Leveraging Ion Pairing and Transport in Localized High-Concentration Electrolytes for Reversible Lithium Metal Anodes at Low Temperatures.

Coulombic efficiency of over 99% is rarely achieved for Li metal anode below -40°C, hindering the practical application of high-energy-density Li metal batteries under extreme conditions. Herein, limiting factors for Li metal reversibility are investigated utilizing ether-based localized high-concentration electrolytes of different solvent-diluent combinations. We find that along with the desolvation barrier, bulk ion transport properties including ionic conductivity, transference number, and diffusivity are also crucial factors for low-temperature Li deposition behavior. Superior Li metal reversibility was observed within the combination of the solvent with moderately weak solvating power and the diluent with minimal viscosity, highlighting the role of ion transport and the necessity for a trade-off with desolvation. The optimized electrolyte composed of lithium bis(fluorosulfonyl)imide, methyl n-propyl ether, and 1,1,2,2-tetrafluoroethyl methyl ether delivers exceptional Coulombic efficiency of 99.34% at -40°C and 98.96% at -60°C under a current density of 0.5 mA cm-2. Furthermore, Li||LiCoO2 (2.7 mAh cm-2) cells demonstrate impressive reversible capacity and cycling stability at these temperatures. This work sheds light on the less-recognized relevance of bulk ion transport to low-temperature performance and provides guidelines for the electrolyte design of Li metal batteries operating in cold environments.

Boosted Mg-CO2 Batteries by Amine-Mediated CO2 Capture Chemistry and Mg2+ -Conducting Solid-electrolyte Interphases.

Mg-CO2 battery has been considered as an ideal system for energy conversion and CO2 fixation. However, its practical application is significantly limited by the poor reversibility and sluggish kinetics of CO2 cathode and Mg anode. Here, a new amine mediated chemistry strategy is proposed to realize a highly reversible and high-rate Mg-CO2 battery in conventional electrolyte. Judiciously combined experimental characterization and theoretical computation unveiled that the introduced amine could simultaneously modify the reactant state of CO2 and Mg2+ to accelerate CO2 cathodic reactions on the thermodynamic-kinetic levels and facilitate the formation of Mg2+ -conductive solid-electrolyte interphase (SEI) to enable highly reversible Mg anode. As a result, the Mg-CO2 battery exhibits boosted stable cyclability (70 cycles, more than 400 h at 200 mA g-1 ) and high-rate capability (from 100 to 2000 mA g-1 with 1.5 V overpotential) even at -15 °C. This work opens a newly promising avenue for advanced metal-CO2 batteries.

A Molecularly Engineered Cathode Lithium Compensation Agent for High Energy Density Batteries.

Prelithiating cathode is considered as one of the most promising lithium compensation strategies for practical high energy density batteries. Whereas most of reported cathode lithium compensation agents are deficient owing to their poor air-stability, residual insulating solid, or formidable Li-extracting barrier. Here, this work proposes molecularly engineered 4-Fluoro-1,2-dihydroxybenzene Li salt (LiDF) with high specific capacity (382.7 mAh g-1 ) and appropriate delithiation potential (3.6-4.2 V) as an air-stable cathode Li compensation agent. More importantly, the charged residue 4-Fluoro-1,2-benzoquinone (BQF) can synergistically work as an electrode/electrolyte interface forming additive to build uniform and robust LiF-riched cathode/anode electrolyte interfaces (CEI/SEI). Consequently, less Li loss and retrained electrolyte decomposition are achieved. With 2 wt% 4-Fluoro-1,2-dihydroxybenzene Li salt initially blended within the cathode, 1.3 Ah pouch cells with NCM (Ni92) cathode and SiO/C (550 mAh g-1 ) anode can keep 91% capacity retention after 350 cycles at 1 C rate. Moreover, the anode free of NCM622+LiDF||Cu cell achieves 78% capacity retention after 100 cycles with the addition of 15 wt% LiDF. This work provides a feasible sight for the rational designing Li compensation agent at molecular level to realize high energy density batteries.

Lithium Foam for Deep Cycling Lithium Metal Batteries.

Li metal anode has been recognized as the most promising anode for its high theoretical capacity and low reduction potential. But its large-scale commercialization is hampered because of the infinite volume expansion, severe side reactions, and uncontrollable dendrite formation. Herein, the self-supporting porous lithium foam anode is obtained by a melt foaming method. The adjustable interpenetrating pore structure and dense Li3 N protective layer coating on the inner surface enable the lithium foam anode with great tolerance to electrode volume variation, parasitic reaction, and dendritic growth during cycling. Full cell using high areal capacity (4.0 mAh cm-2 ) LiNi0.8Co0.1Mn0.1 (NCM811) cathode with the N/P ratio of 2 and E/C ratio of 3 g Ah-1 can stably operate for 200 times with 80% capacity retention. The corresponding pouch cell has <3% pressure fluctuation per cycle and almost zero pressure accumulation.

A Moisture-Assisted Rechargeable Mg-CO2 Battery.

New sustainable energy conversion and storage technologies are required to address the energy crisis and CO2 emission. Among various metal-CO2 batteries that utilize CO2 and offer high energy density, rechargeable Mg-CO2 batteries based on earth-abundant and safe magnesium (Mg) metal have been limited due to the lack of a compatible electrolyte, operation atmosphere, and unambiguous reaction process. Herein, the first rechargeable nonaqueous Mg-CO2 batteries have been proposed with moisture assistance in a CO2 atmosphere. These display more than 250 h cycle life and maintain the discharge voltage over 1 V at 200 mA g-1 . Combining with the experimental observations and theoretical calculations, the reaction in the moisture-assisted Mg-CO2 battery is revealed to be 2 Mg+3 CO2 +6 H2 O↔2 MgCO3 ⋅3 H2 O+C. It is anticipated that the moisture-assisted rechargeable Mg-CO2 batteries would stimulate the development of multivalent metal-CO2 batteries and extend CO2 fixation and utilization for carbon neutrality.

Long Cycling Life Solid-State Li Metal Batteries with Stress Self-Adapted Li/Garnet Interface.

Inorganic solid-state electrolyte (SSE) has offered a promising option for the safe rechargeable Li metal batteries. However, the solid-solid interfacial incompatibility greatly hampers the practical use. The interface becomes even worse during repeated Li plating/stripping, especially under high current density and long cycling operation. To promise an intimate contact and uniform Li deposition during cycling, we herein demonstrate a stress self-adapted Li/Garnet interface by integrating Li foil with a hyperelastic substrate. Consecutive and conformal physical contact was ensured at Li/Garnet interface during Li plating/stripping, therefore dissipating the localized stress, suppressing Li dendrite formation, and preventing Garnet cracks. Record long cycling life over 5000 cycles was achieved with the ultrasmall hysteresis of 55 mV at high current density of 0.2 mA cm-2. Our strategy provides a new way to stabilize Li/Garnet interface from the perspective of anode mechanical regulation and paves the way for the next generation solid-state Li metal batteries.

Dendrites in Lithium Metal Anodes: Suppression, Regulation, and Elimination.

With the increasing diversification of portable electronics and large-scale energy storage systems, conventional lithium-ion batteries (LIBs) with graphite anodes are now approaching their theoretical limits. Lithium metal, as the "Holy Grail" electrode for next-generation rechargeable batteries, is being revisited to meet the booming demand for high energy density electrodes due to its ultrahigh theoretical specific capacity and negative redox potential. Nevertheless, typical issues like notorious dendrite growth still hamper the bulk application of Li metal anodes. Dendrite growth renders increased surface area of the lithium metal, causing persistent depletion of the electrolyte and active materials, facilitating catastrophic failure of the battery, and even inducing fatal safety hazards. The consequences become more serious during operation at high current densities and over long cycling life. Therefore, it is urgent to suppress and even eliminate dendrite formation during the Li plating/stripping process. This Account highlights several innovative strategies for dendrite suppression, dendrite regulation, and dendrite elimination from the perspective of interface energy and bulk stresses. First, we review the fundamental mechanism of dendrite formation and growth in Li metal anodes. We show that the dendrite morphology could be substantially ameliorated, in theory, by homogenizing the electric field distribution, lowering the Li ion concentration gradient, and facilitating mechanical blocking. Next, we address the problem of dendrite suppression by applying two-dimensional (2D) materials to Li metal systems and preventing dendrite penetration through stress release and mechanical blocking. Graphene with a high specific area and vermiculite sheets (VSs) with a large physical rigidity were demonstrated to be efficacious in reinforcing Li anodes and polymer electrolytes separately. However, Li dendrite growth is a continuous process and remains inevitable with increasing current density and cycling life. Instead of suppressing dendrite growth, we focus on how to regulate homogeneous Li dendrite formation and growth. Dendrite regulation means to allow dendrite growth but take steps to transform it into Li with a smooth morphology. We introduce two main strategies to regulate Li growth: (i) guiding Li nucleation and (ii) controlling the Li growth pathways and directions. These processes greatly rely on the interface energy between the substrate and Li atoms. Elimination of the dendrites, which is the most formidable challenge for dendrite control, can also be achieved by dynamically engineering the force, such as deflecting the electric field by Lorentz force in a magnetic field, enhancing the integrated yield stress by the design of bulk nanostructured materials, and reducing the lateral Li diffusion barrier by a biomimetic co-deposition process. Solutions to the challenges of dendrite control in Li metal anodes can provide safe next-generation rechargeable lithium metal batteries that have a long cycling life. We also hope that our strategies presented in this Account can offer promise for other metal batteries.

Controlling Nucleation in Lithium Metal Anodes.

Rechargeable batteries are regarded as the most promising candidates for practical applications in portable electronic devices and electric vehicles. In recent decades, lithium metal batteries (LMBs) have been extensively studied due to their ultrahigh energy densities. However, short lifespan and poor safety caused by uncontrollable dendrite growth hinder their commercial applications. Besides, a clear understanding of Li nucleation and growth has not yet been obtained. In this Review, the failure mechanisms of Li metal anodes are ascribed to high reactivity of lithium, virtually infinite volume changes, and notorious dendrite growth. The principles of Li deposition nucleation and early dendrite growth are discussed and summarized. Correspondingly, four rational strategies of controlling nucleation are proposed to guide Li nucleation and growth. Finally, perspectives for understanding the Li metal deposition process and realizing safe and high-energy rechargeable LMBs are given.

Porous Al Current Collector for Dendrite-Free Na Metal Anodes.

Na-based batteries are proposed as promising energy storage candidates for beyond Li-ion technology due to the higher natural earth of Na metal. For its high capacity and low potential, Na metal may carve itself a niche when directly used as anodes. Similar to or even more problematic than Li, however, uneven plating/stripping of Na leads to dendrite formation. As the plating substrates, current collectors have a paramount influence on the Na plating/stripping behaviors. Here we propose porous Al current collectors as the plating substrate to suppress Na dendrites. Al does not alloy with Na. It is advantageous over Cu current collectors in terms of cost and weight. The interconnected porous structure can increase available surface for Na to nucleate and decrease the Na+ flux distribution, leading to homogeneous plating. The Na metal anodes can run for over 1000 cycles on porous Al with a low and stable voltage hysteresis and their average plating/stripping Coulombic efficiency was above 99.9%, which is greatly improved compared to planar Al. We used the porous Al for Na-O2, Na-Na3V2(PO4)3 cells with low Na amount and anode free Na-TiS2 batteries and anticipate that using this strategy can be combined with further electrolyte and cathodes to develop high performance Na-based batteries.

MXene Aerogel Scaffolds for High-Rate Lithium Metal Anodes.

Li metal is considered to be an ultimate anode for metal batteries owing to its extremely high theoretical capacity and lowest potential. However, numerous issues such as short lifespan and infinite volume expansion caused by the dendrite growth during Li plating/stripping hinder its practical usage. These challenges become more grievous under high current densities. Herein, 3D porous MXene aerogels are proposed as scaffolds for high-rate Li metal anodes using Ti3 C2 as an example. With high metallic electron conductivity, fast Li ion transport capability, and abundant Li nucleation sites, such scaffolds could deliver high cycling stability and low overpotential at current density up to 10 mA cm-2 . High rate performance is also demonstrated in full cells with LiFePO4 as cathodes. This work provides a new type of scaffolds for Li metal anodes and paves the way for the application of non-graphene 2D materials toward high energy density Li metal batteries.

Processable and Moldable Sodium-Metal Anodes.

Sodium-ion batteries are similar in concept and function to lithium-ion batteries, but their development and commercialization lag far behind. One obstacle is the lack of a standard reference electrode. Unlike Li foil reference electrodes, sodium is not easily processable or moldable and it deforms easily. Herein we fabricate a processable and moldable composite Na metal anode made from Na and reduced graphene oxide (r-GO). With only 4.5 % percent r-GO, the composite anodes had improved hardness, strength, and stability to corrosion compared to Na metal, and can be engineered to various shapes and sizes. The plating/stripping cycling of the composite anode was significantly extended in both ether and carbonate electrolytes giving less dendrite formation. We used the composite anode in both Na-O2 and Na-Na3 V2 (PO4 )3 full cells.

Mechano-Electrochemically Promoting Lithium Atom Diffusion and Relieving Accumulative Stress for Deep-Cycling Lithium Metal Anodes.

Lithium metal batteries (LMBs) can double the energy density of lithium-ion batteries. However, the notorious lithium dendrite growth and large volume change are not well addressed, especially under deep cycling. Here, an in-situ mechanical-electrochemical coupling system is built, and it is found that tensile stress can induce smooth lithium deposition. Density functional theory (DFT) calculation and finite element method (FEM) simulation confirm that the lithium atom diffusion energy barrier can be reduced when the lithium foils are under tensile strain. Then tensile stress is incorporated into lithium metal anodes by designing an adhesive copolymer layer attached to lithium in which the copolymer thinning can yield tensile stress to the lithium foil. Elastic lithium metal anode (ELMA) is further prepared via introducing a 3D elastic conductive polyurethane (CPU) host for the copolymer-lithium bilayer to release accumulated internal stresses and resist volume variation. The ELMA can withstand hundreds of compression-release cycles under 10% strain. LMBs paired with ELMA and LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathode can operate beyond 250 cycles with 80% capacity retention under practical condition of 4 mAh cm-2 cathode capacity, 2.86 g Ah-1 electrolyte-to-capacity ratio (E/C) and 1.8 negative-to-cathode capacity ratio (N/P), five times of the lifetime using lithium foils.

Green Protective Geopolymer Coatings: Interface Characterization, Modification and Life-Cycle Analysis.

In the interest of solving the resource and environmental problems of the construction industry, low-carbon geopolymer coating ensures great durability and extends the service life of existing infrastructure. This paper presents a multidisciplinary assessment of the protective performance and environmental impacts of geopolymer coating. Various parameters, such as main substance, water-solid (W/S) ratio, activator type and curing time, were investigated for their effects on interface characterization in terms of contact angle, surface energy, mechanical properties and microstructure. These parameters had negligible effects on the amounts and types of hydrophilic functional groups of geopolymer surfaces. A combination of organic surface modifiers and geopolymer coatings was shown to ensure hydrophobic surface conditions and great durability. Silicon-based modifiers exhibited better wetting performance than capillary crystalline surfactants by eliminating hydroxyl groups and maintaining structural backbone Si-O-T (Si, Al) on geopolymers' surfaces. Finally, life-cycle analysis was conducted to investigate the environmental performance. Geopolymer coating yielded substantially lower environmental impacts (50-80% lower in most impact categories) than ordinary Portland cement (OPC) coating. Silicon-based modifiers had negligible influence due to their minimal usage. Increasing the W/S ratio diluted the geopolymer coating and decreased the environmental impacts, and slag-based geopolymer coating achieved lower environmental impacts than FA-based and MK-based varietie.

Aqueous Stable Ti3C2 MXene Membrane with Fast and Photoswitchable Nanofluidic Transport.

High, stable, and modulatable ionic conductivity is important for many nanofluidic applications of layered two-dimensional (2D) membranes. In this study, we demonstrate a proton and ionic conductivity of the Ti3C2T x membrane that is orders of magnitude higher than that of bulk solution at low concentrations. Importantly, the membrane is highly stable in aqueous solution without any modification, due to the strong and attractive interlayer van der Waals interaction and weak electrostatic repulsive interaction. Furthermore, by exploiting the intrinsic photothermal property of MXene, we demonstrate that the ionic conductivity can be reversely, rapidly, and completely switched on or off with laser light. This study should prove MXene membrane as a suitable platform to study and utilize nanofluidic ion transport. It should also inspire future studies to manipulate the mass transport through 2D membranes using their inherent physicochemical properties.

Bending-Tolerant Anodes for Lithium-Metal Batteries.

Bendable energy-storage systems with high energy density are demanded for conformal electronics. Lithium-metal batteries including lithium-sulfur and lithium-oxygen cells have much higher theoretical energy density than lithium-ion batteries. Reckoned as the ideal anode, however, Li has many challenges when directly used, especially its tendency to form dendrite. Under bending conditions, the Li-dendrite growth can be further aggravated due to bending-induced local plastic deformation and Li-filaments pulverization. Here, the Li-metal anodes are made bending tolerant by integrating Li into bendable scaffolds such as reduced graphene oxide (r-GO) films. In the composites, the bending stress is largely dissipated by the scaffolds. The scaffolds have increased available surface for homogeneous Li plating and minimize volume fluctuation of Li electrodes during cycling. Significantly improved cycling performance under bending conditions is achieved. With the bending-tolerant r-GO/Li-metal anode, bendable lithium-sulfur and lithium-oxygen batteries with long cycling stability are realized. A bendable integrated solar cell-battery system charged by light with stable output and a series connected bendable battery pack with higher voltage is also demonstrated. It is anticipated that this bending-tolerant anode can be combined with further electrolytes and cathodes to develop new bendable energy systems.

Bulk Nanostructured Materials Based on Two-Dimensional Building Blocks: A Roadmap.

The family of two-dimensional (2D) materials, in particular MXenes, can now be greatly expanded based on a new "double metal" strategy as reported by Anasori, Xie, and Beidaghi et al. in this issue of ACS Nano. Now that a diverse array of well-defined nanoscale building blocks, especially the 2D systems, has become available, we are better prepared to think about scaling up nanomaterials in the broader context of materials science and engineering. In this Perspective, we construct a roadmap for assembling nanoscale building blocks into bulk nanostructured materials, and define some of the critical challenges and goals. Two-dimensional sheets are uniquely well-suited in this roadmap for constructing dense, bulk-sized samples with scalable material performance or interesting emergent properties.