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Magnesium batteries are one of the most promising post-lithium technologies. One of the main challenges preventing its commercialization is to find an efficient and safe electrolyte. The electrolyte, playing the role of the blood in a battery, interacts with all battery components and must be highly compatible with all of them. The development of Cl-free electrolyte systems is desired to avoid corrosion issues, and many studies suggest magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)4]2) as one of the best candidates in terms of electrochemical properties and chemical stability. Here we present an in-depth analysis of the unique structure of this salt and the interactions generated in the electrolyte among the dissociated ions. The results show a delicate balance between electron-withdrawing effects and ligand stabilization in B(hfip)4−, crucial from the point of view of magnesium electrolytes. Moreover, the bulk nature of B(hfip)4− limits the anion-cation contacts to infrequent interactions through fluorine atoms. This has consequences not only for ion transport but also for hindering the anion decomposition towards the formation of MgF2. Taken together, we managed to demystify the exclusive nature of B(hfip)4− anion, thereby allowing for further rational development of new anion structures.

Development of practical rechargeable Mg batteries (RMBs) is impeded by their limited cycle life and rate performance of cathodes. As demonstrated herein, a copper‐porphyrin with meso‐functionalized ethynyl groups is capable of reversible two‐ and four‐electron storage at an extremely fast rate (tested up to 53 C). The reversible four‐electron redox process with cationic‐anionic contributions resulted in a specific discharge capacity of 155 mAh g−1 at the high current density of 1000 mA g−1. Even at 4000 mA g−1, it still delivered >70 mAh g−1 after 500 cycles, corresponding to an energy density of >92 Wh kg−1 at a high power of >5100 W kg−1. The ability to provide such high‐rate performance and long‐life opens the way to the development of practical cathodes for multivalent metal batteries., Self‐conditioning: The metalloporphyrin based electrodes exhibit reversible insertion/deinsertion of Mg2+ and bulky anions as well as excellent capacity retention over several hundred cycles at very fast charge‐discharge rates. This opens the pathway for development of new organic electrode materials for stable high‐energy and high‐power multivalent metal‐based energy storage systems.

Summary The need for energy storage technologies that meet the demands for safety, sustainability, and high energy density has spurred increased interests in rechargeable Mg batteries. However, verification of their potential remains hampered by the absence of practical components. Recently, an unconventional research direction that deviates from the mainstream path has emerged and is redefining the boundaries of what can be accomplished in Mg batteries. Herein, we analyze this direction with a focus on fundamental phenomena with an aim to reveal new opportunities that can overcome long-standing challenges and unveil unaddressed issues. The topics covered encompass: (1) realizing efficient single salt Mg electrolytes that mimic those used in typical Li- and Na-ion batteries, (2) discovery of important concomitant mechanisms and previously unanticipated interfacial phenomena, and (3) identification of often overlooked bottlenecks in Mg cathodes and alternative anodes. In closing, we put forward our proposal for R&D focuses to help realize practical Mg batteries.

The realization of sustainable and cheap Mg‐S batteries depends on significant improvements in cycling stability. Building on the immense research on cathode optimization from Li‐S batteries, for the first time a beneficial role of MXenes for Mg‐S batteries is reported. Through a facile, low‐temperature vacuum‐filtration technique, several novel current collector‐ and binder‐free cathode films were developed, with either dipenthamethylene thiuram tetrasulfide (PMTT) or S8 nanoparticles as the source of redox‐active sulfur. The importance of combining MXene with a high surface area co‐host material, such as carbon nanotubes, was demonstrated. A positive effect of MXenes on the average voltage and reduced self‐discharge was also discovered. Ascribed to the rich polar surface chemistry of Ti3C2Tx MXene, an almost doubling of the discharge capacity (530 vs. 290 mA h g−1) was achieved by using MXene as a polysulfide‐confining interlayer, obtaining a capacity retention of 83 % after 25 cycles., Layer by layer: A positive effect of MXenes on capacity retention and reduced self‐discharge for Mg‐S batteries is demonstrated, either as a cathode constituent or as an interlayer between the cathode and the separator. An almost doubling of the discharge capacity (530 vs. 290 mA h g−1) is achieved by using MXene as a polysulfide‐confining interlayer, obtaining a capacity retention of 83 % after 25 cycles.

Li-rich disordered rock-salt structures have, because of their high theoretical capacity, gained a lot of attention as a promising class of cathode materials for battery applications. However, the ...

Put into storage: Cathodes allowing fast cation mobility are demonstrated in a VS4 structure for high-energy, multivalent (Mg and Ca) batteries. The flexible VS4 electronic structure enables cationic and anionic redox processes with multi-electron transfer. The development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistry that shows reversible multi-electron redox reactions. Cationic redox centres in the classical cathodes can only afford stepwise single-electron transfer, which are not ideal for multivalent-ion storage. The charge imbalance during multivalent ion insertion might lead to an additional kinetic barrier for ion mobility. Therefore, multivalent battery cathodes only exhibit slope-like voltage profiles with insertion/extraction redox of less than one electron. Taking VS4 as a model material, reversible two-electron redox with cationic–anionic contributions is verified in both rechargeable Mg batteries (RMBs) and rechargeable Ca batteries (RCBs). The corresponding cells exhibit high capacities of >300 mAh g−1 at a current density of 100 mA g−1 in both RMBs and RCBs, resulting in a high energy density of >300 Wh kg−1 for RMBs and >500 Wh kg−1 for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast Mg2+ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS4., publishedVersion

Rechargeable potassium-ion batteries (KIBs) are promising alternatives to lithium-ion batteries for large-scale electrochemical energy-storage applications because of the abundance and low cost of potassium. However, the development of KIBs is hampered by the lack of stable and high-capacity cathode materials. Herein, a functionalized porphyrin complex, [5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP), was proposed as a new cathode for rechargeable potassium batteries. Spectroscopy and molecular simulation studies were used to show that both PF6- and K+ interact with the porphyrin macrocycle to allow a four-electron transfer. In addition, the electrochemical polymerization of the ethynyl functional groups in CuDEPP resulted in the self-stabilization of the cathode, which was highly stable during cycling. This unique charge storage mechanism enabled CuDEPP to provide a capacity of 181 mAh g-1 with an average potential of 2.8 V (vs. K+ /K). These findings could open a pathway towards the design of new stable organic electrodes for KIBs.

Lithium-excess disordered rock-salt oxides have opened up a new vista in search of high-capacity cathodes, resulting in a variety of new materials with versatile elemental compositions. This work i...

The Li+ transport kinetics at the solid–solid electrode|electrolyte interfaces are crucial for the stable and durable performance of solid-state batteries (SSBs). A poor interface due to mechanical problems and/or (electro-)chemical instabilities will curtail the performance of such batteries. Herein, we present a detailed study on the interfaces of a lithium–sulfur (Li–S) SSB with a Li anode, Li–garnet (LLZO) solid electrolyte (SE), and a sulfur–carbon composite as the cathode. Interlayer gels based on ionic liquids were introduced to improve the interfacial properties of the system. For Li symmetric cells, the strategy resulted in a decrease in cell resistance by about a factor of five and stable voltage profiles with low overpotentials (∼300 mV at 0.4 mA cm−2 after >450 hours). Furthermore, the LLZO SE efficiently blocked the polysulfide shuttle to the Li anode. Due to the advantageous features of the design, good electrochemical performance was obtained, where the Li–S SSB delivered an initial discharge capacity of ca. 1360 mA h gsulfur−1 and a discharge capacity of ca. 570 mA h gsulfur−1 after 100 cycles. Detailed electrochemical and compositional characterization of the interphase layers was performed at the Li anode and sulfur cathode interfaces through X-ray photoelectron spectroscopy (XPS), applying depth-profiling techniques, and scanning transmission electron microscopy (STEM). The results revealed the presence of interphase nano-layers with varying thicknesses on the LLZO surface which contained organic and inorganic species.

A rapid microwave hydrothermal process is adopted for the synthesis of titanium dioxide and reduced graphene oxide nanocomposites as high-performance anode materials for Li-ion batteries. With the assistance of hydrazine hydrate as a reducing agent, graphene oxide was reduced while TiO$_{2}$ nanoparticles were grown in situ on the nanosheets to obtain the nanocomposite material. The morphology of the nanocomposite obtained consisted of TiO$_{2}$ particles with a size of ∼100 nm, uniformly distributed on the reduced graphene oxide nanosheets. The as-prepared TiO$_{2}$–graphene nanocomposite was able to deliver a capacity of 250 mA h g−1 ± 5% at 0.2C for more than 200 cycles with remarkably stable cycle life during the Li+ insertion/extraction process. In terms of high rate capability performance, the nanocomposite delivered discharge capacity of ca. 100 mA h g−1 with >99% coulombic efficiency at C-rates of up to 20C. The enhanced electrochemical performance of the material in terms of high rate capability and cycling stability indicates that the as-developed TiO$_{2}$–rGO nanocomposites are promising electrode materials for future Li-ion batteries.

International audience; This roadmap presents the transformational research ideas proposed by "BATTERY 2030+", the European large-scale research initiative for future battery chemistries. In this paper we outline a "chemistry-neutral" roadmap to advance battery research, particularly at low TRL, with a time horizon of about ten years. The roadmap is centered around six themes: 1) accelerated materials discovery platform, 2) battery interface genome, with the integration of smart functionalities such as 3) sensoring and 4) self-healing processes. Beyond chemistry related aspects we also include crosscutting research regarding 5) manufacturability and 6) recyclability. This roadmap should be seen as an important enabling complement to the many global battery roadmaps which focus on expected ultrahigh battery performance, especially for the future of transports. Batteries are used in many other applications and are considered to be one of Europe's key technologies necessary to reach the climate goals. Currently the market is dominated by lithium-ion batteries, which performs well in most applications, but despite new generations coming in near time, soon will approach their performance limits. Without major breakthroughs, battery performance and production requirements will not be sufficient to enable the building of a climate-neutral society. Through our "chemistry neutral" approach we aim to create a generic toolbox transforming the way we develop and design batteries, which later benefit into the development of specific battery chemistries and technologies. The goal is to integrate modeling and high-through-put experimental results in a closed integrated loop and manage the large amounts of data we generate to learn more about complex processes on different levels affecting the function of a battery cell or a battery system. Based on this we suggest concrete actions with the ambition to be part of and support the implementation of the European Green Deal, the UN Sustainable Development Goals, as well as the European Strategic Action plan on Batteries and the Strategic Energy Technology Plan.

In nonaqueous Mg batteries, inactive adsorbed species and the passivation layer formed from the reactive Mg with impurities in the electrolyte seriously affect the Mg metal/electrolyte interface. These adlayers can impede the passage of Mg$^{2+}$ ions, leading to a high Mg plating/stripping overpotential. Herein, we report the properties of a new additive, bismuth triflate (Bi(OTf)$_{3}$), for synthesizing a chlorine-free Mg electrolyte to enhance Mg plating/stripping from initial cycles. The beneficial effect of Bi(OTf)$_{3}$ can be ascribed to Bi/Mg$_{3}$Bi$_{2}$ formed in situ on the Mg metal surface, which increases the charge transfer during the on–off transition by reducing the adsorption of inactive species on the Mg surface and enhancing the resistance of the reactive surface to passivation. This simple method provides a new avenue to improve the compatibility between the Cl-free Mg electrolyte and the Mg metal anode.

Simple magnesium salts with high electrochemical and chemical stability and adequate ionic conductivity represent a new-generation electrolyte for magnesium (Mg) batteries. Similar to other Mg electrolytes, the simple-salt electrolyte also suffers from high charge-transfer resistance on the Mg surface due to the adsorbed species in the solution. In the current study, we built a model Mg cell system with the Mg[B(hfip)4]2/DME electrolyte and Chevrel phase Mo6S8 cathode, to demonstrate the effect of such anode���electrolyte interfacial properties on the full-cell performance. It was found that the cell required additional activation cycles to achieve its maximal capacity. The activation process is mainly attributed to the conditioning of the anode���electrolyte interface, which could be boosted by introducing an additive amount of Mg(BH4)2 to the Mg[B(hfip)4]2/DME electrolyte. Electrochemical and spectroscopic analyses revealed that the Mg(BH4)2 additive helps to remove the native oxide layer and promotes the formation of a solid electrolyte interphase layer on Mg. As a result, the full cell with the additive-containing electrolyte delivered a stable capacity from the second cycle onward. Further battery tests showed a reversible cycling for 600 cycles and an excellent rate capability, indicating good compatibility of the Mg(BH4)2 additive. The current study not only provides fundamental insights into the interfacial phenomena in Mg batteries but also highlights the facile tunability of the simple-salt Mg electrolytes.

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes promise improved safety, higher energy density, longer cycle life, and lower cost than conventional Li-ion batteries. However, their practical application is hampered by the high resistance arising at the solid-solid electrode-electrolyte interface. Although the exact mechanism of this interface resistance has not been fully understood, various chemical, electrochemical, and chemo-mechanical processes govern the charge transfer phenomenon at the interface. This paper reports the interfacial behavior of the lithium and the cathode in oxide and sulfide inorganic solid-electrolytes and how that affects the overall battery performance. An overview of the recent reports dealing with high resistance at the anodic and cathodic interfaces is presented and the scientific and engineering aspects of the approaches adopted to solve the issue are summarized.

Li-rich disordered rock-salt oxides such as Li1.2Ni1/3Ti1/3Mo2/15O2 are receiving increasing attention as high-capacity cathodes due to their potential as high-energy materials with variable elemen...

The green, sustainable, and versatile nature of using organic compounds that can be derived from biomass makes them extremely interesting materials for use in various rechargeable batteries. Howeve...

Recently, the ternary spinel selenide MgSc2 Se4 was proposed to have high magnesium ion mobility and is therefore an interesting potential candidate as a solid electrolyte in magnesium secondary batteries. To test the properties of the material, a modified solid-state reaction was used to synthesize pure MgSc2 Se4 . Electrochemical characterizations identified detrimental high electronic conductivities, which limit its application as a Mg-conducting solid electrolyte. Two methods were attempted to lower electronic conductivities, including the implementation of Se-rich phases and aliovalent doping, however, with no sufficient improvement. Based on the mixed conducting properties of MgSc2 Se4 , a reversible insertion/extraction of Mg2+ into the spinel structure could be demonstrated.

Rechargeable calcium (Ca) batteries have the prospect of high-energy and low-cost. However, the development of Ca batteries is hindered due to the lack of efficient electrolytes. Herein, we report novel calcium tetrakis(hexafluoroisopropyloxy)borate Ca[B(hfip)4]2 based electrolytes exhibiting reversible Ca deposition at room temperature, a high oxidative stability up to 4.5 V and high ionic conductivity >8 mS cm−1. This finding opens a new approach towards room-temperature rechargeable calcium batteries.

The performance of rechargeable magnesium batteries is strongly dependent on the choice of electrolyte. The desolvation of multivalent cations usually goes along with high energy barriers, which can have a crucial impact on the plating reaction. This can lead to significantly higher overpotentials for magnesium deposition compared to magnesium dissolution. In this work we combine experimental measurements with DFT calculations and continuum modelling to analyze Mg deposition in various solvents. Jointly, these methods provide a better understanding of the electrode reactions and especially the magnesium deposition mechanism. Thereby, a kinetic model for electrochemical reactions at metal electrodes is developed, which explicitly couples desolvation to electron transfer and, furthermore, qualitatively takes into account effects of the electrochemical double layer. The influence of different solvents on the battery performance is studied for the state‐of‐the‐art magnesium tetrakis(hexafluoroisopropyloxy)borate electrolyte salt. It becomes apparent that not necessarily a whole solvent molecule must be stripped from the solvated magnesium cation before the first reduction step can take place. For Mg reduction it seems to be sufficient to have one coordination site available, so that the magnesium cation is able to get closer to the electrode surface. Thereby, the initial desolvation of the magnesium cation determines the deposition reaction for mono‐, tri‐ and tetraglyme, whereas the influence of the desolvation on the plating reaction is minor for diglyme and tetrahydrofuran. Overall, we can give a clear recommendation for diglyme to be applied as solvent in magnesium electrolytes., Desolvation vs. electron transfer: The combination of DFT and continuum modeling techniques with experimental measurements sheds light on the magnesium deposition mechanism and the role of the involved solvent. Thereby, a new kinetic model for the electrochemical reaction is developed and general properties, which are crucial for fast kinetics and low overpotentials, are identified.

Lithium–sulphur (Li–S) batteries are currently considered as next-generation battery technology. Sulphur is an attractive positive electrode for lithium metal batteries, mainly due to its high capacity (1675 mAh g−1) and high specific energy (2600 Wh kg−1). The electrochemical reaction of lithium with sulphur in non-aqueous electrolytes results in the formation of electrolyte soluble intermediate lithium–polysulphides. The dissolved polysulphides shuttle to the anode and get reduced at the anode resulting in Li metal corrosion. The solubility of polysulphide gradually reduces the amount of sulphur in the cathode, thereby limiting the cycle life of Li–S batteries. Several strategies have been proposed to improve the cycling stability of Li–S batteries. A unique approach to eliminate the polysulphide shuttle is to use ultramicroporous carbon (UMC) as a host for sulphur. The pore size of UMC which is below 7 A, is the bottleneck for carbonate solvents to access sulphur/polysulphides confined in the pores, thereby preventing the polysulphide dissolution. This perspective article will emphasise the role of UMC host in directing the lithiation mechanism of sulphur and in inhibiting polysulphide dissolution, including the resulting parasitic reaction on the lithium anode. Further, the challenges that need to be addressed by UMC-S based Li–S batteries, and the strategies to realise high power density, high Coulombic efficiency, and resilient Li–S batteries will be discussed.

The development of new batteries has historically been achieved through discovery and development cycles based on the intuition of the researcher, followed by experimental trial and error—often helped along by serendipitous breakthroughs. Meanwhile, it is evident that new strategies are needed to master the ever-growing complexity in the development of battery systems, and to fast-track the transfer of findings from the laboratory into commercially viable products. This review gives an overview over the future needs and the current state-of-the art of five research pillars of the European Large-Scale Research Initiative BATTERY 2030+, namely 1) Battery Interface Genome in combination with a Materials Acceleration Platform (BIG-MAP), progress toward the development of 2) self-healing battery materials, and methods for operando, 3) sensing to monitor battery health. These subjects are complemented by an overview over current and up-coming strategies to optimize 4) manufacturability of batteries and efforts toward development of a circular battery economy through implementation of 5) recyclability aspects in the design of the battery.

Rechargeable magnesium batteries are gaining attention as promising candidates for large-scale energy storage applications because of their potentially high energy, safety and sustainability. However, the development of Mg batteries is impeded by the lack of efficient cathode materials and compatible electrode-electrolyte combinations. Herein, we demonstrate a new poly(1,4-anthraquinone)/Ketjenblack composite (14PAQ@KB) in combination with non-corrosive magnesium tetrakis(hexafluoroisopropyloxy) borate Mg[B(hfip)(4)](2) (hfip=OC(H)(CF3)(2)) electrolyte towards high-energy and long-lifespan Mg batteries. This combination exhibits prominent electrochemical performance including a maximum discharge capacity of 242 mA h g(-1) (approximately 93 % of the theoretical capacity), superior cycling stability (81 mA h g(-1) after 1000 cycles), and excellent rate capability (120 mA h g(-1) at 5 C).

The current energy transition imposes a rapid implementation of energy storage systems with high energy density and eminent regeneration and cycling efficiency. Metal hydrides are potential candidates for generalized energy storage, when coupled with fuel cell units and/or batteries. An overview of ongoing research is reported and discussed in this review work on the light of application as hydrogen and heat storage matrices, as well as thin films for hydrogen optical sensors. These include a selection of single-metal hydrides, Ti-V(Fe) based intermetallics, multi-principal element alloys (high-entropy alloys), and a series of novel synthetically accessible metal borohydrides. Metal hydride materials can be as well of important usefulness for MH-based electrodes with high capacity (e.g. MgH2 ~ 2000 mAh g-1) and solid-state electrolytes displaying high ionic conductivity suitable, respectively, for Li-ion and Li/Mg battery technologies. To boost further research and development directions some characterization techniques dedicated to the study of M-H interactions, their equilibrium reactions, and additional quantification of hydrogen concentration in thin film and bulk hydrides are presented at the end of this manuscript.

The changes introduced by both ball milling and the addition of small amounts of TiF3 in the kinetics of the hydrogen desorption of three different Ca(BH4)2 polymorphs (α, β and γ) have been systematically investigated. The samples with different polymorphic contents, before and after the addition of TiF3, were characterized by powder X-ray diffraction and vibrational spectroscopy. The hydrogen desorption reaction pathways were monitored by differential scanning calorimetry. The hydrogen desorption of Ca(BH4)2 depends strongly on the amount of coexistent α, β and γ polymorphs as well as additional ball milling and added TiF3 to the sample. The addition of TiF3 increased the hydrogen desorption rate without significant dissociation of the fluoride. The combination of an α-Ca(BH4)2 rich sample with 10 mol% of TiF3 and 8 h of milling led to up to 27 ∘C decrease of the hydrogen desorption peak temperature., publishedVersion

The choice of electrolyte has a crucial influence on the performance of rechargeable magnesium batteries. In multivalent electrolytes an agglomeration of ions to pairs or bigger clusters may affect the transport in the electrolyte and the reaction at the electrodes. In this work the formation of clusters is included in a general model for magnesium batteries. In this model, the effect of cluster formation on transport, thermodynamics and kinetics is consistently taken into account. The model is used to analyze the effect of ion clustering in magnesium tetrakis(hexafluoroisopropyloxy)borate in dimethoxyethane as electrolyte. It becomes apparent that ion agglomeration is able to explain experimentally observed phenomena at high salt concentrations., Raised overpotential by clustering?! A new continuum model that considers ion aggregation is presented and applied to magnesium tetrakis(hexafluoroisopropyloxy)borate in dimethoxyethane as electrolyte. Thereby, the properties of ion clusters and their effects on the battery performance are analyzed. Simulations imply that cluster formation can be essential to explain the concentration‐dependence of experimental data.

Li-garnets are promising inorganic ceramic solid electrolytes for lithium metal batteries, showing good electrochemical stability with Li anode. However, their brittle and stiff nature restricts their intimate contact with both the electrodes, hence presenting high interfacial resistance to the ionic mobility. To address this issue, a strategy employing ionic liquid electrolyte (ILE) thin interlayers at the electrodes/electrolyte interfaces is adopted, which helps overcome the barrier for ion transport. The chemically stable ILE improves the electrodes-solid electrolyte contact, significantly reducing the interfacial resistance at both the positive and negative electrodes interfaces. This results in the more homogeneous deposition of metallic lithium at the negative electrode, suppressing the dendrite growth across the solid electrolyte even at high current densities of 0.3 mA cm−2. Further, the improved interface Li/electrolyte interface results in decreasing the overpotential of symmetric Li/Li cells from 1.35 to 0.35 V. The ILE modified Li/LLZO/LFP cells stacked either in monopolar or bipolar configurations show excellent electrochemical performance. In particular, the bipolar cell operates at a high voltage (≈8 V) and delivers specific capacity as high as 145 mAh g−1 with a coulombic efficiency greater than 99%., publishedVersion

The properties of the mixed system LiBH$_{4}$–LiCl–P$_{2}$S$_{5}$ are studied with respect to all-solid-state batteries. The studied material undergoes an amorphization upon heating above 60 °C, accompanied with increased Li$^{+}$ conductivity beneficial for battery electrolyte applications. The measured ionic conductivity is ∼10$^{-3}$ S cm$^{-1}$ at room temperature with an activation energy of 0.40(2) eV after amorphization. Structural analysis and characterization of the material suggest that BH$_{4}$ groups and PS4 may belong to the same molecular structure, where Cl ions interplay to accommodate the structural unit. Thanks to its conductivity, ductility and electrochemical stability (up to 5 V, Au vs. Li$^{+}$/Li), this new electrolyte is successfully tested in battery cells operated with a cathode material (layered TiS$_{2}$, theo. capacity 239 mA h g$^{-1}$) and Li anode resulting in 93% capacity retention (10 cycles) and notable cycling stability under the current density ∼12 mA g$^{-1}$ (0.05C-rate) at 50 °C. Further advanced characterisation by means of operando synchrotron X-ray diffraction in transmission mode contributes explicitly to a better understanding of the (de)lithiation processes of solid-state battery electrodes operated at moderate temperatures.

Disordered rock salt Li2VO2F cathode material for lithium-ion batteries was investigated using operando X-ray diffraction and total scattering to gain insight into the structural changes of the short-range and long-range orders during electrochemical cycling. The X-ray powder diffraction data show the well-known pattern of the disordered rock salt cubic structure, whereas the pair distribution function (PDF) analysis reveals significant deviations from the ideal cubic structure. During battery operation, a reversible rock salt-to-amorphous phase transformation is observed, upon Li extraction and reinsertion. The X-ray total scattering data show strong indications of the formation of tetrahedrally coordinated V in a nondisordered rock salt phase of the charged electrode material. The results show that the disordered rock salt Li2VO2F material undergoes a hidden structural rearrangement during battery operation.

There is an increasing demand for rechargeable batteries in high-performance energy storage systems. The current dominating Li-ion batteries are limited by price, resource availability, as well as their theoretical capacities. So that the community has started to explore alternative battery chemistries. As a promising multivalent battery type, rechargeable magnesium batteries (RMBs) have attracted increasing attention because of high safety, high volumetric energy density, and low cost thanks to abundant resource of Mg. However, the development of high-performance anodes is still hampered by formation of passivating layers on the Mg surface. Additionally, dendrites can also grow under certain conditions with pure Mg anodes, which requires further studies for reliable operation window and substitutes. Therefore, this review specifically aims to provide an overview on the often overlooked yet very important anode materials of RMBs, with the hope to inspire more attention and research efforts for the achievement of over-all better performance of future RMBs.

Anionic redox activity in Li-rich cathode materials is a topic of intense interest because it presents the potential to increase the energy density of Li-ion batteries. An in-depth understanding of the anionic redox activity such as lithiation/voltage levels in which peroxide and superoxide formations take place and the reversibility of the reactions are important. In this work, we present a combined computational and experimental analysis that probes the formation of anionic redox species during electrochemical cycling. We report the formation of peroxides and superoxides in Li2VO2F when charged up to a relatively low potential of 4.1 V for Li-rich cathode materials. The formation of superoxide is not entirely reversible upon discharge, which is closely linked to the vanadium dissolution and limited reversibility of the vanadium redox couple. This article provides new insights and fundamental understanding of anionic redox activities in disordered Li-rich materials.

Li‐rich disordered rock salt (DRS) materials are new promising high‐capacity cathode candidates for Li‐ion batteries. DRS structures were initially assumed to have a completely random cation and anion distribution, but recent reports suggest that some of these structures can exhibit local atomic arrangements, or short‐range ordering (SRO). Here, we prove the existence of SRO in the Li‐rich DRS material Li$_{2}$VO$_{2}$F by employing Raman spectroscopy supported by density functional theory (DFT) calculations. Our results suggest that this combination of Raman spectroscopy with computational tools is useful for SRO estimation in this new class of Li‐rich DRS cathode materials.

Sodium-ion batteries are an excellent candidate to meet the challenge of grid-level storage because of the abundance and low cost of sodium resources. It is crucial to identify suitable anode material for such batteries in order to replace the current technology involving metallic sodium- or carbon-based anodes. Anodes of metal-ion batteries determine key characteristics, such as safety issues and cyclability. Nickel molybdate (NiMoO4) is an alternative candidate material for anode applications, possessing a number of useful characteristics. In this work, we have produced the phase by solution combustion synthesis. We have investigated the influence of oxidant (NH4NO3) concentration and optimised it to produce desirable material characteristics. The oxidant has a central role in the synthesis, being able to influence the properties of NiMoO4 including the electrochemical performance. At a low concentration of oxidiser (NH4NO3) the product obtained is partly crystalline and contains carbonaceous impurities while at a higher concentration of oxidiser, the reaction is incomplete forming secondary phases. The optimised fuel-to-oxidiser ratio is found to be around 1:1 whereby the oxidant is able to interact and chelate metal cations. This optimised material produces a high initial discharge capacity of NiMoO4vs. Na of 550 mAh g−1 at a current density of 0.05 A g−1. However, the reversible capacity is found to be 245 mAh g−1 but resulted in good capacity retention of 82% after 50 cycles and higher rate capability performance. This anode material is comparable to the capacity and outperforms by the voltage of classical carbon anodes used in sodium-ion battery. In the case of material produced with the lowest and highest concentrations, capacity retention is 45% and 75%, respectively. The electrochemical intercalation of Na ions into nickel molybdate produces a new type of intercalation compound (Na2MoO3) and this is discussed. These results provide insight regarding a versatile methodology based on solution combustion synthesis and an alternative insertion-type anode to metallic sodium or carbon.

The rechargeable magnesium (Mg) battery has been considered a promising candidate for future battery generations due to unique advantages of the Mg metal anode. The combination of Mg with a sulfur cathode is one of the attractive electrochemical energy storage systems that use safe, low-cost, and sustainable materials and could potentially provide a high energy density. To develop a suitable electrolyte remains the key challenge for realization of a magnesium sulfur (Mg–S) battery. Herein, we demonstrate that magnesium tetrakis(hexafluoroisopropyloxy) borate Mg[B(hfip)4]2 (hfip = OC(H)(CF3)2) satisfies a multitude of requirements for an efficient and practical electrolyte, including high anodic stability (>4.5 V), high ionic conductivity (∼11 mS cm–1), and excellent long-term Mg cycling stability with a low polarization. Insightful mechanistic studies verify the reversible redox processes of Mg–S chemistry by utilizing Mg[B(hfip)4]2 electroylte and also unveil the origin of the voltage hysteresis in Mg–S ...

Composites of nanoparticles/graphene sheets show an improved lithium storage capacity and cycling stability compared to bare graphene sheets (GSs). They are therefore considered as one of the promising candidates of anode materials for portable electrochemical energy storage devices requiring lightweight and ultrathin batteries. The practical application of these materials relies on the in-depth understanding of the origin of their improved performance. In this work, a composite of silver nanoparticles and graphene sheets (Ag/GSs) has been used as a model material to investigate the origin of the improved electrochemical performance by impedance, ex situ XPS and in situ Raman spectroscopy. We found that the Ag/GSs composite electrode has higher electrical conductivity than GSs. AgLix alloy was formed during the lithiation. Insertion of Ag nanoparticles into the interlayers between graphene sheets reduced the mean number of graphene stacking layers in the composite and provided a better site accessibility for Li+ insertion. Comparative in situ Raman measurements of them showed a completely reversible structural evolution of graphene sheets in Ag/GSs during the first lithiation/de-lithiation process, while for GSs the structural stability was worse. In combination, these effects are favorable for improving the reversible capacity and retaining the cycle stability of the Ag/GSs composite.

A fundamental electrochemical model is developed, describing the capacity fade of C6/LiFePO4 batteries as a function of calendar time and cycling conditions. At moderate temperatures the capacity losses are mainly attributed to Li immobilization in Solid-Electrolyte-Interface (SEI) layers at the anode surface. The SEI formation model presumes the availability of an outer and inner SEI layers. Electron tunneling through the inner SEI layer is regarded as the rate-determining step. The model also includes high temperature degradation. At elevated temperatures, iron dissolution from the positive electrode and the subsequent metal sedimentation on the negative electrode influence the capacity loss. The SEI formation on the metal-covered graphite surface is faster than the conventional SEI formation. The model predicts that capacity fade during storage is lower than during cycling due to the generation of SEI cracks induced by the volumetric changes during (dis)charging. The model has been validated by cycling and calendar aging experiments and shows that the capacity loss during storage depends on the storage time, the State-of-Charge (SoC), and temperature. The capacity losses during cycling depend on the cycling current, cycling time, temperature and cycle number. All these dependencies can be explained by the single model presented in this paper.

Two approaches of engineering surface structures of V–Ti-based solid solution hydrogen storage alloys are presented, which enable improved tolerance toward gaseous oxygen (O2) impurities in hydrogen (H2) gas. Surface modification is achieved through engineering lanthanum (La)- or nickel (Ni)-rich surface layers with enhanced cyclic stability in an H2/O2 mixture. The formation of a Ni-rich surface layer does not improve the cycling stability in H2/O2 mixtures. Mischmetal (Mm, a mixture of La and Ce) agglomerates are observed within the bulk and surface of the alloy when small amounts of this material are added during arc melting synthesis. These agglomerates provide hydrogen-transparent diffusion pathways into the bulk of the V–Ti–Cr–Fe hydrogen storage alloy when the remaining oxidized surface is already nontransparent for hydrogen. Thus, the cycling stability of the alloy is improved in an O2-containing hydrogen environment as compared to the same alloy without addition of Mm. The obtained surface-engine...

Magnesium molybdate (MgMoO4), which possesses synergistic features combining both hierarchical plate-like nanomaterials and porous architectures, has been successfully synthesized through a facile combustion synthesis at a low temperature. The hierarchical architecture is characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), scanning transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses. The as-obtained MgMoO4 nanoplates showed a porous structure with a pore-size distribution ranging from 50 to 70 nm. This porosity provides an electron transport pathway and enhanced surface reaction kinetics. The binding energies measured for Mg 2p, Mo 3d, 3p and O 1s are consistent with the literature, and with the metal ions being present as M(ii) and M(vi) states, respectively. This indicates that the oxidation states of the metal cations are as expected. The electrochemical behaviour of MgMoO4 was investigated using aqueous (NaOH) and non-aqueous solvents (NaClO4 in EC : DMC : FEC) for supercapacitor and battery applications. The sodium-ion capacitor involves ion absorption and insertion into the MgMoO4 electrodes resulting in superior power and energy densities. However, the cycling stability was found to be stable only for an aqueous system. The formation of a solid electrolyte surface layer restricted the reversible capacity of the MgMoO4 in the sodium-battery. Nevertheless, it does offer some promise as an anode material for storing energy with high rate performance and excellent capacity retention. Detailed comparative analyses of various electrolytes in storage devices such as hybrid sodium-ion capacitors and sodium-ion batteries are vital for the integration of hierarchical structured materials into practical applications. The reaction mechanisms are postulated.

Rechargeable magnesium (Mg) battery has been considered as a promising candidate for future battery generations because of its potential high-energy density, its safety features and low cost. The challenges lying ahead for the realization of Mg battery in general are to develop proper electrolytes fulfilling a multitude of requirements and to discover cathode materials enabling high-energy Mg batteries. The combination of Mg anode with a sulfur cathode is one of the promising electrochemical couples due to its advantages of safety, low costs, and a high theoretical energy density of over 3200 Wh/L. However, the research on magnesium-sulfur (Mg-S) battery is just at its beginning and the development of suitable electrolytes has been the key challenge for further improvement, and, thus, in the focus of recent research. In this review, we highlight the recent progress achieved in Mg electrolytes and Mg-S batteries and discuss the major technical issues, which must be resolved for the improvement of Mg-S batteries.

A hierarchically structured MnF2/carbon nanotubes composite (CNTs) was successfully prepared using a novel sol-gel method. The reaction mechanism of this process was investigated by simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectrometry (TGA-DSC-MS). The composition and morphology were characterized by X-ray diffraction (XRD), Raman spectrometry, and scanning electron microscopy (SEM). The material was built from a bunch of radially arranged nano-rods crystallites with a side length of 40 nm. CNTs were dispersed on the surfaces of MnF2 particles and improved the electrochemical performance. A low discharge plateau around 0.6 V vs. Li/Li+ at 0.2 C was obtained in the first cycle with a reversible discharge capacity of 384 mAh g−1 after 100 cycles. In long-term measurements, a discharge capacity of 203 mAh g−1 at 10 C was delivered after 1000 cycles. Moreover, a first full cell (LiNi1/3Mn1/3Co1/3O2 vs. MnF2) test demonstrates the actual applicability of the MnF2/CNTs composite, as well.

Metallic magnesium represents a promising solution for next generation electrochemical energy storage devices because of its dendrite-free electrodeposition, higher theoretical volumetric energy density, abundancy and safety compared to lithium technology. Nevertheless, Mg2+ is strongly polarized, which slows down its diffusion during discharging/charging. An electrochemical energy storage device containing a metallic Mg anode, a Li intercalation cathode and a Li/Mg-electrolyte can be regarded as a solution to circumvent the intrinsic drawbacks linked to both Li- and Mg-electrochemistry. In this contribution, we report on the performance of a VOCl/Mg cell containing a Li−Mg hybrid electrolyte. The cell delivered 195 mA h g−1 at 100 mA g−1 with a coulombic efficiency of 97.5%. At 200 mA g-1, after 150 cycles, 107 mA h g−1 was measured with 99.5% coulombic efficiency. At 500 mA g−1, after 800 cycles, the cell still produced 75 mA h g−1 with 99.8% coulombic efficiency. Lithium reacted with VOCl first via an intercalation reaction and then in a conversion reaction to LiCl and VO. The co-deposition of Li at the anode, Li−Mg alloy formation or contributions from side reactions could be ruled out.

The novel functionalized porphyrin [5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP) was used as electrodes for rechargeable energy-storage systems with an extraordinary combination of storage capacity, rate capability, and cycling stability. The ability of CuDEPP to serve as an electron donor or acceptor supports various energy-storage applications. Combined with a lithium negative electrode, the CuDEPP electrode exhibited a long cycle life of several thousand cycles and fast charge–discharge rates up to 53 C and a specific energy density of 345 Wh kg−1 at a specific power density of 29 kW kg−1. Coupled with a graphite cathode, the CuDEPP anode delivered a specific power density of 14 kW kg−1. Whereas the capacity is in the range of that of ordinary lithium-ion batteries, the CuDEPP electrode has a power density in the range of that of supercapacitors, thus opening a pathway toward new organic electrodes with excellent rate capability and cyclic stability.

Mg-based batteries, which use the Mg2+ shuttle, offer several advantages compared to the Li technology such as higher theoretical volumetric capacity (3833 mA h cm-3) of the Mg-metal anode, the possibility to be safely handled in air and dendrite-free electrodeposition. In this study, VOCl was employed as electrode material in a Mg-based battery. Since the cell delivered just 45 mA h g-1 in the first cycle, we tried to improve the delivered capacity by a preliminary cycling of the VOCl electrode with Li. The strategy is based on the ability of VOCl to expand its interlayer spacing upon intercalation of ions or molecules within them. In fact, a VOCl-electrode with expanded interlayer spacing should facilitate the intercalation of Mg2+, thus leading to higher specific capacities. The Li-pre-treatment was able to promote the specific capacity by a factor of four (170 mA h g-1) after the first discharge at 298 K. Over 130 mA h g-1 were retained at 5 mA g-1 after 70 cycles. The structural and electrochemical characterization were carried out by means of galvanostatic charge/discharge tests, cyclic voltammetry (CV) and ex-situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) as well as transmission electron microscopy (HR-TEM) coupled with energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the concentration of lithium in the electrode.

Fluorinated alkoxyborate based magnesium electrolytes exhibit a high anodic stability, high ionic conductivity and high coulombic efficiency of magnesium deposition. Owing to the non-corrosive, chemically stable nature and the robust, economic synthesis, this design concept of ion conducting salts opens a promising avenue towards the realization of high-energy magnesium batteries.

Biowaste eggshell can be used as a cathode while in its calcined form and it is found to be suitable as an anode in an electrochemical cell. This not only enables energy to be stored reversibly but also achieves waste management and sustainability goals by redirecting material away from landfill. Biowaste eggshell comprises 94% calcium carbonate (CaCO3; calcite), an attractive divalent ion source as a viable option for energy storage. X-ray diffraction and electron microscopy coupled with energy dispersive analyses of the calcined (thermally decomposed) biowaste eggshell show that CaO has been formed and the reaction is topotactic. Field emission scanning electron microscopy (FESEM) images of the textural relationship show that the thermal decomposition of calcite resulted in a change in morphology. High-resolution XPS spectra of the C 1s core level from the CaCO3 and CaO shows that there is a chemical difference in the carbon environments and the total atomic fraction of Ca for each sample with that of carbonate and oxygen varies. In a three-electrode configuration, a working electrode of CaCO3 is found to be electrochemically active in the positive region, whereas a CaO electrode is active in the negative region. This indicates the potential use of eggshell-derived materials for both cathode and anode. Both the electrodes exhibited a quasi-box-shaped potentiostatic curve implying a capacitor-type behaviour. The CaCO3 cathode possesses a modest discharge capacitance of 10 F g−1 but the CaO anode showed excellent capacitance value of 47.5 F g−1. The CaO electrode in both positive and negative regions, at a current density of 0.15 A g−1 exhibited 55 F g−1 with a retention of nearly 100% after 1000 cycles. At a very low sweep rate of 0.5 mV s−1, the CaO electrode showed typical redox-type behaviour with well-defined peaks illustrating battery-type behaviour. The outcome of the calcite/CaO transformation, exhibiting technological importance for energy storage applications, may help to re-evaluate biowaste before throwing it away. The current work explores the viability of eggshell derived materials as a cathode/anode for use in batteries and capacitors.

Lithium-rich transition metal disordered rock salt (DRS) oxyfluorides have the potential to lessen one large bottleneck for lithium ion batteries by improving the cathode capacity. However, irreversible reactions at the electrode/electrolyte interface have so far led to fast capacity fading during electrochemical cycling. Here, we report the synthesis of two new Li-rich transition metal oxyfluorides Li2V0.5Ti0.5O2F and Li2V0.5Fe0.5O2F using the mechanochemical ball milling procedure. Both materials show substantially improved cycling stability compared to Li2VO2F. Rietveld refinements of synchrotron X-ray diffraction patterns reveal the DRS structure of the materials. Based on density functional theory (DFT) calculations, we demonstrate that substitution of V3+ with Ti3+ and Fe3+ favors disordering of the mixed metastable DRS oxyfluoride phase. Hard X-ray photoelectron spectroscopy shows that the substitution stabilizes the active material electrode particle surface and increases the reversibility of the V3+/V5+ redox couple. This work presents a strategy for stabilization of the DRS structure leading to improved electrochemical cyclability of the materials.

Porphyrin complexes are well‐known for their application in solar‐cell systems and as catalysts; however, their use in electrochemical energy‐storage applications has scarcely been studied. Here, a tetra‐alkenyl‐substituted [5,10,15,20‐tetra(ethynyl)porphinato]copper(II) (CuTEP) complex was used as anode material in a high‐performance lithium‐free CuTEP/PP14TFSI/graphite cell [PP14TFSI=1‐butyl‐1‐methylpiperidinium bis(trifluoromethylsulfonyl)imide]. Thereby, the influence of size and morphology on the electrochemical performance of the cell was thoroughly investigated. Three different nanocrystal CuTEP morphologies, namely nanobricks, nanosheets, and nanoribbons, were studied as anode material, and the best cyclability and highest rate capability were obtained for the nanoribbon samples. A high specific power density of 14 kW kg−1 (based on active material) and excellent rechargeability were achieved with negligible capacity decay over 1000 cycles at a high current density of 5 A g−1. These results indicate that the porphyrin complex CuTEP could be a promising electrode material in high‐performance lithium‐free batteries., Morphology matters: In a Li‐free battery cell with organic electrodes consisting of a tetra‐acetenyl‐substituted porphyrin, the nanobrick‐ and nanoribbon‐morphology porphyrin samples show higher rate cyclability than the nanosheet sample. The organic electrode material demonstrated a specific power density of 14 kW kg−1 and excellent rechargeability.

Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiM2+0.5V0.54+O2 stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiM2+0.5V4+0.5O2 (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g-1, respectively. In comparison, for the disordered rock-salt Li2VO3, an average discharge voltage of ∼2.2 V with V5+/4+ redox couple has been reported. However, detrimental electrode-electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode-electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.

The present study shows the electrochemical performance of a room-temperature magnesium/sulfur (Mg/S) battery with a newly designed sulfur (3–0.5 mgsulfur cm−2) composite cathode. Operando Raman spectroscopy is employed to investigate the formation of polysulfide species at the cathode of Mg/S cells during the charge/discharge process, while density functional theory (DFT) calculations are used to correlate the Raman modes with a series of polysulfide species (Sxn−, x = 1–8). Operando Raman spectroscopy proves the chemical transformation from elemental sulfur (S8) via long and short chain polysulfides to magnesium sulfide upon discharge and conversion back to elemental sulfur during charging. Furthermore, the spectral measurements indicate the formation of a nanocrystalline magnesium sulfide with a cubic zinc blende phase at the end of discharge. The changes in the open circuit potential of the Mg/S cell during the resting period are investigated with the help of various spectroscopic techniques. These studies conclude that cell impedance is dominated by the anode side impedance due to the formation of a passivation layer under static conditions. Finally, the impedance studies under dynamic conditions verify that the applied electrode potential plays a significant role on the evolution of the anode interfacial impedance.

Quasielastic neutron scattering was used to investigate the low energy transfer dynamics of the complex borohydrides Mg(BH4)2 in the α- and β-modifications, LiBH4 in the low and high temperature crystal structure, and an 1 : 1 molar mixture of LiBH4 + α-Mg(BH4)2. All investigated compounds show a rich dynamic behaviour below an energy range of ΔE = 10 meV with the superposition of rotational dynamics of the constituent [BH4]− anions and low lying lattice modes. For Mg(BH4)2, the rotational diffusion of the [BH4] units was found to be much more activated in the metastable β-polymorph compared to the α-phase, and the low lying lattice modes are even softer in the former crystal structure. In Mg(BH4)2, the structural phase transition is mainly governed by the lattice dynamics, while alkaline LiBH4 exhibits a transition of the [BH4] rotations around the phase transition temperature. Ball milled LiBH4 + α-Mg(BH4)2 remains a physical mixture of the parent compounds and each component retains its characteristic dynamic signature up to the melting temperature.