Bilayer Interphase for Air-Stable and Dendrite-Free Lithium Metal Anode Cycling in Carbonate Electrolytes.

The intrinsic reactivity of lithium (Li) toward ambient air, combined with insufficient cycling stability in conventional electrolytes, hinders the practical adoption of Li metal anodes in rechargeable batteries. Here, a bilayer interphase for Li metal is introduced to address both its susceptibility to corrosion in ambient air and its deterioration during cycling in carbonate electrolytes. Initially, the Li metal anode is coated with a conformal bottom layer of polysiloxane bearing methacrylate, followed by further grafting with poly(vinyl ethylene carbonate) (PVEC) to enhance anti-corrosion capability and electrochemical stability. In contrast to single-layer applications of polysiloxane or PVEC, the bilayer design offers a highly uniform coating that effectively resists humid air and prevents dendritic Li growth. Consequently, it demonstrates stable plating/stripping behavior with only a marginal increase in overpotential over 200 cycles in carbonate electrolytes, even after exposure to ambient air with 46% relative humidity. The design concept paves the way for scalable production of high-voltage, long-cycling Li metal batteries.

High-Power and Large-Area Anodes for Safe Lithium-Metal Batteries.

The lithium deposited via the complex electrochemical heterogeneous lithium deposition reaction (LDR) process on a lithium foil-based anode (LFA) forms a high-aspect-ratio shape whenever the reaction kinetics reach its limit, threatening battery safety. Thereby, a research strategy that boosts the LDR kinetics is needed to construct a high-power and safe lithium metal anode. In this study, the kinetic limitations of the LDR process on LFA are elucidated through operando and ex situ observations using in-depth electrochemical analyses. In addition, ultra-thin (≈0.5 µm) and high modulus (≥19 GPa) double-walled carbon nanotube (DWNT) membranes with different surface properties are designed to catalyze high-safety LDRs. The oxygen-functionalized DWNT membranes introduced on the LFA top surface simultaneously induce multitudinous lithium nuclei, leading to film-like lithium deposition even at a high current density of 20 mA cm-2. More importantly, the layer-by-layer assembly of the oxygen-functionalized and pristine DWNT membranes results in different surface energies between the top and bottom surfaces, enabling selective surface LDRs underneath the high-modulus bilayer membranes. The protective LDR on the bilayer-covered LFA guarantees an invulnerable cycling process in large-area pouch cells at high current densities for more than 1000 cycles, demonstrating the practicability of LFA in a conventional liquid electrolyte system.

Electrolyte Design for High-Voltage Lithium-Metal Batteries with Synthetic Sulfonamide-Based Solvent and Electrochemically Active Additives.

Considering practical viability, Li-metal battery electrolytes should be formulated by tuning solvent composition similar to electrolyte systems for Li-ion batteries to enable the facile salt-dissociation, ion-conduction, and introduction of sacrificial additives for building stable electrode-electrolyte interfaces. Although 1,2-dimethoxyethane with a high-donor number enables the implementation of ionic compounds as effective interface modifiers, its ubiquitous usage is limited by its low-oxidation durability and high-volatility. Regulation of the solvation structure and construction of well-structured interfacial layers ensure the potential strength of electrolytes in both Li-metal and LiNi0.8Co0.1Mn0.1O2 (NCM811). This study reports the build-up of multilayer solid-electrolyte interphase by utilizing different electron-accepting tendencies of lithium difluoro(bisoxalato) phosphate (LiDFBP), lithium nitrate, and synthetic 1-((trifluoromethyl)sulfonyl)piperidine. Furthermore, a well-structured cathode-electrolyte interface from LiDFBP effectively addresses the issues with NCM811. The developed electrolyte based on a framework of highly- and weakly-solvating solvents with interface modifiers enables the operation of Li|NCM811 cells with a high areal capacity cathode (4.3 mAh cm-2) at 4.4 V versus Li/Li+.

Solution-Based Deep Prelithiation for Lithium-Ion Capacitors with High Energy Density.

Lithium-ion capacitors (LICs) exhibit superior power density and cyclability compared to lithium-ion batteries. However, the low initial Coulombic efficiency (ICE) of amorphous carbon anodes (e.g., hard carbon (HC) and soft carbon (SC)) limits the energy density of LICs by underutilizing cathode capacity. Here, a solution-based deep prelithiation strategy for carbon anodes is applied using a contact-ion pair dominant solution, offering high energy density based on a systematic electrode balancing based on the cathode capacity increased beyond the original theoretical limit. Increasing the anode ICE to 150% over 100%, the activated carbon (AC) capacity is doubled by activating Li+ cation storage, which unleashes rocking-chair LIC operation alongside the dual-ion-storage mechanism. The increased AC capacity results in an energy density of 106.6 Wh kg-1 AC+SC , equivalent to 281% of that of LICs without prelithiation. Moreover, this process lowers the cathode-anode mass ratio, reducing the cell thickness by 67% without compromising the cell capacity. This solution-based deep chemical prelithiation promises high-energy LICs based on transition metal-free, earth-abundant active materials to meet the practical demands of power-intensive applications.

Improvement of Thermodynamic Phase Stability and High-Rate Capability of Li Layered Oxides.

The use of elemental doping in lithium cobalt oxide (LCO) cathode material at high cutoff voltage is a widely adopted technique in the field of rechargeable batteries to mitigate multiple unfavorable phase transitions. However, there is still a lack of fundamental understanding regarding the rationality of each doping element implemented in this method, specifically considering the various thermodynamic stability and phase transitions. Herein, we investigated the effect of Ti doping on an O2 phase LCO (LCTO) cathode material that exhibited enhanced rate performance. We suggest that the incorporation of Ti into an O2 phase LCO results in the mitigation of multiple-phase transitions and the improvement of phase stability, thereby yielding a high-rate-capable cathode material. Through a combination of experimental and computational calculations, we demonstrate that Ti doping improves the thermodynamic stability and kinetics of Li-ions during the cycling process.

Reversible Magnesium Metal Cycling in Additive-Free Simple Salt Electrolytes Enabled by Spontaneous Chemical Activation.

Rechargeable magnesium (Mg) batteries can offer higher volumetric energy densities and be safer than their conventional counterparts, lithium-ion batteries. However, their practical implementation is impeded due to the passivation of the Mg metal anode or the severe corrosion of the cell parts in conventional electrolyte systems. Here, we present a chemical activation strategy to facilitate the Mg deposition/stripping process in additive-free simple salt electrolytes. By exploiting the simple immersion-triggered spontaneous chemical reaction between reactive organic halides and Mg metal, the activated Mg anode exhibited an overpotential below 0.2 V and a Coulombic efficiency as high as 99.5% in a Mg(TFSI)2 electrolyte. Comprehensive analyses reveal simultaneous evolution of morphology and interphasial chemistry during the activation process, through which stable Mg cycling over 990 cycles was attained. Our activation strategy enabled the efficient cycling of Mg full-cell candidates using commercially available electrolytes, thereby offering possibilities of building practical Mg batteries.

Reversible Mg-Metal Batteries Enabled by a Ga-Rich Protective Layer through One-Step Interface Engineering.

Practical applications of Mg-metal batteries (MMBs) have been plagued by a critical bottleneck─the formation of a native oxide layer on the Mg-metal interface─which inevitably limits the use of conventional nontoxic electrolytes. The major aim of this work was to propose a simple and effective way to reversibly operate MMBs in combination with Mg(TFSI)2-diglyme electrolyte by forming a Ga-rich protective layer on the Mg metal (GPL@Mg). Mg metal was carefully reacted with a GaCl3 solution to trigger a galvanic replacement reaction between Ga3+ and Mg, resulting in the layering of a stable and ion-conducting Ga-rich protective film while preventing the formation of a native insulating layer. Various characterization tools were applied to analyze GPL@Mg, and it was demonstrated to contain inorganic-rich compounds (MgCO3, Mg(OH)2, MgCl2, Ga2O3, GaCl3, and MgO) roughly in a double-layered structure. The artificial GPL on Mg was effective in greatly reducing the high polarization for Mg plating and stripping in diglyme-based electrolyte, and the stable cycling was maintained for over 200 h. The one-step process suggested in this work offers insights into exploring a cost-effective approach to cover the Mg-metal surface with an ion-conducting artificial layer, which will help to practically advance MMBs.

Strong Anionic Repulsion for Fast Na Kinetics in P2-Type Layered Oxides.

An intriguing mechanism for enabling fast Na kinetics during oxygen redox (OR) is proposed to produce high-power-density cathodes for sodium-ion batteries (SIBs) based on the P2-type oxide models, Na2/3 [Mn6/9 Ni3/9 ]O2 (NMNO) and Na2/3 [Ti1/9 Mn5/9 Ni3/9 ]O2 (NTMNO) using the "potential pillar" effect. The critical structural parameter of NTMNO lowers the Na migration barrier in the desodiated state because the electrostatic repulsion of O(2p)O(2p) that occurs between transition metal layers is combined with the chemically stiff Ti4+ (3d)O(2p) bond to locally retain the strong repulsion effect. The NTMNO interlayer distance moderately decreases upon charging with oxygen oxidation, whereas that of NMNO decreases at a much faster rate, which can be explained by the dependence of OR activity on the coordination environment. Fundamental electrochemical experiments clearly indicate that the Ti doping of the bare material significantly improves its rate capability during OR, and detailed electrochemical and structural analyses show much faster Na kinetics for NTMNO than for NMNO. A systematic comparison of the two cathode oxides based on experiments and first-principles calculations establishes the "potential pillar" concept of not only improving the sluggish Na kinetics upon OR reaction but also harnessing the full potential of the anionic redox for high-power-density SIBs.

SiO-induced thermal instability and interplay between graphite and SiO in graphite/SiO composite anode.

Silicon monoxide (SiO), which exhibits better cyclability compared to silicon while delivering higher capacity than that of graphite, is an adequate material for the development of lithium-ion batteries (LIBs) having higher energy densities. However, incorporating silicon-based materials including SiO into stable graphite anode inevitably degrades not only cycle life but also calendar life of LIBs, while little is known about their aging mechanisms. Here, SiO-induced thermal instability of the graphite/SiO composite anode is investigated. We reveal that under thermal exposure, SiO accelerates the loss of lithium inventory and concomitantly facilitates the lithium de-intercalation from graphite. This self-discharge phenomenon, which is weakly observed in the graphite anode without SiO, is the result of preferential parasitic reaction on the SiO interface and spontaneous electron and lithium-ion migration to equilibrate the electron energy imbalance between graphite and SiO. Understanding this underlying electron-level interplay between graphite and SiO in the composite anode will contribute toward improving shelf life of SiO-containing LIBs in actual operating conditions.

Real-Time Visualizing Nucleation and Growth of Electrodes for Post-Lithium-Ion Batteries.

ConspectusUntil recently, most studies on nucleation and growth mechanisms have employed electrochemical transient measurements, and numerous models have been established on various metal electrode elements. Contrary to the conventional tip-induced nucleation and growth model, a base-induced nucleation and growth mode was discovered not so long ago, which highlighted the importance of direct real-time observations such as visualization. As analysis techniques developed, diverse in situ/operando imaging methods have spurred the fundamental understanding of complex and dynamic battery electrochemistry. Experimental observations of alkali Li and Na metals are limited and difficult because their high reactivity makes not only the fabrication but also the analysis itself challenging. Na metal has high reactivity to electrolytes. Accordingly, it is difficult to visualize the Na deposition in real-time due to gas evolution and resolution limitation. Only a few studies have examined the Na deposition and dissolution reactions in operando. It is generally believed that the Mg anode is free from the dendrite growth of Mg metal, and Mg deposition preferentially occurs along the surface direction. However, whether the Mg anode always follows the dendrite-free growth has currently become a controversial topic and is being discussed and redefined based on real-time imaging analyses. In addition, a variety of morphological evolutions in the metal anodes are required to be systematically distinguished by key parameters. Real-time imaging analysis can directly confirm the solid-liquid-solid multiphase conversion reactions of S and Se cathodes. S and Se elements belong to the same chalcogen group, but their crystal structures and morphological changes significantly differ in each electrode during deposition and dissolution reactions. Therefore, it is necessitated to discuss the nucleation and growth behaviors by examining intrinsic properties of each element in chalcogen cathodes. Considering that a mechanistic understanding of the Se cathode is in its infancy, its nucleation and growth behaviors must be further explored through fundamental studies. In this Account, we aim to discuss the nucleation and growth behaviors of metal (Li, Na, and Mg) anodes and chalcogen (S and Se) cathodes. To elucidate their nucleation and growth mechanisms, we overview the morphological evolutions on the electrode surface and interface by in situ/operando visualizations. Our recent studies covering Li, Na, Mg, S, and Se electrodes verified by operando X-ray imaging are used as critical resources in understanding their nucleation and growth behaviors. Overall, with validation of the complex and dynamic nucleation and growth behaviors of metal and chalcogen electrodes by in situ/operando visualization methods, we hope that this Account can contribute to supporting the fundamental knowledge for the development of high-energy-density metal and chalcogen electrodes.

Selective dealloying of chemically disordered Pt-Ni bimetallic nanoparticles for the oxygen reduction reaction.

Changes in electronic and compositional structures of Pt-Ni electrocatalysts with 44% of Ni fraction with repeated chemical dealloying have been studied. By comparing the Pt-enriched surfaces formed using hydroquinone and sulfuric acid as a leaching agent, we found that hydroquinone generated Pt-enriched surfaces exhibit the highest oxygen reduction reaction (ORR) activity after repeating the treatment twice. In particular, it was found that while sulfuric acid causes an uncontrollable dissolution of Ni clusters, the unique selectivity of hydroquinone allows the preferential dissolution of Ni atoms alloyed with Pt. Despite its wide usage in the field, the results show that traditional acid leaching is unsuitable for Pt-Ni alloys with a high Ni content and an incomplete alloying level. We finally proved that the unique and lasting selectivity of hydroquinone enables an incompletely alloyed Pt-Ni catalyst to obtain a highly ORR active Pt shell region without an extensive loss of Ni.

Understanding the lithium-sulfur battery redox reactions via operando confocal Raman microscopy.

The complex interplay and only partial understanding of the multi-step phase transitions and reaction kinetics of redox processes in lithium-sulfur batteries are the main stumbling blocks that hinder the advancement and broad deployment of this electrochemical energy storage system. To better understand these aspects, here we report operando confocal Raman microscopy measurements to investigate the reaction kinetics of Li-S redox processes and provide mechanistic insights into polysulfide generation/evolution and sulfur deposition. Operando visualization and quantification of the reactants and intermediates enabled the characterization of potential-dependent rates during Li-S redox and the linking of the electronic conductivity of the sulfur-based electrode and concentrations of polysulfides to the cell performance. We also report the visualization of the interfacial evolution and diffusion processes of different polysulfides that demonstrate stepwise discharge and parallel recharge mechanisms during cell operation. These results provide fundamental insights into the mechanisms and kinetics of Li-S redox reactions.

Visualization of Sodium Metal Anodes via Operando X-Ray and Optical Microscopy: Controlling the Morphological Evolution of Sodium Metal Plating.

Because of the abundance and cost effectiveness of sodium, rechargeable sodium metal batteries have been widely studied to replace current lithium-ion batteries. However, there are some critical unresolved issues including the high reactivity of sodium, an unstable solid-electrolyte interphase (SEI), and sodium dendrite formation. While several studies have been conducted to understand sodium plating/stripping processes, only a very limited number of studies have been carried out under operando conditions. We have employed operando X-ray and optical imaging techniques to understand the mechanistic behavior of Na metal plating. The morphology of sodium metal plated on a copper electrode depends strongly on the salts and solvents used in the electrolyte. The addition of a fluorine-containing additive to a carbonate-based electrolyte, NaClO4 in propylene carbonate (PC):fluoroethylene carbonate (FEC), results in uniform sodium plating processes and much more stable cycling performance, compared to NaClO4 in PC, because of the formation of a stable SEI containing NaF. A NaF layer, on top of the sodium metal, leads to a much more uniform deposition of sodium and greatly enhanced cyclability.

Unraveling the Dynamic Interfacial Behavior of LiCoO2 at Various Voltages with Lithium Bis(oxalato)borate for Lithium-Ion Batteries.

The electrochemical dynamic behavior of the solid electrolyte interface (SEI) formed on LiCoO2 (LCO) by lithium bis(oxalato)borate (LiBOB) is investigated at various cutoff voltages. Particularly, for layered cathode active materials, various cutoff voltages are used to control the delithiation states; however, systematic investigations of the voltage and SEI are lacking. To increase the practical energy density of the LCO, a high cutoff voltage is pursued to utilize a state of high delithiation. However, this high cutoff voltage causes the electrolyte to undergo side reactions and the crystalline structure changes irreversibly, limiting the cycle life. In a low-voltage environment (<4.7 V), LiBOB improves the initial Coulombic efficiency and cycling performance by forming an effective SEI, which suppresses side reactions. At higher voltage levels (4.7-4.9 V), LiBOB no longer effectively protects the surface, causing the electrochemical performance to decrease rapidly. The main cause of this phenomenon is the decomposition of LiBOB-SEI at a high voltage, as shown by systematic surface and electrochemical analyses comprising linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. In conclusion, LiBOB can suppress side reactions of the electrolyte by SEI formation, but the SEI decomposes at voltage levels higher than 4.7 V.

Dynamic Ligand Screening by Magnetic Nanoassembly Modulates Stem Cell Differentiation.

In native microenvironment, diverse physical barriers exist to dynamically modulate stem cell recruitment and differentiation for tissue repair. In this study, nanoassembly-based magnetic screens of various sizes are utilized, and they are elastically tethered over an RGD ligand (cell-adhesive motif)-presenting material surface to generate various nanogaps between the screens and the RGDs without modulating the RGD density. Large screens exhibiting low RGD distribution stimulate integrin clustering to facilitate focal adhesion, mechanotransduction, and differentiation of stem cells, which are not observed with small screens. Magnetic downward pulling of the large screens decreases the nanogaps, which dynamically suppress the focal adhesion, mechanotransduction, and differentiation of stem cells. Conversely, magnetic upward pulling of the small screens increases the nanogaps, which dynamically activates focal adhesion, mechanotransduction, and differentiation of stem cells. This regulation mechanism is also shown to be effective in the microenvironment in vivo. Further diversifying the geometries of the physical screens can further enable diverse modalities of multifaceted and safe unscreening of the distributed RGDs to unravel and modulate stem cell differentiation for tissue repair.

Understanding the Effects of Interfacial Lithium Ion Concentration on Lithium Metal Anode.

Despite the development of multidimensional state-of-the-art electrode materials for constructing better lithium metal anodes (LMAs), the key factors influencing the electrochemical performance of LMAs are still poorly understood. Herein, it is demonstrated that the local lithium ion concentration at the interface between the electrode and electrolyte exerts significant influence on the electrochemical performance of LMAs. The local ion concentration is multiplied by introducing pseudocapacitive nanocarbons (PNCs) containing numerous heteroatoms, because PNCs can store large numbers of lithium ions in a pseudocapacitive manner, and promote the formation of an electrochemical double layer. The high interfacial lithium ion concentration induces the formation of lithium-rich inorganic solid-electrolyte-interface layers with high ionic conductivities, and facilitates sustainable and stable supplies of lithium ion charge carriers on the overall active surfaces of the PNCs. Accordingly, the PNC-induced LMA exhibits high Coulombic efficiencies, high rate capabilities, and stable cycling performance.

Understanding the Impacts of Li Stripping Overpotentials at the Counter Electrode by Three-Electrode Coin Cell Measurements.

The evaluation of new materials, interfaces, and architectures for battery applications are routinely conducted in two-electrode coin cell experiments, which although convenient, can lead to misrepresentations of the processes occurring in the cell. Few three-electrode coin cell designs have been reported, but those which have involve complex cell assembly, specialized equipment, and/or cell configurations which vary drastically from the standard coin cell environment. Herein, we present a novel, facile three-electrode coin cell design which can be easily assembled with existing coin cell parts and which accurately reproduces the environment of traditional coin cells. Using this design, we systematically investigated the inaccuracies incurred in two-electrode measurements in both symmetric/asymmetric cells and half-cell experiments by galvanostatic charge/discharge, galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry. From our investigation, we reveal that lithium metal stripping contributes larger overpotentials than its nucleation/plating processes, a phenomenon which is often misinterpreted in two-electrode cell measurements.

Structural and Chemical Compatibilities of Li1- x Ni0.5 Co0.2 Mn0.3 O2 Cathode Material with Garnet-Type Solid Electrolyte for All-Solid-State Batteries.

All-solid-state batteries (ASSBs) based on ceramic materials are considered a key technology for automobiles and energy storage systems owing to their high safety and stability. However, contact issues between the electrode and solid-electrolyte materials and undesired chemical reaction occurring at interfaces have hindered their development. Herein, the chemical compatibility and structural stability of composite mixtures of the layered cathode materials Li1- x Ni0.5 Co0.2 Mn0.3 O2 (NCM523) with the garnet-type solid electrolyte Li6.25 Ga0.25 La3 Zr2 O12 (LLZO-Ga) during high-temperature co-sintering under various gas flowing conditions are investigated. In situ high-temperature X-ray diffraction analysis of the composite materials reveals that Li diffusion from LLZO-Ga to NCM523 occurs at high temperature under synthetic air atmosphere, resulting in the decomposition of LLZO-Ga into La2 Zr2 O7 and the recovery of charged NCM523 to the as-prepared state. The structural stability of the composite mixture at high temperature is further investigated under N2 atmosphere, revealing that Li diffuses toward the opposite direction and involves the phase transition of LLZO-Ga from a cubic to tetragonal structure and the reduction of the NCM523 cathode to Ni metal. These findings provide insight into the structural stability of layered cathode and garnet-type solid-electrolyte composite materials and the design of stable interfaces between them via co-sintering for ASSBs.

Remote Control of Time-Regulated Stretching of Ligand-Presenting Nanocoils In Situ Regulates the Cyclic Adhesion and Differentiation of Stem Cells.

Native extracellular matrix (ECM) can exhibit cyclic nanoscale stretching and shrinking of ligands to regulate complex cell-material interactions. Designing materials that allow cyclic control of changes in intrinsic ligand-presenting nanostructures in situ can emulate ECM dynamicity to regulate cellular adhesion. Unprecedented remote control of rapid, cyclic, and mechanical stretching ("ON") and shrinking ("OFF") of cell-adhesive RGD ligand-presenting magnetic nanocoils on a material surface in five repeated cycles are reported, thereby independently increasing and decreasing ligand pitch in nanocoils, respectively, without modulating ligand-presenting surface area per nanocoil. It is demonstrated that cyclic switching "ON" (ligand nanostretching) facilitates time-regulated integrin ligation, focal adhesion, spreading, YAP/TAZ mechanosensing, and differentiation of viable stem cells, both in vitro and in vivo. Fluorescence resonance energy transfer (FRET) imaging reveals magnetic switching "ON" (stretching) and "OFF" (shrinking) of the nanocoils inside animals. Versatile tuning of physical dimensions and elements of nanocoils by regulating electrodeposition conditions is also demonstrated. The study sheds novel insight into designing materials with connected ligand nanostructures that exhibit nanocoil-specific nano-spaced declustering, which is ineffective in nanowires, to facilitate cell adhesion. This unprecedented, independent, remote, and cytocompatible control of ligand nanopitch is promising for regulating the mechanosensing-mediated differentiation of stem cells in vivo.

Electrolyte screening studies for Li metal batteries.

From 60 solvent electrolyte combinations tested, we find that Li metal anodes, tested in 1 M LiFSI in DOL:DME exhibit an outstanding cycling performance (>500 cycles) even at high current densities (3 mA cm-2). The excellent performance is ascribed, at least in part, to a low Li nucleation overpotential and a low charge transfer resistance during cycling.