Powder diffraction methods for studies of borohydride-based energy storage materials

The world today is facing increasing energy demands and a simultaneous demand for cleaner and more environmentally friendly energy technologies. Hydrogen is recognized as a possible renewable energy carrier, but its large-scale utilization is mainly hampered by insufficient hydrogen storage capabilities. In this scenario, powder diffraction has a central position as the most informative and versatile technique available in materials science. This is illustrated in the present review by synthesis, physical, chemical and structural characterisation of novel boron based hydrides for hydrogen storage. Numerous novel BH4- based materials have been investigated during the past few years and this class of materials has a fascinating structural chemistry. The experimental methods presented can be applied to a variety of other material

Materials for hydrogen-based energy storage - past, recent progress and future outlook

Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered; porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage

Phase Transformation Mechanism of Li-Ion Storage in Iron(III) Hydroxide Phosphates

Many ion storage compounds used for electrodes in Li-ion batteries undergo a first order phase transformation between the Li-rich and Li-poor end-members during battery charge and discharge. This often entails large transformation strains due to lattice misfits, which may hamper charge and discharge kinetics. Iron­(III) hydroxide phosphate, Fe_2–<i>y</i>(PO₄)­(OH)_{3–3<i>y</i>}(H₂O)_{3<i>y</i>−2} is a promising new cathode material with high Li-ion storage capacity, low production costs and low toxicity. Previous reports on this material indicate that the Li-ion intercalation and extraction in this material is accompanied by a second-order solid solution transformation. However, direct information about the transformation mechanism in Fe_2–<i>y</i>(PO₄)­(OH)_{3–3<i>y</i>}(H₂O)_{3<i>y</i>−2} is lacking, and several details remain unclear. In this work, Fe_2–<i>y</i>(PO₄)­(OH)_{3–3<i>y</i>}(H₂O)_{3<i>y</i>−2} is prepared by hydrothermal synthesis and characterized structurally, morphologically and by electrochemical analysis (galvostatic cycling and cyclic voltammetry). A wide range of synthesis conditions is screened, which provides information about their correlation with chemical composition, crystallite size, particle morphology and electrochemical performance. The phase transformation mechanism of selected materials is investigated through synchrotron radiation powder X-ray diffraction collected during galvanostatic discharge–charge cycling. This confirms a complete solid solution transformation both during Li-insertion (discharge) and -extraction (charge), but also reveals a highly anisotropic evolution in lattice dimensions, which is linked to an irreversible reaction step and the high vacancy concentration in Fe_2–<i>y</i>(PO₄)­(OH)_{3–3<i>y</i>}(H₂O)_{3<i>y</i>−2}

Structural Evolution of Disordered LixV2O5\mathrm{Li_{x}V_{2}O_{5}} Bronzes in V2O5\mathrm{V_{2}O_{5}} Cathodes for Li-Ion Batteries

Vanadium pentaoxide, $\mathrm{V_{2}O_{5}}$, is an attractive cathode material for Li-ion batteries, which can store up to three Li ion per formula unit. At deep discharge, an irreversible reconstructive phase transition occurs with formation of the disordered ω-LixV2O5 bronze, which, despite the lack of long-range order, exhibits a high reversible capacity (∼310 mAh/g) without regaining the crystallinity upon recharge. Here, we utilize operando powder X-ray diffraction and total scattering (i.e., pair distribution function analysis) to investigate the atomic-scale structures of the deep-discharge phase $ω-\mathrm{Li_{x}V_{2}O_{5}}$ (x ∼ 3) and, for the first time, the highly disordered phase $β-\mathrm{Li_{x}V_{2}O_{5}}$ (x ∼ 0.3) formed during subsequent Li-extraction. Our studies reveal that the deep discharge $ω-\mathrm{Li_{x}V_{2}O_{5}}$ phase consists of ∼60 Å domains rock salt structure with a local cation ordering on an ∼15 Å length scale. The charged $β-\mathrm{Li_{x}V_{2}O_{5}}$ phase only exhibits very short-range ordering (∼10 Å). The phase transition between these phases is structurally reversible and appears unexpectedly to occur via a two-phase transition mechanism

Synthesis, structure and properties of bimetallic sodium rare-earth (RE) borohydrides, NaRE(BH₄)₄,RE = Ce, Pr, Er or Gd

Formation, stability and properties of new metal borohydrides within RE(BH4)3–NaBH4, RE = Ce, Pr, Er or Gd is investigated. Three new bimetallic sodium rare-earth borohydrides, NaCe(BH4)4, NaPr(BH4)4 and NaEr(BH4)4 are formed based on an addition reaction between NaBH4 and halide free rare-earth metal borohydrides RE(BH4)3, RE = Ce, Pr, Er. All the new compounds crystallize in the orthorhombic crystal system. NaCe(BH4)4 has unit cell parameters of a = 6.8028(5), b = 17.5181(13), c = 7.2841(5) Å and space group Pbcn. NaPr(BH4)4 is isostructural to NaCe(BH4)4 with unit cell parameters of a = 6.7617(2), b = 17.4678(7), c = 7.2522(3) Å. NaEr(BH4)4 crystallizes in space group Cmcm with unit cell parameters of a = 8.5379(2), b = 12.1570(4), c = 9.1652(3) Å. The structural relationships, also to the known RE(BH4)3, are discussed in detail and related to the stability and synthesis conditions. Heat treatment of NaBH4–Gd(BH4)3 mixture forms an unstable amorphous phase, which decomposes after one day at RT. NaCe(BH4)4 and NaPr(BH4)4 show reversible hydrogen storage capacity of 1.65 and 1.04 wt% in the fourth H2 release, whereas that of NaEr(BH4)4 continuously decreases. This is mainly assigned to formation of metal hydrides and possibly slower formation of sodium borohydride. The dehydrogenated state clearly contains rare-earth metal borides, which stabilize boron in the dehydrogenated state

Manganese borohydride; synthesis and characterization

Solvent-based synthesis and characterization of α-Mn(BH4)2 and a new nanoporous polymorph of manganese borohydride, γ-Mn(BH4)2, via a new solvate precursor, Mn(BH4)2·1/2S(CH3)2, is presented. Manganese chloride is reacted with lithium borohydride in a toluene/dimethylsulfide mixture at room temperature, which yields halide and solvent-free manganese borohydride after extraction with dimethylsulfide (DMS) and subsequent removal of residual solvent. This work constitutes the first example of establishing a successful, reproducible solvent-based synthesis route for a pure, crystalline, stable transition metal borohydride. The new polymorph, γ-Mn(BH4)2, is shown to be the direct manganese counterpart of the zeolite-like structure, γ-Mg(BH4)2 (cubic, a = 16.209(1) Å, space group Id3¯a). It is verified that large pores (diameter > 6.0 Å) exist in this structure. The solvate, Mn(BH4)2·1/2S(CH3)2, is subsequently shown to be the analogue of Mg(BH4)2·1/2S(CH3)2. As the structural analogies between Mg(BH4)2 and Mn(BH4)2 became evident a new polymorph of Mg(BH4)2 was identified and termed ζ-Mg(BH4)2. ζ-Mg(BH4)2 is the structural counterpart of α-Mn(BH4)2. All synthesis products are characterized employing synchrotron radiation-powder X-ray diffraction, infrared spectroscopy and thermogravimetric analysis in combination with mass spectroscopy. Thermal analysis reveals the decomposition of Mn(BH4)2 to occur at 160 °C, accompanied by a mass loss of 14.8 wt%. A small quantity of the desorbed gaseous species is identified as diborane (ρm(Mn(BH4)2) = 9.5 wt% H2), while the remaining majority is found to be hydrogen