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Solid ion conductors exhibit a frequency dependent conductivity that is well described by the coupling model, which includes stretched-exponential relaxation to describe ion-ion interactions. As yet, materials characterization taking this frequency dependence into account is inaccessible to non-experts in numerical analysis, since it requires computer-assisted evaluation of complex integrals. On the other hand, commonly used impedance models lead to inaccurate results and have limited applicability. The purpose of this article is to provide an approximation to the coupling model in terms of elemental functions. With fits to multiple reference data sets we demonstrate the advantages of our approach over commonly used methods. We then apply our model to the characterization of polyethylene oxide based electrolytes, used in all-solid-state batteries, and show that such a case could not be accurately studied with standard impedance models.

Mg(B3H8)2 is a crucial reaction intermediate in the thermal decomposition of the hydrogen storage material Mg(BH4)2 and is discussed as a potential solid-state Mg-ion conductor. We successfully synthesized unsolvated Mg(B3H8)2 and highlight that Mg(B3H8)2 exists mainly as a low-dimensional solid. In addition, the Mg2+ conductivity was evaluated to be 1.4.10-4 S cm-1 at 80 °C.

Correction for ‘Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications’ by Romain Moury et al., Dalton Trans., 2020, 49, 12168–12173, https://doi.org/10.1039/D0DT02170A.

Hybrid perovskite crystals with organic and inorganic structural components are able to combine desirable properties from both classes of materials. Electronic interactions between the anionic inorganic framework and functional organic cations (such as chromophores or semiconductors) can give rise to unusual photophysical properties. Cyanine dyes are a well known class of cationic organic dyes with high extinction coefficients and tunable absorption maxima all over the visible and near-infrared spectrum. Here we present the synthesis and characterization of an original 1D hybrid perovskite composed of NIR-absorbing cyanine cations and polyanionic lead halide chains. This first demonstration of a cyanine-perovskite hybrid material is paving the way to a new class of compounds with great potential for applications in photonic devices.

closo-Borates, such as Na

closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressurein situpowder X-ray diffraction on Na2B12H12was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.

Separators and electrolytes provide electronic blockage and ion permeability between the electrodes in electrochemical cells. Nowadays, their performance and cost is often even more crucial to the commercial use of common and future electrochemical cells than the chosen electrode materials. Hence, at the present, many efforts are directed towards finding safe and reliable solid electrolytes or liquid electrolyte/separator combinations. With this comprehensive review, the reader is provided with recent approaches on this field and the fundamental knowledge that can be helpful to understand and push forward the developments of new electrolytes for rechargeable batteries. After presenting different types of separators as well as the main hurdles that are associated with them, this work focuses on promising material classes and concepts for next-generation batteries. First, chemical and crystallographic concepts and models for the description and improvement of the ionic conductivity of bulk and composite solid electrolytes are outlined. To demonstrate recent perspectives, research highlights have been included in this work: magnesium borohydride-based complexes for solid-state Mg batteries as well as all-in-one rechargeable SrTiO3 single-crystal energy storage. Furthermore, ionic liquids pose a promising safe alternative for future battery cells. An overview on their basic principles and use is given, demonstrating their applicability for Li-ion systems as well as for so-called post-Li chemistries, such as Mg- and Al-ion batteries.

The physical properties and the hydrogen release of NaBH 4 –Mg(BH 4 ) 2 and NaBH 4 −Ca(BH 4 ) 2 composites are investigated using in situ synchrotron radiation powder X-ray diffraction, thermal analysis and temperature programmed photographic analysis. The composite, x NaBH 4 –(1 − x )Mg(BH 4 ) 2 , x = 0.4 to 0.5, shows melting/frothing between 205 and 220 °C. However, the sample does not become a transparent molten phase. This behavior is similar to other alkali-alkaline earth metal borohydride composites. In the x NaBH 4 –(1 − x )Ca(BH 4 ) 2 system, eutectic melting is not observed. Interestingly, eutectic melting in metal borohydrides systems leads to partial thermolysis and hydrogen release at lower temperatures and the control of sample melting may open new routes for obtaining high-capacity hydrogen storage materials.

[Fe(NH3)6](BH4)2 (I) and [Co(NH3)6](BH4)2 (II) as well as the solid solution [Co(NH3)6](BH4)2-xClx (x ≈1) (III) and the bimetallic [Fe(NH3)6](Li2(BH4)4) (IV) are prepared either in anhydrous Me2S or by a metathesis reaction in liquid NH3 as the solvents which allow for the removal of formed inert metal halides.

Two bimetallic borohydrides, lithium yttrium tetraborohydride, LiY(BH4)4, and sodium yttrium tetraborohydride, NaY(BH4)4, have been synthesized by thermal treatment and quenching of ball-milled precursors. LiY(BH4)4 crystallizes in the space group P2c and NaY(BH4)4 in C2221 and the compounds are isostructural to LiSc(BH4)4 and NaSc(BH4)4, respectively. Both LiY(BH4)4 and NaY(BH4)4 are built from the complex anions of [Y(BH4)4]−. The unit cell volumes per formula unit of the bimetallic compounds LiY(BH4)4 and NaY(BH4)4 are significantly larger compared to those of the constituent components (14% and 7.5% unit cell expansion, respectively), which may contribute to the mobility of the Li+ and Na+ cations. The ionic conductivities of LiY(BH4)4 and NaY(BH4)4 at room temperature, measured by impedance spectroscopy, are σ = 1.26 × 10−6 and 6.92 × 10−7 S cm−1 respectively, and are improved compared to those of the precursors. DFT calculations were used to optimize the structures and identify a possible mechanism for the Na ion mobility. In addition, they provide the basis for assignment of the 11B NMR parameters determined by 11B MAS NMR for the two boron sites in NaY(BH4)4.

Zinc borohydride–ethylenediamine composites with high hydrogen content and moderate decomposition temperatures were synthesized by a solvent based approach in diethyl ether. The complexation with ethylenediamine allows to stabilize zinc borohydride, Zn(BH4)2, which otherwise decomposes at temperatures below RT. The crystal structures of Zn(en)n(BH4)2, n=3 or 4, were determined from synchrotron radiation powder X-ray diffraction data (SR-PXD). The decomposition mechanism of the compounds was analyzed by in situ SR-PXD and thermal analysis. Zn(en)n(BH4)2, n=3 or 4, release ethylenediamine during heating, while the composites Zn(BH4)2-en 1:2 and 1:1 release 8 or 5 wt% of hydrogen at 135 and 110 °C, respectively, in contrast to e. g. alkali metal zinc borohydrides, which release a mix of diborane and hydrogen.

The LiBH4–NaBH4 pseudo-binary system has been investigated by X-ray diffraction, temperature-programmed photographic analysis, and differential scanning calorimetry, in order to establish the phase diagram. The polymorphic orthorhombic-to-hexagonal phase transition of LiBH4 was observed at 94 °C in samples containing NaBH4, i.e. 15 °C lower than for pure LiBH4, which indicates the dissolution of sodium into LiBH4. The formation of solid solutions was confirmed by powder X-ray diffraction measurements performed as a function of temperature. A new eutectic composition between Li0.65Na0.35BH4 and Li0.70Na0.30BH4, with a melting temperature of 216 °C, is observed. Ab initio calculations have been performed to establish the relative stabilities of the pure compounds in orthorhombic, hexagonal and cubic structures. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH4–NaBH4 system by the calphad method. The enthalpy of mixing for solid and liquid solutions has been estimated on the basis of experimental data.

Iron and cobalt borohydrides stabilized by ammonia (NH3), [Fe(NH3)6](BH4)2 and [Co(NH3)6](BH4)2, were synthesized along with a solid solution, [Co(NH3)6](BH4)2-xClx (x ∼ 1), and a bimetallic compound, [Fe(NH3)6](Li2(BH4)4). The compounds were prepared by new low-temperature, solvent-based synthesis methods, using dimethyl sulfide or liquid NH3, which allow for the removal of inert metal halides. The crystal structures were determined from synchrotron radiation powder X-ray diffraction (SR-PXD) data. [M(NH3)6](BH4)2 (M = Fe, Co) and [Co(NH3)6](BH4)2-xClx crystallize in the cubic crystal system, where the transition metals are octahedrally coordinated by NH3. Polymeric chains of lithium coordinated by four bridging BH4(-) anions are found in [Fe(NH3)6](Li2(BH4)4). The new compounds have high hydrogen densities of ∼14 wt % H2 and ∼140 g H2/L and release a mixture of hydrogen and NH3 gas at low temperatures, T < 80 °C. The decomposition mechanisms of the prepared compounds along with the composites [Fe(NH3)6](BH4)2·nNH3BH3 (n = 2, 4, 6) were studied by thermal analysis and in situ SR-PXD.

Eutectic melting, reactive hydride composites, and nanoconfinement have the potential to improve the reversible hydrogen storage properties in metal borohydrides. We study and compare the combined effect of all three methods on reversible hydrogen release and uptake of the eutectic melting lithium potassium borohydride system, 0.725LiBH4–0.275KBH4, with low melting temperature (Tm = 105 °C). The eutectic mixture and reactive hydride composites of the eutectic mixture with Mg or MgH2 are melt infiltrated into a CO2 activated nanoporous carbon scaffold, and their properties are compared to those of bulk samples. The decomposition of 0.725LiBH4–0.275KBH4 and the reactive hydride composites initiates simultaneously with the melting at 105 °C, but the decomposition remains slow until higher temperatures are reached (T > 300 °C). Eutectic melting appears to improve kinetics of hydrogen release and absorption, while nanoconfinement lowers the main hydrogen release temperature in the first cycle by up to 200 °C. ...

The identification of a solid-state electrolyte that fulfills all the requirements to enable a competitive bulk-type all-solid-state battery remains a challenging task. The electrolyte should display high ionic conductivity, high thermal stability, good processability and importantly a large electrochemical stability window that ideally spans the redox potential of the conducted metal ion. Several electrolytes within the class of complex hydrides display such a favorable combination of battery relevant properties. Here, we will discuss our recent results for Li-, Na- and Mg-ion conductors. We identified several compositions within the lithium-amide borohydride (Li(BH4)1-x (NH2) x ) system that display near room temperature conductivities of >6 mS/cm, comparable to the ones of typical carbonate-based liquid electrolytes used in commercial lithium-ion batteries. An Li4Ti5O12 half-cell based on such an electrolyte displays >60% capacity retention at 3.5 mA/cm2 (5C) and stable cycling for 400 cycles.[1] The mixed closo-borate Na2(B12H12)0.5(B10H10)0.5 displays a conductivity of 0.9 mS/cm at 20 °C, excellent thermal stability of up to 300 °C and an electrochemical stability window of 3 V that enables reversible plating of sodium.[2] We will also report on the preparation and characterization of magnesium ethylenediamine borohydride (Mg(en)1(BH4)2), which displays a very high conductivity of 6x10-5 S/cm at 70 °C for a magnesium-ion solid-state electrolyte.[3] [1] Y. Yan, R.-S. Kühnel, A. Remhof, L. Duchêne, E. Cuervo Reyes, D. Rentsch, Z. Łodziana, C. Battaglia, Adv. Energy Mater., accepted. [2] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Commun. 2017, 53, 4195. [3] E. Roedern, R.-S. Kühnel, A. Remhof, C. Battaglia, Sci. Rep. 2017, 7, 46189.

All-solid-state lithium-ion batteries employing solid-state electrolytes promise higher operational safety, enhanced temperature stability, potentially higher cell voltages, and consequently higher energy density compared to traditional batteries with liquid electrolytes based on flammable organic solvents. So far, only a few oxide- and thiophosphate-based solid-state electrolytes exhibit lithium ion conductivities near room temperature comparable to liquid organic electrolytes. However, none of these electrolytes has enabled the fabrication of a competitive all-solid-state battery yet. Here we report the discovery of a superionic phase near room temperature in the class of lithium amide-borohydrides, enabling ionic conductivities of 8 mS/cm near room temperature, comparable to values of common organic liquid electrolytes. The transition into the superionic phase is demonstrated to be triggered by melting of a fully occupied lithium sublattice into a symmetry-equivalent interpenetrating dual sublattice consisting of lithium vacancy sites. We further demonstrate excellent rate performance up to 5C and stable cycling over 400 cycles at 1C of our solid-state electrolytes in half-cell battery configuration representing an important step towards a viable solid-state battery technology.

Efficient magnesium electrolytes are crucial in order to realize rechargeable magnesium batteries, which are an important contender for post-lithium-ion-battery technology.1,2 Due to the divalent positive charge carried by the Mg2+ ion, magnesium mobility is difficult to achieve and so far Mg ion conductivity in inorganic solids has only been reported for few systems and typically, conductivities above 10-5 Scm-1 are only reached above 400 °C, see Figure 1. In this work, we demonstrate that partially chelated ethylenediamine magnesium borohydride complexes show promise as solid-state Mg conductors with unprecedented high conductivity of up to 6x10-5 Scm-1already at 70 °C. The compounds are easily prepared by a mechanochemical reaction and allow for reversible Mg plating/stripping at 60 °C, which is recognized to be a major challenge in magnesium ion batteries. While the electrochemical stability, limited by the ethylenediamine ligand, must be improved to reach competitive energy densities, our results demonstrate that partially chelated Mg2+complexes represent a promising platform for the development of an all-solid-state magnesium battery. References 1 D. Aurbach, G. S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid and M. Brunelli, Adv. Mater., 2007, 19, 4260–4267. 2 H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour and D. Aurbach, Energy Environ. Sci., 2013, 6, 2265. 3 S. Ikeda, M. Takahashi, J. Ishikawa and K. Ito, Solid State Ionics, 1987, 23, 125–129. 4 N. Imanaka, Y. Okazaki and G. Adachi, J. Mater. Chem., 2000, 10, 1431–1435. 5 N. Imanaka, Y. Okazaki and G. Adachi, Ionics, 2001, 7, 440–446. 6 S. Higashi, K. Miwa, M. Aoki and K. Takechi, Chem. Commun., 2014, 50, 1320–2. Figure 1

Eutectic melting in mixtures of alkali and alkali earth metal borohydrides can pave the way for new applications as fast ionic conductors, and facilitate hydrogen release by low temperature chemical reactions and convenient nanoconfinement. Here, we determine the eutectic composition for the lithium potassium borohydride system, 0.725LiBH4–0.275KBH4, with the lowest melting point, Tmelt ∼105 °C, of all known alkali and alkali earth metal borohydride mixtures. Mechanochemistry and manual mixing of LiBH4–KBH4 mixtures facilitate the formation of LiK(BH4)2. However, the melting or heat treatments used in this work do not produce LiK(BH4)2. The bimetallic borohydride dissociates into the monometallic borohydrides at ∼95 °C and partial melting occurs at ∼105 °C. Analysis of the unit cell volumes of LiBH4, KBH4 and LiK(BH4)2 in the temperature range 25 to 90 °C indicates that the formation of the bimetallic borohydride is facilitated by a more dense packing as compared to the reactants. Thus, LiK(BH4)2 is considered metastable and the formation is pressure induced. A phase diagram for the LiBH4–KBH4 system is established, which illustrates the low eutectic melting point and the stability range for the bimetallic borohydride, LiK(BH4)2.

The aim of this study is to investigate the thermal decomposition and hydrogen release of salt-free manganese borohydride composites with metal hydrides and metal borohydrides of lithium, sodium, magnesium, and calcium. Combined thermogravimetry, differential scanning calorimetry, and mass spectrometry were performed to measure and analyze the released gas; in situ synchrotron radiation powder X-ray diffraction was employed to investigate the reaction mechanisms. A new high-pressure polymorph of Mn(BH4)2, denoted δ-Mn(BH4)2, was prepared by ball milling of the halide free borohydrides in the composites Mn(BH4)2–M(BH4)x for M = Li and Na. In the reactive hydride composites, Mn(BH4)2–MHx (M = Li, Na, Mg, Ca), the formation of the more stable M(BH4)x for M = Li, Na, and Ca suppresses the release of diborane and provides higher hydrogen contents in the released gas during the decomposition.

While the size of electronic components has shrunk dramatically over the last few decades, the footprint of autonomous electronic devices is today often limited by the battery. Miniaturization of batteries is challenging in particular when using liquid electrolytes based on organic solvents. All-solid-state batteries employing solid-state electrolytes promise to facilitate miniaturization and integration. Current developments of solid-state electrolytes for lithium ion batteries focus mainly on oxide- and thiophosphate-based compounds, but have not yet enabled the fabrication of a competitive all-solid-state battery. We recently reported the discovery of a superionic phase at room temperature in the class of lithium amide-borohydrides, enabling ionic conductivities comparable to values of common organic liquid electrolytes. We demonstrate good rate performance up to 5C and stable cycling over 400 cycles at 1C in a lithium titanate half-cell battery configuration, indicating high bulk and interfacial stability. Our results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for scalable high-power lithium ion batteries.