Interfacial Ion Research Articles

Amorphous AlOCl Compounds Enabling Nanocrystalline LiCl with Abnormally High Ionic Conductivity.

LiCl is a promising solid electrolyte, providing it possesses high ionic conductivity. Numerous efforts have been made to enhance its ionic conductivity through aliovalent doping. However, aliovalent substitution changes the intrinsic structure of LiCl, compromising its cost-effectiveness and electrochemical stability. Here, we report nanocrystalline LiCl embedded in amorphous AlOCl compounds with a heterogeneous structure to enhance its ionic conductivity. Nanocrystallization enlarges the LiCl unit cell, while amorphization facilitates interfacial ion transport. As a result, the amorphous AlOCl-modified LiCl nanocrystal (AlOCl-nanoLiCl) demonstrates a high ionic conductivity of 1.02 mS cm-1, which is 5 orders of magnitude higher than that of LiCl. Additionally, it exhibits high oxidative stability, low cost ($19.87 US kg-1), and low Young's modulus (2-3 GPa). When AlOCl-nanoLiCl is coupled with Li-rich cathodes (Li1.17Mn0.55Ni0.24Co0.05O2, 4.8 V vs Li+/Li), all-solid-state batteries exhibit remarkable long-term cycling stability (>1000 cycles). This work presents a novel strategy to enhance the ionic conductivity of alkaline chlorides without compromising their intrinsic advantages.

Elucidating Heterogeneous Li Insertion Using Single-Crystalline and Freestanding Layered Oxide Thin Film.

The kinetics of interfacial ion insertion govern the uniformity of electrochemical reactions, playing a crucial role in lithium-ion battery performance. In two-dimensional lithium-conducting layered-oxide battery particles, variation in insertion rates across insertion channels remains unclear due to poorly defined crystal orientation at the solid-liquid interface and solid-state-lithium-diffusion length. This ambiguity complicates understanding inhomogeneous lithium-insertion channels activation. A systematic study requires crystallographically predefined interfaces and in situ lithium-concentration mapping. Here, we fabricated a freestanding, (104)-oriented-LiNi1/3Mn1/3Co1/3O2 single-crystal thin film using dissolution-induced release and performed in situ scanning-transmission-X-ray-microscopy to spatially resolve lithium-insertion at well-defined-interfaces. We observed heterogeneous lithium-concentration evolution due to channel-by-channel insertion rate variation, despite the potential for homogeneous lithium distribution via a solid-solution-phase at equilibrium in NMC111. Increasing current density exacerbates this heterogeneity, highlighting channel-by-channel variation. Our findings provide critical insights into battery electrode utilization and lifetime management, potentially guiding the design of more efficient and durable lithium-ion batteries.

Boosting Li-Ion Conductivity of Fluoride Solid Electrolyte by Low-Temperature Molten Salt Ablation and Particle Boundary Doping.

Halide solid electrolytes (SEs) are attracting great attention, owing to their high ionic conductivity and excellent high-voltage compatibility. However, severe moisture sensitivity, poor thermal stability, and instability at the lithium metal anode interface with chloride and bromide SEs retard their applications in solid-state lithium metal batteries. Fluoride SEs are expected to solve these problems, but they are now plagued by inadequate room-temperature (RT) ionic conductivity. Herein, a low-temperature molten salt (LiCl+1.33AlCl3) ablation method is proposed to enhance the ionic conductivity of monoclinic Li3GaF6 by particle boundary doping. The RT ionic conductivity of Li3GaF6 is correspondingly increased by 2 orders of magnitude, and the conductivity reaches 10-4 S cm-1 at 60 °C. The improved ionic conductivity benefits from the enhancement of interfacial ion transport, with the formation of more conductive chlorine-doped Li3GaF6-xClx and in situ binder LiAlCl4 to cement surrounding nanoparticles. The as-synthesized Li3GaF6 demonstrates outstanding humidity tolerance without conductivity degradation after exposure to a relative humidity of up to 35%. It also exhibits the widest electrochemical stability window experimentally (close to 6 V) compared with other state-of-the-art SEs. The solid-state Li/Li3GaF6/LiFePO4 cell with a stable Li+-conductive polymer interface is successfully driven for at least 200 cycles at 0.5C. Our study provides a solution to various chemical and electrochemical stability issues encountered by the halide SE family.

Quantitative Analysis on Interfacial Charges and Ions’ Speciation, and Identification of Interfacial Species Through Interfacial Waters’ Structure Revealed by Interface-specific Spectroscopy

Quantitative Analysis on Interfacial Charges and Ions’ Speciation, and Identification of Interfacial Species Through Interfacial Waters’ Structure Revealed by Interface-specific Spectroscopy

Carbon matrix NiFeP/C nanoparticles anchored on Ti3C2Tx nanosheets as heterostructure high-performance anode for lithium-ion batteries

Engineered micro-nano heterogeneous architectures for transition metal phosphide (TMPs) anode materials in lithium-ion batteries (LIBs) provide multifunctional active components. These components synergistically interact to address pivotal challenges of pronounced volume expansion and sluggish reaction kinetics within the electrode. Consequently, this engineering design approach significantly enhances the overall battery performance. In this study, a self-assembled approach is employed to integrate hybridized NiFe-PBA nanocubes with two-dimensional (2D) monolayer MXene (Ti3C2Tx), producing a heterostructured NiFeP/C@Ti3C2Tx composite. In this meticulously engineered architecture, the interface between the outer MXene layer and NiFeP embedded within the carbon matrix exhibits several distinct superiorities, such as rapid interfacial ion migration, multidirectional migration pathways, and strong spatial adaptability. The resulting non-uniform phase structure effectively mitigates electrode expansion and enhances electronic conductivity. Consequently, the NiFeP/C@Ti3C2Tx-15 composite demonstrates distinct performance advantages over NiFeP/C without MXene. The composite achieves a specific capacity of 689.4 mAh g⁻¹ at 200 mA g⁻¹ over 300 cycles, double that of NiFeP/C. Furthermore, density functional theory (DFT) calculations validate the electronic conductivity facilitated by the heterointerface between MXene and NiFeP, substantiating the strategic significance of the heterostructure in advancing high-performance anode materials for lithium-ion batteries.

A laboratory-based electrochemical NAP-XPS system for operando electrocatalysis studies

During electrocatalytic reactions, the electrode, adsorbates, electrolyte ions, and solvent molecules at the electrode-electrolyte interface each play an important role. Electrochemical X-ray photoelectron spectroscopy (XPS) holds great promise for deciphering these roles, providing the oxidation state or bonding environment of every element present at the interface. However, combining the vacuum required for XPS with the wet environment needed for electrochemistry constitutes a technical challenge, requiring purpose-built instrumentation and spectro-electrochemical cell design. Here, we present a laboratory-based electrochemical XPS instrument optimized for operando studies on nano-structured electrocatalysts. The core of the system is a 3D printed spectro-electrochemical cell containing a membrane-electrode-graphene assembly. We show that this design enables us to probe the electrode surface, interfacial water, and interfacial ions under well-defined potential control. Meanwhile, the introduction of a mesoporous membrane into the assembly enables the transport of any molecular or ionic reactant towards the working electrode, opening the way to study any aqueous phase electrocatalytic system using laboratory-based electrochemical XPS. We exemplify this for the oxygen reduction reaction.

Slurry Casted Ultrathin Li3Zr2Si2PO12 Electrolyte Film for Solid-State Lithium Metal Batteries.

Thin and flexible solid-state electrolyte (SSE) films with high ionic conductivity and low interfacial resistance are urgently required for lithium metal batteries (LMBs). However, it's still challenging to reduce the film thickness to <20µm, especially for those with high ceramic contents. Herein, a facile slurry casting method is developed to prepare the ultra-thin (14µm) Li3Zr2Si2PO12 (LZSP) films with ceramic content up to 91% using a composite polymer binder, polyvinylidene fluoride (PVDF), and polyethylene oxide (PEO). It shows that PEO not only enhanced the film flexibility but also makes it be easily peeled off to form a freestanding membrane, PLN. To promote the interfacial ion transport, PEO/lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) is introduced to the film surface, and the resultant tri-layer film, PPLN, shows a satisfying room temperature ionic conductivity of 0.116mScm-1, high Li+ transference number of 0.79, and good compatibility with metal lithium. As a result, LMBs using LiFePO4 cathode and PPLN electrolyte exhibit excellent safety as well as electrochemical performances in the wide temperature range between room temperature (RT) and 100°C.

Engineering of charge density at the anode/electrolyte interface for long-life Zn anode in aqueous zinc ion battery.

The aqueous zinc ion battery emerges as the promising candidate applied in large-scale energy storage system. However, Zn anode suffers from the issues including Zn dendrite, Hydrogen evolution reaction and corrosion. These challenges are primarily derived from the instability of anode/electrolyte interface, which is associated with the interfacial charge density distribution. In this context, the recent advancements concentrating on the strategies and mechanism to regulate charge density at the Zn anode/electrolyte interface are summarized. Different characterization techniques for charge density distribution have been analysed, which can be applied to assess the interfacial zinc ion transport. Additionally, the charge density regulations at the Zn anode/electrolyte interface are discussed, elucidating their roles in modulating electrostatic interactions, electric field, structure of solvated zinc ion and electric double layer, respectively. Finally, the perspectives and challenges on the further research are provided to establish the stable anode/electrolyte interface by focusing on charge density modifications, which is expected to facilitate the development of aqueous zinc ion battery.

Asymmetric lithium supercapacitors with solid-state hybrid electrolytes containing zeolite particle and water-soluble polymer

Here we demonstrate that high-performance solid-state hybrid electrolytes are prepared via environment-friendly water-based processes of zeolite Y (Zy) particle, branched-poly(ethylene imine) (bPEI), and lithium hydroxide (LiOH). The hybrid Zy:bPEI:LiOH electrolytes (ZyPL) were successfully applied to fabricate asymmetric-type lithium supercapacitors consisting of graphite-based active electrodes and indium-tin-oxide (ITO) counter electrodes. Results showed that the ion conductivity of electrolytes was greatly dependent on the Zy content and reached the highest value (ca. 0.28 mS/cm) at Zy = 30 wt% due to the formation of interfacial ion transport channels between Zy particles and bPEI chains. The galvanostatic charging/discharging (GCD) test revealed that the ZyPL supercapacitors could deliver the highest potential of 2.28 V at Zy = 30 wt% compared to 1.86 V at Zy = 0 wt%. In particular, the ZyPL supercapacitors, after charging at 0.1 mA/g, exhibited a long-lasting high-potential level of > 1.0 V at Zy = 30 wt% compared to 0.5 V at Zy = 0 wt%. The long-term GCD cycle test disclosed that the ZyPL supercapacitors could work stably with a capacitance retention of 91.7 % even after 3000 cycles. The series connection of six ZyPL supercapacitors delivered ca. 10.94 V with consistent charging/discharging characteristics.

Reversible Li-ion trade-off in ultrathick sulfur cathodes for practical lean Li–S batteries

The use of low electrolyte volume is beneficial to improve energy density but severely limits access over obscured sulfur, along with sluggish ion kinetics and aggravated polarization, as the ion imbalance across the multicomponent interface of thick sulfur cathodes at a lean electrolyte viciously dominates the sulfur electrokinetics. Herein, we demonstrate that an ion imbalance at the interfaces of a thick electrode with a lean electrolyte can be compensated by the ion trade-off strategy utilizing a cationic ion conductive active binder. It ensures sustained lithium-ion donation/release over the vicinity of slow electrolyte percolation to realize an ion-enriched sulfur–binder–electrolyte interface. The in-situ evolved ionic interface essentially activates the inaccessible sulfur, bringing about additional capacity and low ion and charge transfer resistances. The active binder adopts sulfur cathodes housing 8.1 mg cm–2 with an E/S ratio of 6 µL mg–1 electrochemically utilized 60.89 % sulfur, corresponding to a 1020 mAh g–1 capacity. The lean Li–S pouch cell delivers an energy density of 324 Wh kg–1, demonstrating the efficacy of ion trade-off to ease the interfacial barrier. This study would open up a new paradigm in potentially designing thick electrodes for multiple high energy density batteries.

The morphologic dependence of MnO2 electrodes in capacitive deionization process

Manganese dioxide (MnO2) materials are one of promising cathode candidates for capacitive deionization (CDI) applications, with their morphology significantly impacting the performance of the electrode material itself. Therefore, this study systematically investigates the structure-performance relationships and the micro-interface ion storage mechanisms of four morphologies of MnO2 (1D nanorods (NNMO), nanowires (NWMO), 3D microspheres (MMO), and hollow urchin-like spheres (HUMO)) in relation to their capacitive deionization performance with typical heavy metal ions (Cd2+) through experimental and theoretical calculations. In terms of pore morphology, capacitance significantly increases with increasing surface curvature of materials, demonstrating a clear pore structure-dependent characteristic. NNMO, with the smallest pore size, has much lower surface capacitance (∼0.15 μF cm−2) than other materials (∼0.33 μF cm−2). As pore size increases, the capacitance distribution difference driven by pore structure size gradually disappears. Regarding surface and interface characteristics, high-energy crystal facets facilitate electron transfer ({310}>{200}≈{211}>{100}) and increase the proportion of surface active sites (Osur), thus promoting CDI absorption kinetics. During the capacitive deionization process, the pore structure (∼77 %) and surface-interface characteristics (∼23 %) exhibit highly coupled features and the driving force for interfacial ion storage among the four materials is in the order of HUMO>MMO>NWMO>NNMO. This work elucidates the morphology-dependent capacitive processes of MnO2 nanomaterials, enhancing the understanding of structure- electrochemical process at the nanoscale but also providing effective guidance for the design and development of practical, high-performance CDI electrode materials.

Unveiling the Interfacial Species Synergy in Promoting CO2 Tandem Electrocatalysis in Near-Neutral Electrolyte.

The interfacial species-built local environments on Cu surfaces impact the CO2 electroreduction process significantly in producing value-added multicarbon (C2+) products. However, intricate interfacial dynamics leads to a challenge in understanding how these species affect the process. Herein, with ab initio molecular dynamics (AIMD) and finite element method (FEM) simulations, we reveal that the highly concentrated interfacial species, including the *CO, hydroxide, and K+, could synergistically promote the C-C coupling on the one-dimensional (1D) porous hollow structure regulated interfacial environment. The Cu-Ag tandem catalyst was then synthesized with the as-designed structure, exhibiting a high C2+ Faradaic efficiency of 76.0% with a partial current density of 380.0 mA cm-2 in near-neutral electrolytes. Furthermore, in situ Raman spectra validate that the 1D porous structure regulates the concentration of interfacial CO intermediates and ions to increase *CO coverage, local pH value, and ionic field, promoting the CO2-to-C2+ activity. These results provide insights into the design of practical ECR electrocatalysts by regulating interfacial species-induced local environments.

(Invited) A Molecular Understanding of (Interfacial) Ion Solvation, and Its Implications for Electrochemistry

Ions in solution play a central role in most electrochemical systems. Theoretical approaches rooted in continuum models, such as Debye-Hückel theory or the Born solvation model, provide the basis for much of our understanding about processes that involve dissolved ions. Despite the remarkable success of these relatively simple theories, their (drastic) assumptions can sometimes lead to a qualitatively incorrect description of the underlying chemical physics. Here, we will overview recent work from molecular simulations that casts light on the molecular details of both equilibrium and nonequilibrium processes in solution. Implications for relating the details of microscopic relaxation processes to experimental observables such as impedance and capacitance will also be discussed.

2D Porous Frameworks for Sustainable Energy Storage Applications

Two-dimensional (2D) porous frameworks have garnered significant research interest as a new generation of multifunctional materials. Examples of such frameworks include 2D covalent organic frameworks and 2D conjugated metal-organic frameworks. These structures are characterized by their regular porosities, large specific surface areas, and exceptional chemical stability. Moreover, 2D porous frameworks exhibit distinct properties, such as designable topologies and well-defined redox-active sites, which have sparked increased exploration of these materials for electrochemical energy storage applications.[1] In this presentation, I will illustrate how 2D porous frameworks offer promising electrode alternatives for the next generation of energy storage devices. This will be demonstrated through examples including 2D polyarylimide covalent organic frameworks for multivalent metal batteries[2-3] and 2D conjugated metal-organic frameworks, which serve as appealing pseudocapacitive electrodes[4-5]. Besides, I will present our recent endeavors in exploring 2D crystalline polymers as effective interfacial coatings for batteries.[6-7] These coatings facilitate efficient interfacial ion transport and charge transfer, thus effectively enhancing the overall electrochemical reaction kinetics, reversibility, and durability.

Cryogenic Atom Probe Tomograph to Resolve Confined Electrochemical Interfaces

Atom Probe Tomography (APT) is an analysis technique capable of providing a 3-dimensional, element-sensitive map of needle-shaped samples (where the tip radius is about 50 nm) with sub-nanometre resolution [1]. APT analysis is carried out at temperatures in the range of 25-75 K to limit the thermal motion of surface atoms which improves the spatial and mass resolution. As a result, this enables the analysis of frozen liquids, and by extension solid-liquid interfaces, by so-called “cryo-APT”. While this was historically challenging, recent advancements in UHV-cryogenic transfer and sample preparation methods now make it possible for the atomic scale analysis of interfacial species in electrochemical systems [2-4].In this study, we will attempt to explore the effect of nanoscale geometrical confinement on the distribution of interfacial ions for the first time and to resolve the electric double layer, using cryo-APT. We will describe the workflow for cryo-APT sample preparation to study confined solid-liquid interfaces, using nanoporous gold as a template material with a frozen sodium iodide solution inside its pores. APT reconstructions will allow 3-dimensional mapping of aqueous interfacial species, helping to develop our understanding of the structure of the electric double layer under nanoscale geometrical confinement. The attached Figure shows an SEM image of the later stages of cryo-FIB milling of an APT tip of frozen deionised water and 50 mM sodium iodide and a segment of the related APT atom map reconstruction.

Ion Transfer Kinetics at the Tungsten Oxide Interface

Investigating the kinetics of interfacial ion transfer is imperative for the enhancement of electrochemical energy storage technologies, such as cation batteries. Consequently, we study the enigmatic proton transfer mechanisms that occur at solid-liquid interfaces to evaluate parameters that may modulate the optimization of battery efficiency. Utilizing electrochemical techniques, our objective is to deconvolute the mechanistic and rate-limiting factors associated with proton insertion from an acidic aqueous electrolyte into a thin film of tungsten trioxide (WO3). While WO3 has been investigated as a battery anode, the kinetics of cation insertion remain inadequately explored despite their effects on system efficiency. Through the Potentiostatic Intermittent Titration Technique (PITT), a developed application of chronoamperometry, along with Butler-Volmer equation fitting, our research endeavors to define the exchange current density and activation energy pertinent to cation insertion. The broader implications of exploring interfacial ion transfer phenomena manifest in advancing the fundamental understanding of energy storage processes.

Influence of Sulfonates on the Interfacial Ion Transfer Kinetics of Cu at Au(111) Surfaces

Corrosion reactions profoundly impact the operational stability of various electrochemical components, such as electrocatalysts, conductive supports, and semiconducting light absorbers. Understanding the kinetics of ion transfer at electrode/electrolyte interfaces is crucial for deciphering the durability of copper-containing electrochemical devices. This study investigates the corrosion and deposition behavior of copper adlayers on gold(111) surfaces in sulfate and sulfonate electrolytes, with a focus on adsorption kinetics influenced by anion structure and Lewis acidity. At low scan rates, the adsorption and desorption capacitance of the broad adsorption region previously assigned to random adsorption of Cu on Au(111) surface in sulfuric acid, were nearly symmetric in E and invariant with ν, indicating reversible kinetics across all four electrolytes. At faster scan rates, the experiments reveal distinct adsorption/desorption responses for different anions, shedding light on the interplay between ion transfer kinetics, corrosion rates, and the microenvironment of electrocatalysts. The results provide valuable insights into the fundamental processes governing interfacial ion transfer and offer implications for the design and optimization of electrochemical systems. Figure 1

Practical SPAN||Li cells enabled by insitu polymerized electrolyte

Sulfurized polyacrylonitrile (SPAN) is a promising cathode material for long-life lithium-sulfur batteries (LSBs) due to its enhanced electronic conductivity as well as the eliminated shuttle effect. However, the uncontrollable lithium dendrite issue as well as slow kinetics of thick electrodes still hinders its practical application. Herein, an in situ polymerized electrolyte (PGE) based on vinyl carbonate and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is designed and prepared to enhance the plating/stripping stability of the metal lithium while ensuring the cathode/electrolyte interfacial ion transportation. The optimized electrolyte, PGE-D, shows a satisfying ionic conductivity of 0.46 mS/cm at 25 °C; in the meantime, the polymer matrix hinders the diffusion of TFSI− anion and results in a high Li+ transference number (tLi+) of 0.73. Benefiting from the high affinity of the flowable PGE precursor to the thick SPAN cathode as well as enhanced lithium compatibility, the Li||SPAN battery with high areal loading of 14.1 mg/cm2 exhibits a high reversible specific capacity of 556.5 mAh/g and retains 76.3% of its capacity after 90 cycles.

In situ three-dimensional reconstruction and gradient-induced design of ultrastable Zn metal anodes in Zn-Se batteries

Zinc-selenium batteries have high specific and volumetric capacities, but practical development faces key challenge in achieving stable zinc metal anodes, and avoiding zinc dendrite growth, hydrogen evolution reaction (HER) and corrosion. Here, a zinc foam substrate is employed to fabricate three-dimensional gradient electrodes Zn foam@Cu@CuSe2 (ZFCCS) with gradient change of zincophilicity and conductivity. Experimental observations and simulations collectively confirm that the three-dimensional structure and gradient design effectively homogenize interfacial ion flux and diminish local current density. This optimization of the zinc deposition path synergistically fosters high flux and deep deposition of zinc metal while simultaneously preventing dendrite growth. The results demonstrate that the developed ZFCCS electrode produces an excellent Coulombic efficiency of 99.5 % over 1000 plating/stripping cycles, and the corresponding symmetric cell provides an ultra-long dendrite-free cycle life of 800 h at 5 mA cm−2 and 2 mAh cm−2 with a low overpotential of 27.4 mV. In addition, the Zn-Se full cell based on the designed ZFCCS composite anode exerts superior stable cycle life (358.1 mAh g−1 at 2 A g−1 after 1000 cycles). Therefore, the gradient design strategy based on 3D electrodes will be a reference for high performance energy storage devices.

Single-Particle Imaging Reveals the Electrical Double-Layer Modulated Ion Dynamics at Crowded Interface.

The dynamics of ion transport at the interface is the critical factor for determining the performance of an electrochemical energy storage device. While practical applications are realized in concentrated electrolytes and nanopores, there is a limited understanding of their ion dynamic features. Herein, we studied the interfacial ion dynamics in room-temperature ionic liquids by transient single-particle imaging with microsecond-scale resolution. We observed slowed-down dynamics at lower potential while acceleration was observed at higher potential. Combined with simulation, we found that the microstructure evolution of the electric double layer (EDL) results in potential-dependent kinetics. Then, we established a correspondence between the ion dynamics and interfacial ion composition. Besides, the ordered ion orientation within EDL is also an essential factor for accelerating interfacial ion transport. These results inspire us with a new possibility to optimize electrochemical energy storage through the good control of the rational design of the interfacial ion structures.