Fluorine-Substituted Halide Solid Electrolytes with Enhanced Stability toward the Lithium Metal.

The high ionic conductivity and good oxidation stability of halide-based solid electrolytes evoke strong interest in this class of materials. Nonetheless, the superior oxidative stability compared to sulfides comes at the expense of limited stability toward reduction and instability against metallic lithium anodes, which hinders their practical use. In this context, the gradual fluorination of Li2ZrCl6-xFx (0 ≤ x ≤ 1.2) is proposed to enhance the stability toward lithium-metal anodes. The mechanochemically synthesized fluorine-substituted compounds show the expected distorted local structure (M2-M3 site disorder) and significant change in the overall Li-ion migration barrier. Theoretical calculations reveal an approximate minimum energy path for Li2ZrCl6-xFx (x = 0 and 0.5) with an increase in the Li+ migration energy barrier for Li2ZrCl5.5F0.5 in comparison to Li2ZrCl6. However, it is found that the fluorine-substituted compound exhibits substantially lower polarization after 800 h of lithium stripping and plating owing to enhanced interfacial stability against the lithium metal, as revealed by density functional theory and ex situ X-ray photoelectron spectroscopy, thanks to the formation of a fluorine-rich passivating interphase.

Disclosing the Redox Pathway Behind the Excellent Performance of CuS in Solid-State Batteries.

Copper sulfide has attracted increasing attention as conversion-type cathode material for, especially, solid-state lithium-based batteries. However, the reaction mechanism behind its extraordinary electroactivity is not well understood, and the various explanations given by the scientific community are diverging. Herein, the CuS reaction dynamics are highlighted by examining the occurring redox processes via a cutting-edge methodology combining X-ray absorption fine structure spectroscopy, and chemometrics to overcome X-ray diffraction limitations posed by the poor material's crystallinity. The mathematical approach rules out the formation of intermediates and clarifies the direct conversion of CuS to Cu in a two-electron process during discharge and reversible oxidation upon delithiation. Two distinct voltage regions are identified corresponding to Cu- as well as the S-redox mechanisms occurring in the material.

Hybrid Organic/Inorganic Interphase for Stabilizing a Zinc Metal Anode in a Mild Aqueous Electrolyte.

Aqueous rechargeable zinc-based batteries have recently gained tremendous attention because of their low cost and high safety. However, the issues associated with the zinc metal anode, including corrosion, H2 evolution, and dendrite growth, hinder their practical applications. Herein, we design a hybrid organic/inorganic interphase composed of poly(vinylidene fluoride-co-hexafluoropropylene), silica, and zinc triflate to stabilize the zinc metal anode in a mild aqueous electrolyte. It is proven that the artificial interphase reduces corrosion of the Zn metal in the ZnSO4 electrolyte and suppresses dendrite growth by regulating Zn2+ deposition. Therefore, the lifespan of symmetrical cells with coated Zn could be enhanced to over 960 h with a stripping/plating capacity of 0.5 mAh cm-2. In addition, zinc-ion batteries including a sodium vanadate cathode and a coated Zn anode could achieve 3000 cycles with nearly no capacity fading at 5 A g-1.

Novel sulfur-doped single-ion conducting multi-block copolymer electrolyte.

Solid-state lithium batteries are considered one of the most promising candidates for future electrochemical energy storage. However, both inorganic solid electrolytes (such as oxide-based or sulfide-based materials) and polymer electrolytes still have to overcome several challenges to replace the currently used liquid organic electrolytes. An increasingly adopted approach to overcome these challenges relies on the combination of different electrolyte systems. Herein, we report the synthesis and characterization of a novel sulfur-doped single-ion conducting multi-block copolymer (SIC-BCE) system. This SIC-BCE may serve as interlayer between the electrodes and the sulfidic electrolyte such as Li6PS5Cl, thus benefitting of the high ionic conductivity of the latter and the favorable interfacial contact and electrochemical stability of the polymer. The polymer shows excellent ionic conductivity when swollen with ethylene carbonate and allows for stable stripping/plating of lithium, accompanied by a suitable electrochemical stability towards reduction and oxidation. First tests in symmetric Cu|SIC-BCE|Li6PS5Cl|SIC-BCE|Cu cells confirm the general suitability of the polymer to stabilize the electrode|electrolyte interface by preventing the direct contact of the sulfidic electrolyte with, e.g., metallic copper foils.

Zinc-Ion Hybrid Supercapacitors Employing Acetate-Based Water-in-Salt Electrolytes.

Halide-free, water-in-salt electrolytes (WiSEs) composed of potassium acetate (KAc) and zinc acetate (ZnAc2 ) are investigated as electrolytes in zinc-ion hybrid supercapacitors (ZHSs). Molecular dynamics simulations demonstrate that water molecules are mostly non-interacting with each other in the highly concentrated WiSEs, while "bulk-like water" regions are present in the dilute electrolyte. Among the various concentrated electrolytes investigated, the 30 m KAc and 1 m ZnAc2 electrolyte (30K1Zn) grants the best performance in terms of reversibility and stability of Zn plating/stripping while the less concentrated electrolyte cannot suppress corrosion of Zn and hydrogen evolution. The ZHSs utilizing 30K1Zn, in combination with a commercial activated carbon (AC) positive electrode and Zn as the negative electrode, deliver a capacity of 65 mAh g-1 (based on the AC weight) at a current density of 5 A g-1 . They also offer an excellent capacity retention over 10 000 cycles and an impressive coulombic efficiency (≈100%).

Concentrated Electrolytes Enabling Stable Aqueous Ammonium-Ion Batteries.

Rechargeable aqueous batteries are promising devices for large-scale energy-storage applications because of their low-cost, inherent safety, and environmental friendliness. Among them, aqueous ammonium-ion (NH4 + ) batteries (AAIB) are currently emerging owing to the fast diffusion kinetics of NH4 + . Nevertheless, it is still a challenge to obtain stable AAIB with relatively high output potential, considering the instability of many electrode materials in an aqueous environment. Herein, a cell based on a concentrated (5.8 m) aqueous (NH4 )2 SO4 electrolyte, ammonium copper hexacyanoferrate (N-CuHCF) as the positive electrode (cathode), and 3,4,9,10-perylene-bis(dicarboximide) (PTCDI) as the negative electrode (anode) is reported. The solvation structure, electrochemical properties, as well as the electrode-electrolyte interface and interphase are systematically investigated by the combination of theoretical and experimental methods. The results indicate a remarkable cycling performance of the low-cost rocking-chair AAIB, which offers a capacity retention of ≈72% after 1000 cycles and an average output potential of ≈1.0 V.

Elucidating the Role of Microstructure in Thiophosphate Electrolytes - a Combined Experimental and Theoretical Study of ß-Li3 PS4.

Solid-state batteries (SSBs) are promising candidates to significantly exceed the energy densities of today's state-of-the-art technology, lithium-ion batteries (LIBs). To enable this advancement, optimizing the solid electrolyte (SE) is the key. ß-Li3 PS4 (ß-LPS) is the most studied member of the Li2 S-P2 S5 family, offering promising properties for implementation in electric vehicles. In this work, the microstructure of this SE and how it influences the electrochemical performance are systematically investigated. To figure this out, four batches of ß-LPS electrolyte with different particle size, shape, and porosity are investigated in detail. It is found that differences in pellet porosities mostly originate from single-particle intrinsic features and less from interparticle voids. Surprisingly, the ß-LPS electrolyte pellets with the highest porosity and larger particle size not only show the highest ionic conductivity (up to 0.049 mS cm-1 at RT), but also the most stable cycling performance in symmetrical Li cells. This behavior is traced back to the grain boundary resistance. Larger SE particles seem to be more attractive, as their grain boundary contribution is lower than that of denser pellets prepared using smaller ß-LPS particles.

Investigation of a Fluorine-Free Phosphonium-Based Ionic Liquid Electrolyte and Its Compatibility with Lithium Metal.

A novel fluorine-free ionic liquid electrolyte comprising lithium dicyanamide (LiDCA) and trimethyl(isobutyl)phosphonium tricyanomethanide (P111i4TCM) in a 1:9 molar ratio is studied as an electrolyte for lithium metal batteries. At room temperature, it demonstrates high ionic conductivity and viscosity of about 4.5 mS cm-1 and 64.9 mPa s, respectively, as well as a 4 V electrochemical stability window (ESW). Li stripping/plating tests prove the excellent electrolyte compatibility with Li metal, evidenced by the remarkable cycling stability over 800 cycles. The evolution of the Li-electrolyte interface upon cycling was investigated via electrochemical impedance spectroscopy, displaying a relatively low impedance increase after the initial formation cycles. Finally, the solid electrolyte interphase (SEI) formed on Li metal appeared to have a bilayer structure mostly consisting of DCA and TCM reduction products. Additionally, decomposition products of the phosphonium cation were also detected, despite prior studies reporting its stability against Li metal.

The Emergence of Aqueous Ammonium-Ion Batteries.

Aqueous ammonium-ion (NH4 + ) batteries (AAIB) are a recently emerging technology that utilize the abundant electrode resources and the fast diffusion kinetics of NH4 + to deliver an excellent rate performance at a low cost. Although significant progress has been made on AAIBs, the technology is still limited by various challenges. In this Minireview, the most recent advances are comprehensively summarized and discussed, including cathode and anode materials as well as the electrolytes. Finally, a perspective on possible solutions for the current limitations of AAIBs is provided.

On the nanoscopic structural heterogeneity of liquid n-alkyl carboxylic acids.

Herein we report the first in-depth structural characterisation of simple linear carboxylic acids with alkyl tail length ranging from one to six carbon atoms. By means of the SWAXS technique, a pronounced nanoscopic heterogeneity evolving along the aliphatic portion of the molecule is highlighted. Via classical molecular dynamics, the origin of such heterogeneity is unambiguously assigned to the existence of aliphatic domains resulting from the self-segregation of the polar and apolar portions of the molecules. Furthermore, the structural correlation of aliphatic-separated polar domains is responsible for observing the so-called "pre-peak" in the SAXS region.

Tragacanth Gum as Green Binder for Sustainable Water-Processable Electrochemical Capacitor.

Enabling green fabrication processes for energy storage devices is becoming a key aspect in order to achieve a sustainable fabrication cycle. Here, the focus was on the exploitation of the tragacanth gum, an exudated gum like arabic and karaya gums, as green binder for the preparation of carbon-based materials for electrochemical capacitors. The electrochemical performance of tragacanth (TRGC)-based electrodes was thoroughly investigated and compared with another water-soluble binder largely used in this field, sodium-carboxymethyl cellulose (CMC). Apart from the higher sustainability both in production and processing, TRGC exhibited a lower impact on the obstruction of pores in the final active material film with respect to CMC, allowing for more available surface area. This directly impacted the electrochemical performance, resulting in a higher specific capacitance and better rate capability. Moreover, the TRGC-based supercapacitor showed a superior thermal stability compared with CMC, with a capacity retention of about 80 % after 10000 cycles at 70 °C.

Artificial Solid Electrolyte Interphases for Lithium Metal Electrodes by Wet Processing: The Role of Metal Salt Concentration and Solvent Choice.

In this study, the artificial solid electrolyte interphase (SEI) formed on lithium metal when treated in ZnCl2 solutions is thoroughly investigated. The artificial SEI on lithium metal electrodes substantially decreases the interfacial resistance by ca. 80% and improves cycling stability in comparison to untreated lithium. The presence of a native SEI negatively affects the morphology and interfacial resistance of the artificial SEI. Increasing the ZnCl2 concentration in tetrahydrofuran (THF) (precursor solution) results in higher homogeneity of the surface morphology. Independent of the ZnCl2 concentrations, the artificial SEI is composed of Cx, CO, LiCl, Li2CO3, ZnCl2, and LixZny alloys. ZnCl2 (1 M) produces the most homogenous surface and additional surface species with carbonyl side groups. Nonetheless, the ZnCl2 concentration only has a small effect on the interfacial resistance or cycling stability. Using ethyl methyl carbonate (EMC) as the solvent significantly reduces the interfacial resistance to 7 Ω cm2, in comparison to 25 Ω cm2 for THF. The composition of the artificial SEIs varies depending on the solvent. Either way, the SEI consists of Cx LixC, LiCl, Li2CO3, ZnCl2, and LiZn alloys. The THF-based SEI additionally features ether and carbonyl groups, LiZnO, and Zn metal. For the artificial SEI formed with both solvents, the atomic percentage of the LiZn alloy increases close to the Li surface.

Highly Reversible Sodiation of Tin in Glyme Electrolytes: The Critical Role of the Solid Electrolyte Interphase and Its Formation Mechanism.

Utilization of high-capacity alloying anodes is a promising yet extremely challenging strategy in building high energy density alkali-ion batteries (AIBs). Excitingly, it was very recently found that the (de-)sodiation of tin (Sn) can be a highly reversible process in specific glyme electrolytes, enabling high specific capacities close to the theoretical value of 847 mA h g-1. The unique solid electrolyte interphase (SEI) formed on Sn electrodes, which allows highly reversible sodiation regardless of the huge volume expansion, is herein demonstrated according to a series of in situ and ex situ characterization techniques. The SEI formation process mainly involves NaPF6 decomposition and the polymerization/oligomerization of the glyme solvent, which is induced by the catalytic effect of tin, specifically. This work provides a paradigm showing how solvent, salt, and electrode materials synergistically mediate the SEI formation process and obtains new insights into the unique interfacial chemistry between Na-alloying electrodes and glyme electrolytes, which is highly enlightening in building high energy density AIBs.

Natural Polymers as Green Binders for High-Loading Supercapacitor Electrodes.

The state-of-the-art aqueous binder for supercapacitors is carboxymethyl cellulose (CMC). However, it limits the mass loading of the coatings owing to shrinkage upon drying. In this work, natural polymers, that is, guar gum (GG), wheat starch (WS), and potato starch (PS), were studied as alternatives. The flexibility and adhesion of the resulting coatings and electrochemical performance was tested. The combination of 75:25 (w/w) ratio PS/GG showed a promising performance. Electrodes were characterized by SEM, thermal, adhesion, and bending tests. Their electrochemical properties were determined by cyclic voltammetry, electrochemical impedance spectroscopy, and cycling experiments. The PS/GG mixture conformed well to criteria for industrial production, enabling mass loadings higher than CMC (7.0 mg cm-2 ) while granting the same specific capacitance (26 F g-1 ) and power performance (20 F g-1 at 10 A g-1 ). Including the mass of the current collector, this represents a +45 % increase in specific energy at the electrode level.

Revisiting the Electrochemical Lithiation Mechanism of Aluminum and the Role of Li-rich Phases (Li1+x Al) on Capacity Fading.

Aluminum is an appealing anode material for high-energy-density lithium-ion batteries (LIBs), owing to its low cost, environmental benignity, high specific capacity, and lower relative volume expansion compared with other alloying materials. However, both, the working and capacity fading processes are not yet consistently and comprehensively understood, which has largely hindered its development. In this study, the electrochemical alloying process of aluminum anodes with lithium is systematically studied by the combination of several in situ and ex situ techniques, providing new insights into phase transitions, electrode dynamics, and surface chemistry. Particular attention is paid to the role of the Li-rich alloys (Li1+x Al). Its existence on the surface of the Al electrode is unexpectedly observed, and its growth in the electrode bulk is found to be strictly correlated with cell failure. Interestingly, cell failure can be delayed by choosing an appropriate electrolyte. This work contributes to a solid and comprehensive understanding of the puzzling Al (de-)lithiation processes, which is fundamental and highly enlightening for future research work on Al and other alloyed anodes.

Calcium vanadate sub-microfibers as highly reversible host cathode material for aqueous zinc-ion batteries.

For the first time, CaV6O16·3H2O (CVO), synthesized via a highly efficient and fast microwave reaction, is used as a cathode material for aqueous zinc-ion batteries. Ex situ X-ray diffraction confirms the structure of this material to be stable upon reversible Zn2+ intercalation, due to its large interlayer distance (8.08 Å). The pillaring effect of calcium makes the as-prepared CVO an excellent Zn2+ cation host.

Effect of Aging-Induced Dioxolane Polymerization on the Electrochemistry of Carbon-Coated Lithium Sulfide.

Lithium sulfide-based materials have been considered as potential positive electrodes for the next generation batteries. Lithium sulfide is the fully lithiated form of sulfur, i.e., they share the same high theoretical capacity. However, it has the benefit of already containing lithium, which allows making cells with lithium-free negative electrodes. Lithium sulfide, however, shares with sulfur the polysulfide dissolution drawback upon cycling. One possible solution to this problem is to envelop the active material particles with carbonaceous materials. In this work, we investigate the effect of a nitrogen-rich carbon coating on lithium sulfide particles. The effect of such coating on the surface properties and electrochemistry of lithium sulfide cathodes is investigated in details, in particular, regarding its interaction with fresh vs. aged electrolyte. The polymerization of dioxalane (DOL) due to aging is found to affect the electrochemistry of lithium sulfide and, interestingly, to improve the cycling performance.

Enabling Reversible (De)Lithiation of Aluminum by using Bis(fluorosulfonyl)imide-Based Electrolytes.

Aluminum, a cost-effective and abundant metal capable of alloying with Li up to around 1000 mAh g-1 , is a very appealing anode material for high energy density lithium-ion batteries (LIBs). However, despite repeated efforts in the past three decades, reports presenting stable cycling performance are extremely rare. This study concerns recent findings on the highly reversible (de)lithiation of a micro-sized Al anode (m-Al) by using bis(fluorosulfonyl)imide (FSI)-based electrolytes. By using this kind of electrolyte, m-Al can deliver a specific capacity over 900 mAh g-1 and superior Coulombic efficiency (96.8 %) to traditional carbonate- and glyme-based electrolytes (87.8 % and 88.1 %, respectively), which represents the best performance ever obtained for an Al anode without sophisticated structure design. The significantly improved electrochemical performance, which paves the way to realizing high-performance Al-based high energy density LIBs, can be attributed the peculiar solid-electrolyte interphase (SEI) formed by the FSI-containing electrolyte.

Portable High Voltage Integrated Harvesting-Storage Device Employing Dye-Sensitized Solar Module and All-Solid-State Electrochemical Double Layer Capacitor.

A dye-sensitized solar module (DSSM) and a high voltage all-solid-state electrochemical double layer capacitor (EDLC) are, for the first time, implemented in a compact Harvesting-Storage (HS) device. Conductive glass is employed as current collecting substrate for both DSSM and EDLC, leading to a robust and portable final structure. The photovoltaic section is constituted by a 4 series cells W-type module, while in the storage section an EDLC employing an ionic liquid-based polymeric electrolyte (a mixture of polyethylene oxide and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, PEO-Pyr14TFSI) and activated carbon electrodes is used. The solid state EDLC is first characterized individually to determine its electrochemical performance before successfully proving the integration with the DSSM. The harvesting-storage properties of the integrated photo-capacitor are evaluated through photo-charge and subsequent discharge protocols performed at two different discharge currents, showing that in this configuration the EDLC unit can be effectively charged up to 2.45 V.

Fluorine-Free Water-in-Salt Electrolyte for Green and Low-Cost Aqueous Sodium-Ion Batteries.

The highly concentrated "water-in-salt" electrolyte (WiSE) containing sodium acetate and potassium acetate demonstrates surprising performance (specific capacity of 37 mA h-1 g-1 at the 5th cycle and average discharge voltage of 0.82 V) in aqueous sodium-ion batteries (SIBs) based on Na2 MnFe(CN)6 and NaTi2 (PO4 )3 . The fluorine-free electrolyte offers a wide electrochemical stability window and compatibility with Al current collector. The electrolyte, current collector, and the electrode materials based on abundant elements make the proposed battery chemistry safe, low-cost, and environmentally friendly.