Emerging electrolytes with fluorinated solvents for rechargeable lithium-based batteries.

Electrolytes that can ensure the movement of ions and regulate interfacial chemistries for fast mass and charge transfer are essential in many types of electrochemical energy storage devices. However, in the emerging energy-dense lithium-based batteries, the uncontrollable side-reactions and consumption of the electrolyte result in poor electrochemical performances and severe safety concerns. In this case, fluorination has been demonstrated to be one of the most effective strategies to overcome the above-mentioned issues without significantly contributing to engineering and technical difficulties. Herein, we present a comprehensive overview of the fluorinated solvents that can be employed in lithium-based batteries. Firstly, the basic parameters that dictate the properties of solvents/electrolytes are elaborated, including physical properties, solvation structure, interface chemistry, and safety. Specifically, we focus on the advances and scientific challenges associated with different solvents and the enhancement in their performance after fluorination. Secondly, we discuss the synthetic methods for new fluorinated solvents and their reaction mechanisms in depth. Thirdly, the progress, structure-performance relationship, and applications of fluorinated solvents are reviewed. Subsequently, we provide suggestions on the solvent selection for different battery chemistries. Finally, the existing challenges and further efforts on fluorinated solvents are summarized. The combination of advanced synthesis and characterization approaches with the assistance of machine learning will enable the design of new fluorinated solvents for advanced lithium-based batteries.

The Dependence of Solid Electrolyte Interphase on the Crystal Facet of Current Collector in Li Metal Battery.

The continuous electrolyte decomposition and uncontrolled dendrite growth caused by the unstable solid electrolyte interphase (SEI) have largely hindered the development of Li metal batteries. Here, we demonstrate that tuning the facet of current collector can regulate the composition of SEI and the subsequent Li deposition behavior using single-crystal Cu foils as an ideal platform. The theoretical and experimental studies reveal that the (100) facet of Cu possesses strong adsorption to anions, guiding more anions to participate preferentially in the inner Helmholtz plane and further promoting the formation of the stable inorganic-rich SEI. Consequently, the single-crystal Cu foils with a single [100] orientation (s-Cu(100)) achieve the dendrite-free Li deposition with enhanced Li plating/stripping reversibility. Moreover, the Li anode deposited on s-Cu(100) can stabilize the operation of an Ah-level pouch cell (350 Wh kg-1) with a low negative/positive capacity ratio (~2) and lean electrolyte (2.4 g Ah-1) for 150 cycles. Impressively, this strategy demonstrates universality in a series of electrolytes employed different anions. This work provides new insights into the correlation between the SEI and current collector, opening a universal avenue towards high-performance Li metal batteries.

An Ionic Liquid Electrolyte with Enhanced Li+ Transport Ability Enables Stable Li Deposition for High-Performance Li-O2 Batteries.

Bis(trifluoromethanesulfonyl)imide-based ionic liquid (TFSI-IL) electrolyte could endow Li-O2 batteries with low charging overpotential. However, their weak Li+ transport ability (LTA) leads to non-uniform Li deposition. Herein, guided by Sand formula, the LTA of TFSI-IL electrolyte is greatly enhanced to realize robust Li deposition through introducing hydrofluoroether (HFE) and optimizing electrolyte component ratios to regulate solvation environment. The solvation environment changes from Li(TFSI)2 - ion pair into ionic aggregate clusters in the optimal electrolyte thanks to the slicing function of HFE toward ionic aggregate network. The transport parameters of Sand formula are synchronously enhanced, resulting in highly robust Li deposition behavior with greatly improved Coulombic efficiency (ca. 97.5 %) and cycling rate (1 mA cm-2 ). Cycling stability of Li-O2 batteries was greatly improved (a tiny overpotential rise of 64 mV after 75 cycles).

Advances and Challenges for the Electrochemical Reduction of CO2 to CO: From Fundamentals to Industrialization.

The electrochemical carbon dioxide reduction reaction (CO2 RR) provides an attractive approach to convert renewable electricity into fuels and feedstocks in the form of chemical bonds. Among the different CO2 RR pathways, the conversion of CO2 into CO is considered one of the most promising candidate reactions because of its high technological and economic feasibility. Integrating catalyst and electrolyte design with an understanding of the catalytic mechanism will yield scientific insights and promote this technology towards industrial implementation. Herein, we give an overview of recent advances and challenges for the selective conversion of CO2 into CO. Multidimensional catalyst and electrolyte engineering for the CO2 RR are also summarized. Furthermore, recent studies on the large-scale production of CO are highlighted to facilitate industrialization of the electrochemical reduction of CO2 . To conclude, the remaining technological challenges and future directions for the industrial application of the CO2 RR to generate CO are highlighted.

A Universal Graphene Quantum Dot Tethering Design Strategy to Synthesize Single-Atom Catalysts.

A general graphene quantum dot-tethering design strategy to synthesize single-atom catalysts (SACs) is presented. The strategy is applicable to different metals (Cr, Mn, Fe, Co, Ni, Cu, and Zn) and supports (0D carbon nanosphere, 1D carbon nanotube, 2D graphene nanosheet, and 3D graphite foam) with the metal loading of 3.0-4.5 wt %. The direct transmission electron microscopy imaging and X-ray absorption spectra analyses confirm the atomic dispersed metal in carbon supports. Our study reveals that the abundant oxygenated groups for complexing metal ions and the rich defective sites for incorporating nitrogen are essential to realize the synthesis of SACs. Furthermore, the carbon nanotube supported Ni SACs exhibits high electrocatalytic activity for CO2 reduction with nearly 100 % CO selectivity. This universal strategy is expected to open up new research avenues to produce SACs for diverse electrocatalytic applications.

Langmuir-Blodgett Nanowire Devices for In Situ Probing of Zinc-Ion Batteries.

In situ monitoring the evolution of electrode materials in micro/nano scale is crucial to understand the intrinsic mechanism of rechargeable batteries. Here a novel on-chip Langmuir-Blodgett nanowire (LBNW) microdevice is designed based on aligned and assembled MnO2 nanowire quasimonolayer films for directly probing Zn-ion batteries (ZIBs) in real-time. With an interdigital device configuration, a splendid Ohmic contact between MnO2 LBNWs and pyrolytic carbon current collector is demonstrated here, enabling a small polarization voltage. In addition, this work reveals, for the first time, that the conductance of MnO2 LBNWs monotonically increases/decreases when the ZIBs are charged/discharged. Multistep phase transition is mainly responsible for the mechanism of the ZIBs, as evidenced by combined high-resolution transmission electron microscopy and in situ Raman spectroscopy. This work provides a new and adaptable platform for microchip-based in situ simultaneous electrochemical and physical detection of batteries, which would promote the fundamental and practical research of nanowire electrode materials in energy storage applications.

Carbon-MEMS-Based Alternating Stacked MoS2 @rGO-CNT Micro-Supercapacitor with High Capacitance and Energy Density.

A novel process to fabricate a carbon-microelectromechanical-system-based alternating stacked MoS2 @rGO-carbon-nanotube (CNT) micro-supercapacitor (MSC) is reported. The MSC is fabricated by successively repeated spin-coating of MoS2 @rGO/photoresist and CNT/photoresist composites twice, followed by photoetching, developing, and pyrolysis. MoS2 @rGO and CNTs are embedded in the carbon microelectrodes, which cooperatively enhance the performance of the MSC. The fabricated MSC exhibits a high areal capacitance of 13.7 mF cm-2 and an energy density of 1.9 µWh cm-2 (5.6 mWh cm-3 ), which exceed many reported carbon- and MoS2 -based MSCs. The MSC also retains 68% of capacitance at a current density of 2 mA cm-2 (5.9 A cm-3 ) and an outstanding cycling performance (96.6% after 10 000 cycles, at a scan rate of 1 V s-1 ). Compared with other MSCs, the MSC in this study is fabricated by a low-cost and facile process, and it achieves an excellent and stable electrochemical performance. This approach could be highly promising for applications in integration of micro/nanostructures into microdevices/systems.

Suppressing Bulk Strain and Surface O2 Release in Li-Rich Cathodes by Just Tuning the Li Content.

Layered oxides represent a prominent class of cathodes employed in lithium-ion batteries. The structural degradation of layered cathodes causes capacity decay during cycling, which is generally induced by anisotropic lattice strain in the bulk of cathode particle and oxygen release at the surface. However, particularly in lithium-rich layered oxides (LLOs) that undergo intense oxygen redox reactions, the challenge of simultaneously addressing bulk and surface issues through a singular modification technique remains arduous. Here a thin (1-nm) and coherent spinel-like phase is constructed on the surface of LLOs particle to suppress bulk strain and surface O2 release by just adjusting the amount of lithium source during synthesis. The spinel-like phase hinders the surface O2 release by accommodating O2 inside the surface layer, while the trapped O2 in the bulk impedes strain evolution by ≈70% at high voltages compared with unmodified LLOs. Consequently, the enhanced structural stability leads to an improved capacity retention of 97.6% and a high Coulombic efficiency of ≈99.5% after 100 cycles at 0.1°C. These findings provide profound mechanistic insights into the functioning of surface structure and offer guidance for synthesizing high-capacity cathodes with superior cyclability.

Organic Electroactive Materials for Aqueous Redox Flow Batteries.

Organic electroactive materials take advantage of potentially sustainable production and structural tunability compared to present commercial inorganic materials. Unfortunately, traditional redox flow batteries based on toxic redox-active metal ions have certain deficiencies in resource utilization and environmental protection. In comparison, organic electroactive materials in aqueous redox flow batteries (ARFBs) have received extensive attention in recent years for low-cost and sustainable energy storage systems due to their inherent safety. This review aims to provide the recent progress in organic electroactive materials for ARFBs. The main reaction types of organic electroactive materials are classified in ARFBs to provide an overview of how to regulate their solubility, potential, stability, and viscosity. Then, the organic anolyte and catholyte in ARFBs are summarized according to the types of quinones, viologens, nitroxide radicals, hydroquinones, etc, and how to increase the solubility by designing various functional groups is emphasized. The research advances are presented next in the characterization of organic electroactive materials for ARFBs. Future efforts are finally suggested to focus on building neutral ARFBs, designing advanced electroactive materials through molecular engineering, and resolving problems of commercial applications.

Metal Anodes with Ultrahigh Reversibility Enabled by the Closest Packing Crystallography for Sustainable Batteries.

The ever-growing annual electricity generated from sustainable and intermittent energy such as wind and solar power requires cost effective and reliable electrochemical energy storage. Rechargeable batteries based on multivalent metal anodes such as Zn, Al, and Fe, taking advantage of large-scale production and affordable cost, have emerged as promising candidates. However, the uncontrollable dendrite-like metal deposition on regular substrate caused by disordered metal crystallization usually leads to premature failure of batteries and even safety concerns when the dendrite bridges the electrodes. Here it is reported that a series of metal anodes (Zn, Co, Al, Ni, and Fe) with multiple crystal structures (hexagonal close-packed, face-centered cubic, and body-centered cubic) can achieve dendrite-free and epitaxial deposition on single-crystal Cu(111) substrates enabled by the closest packing crystallography. Moreover, the closest packed facets are aligned horizontally with the substrates, resulting in compact planar construction and excellent chemical stability even at an unprecedented current density of 1 A cm-2 . The full cells under a practical anode-to-cathode capacity ratio of 2.3 show a cycling life span of over 800 cycles with Coulombic efficiency of > 99.9%. The universal approach of regulating metal electrodeposition in this work is expected to boost the development of emerging sustainable energy storage/conversion devices.

Unlocking the Allometric Growth and Dissolution of Zn Anodes at Initial Nucleation and an Early Stage with Atomic Force Microscopy.

Zn anodes have gained intensive attention for their environmental-friendliness and high volumetric capacity but are limited by their severe dendrite formation. Understanding the initial nucleation behavior is critical for manipulating the uniform deposition of Zn. Herein, the allometric growth and dissolution of Zn in the initial nucleation and early stages are visualized with in situ atomic force microscopy in aqueous ZnCl2 electrolytes. Zn nuclei grow via a horizontal radial direction and dissolve reversibly in a top-down process. The critical nucleation radius and density are dependent on the electrolyte concentration of ZnCl2, namely, the initial nucleus size is proportional to the ratio of surface free energy between deposited Zn and the electrolyte and overpotentials for Zn electrodeposition, and the density is inversely proportional to the cube of this ratio. This investigation provides guidelines for regulating uniform metal electrodeposition and yields benefits for the development of anode-free batteries.

A Universal Compensation Strategy to Anchor Polar Organic Molecules in Bilayered Hydrated Vanadates for Promoting Aqueous Zinc-Ion Storage.

The electrochemical performance of layered vanadium oxides is often improved by introducing guest species into their interlayer. Guest species with high stability in the interlayer and weak interaction with Zn2+ during charge/discharge process are desired to promoting reversible Zn2+ transfer. Herein, a universal compensation strategy was developed to introduce various polar organic molecules into the interlayer of Alx V2 O5 ·nH2 O by replacing partial crystal water. The high-polar groups in the organic molecules have a strong electrostatic attraction with pre-intercalated Al3+ , which ensures that organic molecules can be anchored in the interlayer of hydrated vanadates. Simultaneously, the low-polar groups endow organic molecules with a weak interaction with Zn2+ during cycling, thus liberalizing reversible Zn2+ transfer. As a result, Alx V2 O5 with polar organic molecules displays enhanced electrochemical performance. Furthermore, based on above cathode material, a pouch cell was assembled by further integrating a dendrite-free N-doped carbon nanofiber@Zn anode, displaying an energy density of 50 Wh kg-1 . This work provides a path for designing stable guest species with a weak interaction with Zn2+ in the interlayer of layered vanadium oxide towards high-performance cathode materials of aqueous Zn batteries.

Two-Phase Transition Induced Amorphous Metal Phosphides Enabling Rapid, Reversible Alkali-Metal Ion Storage.

Metal phosphides as anode materials for alkali-metal ion batteries have captured considerable interest due to their high theoretical capacities and electronic conductivity. However, they suffer from huge volume expansion and element segregation during repetitive insertion/extraction of guest ions, leading to structure deterioration and rapid capacity decay. Herein, an amorphous Sn0.5Ge0.5P3 was constructed through a two-phase intermediate strategy based on the elemental composition modulation from two crystalline counterparts and applied in alkali-metal ion batteries. Differing from crystalline P-based compounds, the amorphous structure of Sn0.5Ge0.5P3 effectively reduces the volume variation from above 300% to 225% during cycling. The ordered distribution of cations and anions in the short-range ensures the uniform distribution of each element during cycles and thus contributes to durable cycling stability. Moreover, the long-range disordered structure of amorphous material shortens the ion transport distance, which facilitates diffusion kinetics. Benefiting from the aforementioned effects, the amorphous Sn0.5Ge0.5P3 delivers a high Na storage capacity of 1132 mAh g-1 at 0.1 A g-1 over 100 cycles. Even at high current densities of 2 and 10 A g-1, its capacities still reach 666 and 321 mAh g-1, respectively. As an anode for Li storage, the Sn0.5Ge0.5P3 similarly also exhibits better cycling stability and rate performance compared to its crystalline counterparts. Significantly, the two-phase transition strategy is generally applicable to achieving other amorphous metal phosphides such as GeP2. This work would be helpful for constructing high-performance amorphous anode materials for alkali-metal ion batteries.

Electrodeposition of (hydro)oxides for an oxygen evolution electrode.

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the electrolyzers that are currently available do not have anodic electrodes that are robust enough and highly active for the oxygen evolution reaction (OER). Electrodeposition provides a feasible route for preparing freestanding OER electrodes with high active site utilization, fast mass transport and a simple fabrication process, which is highly attractive from both academic and commercial points of view. This minireview focuses on the recent electrodeposition strategies for metal (hydro)oxide design and water oxidation applications. First, the intrinsic advantages of electrodeposition in comparison with traditional technologies are introduced. Then, the unique properties and underlying principles of electrodeposited metal (hydro)oxides in the OER are unveiled. In parallel, illustrative examples of the latest advances in materials structural design, controllable synthesis, and mechanism understanding through the electrochemical synthesis of (hydro)oxides are presented. Finally, the latest representative OER mechanism and electrodeposition routes for OER catalysts are briefly overviewed. Such observations provide new insights into freestanding (hydro)oxides electrodes prepared via electrodeposition, which show significant practical application potential in water splitting devices. We hope that this review will provide inspiration for researchers and stimulate the development of water splitting technology.