO3 and LiMn2O4.
This work represents a rare example of using single-source precursors for making previously unknown, metastable, low-temperature phases. Particularly noteworthy is the successful isolation of the phase in its pure form. This achievement was made possible by employing a well-defined molecular precursor with specific ligands that facilitate low-temperature decomposition. Prior to this work, we designed several single-source precursors in the Li-Mn system with 1:1 and 1:2[26] metal ratios, but those exhibited relatively high decomposition temperatures, not resulting in new phases.
We believe that other unknown or unusual metastable phases can still be discovered in the Li-Mn-O system, particularly those with high Li content. The key to isolating these phases lies in using specific metal ratios, low-temperature synthetic approaches, and the right decomposition conditions to effectively and precisely control the oxygen content.
DECLARATIONS
Authors’ contributions
Writing: Han, H.; Zhang, Y.; Dikarev, E. V.; Abakumov, A. M.
Experimentation: Han, H.; Zhang, Y.; Chang, J.
Materials characterization: Zhang, Y.; Han, H.; Filatov, A. S.; Wei, Z.; Batuk, M.; Hadermann, J.; Chang, J.
Manuscript review: Dikarev, E. V.; Abakumov, A. M.; Hadermann, J.
Availability of data and materials
The data are available upon request from the authors.
Financial support and sponsorship
Financial support from the National Science Foundation is gratefully acknowledged for grants CHE-1955585 and CHE-2400091 (E.V.D.). NSF’s ChemMatCARS, Sector 15 at the Advanced Photon Source (APS), Argonne National Laboratory (ANL), is supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number CHE-1834750. This research used resources from the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. H.H. acknowledges the Fundamental Research Funds for the Central Universities (22120240204) and the National Natural Science Foundation of China (22101205).
Conflicts of interest
All authors declared no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
Supplementary Materials
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