Volatile single-source molecular precursor for the lithium ion battery cathode

Protective Spinel Coating for Li1.17Ni0.17Mn0.50Co0.17O2 Cathode for Li-Ion Batteries through Single-Source Precursor Approach

The Li_1.17Ni_0.17Mn_0.50Co_0.17O₂ Li-rich NMC positive electrode (cathode) for lithium-ion batteries has been coated with nanocrystals of the LiMn_1.5Co_0.5O₄ high-voltage spinel cathode material. The coating was applied through a single-source precursor approach by a deposition of the molecular precursor LiMn_1.5Co_0.5(thd)₅ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) dissolved in diethyl ether, followed by thermal decomposition at 400 °C inair resulting in a chemically homogeneous cubic spinel. The structure and chemical composition of the coatings, deposited on the model SiO₂ spheres and Li-rich NMC crystallites, were analyzed using powder X-ray diffraction, electron diffraction, high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray (EDX) mapping. The coated material containing 12 wt.% of spinel demonstrates a significantly improved first cycle Coulombic efficiency of 92% with a high first cycle discharge capacity of 290 mAhg^-1. The coating also improves the capacity and voltage retention monitored over 25 galvanostatic charge-discharge cycles, although a complete suppression of the capacity and voltage fade is not achieved.

Heterotrimetallic Precursor with 2:2:1 Metal Ratio Requiring at Least a Pentanuclear Molecular Assembly

This work represents an important step in the quest to make heteromultimetallic molecules featuring specific metal types and complicated metal ratios. The rational design, synthesis, and characterization of a complex heterotrimetallic single-source molecular precursor for the next generation sodium-ion battery cathode material, Na₂Mn₂FeO₆, is described. A unique pentametallic platform [Mn^II(ptac)₃-Na-Mn^III(acac)₃-Na-Mn^II(ptac)₃] (1) was derived from the known polymeric structure of [NaMn^II(acac)₃]_∞, through a series of elaborate design procedures, such as mixed-ligand, unsymmetric ligand, and mixed-valent approaches. Importantly, the application of those techniques results in a molecule with distinctively different transition metal positions in terms of ligand environment and oxidation states. An isovalent substitution of Fe^III for the central Mn^III ion forms the target heterotrimetallic precursor [Mn^II(ptac)₃-Na-Fe^III(acac)₃-Na-Mn^II(ptac)₃] (3) with an appropriate metal ratio of Na:Mn:Fe = 2:2:1. The arrangement of metal ions and ligands in this pentametallic assembly was confirmed by single crystal X-ray investigation. The unambiguous assignment of the positions and oxidation states of the Periodic Table neighbors Fe and Mn in 3 has been achieved by a combination of investigative techniques that include synchrotron resonant diffraction, X-ray multiwavelength anomalous diffraction, X-ray fluorescence spectroscopy, Mössbauer spectroscopy, and gas-phase DART mass spectrometry. The heterotrimetallic single-source precursor 3 was shown to exhibit a clean decomposition pattern yielding the phase-pure P2-Na₂Mn₂FeO₆ quaternary oxide with high uniformity of metal ion distribution as confirmed by electron microscopy.

The use of mixed-metal single source precursors for the synthesis of complex metal oxides

This Feature Article highlights the use of mixed-metal single source precursors to directly access useful complex metal oxide materials.

Facile one step synthesis method of spinel LiMn2O4 cathode material for lithium batteries

This study succeeded to prepare three pure phases of Mn₂O₃, Mn₃O₄ beside one of the best cathode materials, spinel LiMn₂O₄. LiMn₂O₄ with high phase purity and crystallinity was synthesized by a facile, cost effective and one step synthesis method. The structure and morphology of the powders were studied in detail by means of X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM) and surface area. The X-ray diffraction shows that the post-annealing process reveals the formation of pure crystalline spinel LiMn₂O₄ with small particle size and lower lattice strain. The thermogravimetric analysis threw the light on the role of the evaporation technique in producing LiMn₂O₄ by following the different phases on the thermal performance of the precursor. The morphological characterization shows the clear appearance of the octahedral particles of LiMn₂O₄ calcined at high temperature with microporous nanosized structure. Electrochemical testing of the as prepared spinel at 900 °C showed promising results in terms of high initial capacity and good cycle stability. The as prepared spinel sample shows also good rate performance.

Three to tango requires a site-specific substitution: heterotrimetallic molecular precursors for high-voltage rechargeable batteries

Design of heterotrimetallic molecules, especially those containing at least two different metals with close atomic numbers, radii, and the same coordination number/environment is a challenging task. This quest is greatly facilitated by having a heterobimetallic parent molecule that features multiple metal sites with only some of those displaying substitutional flexibility. Recently, a unique heterobimetallic complex LiMn2(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) has been introduced as a single-source precursor for the preparation of a popular spinel cathode material, LiMn2O4. Theoretical calculations convincingly predict that in the above trinuclear molecule only one of the Mn sites is sufficiently flexible to be substituted with another 3d transition metal. Following those predictions, two heterotrimetallic complexes, LiMn2-x Co x (thd)5 (x = 1 (1a) and 0.5 (1b)), that represent full and partial substitution, respectively, of Co for Mn in the parent molecule, have been synthesized. X-ray structural elucidation clearly showed that only one transition metal position in the trinuclear molecule contains Co, while the other site remains fully occupied by Mn. A number of techniques have been employed for deciphering the structure and composition of heterotrimetallic compounds. Synchrotron resonant diffraction experiments unambiguously assigned 3d transition metal positions as well as provided a precise "site-specific Mn/Co elemental analysis" in a single crystal, even in an extremely difficult case of severely disordered structure formed by the superposition of two enantiomers. DART mass spectrometry and magnetic measurements clearly confirmed the presence of heterotrimetallic species LiMnCo(thd)5 rather than a statistical mixture of two heterobimetallic LiMn2(thd)5 and LiCo2(thd)5 molecules. Heterometallic precursors 1a and 1b were found to exhibit a clean decomposition yielding phase-pure LiMnCoO4 and LiMn1.5Co0.5O4 spinels, respectively, at the relatively low temperature of 400 °C. The latter oxide represents an important "5V spinel" cathode material for the lithium ion batteries. Transmission electron microscopy confirmed a homogeneous distribution of transition metals in quaternary oxides obtained by pyrolysis of single-source precursors.

Dinuclear lanthanide–lithium complexes based on fluorinated β-diketonate with acetal group: magnetism and effect of crystal packing on mechanoluminescence

A convenient and straightforward synthesis of a novel type of Ln–Li β-diketonates with a discrete molecular structure is presented.

From lithium to sodium: design of heterometallic molecular precursors for the NaMO2 cathode materials

Based on the Li₂M₂L₆ model structure, an addition of extra neutral donor ligand enables the formation of Na₂M₂L₆(THF)₂ molecular precursors for the NaMO₂ oxide cathode materials.

A three body problem: a genuine heterotrimetallic molecule vs. a mixture of two parent heterobimetallic molecules

Tetranuclear molecular precursor Li₂CoNiL₆ for oxide cathode materials has been shown to contain heterotrimetallic and both parent heterobimetallic molecules.

This work raises a fundamental question about the “real” structure of molecular compounds containing three different metals: whether they consist of genuine heterotrimetallic species or of a mixture of parent heterobimetallic species. Heterotrimetallic complex Li₂CoNi(tbaoac)₆ (1, tbaoac = tert-butyl acetoacetate) has been designed based on the model tetranuclear structure featuring two transition metal sites in order to be utilized as a molecular precursor for the low-temperature preparation of the LiCo_0.5Ni_0.5O₂ battery cathode material. An investigation of the structure of 1 appeared to be very challenging, since the Co and Ni atoms have very similar atomic numbers, monoisotopic masses, and radii as well as the same oxidation state and coordination number/environment. Using a statistical analysis of heavily overlaid isotope distribution patterns of the [Li₂MM′L₅]⁺ (M/M′ = Co₂, Ni₂, and CoNi) ions in DART mass spectra, it was concluded that the reaction product 1 contains both heterotrimetallic and bimetallic species. A structural analogue approach has been applied to obtain Li₂MMg(tbaoac)₆ (M = Co (2) and Ni (3)) complexes that contain lighter, diamagnetic magnesium in the place of one of the 3d transition metals. X-ray crystallography, mass spectrometry, and NMR spectroscopy unambiguously confirmed the presence of three types of molecules in the reaction mixture that reaches an equilibrium, Li₂M₂L₆ + Li₂Mg₂L₆ ↔ 2Li₂MMgL₆, upon prolonged reflux in solution. The equilibrium mixture was shown to have a nearly statistical distribution of the three molecules, and this is fully supported by the results of theoretical calculations revealing that the stabilization energies of heterotrimetallic assemblies fall exactly in between those for the parent heterobimetallic species. The LiCo_0.5Ni_0.5O₂ quaternary oxide has been obtained in its phase-pure form by thermal decomposition of heterometallic precursor 1 at temperatures as low as 450 °C. Its chemical composition, structure, morphology, and transition metal distribution have been studied by X-ray and electron diffraction techniques and compositional energy-dispersive X-ray mapping with nanometer resolution. The work clearly illustrates the advantages of heterometallic single-source precursors over the corresponding multi-source precursors.

Heterometallic molecular precursors for a lithium-iron oxide material: synthesis, solid state structure, solution and gas-phase behaviour, and thermal decomposition

Three heterometallic single-source precursors with a Li : Fe = 1 : 1 ratio for a LiFeO₂ oxide material are reported. Heterometallic compounds LiFeL₃ (L = tbaoac (1), ptac (2), and acac(3)) have been obtained on a large scale, in nearly quantitative yields by one-step reactions that employ readily available reagents. The heterometallic precursor LiFe(acac)₃ (3) with small, symmetric substituents on the ligand (acac = pentane-2,4-dionate), maintains a 1D polymeric structure in the solid state that limits its volatility and prevents solubility in non-coordinating solvents. The application of the unsymmetrical ligands, tbaoac (tert-butyl acetoacetate) and ptac (1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionate), that exhibit different bridging properties at the two ends of the ligand, allowed us to change the connectivity pattern within the heterometallic assembly. The latter was demonstrated by structural characterization of heterometallic complexes LiFe(tbaoac)₃ (1) and LiFe(ptac)₃ (2) that consist of discrete heterocyclic tetranuclear molecules Li₂Fe₂L₆. The compounds are highly volatile and exhibit a congruent sublimation character. DART mass spectrometric investigation revealed the presence of heterometallic molecules in the gas phase. The positive mode spectra are dominated by the presence of [M - L]⁺ peaks (M = Li₂Fe₂L₆). In accord with their discrete molecular structure, complexes 1 and 2 are highly soluble in nearly all common solvents. In order to test the retention of the heterometallic structure in solution, the diamagnetic analog of 1, LiMg(tbaoac)₃ (4), has been isolated. Its tetranuclear molecular structure was found to be isomorphous to that of the iron counterpart. ¹H and ⁷Li NMR spectroscopy unambiguously confirmed the presence of heterometallic molecules in solutions of non-coordinating solvents. The heterometallic precursor 1 was shown to exhibit clean thermal decomposition in air that results in phase-pure α-modification of layered oxide LiFeO₂, the prospective cathode material for lithium ion batteries.

A versatile single molecular precursor for the synthesis of layered oxide cathode materials for Li-ion batteries

A versatile single molecular precursor, LiTM(acac)₃, featuring a 1D chain structure, was developed to achieve the layered oxides LiTMO₂(TM = Ni/Mn/Co, NMC) for Li-ion batteries.

A heterobimetallic single-source precursor enabled layered oxide cathode for sodium-ion batteries

A heterobimetallic single-source precursor, NaCo(acac)₃, featuring a 1D chain structure was for the first time designed to achieve layered oxide P2-Na_xCoO₂ with superior rate capability for Na-ion batteries (SIBs). It opens a new avenue to synthesize high-performance layered oxide cathode materials for SIBs.

Volatile Heterobimetallic Complexes from PdII and CuII β-Diketonates: Structure, Magnetic Anisotropy, and Thermal Properties Related to the Chemical Vapor Deposition of CuPd Thin Films

A novel approach for preparing volatile heterometallic complexes for use as precursors for the chemical vapor deposition of various materials is reported. New CuPd complexes based on β-diketonate units were prepared, and their structures and compositions were determined. [PdL₂ *CuL₂ ] (1) and [PdL₂ *Cu(tmhd)₂ ] (2) (L=2-methoxy-2,6,6-trimethylheptane-3,5-dionate; tmhd=2,2,6,6- tetramethylheptane-3,5-dionate) are 1D coordination polymers with alternating metal complexes, which are connected through weak interactions between the Cu atoms and the OCH₃ groups from the ligand of the Pd complexes. The volatility and thermal stability were studied using thermogravimetric and differential thermal analyses and mass spectrometry. Compound 1 vaporizes without decomposition into monometallic complexes. It exhibits magnetic anisotropy, which was revealed from the angular variations in the EPR spectrum of a single crystal. The vapor thermolysis process for 1 was investigated using mass spectrometry, allowing the process to be framed within the temperature range of 200-350 °C. The experimental data, supported by QTAIM calculations of the allowed intermolecular interactions, suggest that 1 likely exists in the gas phase as bimetallic molecules. Compound 1 proved to be suitable as a single-source precursor for the efficient preparation of CuPd alloy films with tunable Cu/Pd ratio. A possible mechanism for the film growth is proposed based on the reported data.

Mixed-valent, heteroleptic homometallic diketonates as templates for the design of volatile heterometallic precursors

A novel series of mixed-valent, heteroleptic transition metal diketonates that can be utilized as prospective single-source precursors for the low-temperature preparation of oxide materials are reported. The first mixed-valent iron β-diketonates with different FeIII/FeII ratios have been synthesized by applying the mixed-ligand approach. Based on nearly quantitative reaction yields and analysis of iron-oxygen bonds, these compounds were formulated as [FeIII(acac)3][FeII(hfac)2] (1) and [FeII(hfac)2][FeIII(acac)3][FeII(hfac)2] (2). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated FeII centers chelated by two hfac (hexafluoroacetylacetonate) ligands with electron-withdrawing substituents maintain bridging interactions with oxygen atoms of electron-donating acac (acetylacetonate) groups that chelate the neighboring FeIII atoms. Switching the ligands on FeIII and FeII atoms in starting reagents resulted in the instant ligand exchange between iron centers and in yet another polynuclear homometallic diketonate [FeII(hfac)2][FeIII(acac)2(hfac)][FeII(hfac)2] (3) that adheres to the same bonding pattern as in complexes 1 and 2. The proposed synthetic methodology has been extended to design heterometallic diketonates with different M : M' ratios. Homometallic parent molecules have been used as templates to obtain heterometallic mixed-valent [FeIII(acac)3][MnII(hfac)2] (4) and [NiII(hfac)2][FeIII(acac)3][NiII(hfac)2] (5) complexes. The combination of two different diketonate ligands with electron-donating and electron-withdrawing substituents was found to be crucial for maintaining the above mixed-valent heterometallic assemblies. Theoretical investigation of two possible "isomers", [FeIII(acac)3][MnII(hfac)2] (4) and [MnIII(acac)3][FeII(hfac)2] (4') provided an additional support for the metal site assignment giving a preference of 9.78 kcal mol-1 for the molecule 4. Heterometallic complexes obtained in the course of this study have been found to act as effective single-source precursors for the synthesis of mixed-transition metal oxide materials M x M'2-xO3 and M x M'1-xO. The title highly volatile precursors can be used for the low-temperature preparation of both amorphous and crystalline heterometallic oxides in the form of thin films or nanosized particles that are known to operate as efficient catalysts in oxygen evolution reaction.