Metal Hydrides with In Situ Built Electron/Ion Dual-Conductive Framework for Stable All-Solid-State Li-Ion Batteries

Due to their high theoretical specific capacity, metal hydrides are considered to be one of the most promising anode material for all-solid-state Li-ion batteries. Their practical application suffers, however, from the poor cycling stability and sluggish kinetics. Herein, we report the in situ fabrication of MgH₂ and Mg₂NiH₄ that are uniformly space-confined by inactive Nd₂H₅ frameworks with high Li-ion and electron conductivity through facile hydrogenation of single-phase Nd₄Mg₈₀Ni₈ alloys. The formation of MgH₂ and Mg₂NiH₄ nanocrystals could not only shorten Li-ion and electron diffusion pathways of the whole electrode but also relieve the induced stress upon volume changes. Additionally, the robust frameworks constructed by homogeneous distribution of inactive Nd₂H₅ based on a molecular level could effectively alleviate the volume expansion and phase separation of thus-confined MgH₂ and Mg₂NiH₄. More importantly, it is theoretically and experimentally verified that the uniform distribution of Nd₂H₅, which is an electronic conductor with a Li-ion diffusion barrier that is much lower than that of MgH₂ and Mg₂NiH₄, could further facilitate the electron and Li-ion transfer of MgH₂ and Mg₂NiH₄. Consequently, the space-confined MgH₂ and Mg₂NiH₄ deliver a reversible capacity of 997 mAh g^-1 at 2038 mA g^-1 after 100 cycles.

High capacity all-solid-state lithium battery enabled by in situ formation of an ionic conduction path by lithiation of MgH2

All-solid-state Li batteries have attracted significant attention because of their high energy density and high level of safety. In a solid-state Li-ion battery, the electrodes contain a solid electrolyte that does not contribute directly to the capacity. Therefore, a battery that does not require a solid electrolyte in its electrode mixture should exhibit a higher energy density. In this study, a MgH₂ electrode was used as the negative electrode material without a solid electrolyte in its mixture. The resultant battery demonstrated excellent performance because of the formation of an ionic conduction path based on LiH in the electrode mixture. LiH and Mg clearly formed upon lithiation and returned to MgH₂ upon delithiation as revealed by TEM-EELS analysis. This mechanism of in situ electrolyte formation enables the development of a solid-state battery with a high energy density.

MgH₂ electrode in an all-solid-state battery reversibly operated without solid electrolyte in the electrode mixture.

The in situ formation of an electrolyte via the lithiation of Mg(BH4)2 in an all-solid-state lithium battery

The present work proposes a new approach to increasing the capacity of all-solid-state batteries, based on the in situ formation of an electrolyte in a Mg(BH4)2 electrode. Charge/discharge assessments of the electrode composed of Mg(BH4)2 and acetylene black showed an initial reversible capacity of 563 mA h g-1-Mg(BH4)2.

A Review of the MSCA ITN ECOSTORE—Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity

Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.

Hydrangea-Shaped 3D Hierarchical Porous Magnesium Hydride-Carbon Framework with High Rate Performance for Lithium Storage

Magnesium hydride (MgH₂) is a promising anode material for lithium-ion batteries (LIBs) by virtue of its high theoretical specific capacity, suitable potential, and abundant source. However, the electrochemical performance of the MgH₂ electrode is still far from satisfactory due to its poor electronic conductivity and fast capacity decay. In this paper, a hydrangea-shaped three-dimensional (3D) hierarchical magnesium hydride-carbon framework (MH@HyC) comprising MgH₂ nanoparticles (NPs) uniformly self-assembled on hierarchical porous carbon (HyC) is fabricated for advanced lithium storage. Featuring high surface area and a well-defined macro-meso-micropore structure, HyC plays an ideal structure-directing role for the growth of MgH₂ NPs with size control, high loading, and a hydrangea-shape array. Taking advantage of the robust 3D hierarchical porous structure and the derived interactions, MH@HyC not only provides sufficient electrochemically active sites and enhances the electronic conductivity and channels for rapid transfer of electrons/Li ions but also relieves the agglomeration and accommodates the volumetric effects during cycling, leading to high capacity utilization, fast electrochemical kinetics, and well-sustained structural integrity. As a result, MH@HyC delivers a high reversible capacity of 554 mAh g^-1 after 1000 cycles at a high current rate of 2 A g^-1, enabling it a potential anode candidate for LIBs.

Borohydride-Scaffolded Li/Na/Mg Fast Ionic Conductors for Promising Solid-State Electrolytes

Borohydride solid-state electrolytes with room-temperature ionic conductivity up to ≈70 mS cm^-1 have achieved impressive progress and quickly taken their place among the superionic conductive solid-state electrolytes. Here, the focus is on state-of-the-art developments in borohydride solid-state electrolytes, including their competitive ionic-conductive performance, current limitations for practical applications in solid-state batteries, and the strategies to address their problems. To open, fast Li/Na/Mg ionic conductivity in electrolytes with BH₄ ^- groups, approaches to engineering borohydrides with enhanced ionic conductivity, and later on the superionic conductivity of polyhedral borohydrides, their correlated conductive kinetics/thermodynamics, and the theoretically predicted high conductive derivatives are discussed. Furthermore, the validity of borohydride pairing with coated oxides, sulfur, organic electrodes, MgH₂ , TiS₂ , Li₄ Ti₅ O₁₂ , electrode materials, etc., is surveyed in solid-state batteries. From the viewpoint of compatible cathodes, the stable electrochemical windows of borohydride solid-state electrolytes, the electrode/electrolyte interface behavior and battery device design, and the performance optimization of borohydride-based solid-state batteries are also discussed in detail. A comprehensive coverage of emerging trends in borohydride solid-state electrolytes is provided and future maps to promote better performance of borohydride SSEs are sketched out, which will pave the way for their further development in the field of energy storage.

Controlled-Size Hollow Magnesium Sulfide Nanocrystals Anchored on Graphene for Advanced Lithium Storage

Magnesium sulfide (MgS), representative of alkaline-earth metal chalcogenides (AEMCs), is a potential conversion/alloy-type electrode material for lithium ion batteries (LIBs), by virtue of its low potential, high theoretical capacity, and abundant magnesium resource. However, the limited capacity utilization and inferior rate performance still hinder its practical application, and the progress is rather slow due to the challenging fabrication technique for MgS. Herein, we report a series of controlled-size hollow MgS nanocrystals (NCs) homogeneously distributed on graphene (MgS@G), fabricated through a metal hydride framework (MHF) strategy, and its application as advanced electrode material for LIBs. The hollow structure of MgS NCs is mainly attributed to the Kirkendall effect and the escape of hydrogen atoms from metal hydride during sulfuration. The as-synthesized MgS@G demonstrates robust nanoarchitecture and admirable interactions, which ensure a spatially confined lithiation/delithiation process, optimize the dynamics of two-steps conversion/alloying reactions, and induce a synergetic pseudocapacitive storage contribution. As a result, a representative MgS@G composite delivers a largely enhanced capacity of >1208 mAh g^-1 at a current density of 100 mA g^-1 and a long-term cycle stability at a high current density of 5 A g^-1 with a capacity of 838 mAh g^-1 over 3000 cycles, indicating well-sustained structural integrity. This work presents an effective route toward the development of high-performance magnesium-based material for energy storage.

Molecular-Scale Functionality on Graphene To Unlock the Energy Capabilities of Metal Hydrides for High-Capacity Lithium-Ion Batteries

Metal hydrides have attracted great intentions as anodes for lithium-ion batteries (LIBs) due to their extraordinary theoretical capacity. It is an unsolved challenge, however, to achieve high capacity with stable cyclability, owing to their insulating property and large volume expansion upon lithium storage. Here, we introduce self-initiated polymerization to realize molecular-scale functionality of metal hydrides with conductive polymer, that is, polythiophene (PTh), on graphene, leading to the formation of MgH₂@PTh core-shell nanoparticles on graphene. The nanoscale characteristics of MgH₂ not only relieve the induced stress upon volume changes but also allow fast diffusivity and high reactivity for Li-ion transport. More importantly, the conformal coating of ultrathin PTh membrane can effectively suppress the detrimental reactions between MgH₂ and electrolyte, provide enhanced performance with facile electron and Li⁺ transport, and preserve its structural integrity, attributed to the strong molecular interaction between PTh and MgH₂ as well as its various products during electrochemical reactions. With this structure, a high reversible specific capacity of 1311 mAh g^-1 at 100 mA g^-1, excellent rate performance of 1025 mAh g^-1 at 2000 mA g^-1, and a capacity retention of 84.5% at 2000 mA g^-1 after 500 cycles are observed for MgH₂@PTh nanoparticles as anode for LIBs.

Extremely Pure Mg2FeH6 as a Negative Electrode for Lithium Batteries

Nanocrystalline samples of Mg-Fe-H were synthesized by mixing of MgH2 and Fe in a 2:1 molar ratio by hand grinding (MIX) or by reactive ball milling (RBM) in a high-pressure vial. Hydrogenation procedures were performed at various temperatures in order to promote the full conversion to Mg2FeH6. Pure Mg2FeH6 was obtained only for the RBM material cycled at 485 °C. This extremely pure Mg2FeH6 sample was investigated as an anode for lithium batteries. The reversible electrochemical lithium incorporation and de-incorporation reactions were analyzed in view of thermodynamic evaluations, potentiodynamic cycling with galvanostatic acceleration (PCGA), and ex situ X-ray Diffraction (XRD) tests. The Mg2FeH6 phase underwent a conversion reaction; the Mg metal produced in this reaction was alloyed upon further reduction. The back conversion reaction in a lithium cell was here demonstrated for the first time in a stoichiometric extremely pure Mg2FeH6 phase: the reversibility of the overall conversion process was only partial with an overall coulombic yield of 17% under quasi-thermodynamic control. Ex situ XRD analysis highlighted that the material after a full discharge/charge in a lithium cell was strongly amorphized. Under galvanostatic cycling at C/20, C/5 and 1 C, the Mg2FeH6 electrodes were able to supply a reversible capacity with increasing coulombic efficiency and decreasing specific capacity as the current rate increased.

Magnesium Hydride Nanoparticles Self-Assembled on Graphene as Anode Material for High-Performance Lithium-Ion Batteries

MgH₂ nanoparticles (NPs) uniformly anchored on graphene (GR) are fabricated based on a bottom-up self-assembly strategy as anode materials for lithium-ion batteries (LIBs). Monodisperse MgH₂ NPs with an average particle size of ∼13.8 nm are self-assembled on the flexible GR, forming interleaved MgH₂/GR (GMH) composite architectures. Such nanoarchitecture could effectively constrain the aggregation of active materials, buffer the strain of volume changes, and facilitate the electron/lithium ion transfer of the whole electrode, leading to a significant enhancement of the lithium storage capacity of the GMH composite. Furthermore, the performances of GMH composite as anode materials for LIBs are enabled largely through robust interfacial interactions with poly(methyl methacrylate) (PMMA) binder, which plays multifunctional roles in forming a favorable solid-electrolyte interphase (SEI) film, alleviating the volume expansion and detachment of active materials, and maintaining the structural integrity of the whole electrode. As a result, these synergistic effects endow the obtained GMH composite with a significantly enhanced reversible capacity and cyclability as well as a good rate capability. The GMH composite with 50 wt % MgH₂ delivers a high reversible capacity of 946 mA h g^-1 at 100 mA g ^-1 after 100 cycles and a capacity of 395 mAh g^-1 at a high current density of 2000 mA g^-1 after 1000 cycles.

Synergism of Rare Earth Trihydrides and Graphite in Lithium Storage: Evidence of Hydrogen-Enhanced Lithiation

The lithium storage capacity of graphite can be significantly promoted by rare earth trihydrides (REH₃ , RE = Y, La, and Gd) through a synergetic mechanism. High reversible capacity of 720 mA h g^-1 after 250 cycles is achieved in YH₃ -graphite nanocomposite, far exceeding the total contribution from the individual components and the effect of ball milling. Comparative study on LaH₃ -graphite and GdH₃ -graphite composites suggests that the enhancement factor is 3.1-3.4 Li per active H in REH₃ , almost independent of the RE metal, which is evident of a hydrogen-enhanced lithium storage mechanism. Theoretical calculation suggests that the active H from REH₃ can enhance the Li⁺ binding to the graphene layer by introducing negatively charged sites, leading to energetically favorable lithiation up to a composition Li₅ C₁₆ H instead of LiC₆ for conventional graphite anode.

MgH2–CoO: a conversion-type composite electrode for LiBH4-based all-solid-state lithium ion batteries

Several studies have demonstrated that MgH₂ is a promising conversion-type anode toward Li. A major obstacle is the reversible capacity during cycling. Electrochemical co-existence of a mixed metal hydride-oxide conversion type anode is demonstrated for lithium ion batteries using a solid-state electrolyte. 75MgH₂·25CoO anodes are obtained from optimized mixing conditions avoiding reactions occurring during high-energy ball-milling. Electrochemical tests are carried out to investigate the cycling capability and reversibility of the on-going conversion reactions. The cycling led to formation of a single-plateau nanocomposite electrode with higher reversibility yield, lowered discharge–charge hysteresis and mitigated kinetic effect at high C-rate compared to MgH₂ anodes. It is believed that reduced diffusion pathways and less polarized electrodes are the origin of the improved properties. The designed composite-electrode shows good preservation and suitability with LiBH₄ solid electrolyte as revealed from electron microscopy analyses and X-ray photoelectron spectroscopy.

The findings point to a means of guided formation of MgH₂–CoO conversion-type nanocomposite electrode for all-solid-state Li-ion batteries.

Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of a Carbon Nanofiber-Supported MgH2 Composite Electrode

Magnesium hydride, MgH₂, a recently developed compound for lithium-ion batteries, is considered to be a promising conversion-type negative electrode material due to its high theoretical lithium storage capacity of over 2000 mA h g^-1, suitable working potential, and relatively small volume expansion. Nevertheless, it suffers from unsatisfactory cyclability, poor reversibility, and slow kinetics in conventional nonaqueous electrolyte systems, which greatly limit the practical application of MgH₂. In this work, a vapor-grown carbon nanofiber was used to enhance the electrical conductivity of MgH₂ using LiBH₄ as the solid-state electrolyte. It shows that a reversible capacity of over 1200 mA h g^-1 with an average voltage of 0.5 V (vs Li/Li⁺) can be obtained after 50 cycles at a current density of 1000 mA g^-1. In addition, the capacity of MgH₂ retains over 1100 mA h g^-1 at a high current density of 8000 mA g^-1, which indicates the possibility of using MgH₂ as a negative electrode material for high power and high capacity lithium-ion batteries in future practical applications. Moreover, the widely studied sulfide-based solid electrolyte was also used to assemble battery cells with MgH₂ electrode in the same system, and the electrochemical performance was as good as that using LiBH₄ electrolyte.

Synthetic methods and electrochemical applications for transition metal phosphide nanomaterials

Recent developments and challenges in transition metal phosphide nanomaterials, with a focus on synthetic methods and their electrochemical applications, have been stated carefully.

Metal hydride-based materials towards high performance negative electrodes for all-solid-state lithium-ion batteries

Electrode performances of MgH2-LiBH4 composite materials for lithium-ion batteries have been studied using LiBH4 as the solid-state electrolyte, which shows a high reversible capacity of 1650 mA h g(-1) with an extremely low polarization of 0.05 V, durable cyclability and robust rate capability.

Recent achievements on inorganic electrode materials for lithium-ion batteries

The lithium-ion battery technology is rooted in the studies of intercalation of guest ions into inorganic host materials developed ca. 40 years ago. It further turned into a commercial product, which will soon blow its 25th candle. Intense research efforts during this time have resulted in the development of a large spectrum of electrode materials together with deep understanding of the underlying structure-property relationships that govern their performance. This has enabled an ever increasing electrochemical yield together with the diversification of the technology into several subfamilies, tailoring materials to application requirements. The present paper aims at providing a global and critical perspective on inorganic electrode materials for lithium-ion batteries categorized by their reaction mechanism and structural dimensionality. Specific emphasis is put on recent research in the field, which beyond the chemistry and microstructure of the materials themselves also involves considering interfacial chemistry concepts alongside progress in characterization techniques. Finally a short personal perspective is provided on some plausible development of the field.

Bottom-up preparation of MgH₂ nanoparticles with enhanced cycle life stability during electrochemical conversion in Li-ion batteries

A promising anode material for Li-ion batteries based on MgH₂ with around 5 nm average particles size was synthesized by a bottom-up method. A series of several composites containing MgH₂ nanoparticles well dispersed into a porous carbon host has been prepared with different metal content up to 70 wt%. A narrow particle size distribution (1-10 nm) of the MgH₂ nanospecies with around 5.5 nm average size can be controlled up to 50 wt% Mg. After a ball milling treatment under Ar, the composite containing 50 wt% Mg shows an impressive cycle life stability with a good electrochemical capacity of around 500 mA h g(-1). Moreover, the nanoparticles' size distribution is stable during cycling.