Mechanistic Study on Artificial Stabilization of Lithium Metal Anode via Thermal Pyrolysis of Ammonium Fluoride in Lithium Metal Batteries

The use of the "Holy Grail" lithium metal anode is pivotal to achieve superior energy density. However, the practice of a lithium metal anode faces practical challenges due to the thermodynamic instability of lithium metal and dendrite growth. Herein, an artificial stabilization of lithium metal was carried out via the thermal pyrolysis of the NH4F salt, which generates HF(g) and NH3(g). An exposure of lithium metal to the generated gas induces a spontaneous reaction that forms multiple solid electrolyte interface (SEI) components, such as LiF, Li3N, Li2NH, LiNH2, and LiH, from a single salt. The artificially multilayered protection on lithium metal (AF-Li) sustains stable lithium stripping/plating. It suppresses the Li dendrite under the Li||Li symmetric cell. The half-cell Li||Cu and Li||MCMB systems depicted the attributions of the protective layer. We demonstrate that the desirable protective layer in AF-Li exhibited remarkable capacity retention (CR) results. LiFePO4 (LFP) showed a CR of 90.6% at 0.5 mA cm-2 after 280 cycles, and LiNi0.5Mn0.3Co0.2O2 (NCM523) showed 58.7% at 3 mA cm-2 after 410 cycles. Formulating the multilayered protection, with the simultaneous formation of multiple SEI components in a facile and cost-effective approach from NH4F as a single salt, made the system competent.

Chemical Composition of Lithiated Nitrodonickelates Li3-xyNixN: Evidence of the Intermediate Valence of Nickel Ions from Ion Beam Analysis and Ab Initio Calculations

Lamellar lithiated nitridonickelates have been investigated from both experimental and theoretical points of view in a wide range of compositions. In this study, we show that the nickel ion in lamellar lithiated nitridonickelates adopts an intermediate valence close to +1.5. This solid solution can therefore be written Li3-1.5xNixN with 0 ≤ x ≤ 0.68. Attempts to introduce more nickel into these phases systematically lead to the presence of the endmember of the solid solution, Li1.97Ni0.68N, with metallic nickel as an impurity. The LiNiN phase has never been observed, and first-principles calculations suggested that all the structural configurations tested were mechanically unstable.

Compositional flexibility in Li-N-H materials: implications for ammonia catalysis and hydrogen storage

Li–N–H materials, particularly lithium amide and lithium imide, have been explored for use in a variety of energy storage applications in recent years. Compositional variation within the parent lithium imide, anti-fluorite crystal structure has been related to both its facile storage of hydrogen and impressive catalytic performance for the decomposition of ammonia. Here, we explore the controlled solid-state synthesis of Li–N–H solid-solution anti-fluorite structures ranging from amide-dominated (Li_4/3(NH₂)_2/3(NH)_1/3 or Li_1.333NH_1.667) through lithium imide to majority incorporation of lithium nitride–hydride (Li_3.167(NH)_0.416N_0.584H_0.584 or Li_3.167NH). Formation of these solid solutions is demonstrated to cause significant changes to the thermal stability and ammonia reactivity of the samples, highlighting the potential use of compositional variation to control the properties of the material in gas storage and catalytic applications.

A wide solid solution based on the lithium imide anti-fluorite structure is demonstrated and related to its energy storage functions.

Nitrogen Dissociation via Reaction with Lithium Alloys

Lithium alloys are synthesized by reactions between lithium metal and group 14 elements, such as carbon, silicon, germanium, and tin. The nitrogenation and denitrogenation properties are investigated by thermal and structural analyses. All alloys dissociate the nitrogen triple bond of gaseous molecules to form atomic state as nitrides below 500 °C, which is lower than those required for conventional thermochemical and catalytic processes on nitride syntheses. For all alloys except for germanium, it is indicated that nanosized lithium nitride is formed as the product. The denitrogenation (nitrogen desorption) reaction by lithium nitride and metals, which is an ideal opposite reaction of nitrogenation, occurs by heating up to 600 °C to form lithium alloys. Among them, the lithium-tin alloy is a potential material to control the dissociation and recombination of nitrogen below 500 °C by the reversible reaction with the largest amount of utilizable lithium in the alloy phase. The nitrogenation and denitrogenation reactions of the lithium alloys at lower temperature are realized by the high reactivity with nitrogen and mobility of lithium. The above reactions based on lithium alloys are adapted to the ammonia synthesis. As a result, ammonia can be synthesized below 500 °C under 0.5 MPa of pressure. Therefore, the reaction using lithium alloys is recognized as a pseudocatalyst for the ammonia synthesis.

Ammonia decomposition catalysis using non-stoichiometric lithium imide

The non-stoichiometric lithium imide–amide system effectively decomposes ammonia to its constituents, hydrogen and nitrogen. Isotopic studies show that this bulk catalytic reaction has the potential to generate high-purity hydrogen for future energy and transport applications.

We demonstrate that non-stoichiometric lithium imide is a highly active catalyst for the production of high-purity hydrogen from ammonia, with superior ammonia decomposition activity to a number of other catalyst materials. Neutron powder diffraction measurements reveal that the catalyst deviates from pure imide stoichiometry under ammonia flow, with active catalytic behaviour observed across a range of stoichiometry values near the imide. These measurements also show that hydrogen from the ammonia is exchanged with, and incorporated into, the bulk catalyst material, in a significant departure from existing ammonia decomposition catalysts. The efficacy of the lithium imide–amide system not only represents a more promising catalyst system, but also broadens the range of candidates for amide-based ammonia decomposition to include those that form imides.

Ultra-rapid microwave synthesis of Li3−x−yMxN (M = Co, Ni and Cu) nitridometallates

Phase-pure ternary lithium nitrides with demonstrable Li⁺ ion vacancy concentrations can be synthesised by low power microwave reactions in times reduced by orders of magnitude over conventional heating approaches; the electrochemical performance of the materials has been determined.

Rapid Microwave Synthesis, Characterization and Reactivity of Lithium Nitride Hydride, Li₄NH

Lithium nitride hydride, Li₄NH, was synthesised from lithium nitride and lithium hydride over minute timescales, using microwave synthesis methods in the solid state for the first time. The structure of the microwave-synthesised powders was confirmed by powder X-ray diffraction [tetragonal space group I4₁/a; a = 4.8864(1) Å, c = 9.9183(2) Å] and the nitride hydride reacts with moist air under ambient conditions to produce lithium hydroxide and subsequently lithium carbonate. Li₄NH undergoes no dehydrogenation or decomposition [under Ar_(g)] below 773 K. A tetragonal-cubic phase transition, however, occurs for the compound at ca. 770 K. The new high temperature (HT) phase adopts an anti-fluorite structure (space group Fm 3̅ m; a = 4.9462(3) Å) with N^3- and H^- ions disordered on the 4a sites. Thermal treatment of Li₄NH under nitrogen yields a stoichiometric mixture of lithium nitride and lithium imide (Li₃N and Li₂NH respectively).

The Challenge of Storage in the Hydrogen Energy Cycle: Nanostructured Hydrides as a Potential Solution

Hydrogen has the capacity to provide society with the means to carry ‘green’ energy between the point of generation and the point of use. A sustainable energy society in which a hydrogen economy predominates will require renewable generation provided, for example, by artificial photosynthesis and clean, efficient energy conversion effected, for example, by hydrogen fuel cells. Vital in the hydrogen cycle is the ability to store hydrogen safely and effectively. Solid-state storage in hydrides enables this but no material yet satisfies all the demands associated with storage density and hydrogen release and uptake; particularly for mobile power. Nanochemical design methods present potential routes to overcome the thermodynamic and kinetic hurdles associated with solid state storage in hydrides. In this review we discuss strategies of nanosizing, nanoconfinement, morphological/dimensional control, and application of nanoadditives on the hydrogen storage performance of metal hydrides. We present recent examples of how such approaches can begin to address the challenges and an evaluation of prospects for further development.

Generation of high-purity hydrogen from cellulose by its mechanochemical treatment

Cellulose was mixed with the hydroxides of lithium and nickel and the mixture was milled, followed by heating to produce hydrogen. Several analytical methods of X-ray diffraction (XRD), thermogravimetry/mass spectrometry (TG/MS) and gas chromatography (GC) were used to characterize the samples. Hydrogen was emitted when heating the milled sample around 400 degrees C together with low concentrations of methane, carbon monoxide and carbon dioxide. It is understood that an interaction occurs between cellulose and lithium hydroxide to convert the carbon of cellulose into lithium carbonate and to emit hydrogen correspondingly. It is also found that nickel catalyst is required to facilitate the interaction and the behaviours of three different nickel compounds were compared. When high yield of hydrogen emission is available, the prepared samples can also serve the purpose of hydrogen storage.

Ab initio study on the electronic structure and vibration modes of alkali and alkaline-earth amides and alanates

We study the electronic structure and vibrational modes of several amides M(NH(2))(n) and alanates M(AlH(4))(n) (M = K, Na, Li, Ca and Mg), focusing on the role of cation states. Calculated breathing stretching vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing mode frequencies and the structural parameters of NH(2) vary in accordance with the ionization energy of cation. The tendency may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH(2))(-). The microscopic mechanism of correlations between the vibration frequencies and structural parameters is elucidated in relation to the electronic structure. A possible similar tendency in the alanates is also discussed.