Self-Purifying Primary Solvation Sheath Enables Stable Electrode-Electrolyte Interfaces for Nickel-Rich Cathodes

Herein, we optimize the primary solvation sheath to investigate the fundamental correlation between battery performance and electrode-electrolyte interfacial properties through electrolyte solvation chemistry. Experimental and theoretical analyses reveal that the primary solvation sheath with a self-purifying feature can "positively" scavenge both the HF and PF5 (hydrolysis of ion-paired LiPF6), stabilize the PF6 anion-derived electrode-electrolyte interfaces, and thus boost the cycling performances. Being attributed with these superiorities, the NCM811//Li Li metal battery (LMB) with the electrolyte containing the optimized solvation sheath delivers 99.9% capacity retention at 2.5 C after 250 cycles. To circumvent the impact of excess Li content of Li metal on the performance of NCM811 cathode, the as-fabricated NCM811//graphite Li ion battery (LIB) also delivers a high-capacity retention of 90.1% from the 5th to the 100th cycle at 1 C. This work sheds light on the strong ability of the primary solvation sheath to regulate cathode interfacial properties.

Effect of Salt Concentration on the Interfacial Solvation Structure and Early Stage of Solid-Electrolyte Interphase Formation in Ca(BH4)2/THF for Ca Batteries

The Ca²⁺ solvation structure at the electrolyte/electrode interface is of central importance to understand electroreduction stability and solid-electrolyte interphase (SEI) formation for the novel multivalent Ca battery systems. Using an exemplar electrolyte, the concentration-dependent solvation structure of Ca(BH₄)₂-tetrahydrofuran on a gold model electrode has been investigated with various electrolyte concentrations via electrochemical quartz crystal microbalance with dissipation (EQCM-D) and X-ray photoelectron spectroscopy (XPS). For the first time, in situ EQCM-D results prove that the prevalent species adsorbed at the interface is CaBH₄⁺ across all concentrations. As the salt concentration increases, the number of BH₄^- anions associated with Ca²⁺ increases, and much larger solvated complexes such as CaBH₄⁺·4THF or Ca(BH₄)₃^-·4THF form at the interface at high concentrations prior to Ca plating. Different interfacial chemistries lead to the formation of SEIs with different components demonstrated by XPS. High electrolyte concentrations reduce the solvent decomposition and promote the formation of thick, uniform, and inorganic-rich (i.e., CaO) SEI layers, which contribute to improved Ca plating efficiency and current density in electrochemical measurements.

Tuning Electrochemical Performance by Microstructural Optimization of the Nanocrystalline Functional Oxygen Electrode Layer for Solid Oxide Cells

Further development of solid oxide fuel cell (SOFC) oxygen electrodes can be achieved through improvements in oxygen electrode design by microstructure miniaturization alongside nanomaterial implementation. In this work, improved electrochemical performance of an La_0.6Sr_0.4Co_0.2Fe_0.8O_3-d (LSCF) cathode was achieved by the controlled modification of the La_0.6Sr_0.4CoO_3-d (LSC) nanocrystalline interlayer introduced between a porous oxygen electrode and dense electrolyte. The evaluation was carried out for various LSC layer thicknesses, annealing temperatures, oxygen partial pressures, and temperatures as well as subjected to long-term stability tests and evaluated in typical operating conditions in an intermediate temperature SOFC. Electrochemical impedance spectroscopy and a distribution of relaxation times analysis were performed to reveal the rate-limiting electrochemical processes that limit the overall electrode performance. The main processes with an impact on the electrode performance were the adsorption of gaseous oxygen O₂, dissociation of O₂, and charge transfer-diffusion (O^2-). The introduction of a nanoporous and nanocrystalline interlayer with extended electrochemically active surface area accelerates the oxygen surface exchange kinetics and oxygen ion diffusions, reducing polarization resistances. The polarization resistance of the reference LSCF was lowered by one order of magnitude from 0.77 to 0.076 Ω·cm² at 600 °C by the deposition of a 400 nm LSC interlayer at the interface. The developed electrode tested in the anode-supported fuel cell configuration showed a higher cell performance by 20% compared to the cell with the reference electrode. The maximum power density at 700 °C reaches 675 and 820 mW·cm^-2 for the reference cell and the cell with the LSC interlayer, respectively. Aging tests at 700 °C under a high load of 1 A·cm² were performed.

Single Particle Hopping as an Indicator for Evaluating Electrocatalysts

The design and screening of electrocatalysts for gas evolution reactions suffer from little understanding of multiphase processes at the electrode-electrolyte interface. Due to the complexity of the multiphase interface, it is still a great challenge to capture gas evolution dynamics under operando conditions to precisely portray the intrinsic catalytic performance of the interface. Here, we establish a single particle imaging method to real-time monitor a potential-dependent vertical motion or hopping of electrocatalysts induced by electrogenerated gas nanobubbles. The hopping feature of a single particle is closely correlated with intrinsic activities of electrocatalysts and thus is developed as an indicator to evaluate gas evolution performance of various electrocatalysts. This optical indicator diminishes interference from heterogeneous morphologies, non-Faradaic processes, and parasitic side reactions that are unavoidable in conventional electrochemical measurements, therefore enabling precise evaluation and high-throughput screening of catalysts for gas evolution systems.

Mechanistic Study of Controlled Zinc Electrodeposition Behaviors Facilitated by Nanoscale Electrolyte Additives at the Electrode Interface

Nanoparticle organic hybrid materials (NOHMs) are liquid-like materials composed of an inorganic core to which a polymeric canopy is ionically tethered. NOHMs have unique properties including negligible vapor pressure, high oxidative thermal stability, and the ability to bind to reactive species of interest due to the tunability of their polymeric canopy. This makes them promising multifunctional materials for a wide range of energy and environmental technologies, including electrolyte additives for electrochemical energy storage (e.g., flow batteries) and the electrochemical conversion of CO₂ to chemicals and fuels. Due to their unique transport behaviors in fluid systems, an understanding of the near-electrode surface behavior of NOHMs in electrolyte solutions and their effect on electrochemical reactions is still lacking. In this work, the complexation of zinc (Zn) by NOHMs with an ionically tethered polyetheramine canopy (HPE) (NOHM-I-HPE) was studied using attenuated total reflectance Fourier transform infrared and Carbon-13 nuclear magnetic resonance spectroscopy. Additionally, various electrochemical techniques were employed to discern the role of NOHM-I-HPE during zinc electrodeposition, and the results were compared to those of the electrochemical system containing untethered HPE polymers. Our findings confirmed that NOHM-I-HPE and HPE reversibly complex zinc in the aqueous electrolyte. NOHM-I-HPE and HPE were found to block some of the electrode active sites, reducing the overall current density during electrodeposition, while facilitating the formation of smooth zinc deposits, as revealed by surface imaging and diffraction techniques. Observed variations in the current density responses and the degree of passivation created by the NOHM-I-HPE and HPE adsorbed on the electrode surface revealed that their different packing behaviors at the electrode-electrolyte interface influence the zinc deposition mechanism. The presence of the nanoparticle and ordering offered by the NOHMs as well as the structured conformation of the polymeric canopy allowed the formation of void spaces and free volumes for enhanced transport behaviors. These findings provided insights into how structured electrolyte additives such as NOHMs can allow for advancements in electrolyte design for controlled deposition of metal species from energy-dense electrolytes or for other electrochemical reactions.

Interfacial Proton Transfer for Hydrogen Evolution at the Sub-Nanometric Platinum/Electrolyte Interface

Understanding the dynamic process of interfacial charge transfer prior to chemisorption is crucial to the development of electrocatalysis. Recently, interfacial water has been highlighted in transferring protons through the electrode/electrolyte interface; however, the identification of the related structural configurations and their influences on the catalytic mechanism is largely complicated by the amorphous and mutable structure of the electrical double layer (EDL). To this end, sub-nanometric Pt electrocatalysts, potentially offering intriguing activity and featuring fully exposed atoms, are studied to uncover the elusive electrode/electrolyte interface via operando X-ray absorption spectroscopy during the hydrogen evolution reaction (HER). Our results show that the metallic Pt clusters derived from the reduction of sub-nanometric Pt clusters (SNM-Pt) exhibit excellent HER activity, with an only 18 mV overpotential at 10 mA/cm² and one-magnitude-higher mass activity than commercial Pt/C. More importantly, a unique Pt-interfacial water configuration with a Pt (from Pt clusters)-O (from water) radial distance of approximately 2.5 Å is experimentally identified as the structural foundation for the interfacial proton transfer. Toward high overpotentials, the interfacial water that structurally evolves from "O-close" to "O-far" accelerates the proton transfer and is responsible for the improved reaction rate by increasing the hydrogen coverage.

Tailoring Electrode-Electrolyte Interfaces in Lithium-Ion Batteries Using Molecularly Engineered Functional Polymers

Electrode-electrolyte interfaces (EEIs) affect the rate capability, cycling stability, and thermal safety of lithium-ion batteries (LIBs). Designing stable EEIs with fast Li⁺ transport is crucial for developing advanced LIBs. Here, we study Li⁺ kinetics at EEIs tailored by three nanoscale polymer thin films via chemical vapor deposition (CVD) polymerization. Small binding energy with Li⁺ and the presence of sufficient binding sites for Li⁺ allow poly(3,4-ethylenedioxythiophene) (PEDOT) based artificial coatings to enable fast charging of LiCoO₂. Operando synchrotron X-ray diffraction experiments suggest that the superior Li⁺ transport property in PEDOT further improves current homogeneity in the LiCoO₂ electrode during cycling. PEDOT also forms chemical bonds with LiCoO₂, which reduces Co dissolution and inhibits electrolyte decomposition. As a result, the LiCoO₂ 4.5 V cycle life tested at C/2 increases over 1700% after PEDOT coating. In comparison, the other two polymer coatings show undesirable effects on LiCoO₂ performance. These insights provide us with rules for selecting/designing polymers to engineer EEIs in advanced LIBs.

Electrogravimetry and Structural Properties of Thin Silicon Layers Deposited in Sulfolane and Ionic Liquid Electrolytes

Potentiostatic deposition of silicon is performed in sulfolane (SL) and ionic liquid (IL) electrolytes. Electrochemical quartz crystal microbalance with damping monitoring (EQCM-D) is used as main analytical tool for the characterization of the reduction process. The apparent molar mass (M_app) is applied for in situ estimation of the layer contamination. By means of this approach, appropriate electrolyte composition and substrate type are selected to optimize the structural properties of the layers. The application of SL electrolyte results in silicon deposition with higher efficiency compared to the IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSI]. This has been associated with the instability of the IL in the presence of silicon tetrachloride and the enhanced incorporation of IL decomposition products into the growing silicon deposit. X-ray photoelectron spectroscopy (XPS) analysis supports the results about the layer composition, as suggested from the microgravimetric experiments. Attention has been given to the impact of practically relevant substrates (i.e., Cu, Ni, and vitreous carbon) on the reduction process. An effective deposition can be carried out on the metal electrodes in both electrolytes due to accelerated reaction kinetics for these types of substrates. However, on vitreous carbon (VC), a successful reduction of SiCl₄ can only be accomplished in the IL, while the electroreduction process in SL is dominated by the decomposition of the electrolyte. For short deposition times, the scanning electron microscopy (SEM) images display rough morphologies in the nanometer range, which evolve further to structures with increased length scale of the surface roughness. The development of a rough interface during deposition, resulting in QCM damping at advanced stages of the process, is interpreted by a model accounting for the resistive force caused by the interaction of the liquid with a nonuniform layer interface. By using this approach, the individual contributions of the surface roughness and viscoelastic effects to the measured damping values are estimated.

Three-Dimensional Molecular Mapping of Ionic Liquids at Electrified Interfaces

Electric double layers (EDLs), occurring ubiquitously at solid-liquid interfaces, are critical for electrochemical energy conversion and storage processes such as capacitive charging and redox reactions. However, to date the molecular-scale structure of EDLs remains elusive. Here we report an advanced technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), and use it to directly image the molecular-scale EDL structure of an ionic liquid under different electrode potentials. We observe not only multiple discrete ionic layers in the EDL on a graphite electrode but also a quasi-periodic molecular density distribution within each layer. Furthermore, we find pronounced 3D reconfiguration of the EDL at different voltages, especially in the first layer. Combining the experimental results with molecular dynamics simulations, we find potential-dependent molecular redistribution and reorientation in the innermost EDL layer, both of which are critical to EDL capacitive charging. We expect this mechanistic understanding to have profound impacts on the rational design of electrode-electrolyte interfaces for energy conversion and storage.

Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries

Layered lithium nickel, manganese, and cobalt oxides (NMC) are among the most promising commercial positive electrodes in the past decades. Understanding the detailed surface and bulk redox processes of Ni-rich NMC can provide useful insights into material design options to boost reversible capacity and cycle life. Both hard X-ray absorption (XAS) of metal K-edges and soft XAS of metal L-edges collected from charged LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) showed that the charge capacity up to removing ∼0.7 Li/f.u. was accompanied with Ni oxidation in bulk and near the surface (up to 100 nm). Of significance to note is that nickel oxidation is primarily responsible for the charge capacity of NMC622 and 811 up to similar lithium removal (∼0.7 Li/f.u.) albeit charged to different potentials, beyond which was followed by Ni reduction near the surface (up to 100 nm) due to oxygen release and electrolyte parasitic reactions. This observation points toward several new strategies to enhance reversible redox capacities of Ni-rich and/or Co-free electrodes for high-energy Li-ion batteries.

Non-Innocent Role of Porous Carbon Toward Enhancing C2-3 Products in the Electroreduction of Carbon Dioxide

Selectivity for C-C coupled products remains a major challenge for electrochemical CO₂ reduction. Herein, we report a facile method by modifying a Cu foil surface with a layer of porous carbon. The structure of carbon has a major influence on C₁ and C_2,3 product selectivity. A carbon aerogel modifier leads to higher C_2,3 product formation than that of a carbon black modifier, demonstrating the non-innocent role of carbon materials. In both cases, major surface reconstruction on the Cu foil-such as pitting and particle formation-is observed during electrocatalysis. In addition, the restructured Cu surface shows distinctly lower activity toward CO₂ reduction when the carbon modifier is removed. This is likely due to the fact that the carbon modifiers influence the product distribution by (i) modulating the local pH and CO₂ concentration by serving as a highly porous and hydrophobic barrier, and (ii) restructuring the metal surface that generates more active sites. Our findings illustrate that the carbon in carbon-based catalysts can have an disproportionate role in directing product formation in electrocatalytic carbon dioxide reduction.

Elucidating the Origin of the Electrochemical Capacity in a Proton-Based Battery HxIrO4 via Advanced Electrogravimetry

Recently, because of sustainability issues dictated by societal demands, more importance has been given to aqueous systems and especially to proton-based batteries. However, the mechanisms behind the processes leading to energy storage in such systems are still not elucidated. Under this scope, our study is structured on the selection of a model electrode material, the protonic phase H_xIrO₄, and the scrutiny of the interfacial processes through suitable analytical tools. Herein, we employed operando electrochemical quartz crystal microbalance (EQCM) combined with electrochemical impedance spectroscopy (EIS) to provide new insights into the mechanism intervening at the electrode-electrolyte interface. First, we demonstrated that not only the surface or near surface but the whole particle participates in the cationic redox process. Second, we proved that the contribution of the proton on the overall potential window together with the incorporation of water at low potentials solely. This is explained by the fact that water molecules permit a further insertion of protons in the material by shielding the proton charge but at the expense of the proton kinetic properties. These findings shed a new light on the importance of water molecules in the ion-insertion mechanisms taking place at the electrode-electrolyte interface of aqueous proton-based batteries. Overall, the present results further highlight the richness of the EQCM-based methods for the battery field in offering mechanistic insights that are crucial for the understanding of interfaces and charge storage in insertion compounds.

Advanced Lithium Metal-Carbon Nanotube Composite Anode for High-Performance Lithium-Oxygen Batteries

The low Coulombic efficiency and hazardous dendrite growth hinder the adoption of lithium anode in high-energy density batteries. Herein, we report a lithium metal-carbon nanotube (Li-CNT) composite as an alternative to the long-term untamed lithium electrode to address the critical issues associated with the lithium anode in Li-O₂ batteries, where the lithium metal is impregnated in a porous carbon nanotube microsphere matrix (CNTm) and surface-passivated with a self-assembled monolayer of octadecylphosphonic acid as a tailor-designed solid electrolyte interphase (SEI). The high specific surface area of the Li-CNT composite reduces the local current density and thus suppresses the lithium dendrite formation upon cycling. Moreover, the tailor-designed SEI effectively separates the Li-CNT composite from the electrolyte solution and prevents the latter's further decomposition. When the Li-CNT composite anode is coupled with another CNTm-based O₂ cathode, the reversibility and cycle life of the resultant Li-O₂ batteries are drastically elevated.

Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species

Understanding the molecular-level properties of electrochemically active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of high-performance nanostructured surfaces for applications in energy technology. Herein, an electrochemical cell coupled with ion soft landing is employed to examine the effect of "atom-by-atom" metal substitution on the activity and stability of well-defined redox-active anions, PMo _xW_{12- x}O₄₀^3- ( x = 0, 1, 2, 3, 6, 9, or 12) at nanostructured ionic liquid EEI. A striking observation made by in situ electrochemical measurements and further supported by theoretical calculations is that the substitution of only one to three tungsten atoms by molybdenum atoms in the PW₁₂O₄₀^3- anions results in a substantial spike in their first reduction potential. Specifically, PMo₃W₉O₄₀^3- showed the highest redox activity in both in situ electrochemical measurements and as part of a functional redox supercapacitor device, making it a "super-active redox anion" compared with all other PMo _xW_{12- x}O₄₀^3- species. Electronic structure calculations showed that metal substitution in PMo _xW_{12- x}O₄₀^3- causes the lowest unoccupied molecular orbital (LUMO) to protrude locally, making it the "active site" for reduction of the anion. Several critical factors contribute to the observed trend in redox activity including (i) multiple isomeric structures populated at room temperature, which affect the experimentally determined reduction potential; (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more-electronegative Mo atoms; and (iii) structural relaxation of the reduced species produced after the first reduction step. Our results illustrate a path to achieving superior performance of technologically relevant EEIs in functional nanoscale devices through understanding of the molecular-level electronic properties of specific electroactive species with "atom-by-atom" precision.

Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage Cathode-Electrolyte Interface in Lithium-Ion Batteries

Current developments of electrolyte additives to stabilize electrode-electrolyte interface in lithium-ion batteries highly rely on a trial-and-error search, which involves repetitive testing and intensive amount of resources. The lack of understandings on the fundamental protection mechanisms of the additives significantly increases the difficulty for the transformational development of new additives. In this study, we investigated two types of individual protection routes to build a robust cathode-electrolyte interphase at high potentials: (i) a direct reduction in the catalytic decomposition of the electrolyte solvent; and (ii) formation of a "corrosion inhibitor film" that prevents severely attack and passivation from protons that generated from the solvent oxidation, even the decomposition of solvent cannot be mitigated. Effect of two exemplary electrolyte additives, lithium difluoro(oxalato)borate (LiDFOB) and 3-hexylthiophene (3HT), on LiNi_0.6Mn_0.2Co_0.2O₂ (NMC 622) cathode were investigated to validate our hypothesis. It is demonstrated that understandings of both electrolyte additives and solvent are essential and careful balance between the cathode protection mechanism of additives and their side effects is critical to obtain optimum results. More importantly, this study opens up new directions of rational design of functional electrolyte additives for the next-generation high-energy-density lithium-ion chemistries.

Electrode-Impregnable and Cross-Linkable Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) Triblock Polymer Electrolytes with High Ionic Conductivity and a Large Voltage Window for Flexible Solid-State Supercapacitors

We present cross-linkable precursor-type gel polymer electrolytes (GPEs) that have large ionic liquid uptake capability, can easily penetrate electrodes, have high ion conductivity, and are mechanically strong as high-performance, flexible all-solid-state supercapacitors (SC). Our polymer precursors feature a hydrophilic-hydrophobic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock main-chain structure and trifunctional silane end groups that can be multi-cross-linked with each other through a sol-gel process. The cross-linked solid-state electrolyte film with moderate IL content (200 wt %) shows a well-balanced combination of excellent ionic conductivity (5.0 × 10^-3 S cm^-1) and good mechanical stability (maximum strain = 194%). Moreover, our polymer electrolytes have various advantages including high thermal stability (decomposition temperature > 330 °C) and the capability to impregnate electrodes to form an excellent electrode-electrolyte interface due to the very low viscosity of the precursors. By assembling our GPE-impregnated electrodes and solid-state GPE film, we demonstrate an all-solid-state SC that can operate at 3 V and provides an improved specific capacitance (112.3 F g^-1 at 0.1 A g^-1), better rate capability (64% capacity retention until 20 A g^-1), and excellent cycle stability (95% capacitance decay over 10 000 charge/discharge cycles) compared with those of a reference SC using a conventional PEO electrolyte. Finally, flexible SCs with a high energy density (22.6 W h kg^-1 at 1 A g^-1) and an excellent flexibility (>93% capacitance retention after 5000 bending cycles) can successfully be obtained.