Porous Azatruxene Covalent Organic Frameworks for Anion Insertion in Battery Cells.

Covalent organic frameworks (COFs) containing well-defined redox-active groups have become competitive materials for next-generation batteries. Although high potentials and rate performance can be expected, only a few examples of p-type COFs have been reported for charge storage to date with even fewer examples on the use of COFs in multivalent ion batteries. Herein, we report the synthesis of a p-type highly porous and crystalline azatruxene-based COF and its application as a positive electrode material in Li- and Mg-based batteries. When this material is used in Li-based half cells as a COF/carbon nanotube (CNT) electrode, a discharge potential of 3.9 V is obtained with discharge capacities of up to 70 mAh g-1 at a 2 C rate. In Mg batteries using a tetrakis(hexafluoroisopropyloxy)borate electrolyte, cycling proceeds with an average discharge voltage of 2.9 V. Even at a fast current rate of 5 C, the capacity retention amounts to 84% over 1000 cycles.

Improving rechargeable magnesium batteries through dual cation co-intercalation strategy.

The development of competitive rechargeable Mg batteries is hindered by the poor mobility of divalent Mg ions in cathode host materials. In this work, we explore the dual cation co-intercalation strategy to mitigate the sluggishness of Mg2+ in model TiS2 material. The strategy involves pairing Mg2+ with Li+ or Na+ in dual-salt electrolytes in order to exploit the faster mobility of the latter with the aim to reach better electrochemical performance. A combination of experiments and theoretical calculations details the charge storage and redox mechanism of co-intercalating cationic charge carriers. Comparative evaluation reveals that the redox activity of Mg2+ can be improved significantly with the help of the dual cation co-intercalation strategy, although the ionic radius of the accompanying monovalent ion plays a critical role on the viability of the strategy. More specifically, a significantly higher Mg2+ quantity intercalates with Li+ than with Na+ in TiS2. The reason being the absence of phase transition in the former case, which enables improved Mg2+ storage. Our results highlight dual cation co-intercalation strategy as an alternative approach to improve the electrochemical performance of rechargeable Mg batteries by opening the pathway to a rich playground of advanced cathode materials for multivalent battery applications.

Greener, Safer and Better Performing Aqueous Binder for Positive Electrode Manufacturing of Sodium Ion Batteries.

P2-type cobalt-free MnNi-based layered oxides are promising cathode materials for sodium-ion batteries (SIBs) due to their high reversible capacity and well chemical stability. However, the phase transformations during repeated (dis)charge steps lead to rapid capacity decay and deteriorated Na+ diffusion kinetics. Moreover, the electrode manufacturing based on polyvinylidene difluoride (PVDF) binder system has been reported with severely defluorination issue as well as the energy intensive and expensive process due to the use of toxic and volatile N-methyl-2-pyrrolidone (NMP) solvent. It calls for designing a sustainable, better performing, and cost-effective binder for positive electrode manufacturing. In this work, we investigated inorganic sodium metasilicate (SMS) as a viable binder in conjunction with P2-Na0.67Mn0.55Ni0.25Fe0.1Ti0.1O2 (NMNFT) cathode material for SIBs. The NMNFT-SMS electrode delivered a superior electrochemical performance compared to carboxy methylcellulose (CMC) and PVDF based electrodes with a reversible capacity of ~161 mAh/g and retaining ~83 % after 200 cycles. Lower cell impedance and faster Na+ diffusion was also observed in this binder system. Meanwhile, with the assistance of TEM technique, SMS is suggested to form a uniform and stable nanoscale layer over the cathode particle surface, protecting the particle from exfoliation/cracking due to electrolyte attack. It effectively maintained the electrode connectivity and suppressed early phase transitions during cycling as confirmed by operando XRD study. With these findings, SMS binder can be proposed as a powerful multifunctional binder to enable positive electrode manufacturing of SIBs and to overall reduce battery manufacturing costs.

Long Cycle-Life Ca Batteries with Poly(anthraquinonylsulfide) Cathodes and Ca-Sn Alloy Anodes.

Calcium (Ca) batteries are attractive post-lithium battery technologies, due to their potential to provide high-voltage and high-energy systems in a sustainable manner. We investigated herein 1,5-poly(anthraquinonylsulfide) (PAQS) for Ca-ion storage with calcium tetrakis(hexafluoroisopropyloxy)borate Ca[B(hfip)4 ]2 [hfip=OCH(CF3 )2 ] electrolytes. It is demonstrated that PAQS could be synthesized in a cost-effective approach and be processed environmentally friendly into the electrodes. The PAQS cathodes could provide 94 mAh g-1 capacity at 2.2 V vs. Ca at 0.5C (1C=225 mAh g-1 ). However, cycling of the cells was severely hindered due to the fast degradation of the metal anode. Replacing the Ca metal anode with a calcium-tin (Ca-Sn) alloy anode, the PAQS cathodes exhibited long cycling performance (45 mAh g-1 at 0.5C after 1000 cycles) and superior rate capability (52 mAh g-1 at 5C). This is mainly ascribed to the flexible structure of PAQS and good compatibility of the alloy anodes with the electrolyte solutions, which allow reversible quinone carbonyl redox chemistry in the Ca battery systems. The promising properties of PAQS indicate that further exploration of the organic cathode materials could be a feasible direction towards green Ca batteries.

Magnesium Anode Protection by an Organic Artificial Solid Electrolyte Interphase for Magnesium-Sulfur Batteries.

In the search for post-lithium battery systems, magnesium-sulfur batteries have attracted research attention in recent years due to their high potential energy density, raw material abundance, and low cost. Despite significant progress, the system still lacks cycling stability mainly associated with the ongoing parasitic reduction of sulfur at the anode surface, resulting in the loss of active materials and passivating surface layer formation on the anode. In addition to sulfur retention approaches on the cathode side, the protection of the reductive anode surface by an artificial solid electrolyte interphase (SEI) represents a promising approach, which contrarily does not impede the sulfur cathode kinetics. In this study, an organic coating approach based on ionomers and polymers is pursued to combine the desired properties of mechanical flexibility and high ionic conductivity while enabling a facile and energy-efficient preparation. Despite exhibiting higher polarization overpotentials in Mg-Mg cells, the charge overpotential in Mg-S cells was decreased by the coated anodes with the initial Coulombic efficiency being significantly increased. Consequently, the discharge capacity after 300 cycles applying an Aquivion/PVDF-coated Mg anode was twice that of a pristine Mg anode, indicating effective polysulfide repulsion from the Mg surface by the artificial SEI. This was backed by operando imaging during long-term OCV revealing a non-colored separator, i.e. mitigated self-discharge. While SEM, AFM, IR and XPS were applied to gain further insights into the surface morphology and composition, scalable coating techniques were investigated in addition to ensure practical relevance. Remarkably therein, the Mg anode preparation and all surface coatings were prepared under ambient conditions, which facilitates future electrode and cell assembly. Overall, this study highlights the important role of Mg anode coatings to improve the electrochemical performance of magnesium-sulfur batteries.

Anion Storage Chemistry of Organic Cathodes for High-Energy and High-Power Density Divalent Metal Batteries.

Multivalent batteries show promising prospects for next-generation sustainable energy storage applications. Herein, we report a polytriphenylamine (PTPAn) composite cathode capable of highly reversible storage of tetrakis(hexafluoroisopropyloxy) borate [B(hfip)4 ] anions in both Magnesium (Mg) and calcium (Ca) battery systems. Spectroscopic and computational studies reveal the redox reaction mechanism of the PTPAn cathode material. The Mg and Ca cells exhibit a cell voltage >3 V, a high-power density of ∼∼3000 W kg-1 and a high-energy density of ∼∼300 Wh kg-1 , respectively. Moreover, the combination of the PTPAn cathode with a calcium-tin (Ca-Sn) alloy anode could enable a long battery-life of 3000 cycles with a capacity retention of 60 %. The anion storage chemistry associated with dual-ion electrochemical concept demonstrates a new feasible pathway towards high-performance divalent ion batteries.

Long-Cycle-Life Calcium Battery with a High-Capacity Conversion Cathode Enabled by a Ca2+/Li+ Hybrid Electrolyte.

Calcium (Ca) batteries represent an attractive option for electrochemical energy storage due to physicochemical and economic reasons. The standard reduction potential of Ca (-2.87 V) is close to Li and promises a wide voltage window for Ca full batteries, while the high abundance of Ca in the earth's crust implicates low material costs. However, the development of Ca batteries is currently hindered by technical issues such as the lack of compatible electrolytes for reversible Ca2+ plating/stripping and high-capacity cathodes with fast kinetics. Herein, we employed FeS2 as a conversion cathode material and combined it with a Li+/Ca2+ hybrid electrolyte for Ca batteries. We demonstrate that Li+ ions ensured reversible Ca2+ plating/stripping on the Ca metal anode with a small overpotential. At the same time, they enable the conversion of FeS2, offering high discharge capacity. As a result, the Ca/FeS2 cell demonstrated an excellent long-term cycling performance with a high discharge capacity of 303 mAh g-1 over 200 cycles. Even though the practical application of such an approach is questionable due to the high quantity of electrolytes, we believe that our scientific findings still provide new directions for studying Ca batteries with long-term cycling.

Calcium-tin alloys as anodes for rechargeable non-aqueous calcium-ion batteries at room temperature.

Rechargeable calcium batteries possess attractive features for sustainable energy-storage solutions owing to their high theoretical energy densities, safety aspects and abundant natural resources. However, divalent Ca-ions and reactive Ca metal strongly interact with cathode materials and non-aqueous electrolyte solutions, leading to high charge-transfer barriers at the electrode-electrolyte interface and consequently low electrochemical performance. Here, we demonstrate the feasibility and elucidate the electrochemical properties of calcium-tin (Ca-Sn) alloy anodes for Ca-ion chemistries. Crystallographic and microstructural characterizations reveal that Sn formed from electrochemically dealloying the Ca-Sn alloy possesses unique properties, and that this in-situ formed Sn undergoes subsequent reversible calciation/decalciation as CaSn3. As demonstration of the suitability of Ca-Sn alloys as anodes for Ca-ion batteries, we assemble coin cells with an organic cathode (1,4-polyanthraquinone) in an electrolyte of 0.25 M calcium tetrakis(hexafluoroisopropyloxy)borate in dimethoxyethane. These electrochemical cells are charged/discharged for 5000 cycles at 260 mA g-1, retaining a capacity of 78 mAh g-1 with respect to the organic cathode. The discovery of new class of Ca-Sn alloy anodes opens a promising avenue towards viable high-performance Ca-ion batteries.

Dual Role of Mo6 S8 in Polysulfide Conversion and Shuttle for Mg-S Batteries.

Magnesium-Sulfur batteries are one of most appealing options among the post-lithium battery systems due to its potentially high energy density, safe and sustainable electrode materials. The major practical challenges are originated from the soluble magnesium polysulfide intermediates and their shuttling between the electrodes, which cause high overpotentials, low sulfur utilization, and poor Coulombic efficiency. Herein, a functional Mo6 S8 modified separator is designed to effectively address these issues. Both the experimental results and density functional theory calculations show that the electrochemically active Mo6 S8 layer has a superior adsorption capability of polysulfides and simultaneously acts as a mediator to accelerate the polysulfide conversion kinetics. Remarkably, the magnesium-sulfur cell assembled with the functional separator delivers a high specific energy density (942.9 mA h g-1 in the 1st cycle) and can be cycled at 0.2 C for 200 cycles with a Coulombic efficiency of 96%. This work demonstrates a new design concept toward high-performance metal-sulfur batteries.

Modeling of Electron-Transfer Kinetics in Magnesium Electrolytes: Influence of the Solvent on the Battery Performance.

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.

Establishing a Stable Anode-Electrolyte Interface in Mg Batteries by Electrolyte Additive.

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.

Surface Engineering of a Mg Electrode via a New Additive to Reduce Overpotential.

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 Mg2+ 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/Mg3Bi2 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.

A Self-Conditioned Metalloporphyrin as a Highly Stable Cathode for Fast Rechargeable Magnesium Batteries.

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.

Performance Study of MXene/Carbon Nanotube Composites for Current Collector- and Binder-Free Mg-S 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 Ti3 C2 Tx 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.

Rechargeable Calcium-Sulfur Batteries Enabled by an Efficient Borate-Based Electrolyte.

Rechargeable metal-sulfur batteries show great promise for energy storage applications because of their potentially high energy and low cost. The multivalent-metal based electrochemical system exhibits the particular advantage of the feasibility of dendrite-free metal anode. Calcium (Ca) represents a promising anode material owing to the low reductive potential, high capacity, and abundant natural resources. However, calcium-sulfur (Ca-S) battery technology is in an early R&D stage, facing the fundamental challenge to develop a suitable electrolyte enabling reversible electrochemical Ca deposition, and at the same time, sulfur redox reactions in the system. Herein, a study of a room-temperature Ca-S battery by employing a stable and efficient calcium tetrakis(hexafluoroisopropyloxy) borate Ca[B(hfip)4 ]2 electrolyte is presented. The Ca-S batteries exhibit a cell voltage of ≈2.1 V (close to its thermodynamic value) and good reversibility. The mechanistic studies hint at a redox chemistry of sulfur with polysulfide/sulfide species involved in the Ca-based system.

Modeling of Ion Agglomeration in Magnesium Electrolytes and its Impacts on Battery Performance.

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.

Copper Porphyrin as a Stable Cathode for High-Performance Rechargeable Potassium Organic Batteries.

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.

Multi-Electron Reactions Enabled by Anion-Based Redox Chemistry for High-Energy Multivalent Rechargeable Batteries.

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 .

Halide-Based Materials and Chemistry for Rechargeable Batteries.

Rechargeable batteries are considered one of the most effective energy storage technologies to bridge the production and consumption of renewable energy. The further development of rechargeable batteries with characteristics such as high energy density, low cost, safety, and a long cycle life is required to meet the ever-increasing energy-storage demands. This Review highlights the progress achieved with halide-based materials in rechargeable batteries, including the use of halide electrodes, bulk and/or surface halogen-doping of electrodes, electrolyte design, and additives that enable fast ion shuttling and stable electrode/electrolyte interfaces, as well as realization of new battery chemistry. Battery chemistry based on monovalent cation, multivalent cation, anion, and dual-ion transfer is covered. This Review aims to promote the understanding of halide-based materials to stimulate further research and development in the area of high-performance rechargeable batteries. It also offers a perspective on the exploration of new materials and systems for electrochemical energy storage.