Unlocking the potential of weberite-type metal fluorides in electrochemical energy storage

Sodium-ion batteries (NIBs) are a front-runner among the alternative battery technologies suggested for substituting the state-of-the-art lithium-ion batteries (LIBs). The specific energy of Na-ion batteries is significantly lower than that of LIBs, which is mainly due to the lower operating potentials and higher molecular weight of sodium insertion cathode materials. To compete with the high energy density of LIBs, high voltage cathode materials are required for NIBs. Here we report a theoretical investigation on weberite-type sodium metal fluorides (SMFs), a new class of high voltage and high energy density materials which are so far unexplored as cathode materials for NIBs. The weberite structure type is highly favorable for sodium-containing transition metal fluorides, with a large variety of transition metal combinations (M, M’) adopting the corresponding Na2MM’F7 structure. A series of known and hypothetical compounds with weberite-type structure were computationally investigated to evaluate their potential as cathode materials for NIBs. Weberite-type SMFs show two-dimensional pathways for Na+ diffusion with surprisingly low activation barriers. The high energy density combined with low diffusion barriers for Na+ makes this type of compounds promising candidates for cathode materials in NIBs

Effect of additives on the synthesis and reversibility of Ca(BH4)2

Metal borohydrides are potential materials for solid state hydrogen due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH4)2 is particularly interesting because of the predicted suitable thermodynamic properties. In this work, we investigate a new synthesis route using high pressure reactive ball milling. Starting from CaH2 and CaB6 with a TiCl3 or TiF3 as additive, a reaction yield of 19% is obtained after 24 h milling at room temperature and 140 bar H2. The presence of Ca(BH4)2 is confirmed by the presence of the stretching mode of the [BH4]- group in the infrared spectra of the as-milled samples. Using in-situ XRD, we observe the recrystallisation of a poorly crystallised Ca(BH4)2 phase present after milling. The reversible decomposition/formation of Ca(BH4)2 is obtained with higher yield (57%) using higher temperature and TiF3 as additive but not with TiCl3 despite its similar electronic structure. The differences observed using different additives and the influence of the anion are discussed

Synthesis of LiNH₂ + LiH by reactive milling of Li₃N

The hydrogen sorption properties of Li<sub>3</sub>N under reactive milling conditions have been investigated <i>in</i>- and <i>ex-situ</i> as a function of polytype structure (α vs. β), focusing on the influence of the micro-structure and/or the crystal structure upon hydrogen uptake. LiNH<sub>2</sub> and LiH were synthesized by reactive milling of Li<sub>3</sub>N at 20 bar hydrogen pressure for 4 h. Reactive milling represents a quick and effective technique to produce LiNH<sub>2</sub> by hydrogenation of Li<sub>3</sub>N at low hydrogen pressure and without any need for heating. As to our knowledge, we present a full hydrogenation of Li<sub>3</sub>N under the aforementioned conditions for the first time. The (de)hydrogenation and rehydrogenation behaviour of milled amides was evaluated using a combination of powder X-ray diffraction, differential scanning calorimetry, thermogravimetry and <i>in situ</i> Raman spectroscopy. <i>In situ</i> Raman spectroscopy showed a shift in the lithium amide stretching modes upon hydrogenation supporting a non-stoichiometric storage mechanism consistent with the literature. The microstructure and polytype composition of the Li<sub>3</sub>N dehydrogenated materials had no effect on the hydrogenation products and only minor effects on the hydrogen uptake profile during milling

Study of all solid-state rechargeable fluoride ion batteries based on thin-film electrolyte

In this work, a solid-state fluoride ion battery based on a thin-film electrolyte with 10 μm thickness was built and tested. The electrochemical performance was examined using Bi or Cu metals as the active cathode materials and MgF2 as the active anode material, respectively. X-ray diffraction and X-ray photoelectron spectroscopy data showed that the charge transfer ions between the cathode and anode were fluoride ions. Initial discharge capacities of 66 and 76 mAh g−1 were obtained at 160 °C when Bi and Cu were used as cathodes, respectively. Furthermore, this type of fluoride ion battery was rechargeable, but the capacity faded during the subsequent cycles, similar to the bulk-type systems

Electrochemical performance of all solid-state fluoride-ion batteries based on thin-film electrolyte using alternative conductive additives and anodes

CaF2 and MgF2 were tested as active anode materials for solid-state fluoride-ion battery based on thin-film electrolyte. Tin oxide, indium tin oxide, and carbon nanotubes were applied as electronically conductive additives in the anode materials to increase the electronic conductivities. X-ray diffraction demonstrated that these conductive additives were stable during the processes of electrode preparation and charge/discharge. The electrochemical measurements indicated that the batteries using CaF2 as the active anode material and carbon nanotubes as the conductive additive possessed the best electrochemical performance. The 1st and 30th discharge capacities of 114 and 67 mAh g−1 were obtained at 160 °C when Bi metal was used as the active cathode material. Furthermore, possible reasons for voltage hysteresis and the capacity losses were studied

Comprehensive Study of Melt Infiltration for the Synthesis of NaAlH4/C Nanocomposites

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Confinement of NaAIH(4) in Nanoporous Carbon: Impact on H-2 Release, Reversibility, and Thermodynamics

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Confinement of NaAIH4 in Nanoporous Carbon: Impact on H2 Release, Reversibility, and Thermodynamics

Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH4 is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H2. The slow kinetics and poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH4 particle size and the presence of a carbon support. We focused on NaAlH4/porous carbon nanocomposites prepared by melt infiltration. The NaAlH4 was confined in the mainly 2-3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH4 structure. The hydrogen release profile was modified by contact with the carbon; even for ∼10 nm NaAlH4 on a nonporous carbon material the decomposition of NaAlH4 to NaH, Al, and H2 now led to hydrogen release in a single step. This was a kinetic effect, with the temperature at which the hydrogen was released depending on the NaAlH4 feature size. However, confinement in a nanoporous carbon material was essential to not only achieve low H2 release temperatures, but also rehydrogenation at mild conditions (e.g., 24 bar H2 at 150 °C). Not only had the kinetics of hydrogen sorption improved, but the thermodynamics had also changed. When hydrogenating at conditions at which Na3AlH6 would be expected to be the stable phase (e.g., 40 bar H2 at 160 °C), instead nanoconfined NaAlH4 was formed, indicating a shift of the NaAlH4TNa3AlH6 thermodynamic equilibrium in these nanocomposites compared to bulk materials

Comprehensive Study of Melt Infiltration for the Synthesis of NaAlH4/C Nanocomposites

In the search for suitable solid state hydrogen storage systems, NaAlH4 (7.4 wt % H2) holds great promise due to its suitable thermodynamical properties. However, hydrogen release and uptake are hampered by high activation energies, most likely due to solid state mass transfer limitations. A recent strategy to improve the hydrogen de- and rehydrogenation properties of NaAlH4 is to reduce the particle size to the nanometer scale. We prepared high loadings of nanosized NaAlH4 confined in the pores of a carbon support by melt infiltration. XRD, nitrogen physisorption, high pressure DSC and solid-state NMR are used to evidence that the molten NaAlH4 infiltrates the carbon support, and forms a nanosized NaAlH4 phase lacking long-range order. The confined NaAlH4 shows enhanced hydrogen dehydrogenation properties and rehydrogenation under mild conditions that is attributed to the nanosize and close contact to the carbon matrix