Anion Diffusion Research Articles

Hexagonal liquid crystals as emerging quasi solid-state electrolytes for aqueous lithium-ion batteries

The increasing market demand ranging from portable electronics to electric vehicles drives the advancement of lithium-ion batteries. However, traditional liquid electrolytes used in LIBs are plagued by issues such as leakage, vaporization, and the degradation of active materials, which compromise performance and pose safety risks. To address these challenges, herein, the liquid-crystalline electrolytes with hexagonal phase were designed based on the self-assembly of amphiphilic molecules, which exhibit both high ionic conductivity and a high Li+ transference number. The hexagonal liquid crystal structure reconfigures the Li+ solvation structure and provides a transport pathway for the Li+ structural diffusion, facilitating the efficient transport of Li+ and contributing to the high ionic conductivity. Furthermore, the ordered arrangement of amphiphilic anion significantly restricts anion diffusion, resulting in an exceptionally high Li+ transference number of 0.92. Additionally, when utilizing NaV3O8 (NVO) as the anode and LiMn2O4 (LMO) as the cathode, the resultant full cell delivers impressive rate performance and stable cycling performance. This work highlights the potential of lyotropic liquid crystals in the development of high-performance quasi solid-state electrolytes for aqueous lithium-ion batteries and beyond.

Anion‐repulsive polyoxometalate@MOF‐modified separators for dendrite‐free and high‐rate lithium batteries

AbstractCommercial polyolefin separators in lithium batteries encounter issues of uncontrolled lithium‐dendrite growth and safety incidents due to their low Li+ transference numbers () and low melting points. To address these challenges, this study proposes an innovative approach by upgrading conventional separators through the incorporation of metal‐organic framework (MOF)‐confined polyoxometalate (POM). The presence of POM restricts anion diffusion through electrostatic repulsion while facilitating Li+ transport within MOF nanochannels through their affinity for lithium ions. Moreover, MOF confinement effectively mitigates the acidification of electrolytes induced by POM. As a proof‐of‐concept, the polypropylene separators decorated with phosphotungstic acid@UIO66 (denoted as PW12@UIO66‐PP) exhibit remarkable lithium‐ion conductivity of 0.78 mS cm−1 with a high of 0.75 at room temperature. The modified separators also display excellent thermal stability, preventing significant shrinkage even at 150°C. Furthermore, Li symmetric cells employing PW12@UIO66‐PP separators exhibit stable cycling for 1000 h, benefiting from rapid Li‐ion transport and uniform deposition. Additionally, the modified separator shows promising adaptability to industrial manufacturing of lithium‐ion batteries, as evidenced by the assembly of a 4 Ah NCM811/graphite pouch cell that retains 97% capacity after 350 cycles at C/3, thus highlighting its potential for practical applications.

Charge Separation Induced by Asymmetric Surface Charge Effects in Quasi‐Solid State Electrolyte for Sustainable Anion Storage

AbstractCompared with conventional lithium‐ion battery systems, anion‐intercalation in graphite cathodes opens avenues for the development of batteries with groundbreaking power density. This study explores the enhancement of dual‐ion batteries (DIBs) by gel polymer electrolyte (GPE) enhanced with surface‐charged nanoclays, focusing on overcoming traditional challenges such as electrolyte decomposition, anion‐solvent co‐intercalation, and interfacial instability at the graphite cathode. Three nanoclays including montmorillonite, kaolinite, and halloysite are compared by incorporating them into GPE and evaluating their effect on the electrochemical properties, ion conduction, and mechanical integrity of DIBs. Particular attention is paid to the halloysite because of its differential internal and external surface charges, which generate charge separation, facilitate optimal lithium‐ion transport, and mitigate undesirable anion‐solvent interactions. These results indicate that halloysite‐incorporated GPE (HS‐G) significantly improves DIB performance, as demonstrated by extended cycle life, robust capacity retention at fast charge/discharge rates of 20 C, and operational stability over a wide temperature range (0–60 °C). These improvements are attributed to the unique structural advantages of HS‐G, including effective charge separation, enhanced anion diffusion, and reduced solvent co‐intercalation, which provide an environmentally friendly and cost‐effective approach to advanced DIB applications.

Anion Selectivities in Zwitterion Grafted Nanopores: Effect of Zwitterion Architecture.

The separation of ions of similar charge is a crucial challenge in many applications, from water treatment to precious metal recovery. Membranes with cross-linked zwitterionic amphiphilic copolymer (ZAC-X) selective layers, which feature self-assembled, zwitterion-lined nanodomains for permeation, offer unique permselectivity between monovalent anions (e.g., Cl-/F-). This has motivated studies on the mechanisms of transport and selectivity in this family of materials. In this study, we conducted molecular dynamics simulations of aqueous salt solutions within zwitterion-functionalized nanopores to elucidate the influence of dipole orientation of the ZI ligands on anion diffusivities, partitioning, and permeabilities. Our model compares systems with contrasting ZI organization: surface-cation-anion (S-ZI+-ZI-, Motif A) and surface-anion-cation (S-ZI--ZI+, Motif B). Our results reveal that Motif A exhibits less pronounced ion pairing due to a spatial separation in the radial profiles of cations and anions. Motif B demonstrates prominent ion pairing for smaller anions owing to their overlap with cation distributions. Further, our potential of mean force profiles reveals that anion partitioning increases with anion size in both ligand motifs, whereas Motif B exhibits significantly higher partitioning selectivity toward larger anions compared to Motif A. Our results for ion diffusivities show that the self-diffusivities of both anions and cations are lower for Motif B compared to Motif A. Such trends in anion partitioning and diffusivities can be explained by differences in the interactions and steric hindrance experienced by the anionic species in Motifs A and B. Finally, our results for anion permselectivity, obtained by combining partitioning and diffusivity, indicate that partitioning trends dominate over diffusivity trends. Consequently, anion permeability increases with anion size, and ligand Motif B yields much higher permselectivity toward larger anions compared to ligand Motif A.

Anode process on gold in KF–AlF<sub>3</sub>–Al<sub>2</sub>O<sub>3</sub> melt

In the conditions of resource saving and carbon footprint reduction, the development of oxygen–releasing anodes for technologies of production of important metals and alloys by electrolysis of molten salts seems to be an urgent task. To determine the degree of “inertness” of a particular anode material, data on the kinetics and mechanism of the anode process on a material not subject to oxidation are required. In this connection, the anodic process on gold in the KF–AlF3–Al2O3 melt for electrolytic aluminum production was investigated by cyclic and square–wave voltammetry methods. The influence of temperature (715 and 775 оC) of the melt, the content of Al2O3 in it (from 0.1 to saturation), as well as the polarization rate (0.05–1 V/s) on the kinetics and some features of the mechanism of the investigated process was determined. An assumption is made that oxygen release on gold without dissolution of the substrate takes place in the region of overvoltages from 0 to 0.8 V. It is shown that the process includes the stages of electrochemical adsorption and desorption of the intermediate product, the first of which is limited by the diffusion of electroactive anions to the anode.

Investigation of Protective Coatings for Reducing High-Temperature Oxidation of Steels

AbstractOxidation of steel billets during pre-heating at elevated temperatures is a challenge, particularly in the forging industry. There is a need to address this problem by applying effective oxidation-resistant coatings on forging billets. In this investigation, different types of commercially available high-temperature, oxidation-resistant coatings, termed Coating A, B, C, D, E, F, and G, were applied on AISI 4340 low-alloy steel by spraying and dipping at room temperature. The oxidation protection behavior of each coating was studied over a wide range of temperatures and holding times in atmospheric conditions. The weight and height losses caused by oxidation were measured, and the samples were subsequently characterized using XRD and SEM-EDS. The protection mechanisms and anti-oxidation properties of the coatings are discussed in detail. The sample coated with Coating A showed the best protection at all temperatures compared to other coatings. The weight/height loss of Coating A-coated steel samples was reduced by 80–90% compared to an uncoated/bare sample. Coating A not only limited the outward diffusion of iron cations from the substrate but also prevented the inward diffusion of oxygen anions from the atmosphere, resulting in better oxidation resistance. The findings of the study should apply to other steels with similar compositions.

Diffusion and structure of propylene carbonate-metal salt electrolyte solutions for post-lithium-ion batteries: From experiment to simulation.

The diffusion of cations in organic solvent solutions is important for the performance of metal-ion batteries. In this article, pulsed field gradient nuclear magnetic resonance experiments and fully atomistic molecular dynamic simulations were employed to study the temperature-dependent diffusive behavior of various liquid electrolytes representing 1M propylene carbonate solutions of metal salts with bis(trifluoromethylsulfonyl)imide (TFSI-) or hexafluorophosphate (PF6-) anions commonly used in lithium-ion batteries and beyond. The experimental studies revealed the temperature dependence of the diffusion coefficients for the propylene carbonate (PC) solvent and for the anions following an Arrhenius type of behavior. It was observed that the PC molecules are the faster species. For the monovalent cations (Li+, Na+, K+), the PC solvent diffusion was enhanced as the cation size increased, while for the divalent cations (Mg2+, Ca2+, Sr2+, Ba2+), the opposite trend was observed, i.e., the diffusion coefficients decreased as the cation size increased. The anion diffusion in LiTFSI and NaTFSI solutions was found to be similar, while in electrolytes with divalent cations, a decrease in anion diffusion with increasing cation size was observed. It was shown that non-polarizable charge-scaled force fields could correspond perfectly to the experimental values of the anion and PC solvent diffusion coefficients in salt solutions of both monovalent (Li+, Na+, K+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations at a range of operational temperatures. Finally, after calculating the radial distribution functions between cations, anions, and solvent molecules, the increase in the PC diffusion coefficient established with the increase in cation size for monovalent cations was clearly explained by the large hydration shell of small Li+ cations, due to their strong interaction with the PC solvent. In solutions with larger monovalent cations, such as Na+, and with a smaller solvation shell of PC, the PC diffusion is faster due to more liberated solvent molecules. In the salt solutions with divalent cations, both the anion and the PC diffusion coefficients decreased as the cation size increased due to an enhanced cation-anion coordination, which was accompanied by an increase in the amount of PC in the cation solvation shell due to the presence of anions.

Predicting anion diffusion in bentonite using hybrid machine learning model and correlation of physical quantities

Radionuclide diffusion will be influenced by numerous factors. Establishing a model that can elucidate the internal correlation between mesoscopic diffusion and the microscopic structure of bentonite can enhance the comprehension of radionuclide diffusion mechanisms. In this study, a light gradient boosting machine (LightGBM) was employed to predict the effective diffusion coefficients of HCrO4−, I−, and CoEDTA2− in bentonite. The model's hyperparameters were optimized using the particle swarm optimization (PSO) algorithm. Several correlated physical quantities, such as mesoscopic parameters (total porosity, rock capacity factor, and ion molar conductivity) and microscopic parameters (ionic radius and montmorillonite stacking number) were incorporated to develop a machine learning model that incorporated micro- and meso-scale features. The predictive performance of PSO-LightGBM was verified using diffusion experiments, which investigated the diffusion of HCrO4−, I−, and CoEDTA2− at compacted dry densities of 1200–1800 kg/m3 using a through-diffusion method. Spearman correlation and Shapley additive explanation analyses revealed that the compacted dry density, ionic diffusion coefficient in water, ionic radius, and total porosity were the top-four influencing factors among the 16 input features. Partial dependence plot analysis elucidated the relationship between the effective diffusion coefficient and each input feature. The analysis results were consistent with the experimental findings, demonstrating the reliability of machine learning. Due to the incorporation of multi-scale features, the PSO-LightGBM model demonstrated enhanced predictive accuracy, linking the microstructure of bentonite to radionuclide diffusion, and providing a comprehensive interpretation of the diffusion mechanism.

Coupling of porosity and diffusive transport in highly reactive systems: Open issues of reactive transport modelling

Mineral precipitation at interfaces between contrasting materials and porewaters decreases the local porosity and consequently reduces transport across the interface. The near-field of waste repositories usually contains reactive interfaces, and the long-term evolution of diffusive transport away from the waste across the near-field is of special interest in this context. Diffusion as well as further mineral reactions significantly slow down when complete porosity clogging is approached, such that any further development is difficult to detect in experiments. But ongoing diffusion, even if very limited, might be significant for relevant times of up to 1 million years. If clogging occurs in clay materials, experimental studies report a much stronger decrease in anion diffusion compared to that of cations or neutral species. Continuum-scale reactive transport codes can predict experimentally observed clogging, but rely on specific implementations based on conceptual assumptions. Typically, the codes calculate porewater-mineral reactions only within a single cell, and they neglect the physical contact of the clogging mineral in one cell with the porewater in an adjacent cell (no inter-cell reactions considered). This can lead to unrealistic model predictions if porosity approaches zero. For instance, neutral pH porewater is permitted to be in contact with clogging cement hydrates in the adjacent cell, and the implied dissolution of the high-pH minerals is not considered anymore. Thus, intrinsically reactive minerals can be converted erroneously into inert, relict phases by such models. We present a new approach that mimics inter-cell reactions. In doing so, clogging is substantially delayed, and no real steady state with the boundary conditions can be attained, in contrast to conventional approaches. Furthermore, we use a dual-porosity reactive transport approach that takes into account the electrostatic effects of charged clay surfaces on diffusion and precipitation, and apply it to concrete-clay interaction. No complete clogging of the pore space occurs, and the predictions well fit experimental data. Modelling studies that predict the separation of a reactive system into two domains by localised full porosity clogging, followed by separate internal equilibration of the domains, are widespread in the literature. This prediction is interpreted as a conceptual artefact of continuum-scale modelling, which should be avoided by allowing for reactions of completely clogged cells with adjacent porewater. For clays, the dual-porosity model may be a viable option in this respect.

Boosting anions storage via union of in-situ N doping and mesoporous hollow hard carbon nanospheres for advanced dual-ion capacitors

High structural strength in-situ N-doped mesoporous hollow hard carbon nanospheres (NMHS) is prepared to overcome the low anions storage capacity (93 mAh g−1) of easy exfoliated graphite for dual-ion capacitors (DICs). The rich mesoporous structure facilitates the anions diffusion, and the hollow cavity provides expansion space in storage and weakens the high diffusion resistance in the nuclear bulge. With the synergistic effect, The NMHS delivers a high reversible capacity of 100 mAh g−1 with a low decay of 8 % over 5000 cycles at 2 A g−1. In-situ Raman reveals the storage mechanism on hard carbon is “adsorption (major)-intercalation (minor)”. DFT calculations decode the effective storage action of graphitic N and pyridinic N. Consequently, the assembled NMHS-DICs exhibit good long-cycle performance and heterotherm stability, providing a high energy/power density of 200 Wh kg−1 and 11858 W kg−1. This work verifies a feasible cathode design strategy with future application potential on DICs.

In Situ Electronic Structure Analysis of Water-Corroded UNx in High Vacuum

Nitriding technologies are promising surface modification techniques of uranium based on pulsed laser irradiating and glow plasma treatment. Nitrided layers with different nitrogen contents (UN0.35, UN0.75, UN1.08 and UN1.5) were prepared on the surface of uranium. The present study aims to investigate the microstructure and corrosion properties of the reaction of the UNx layers with ultra-low water vapor at room temperature. The electronic structures were analyzed in situ by X-ray photoelectron spectroscopy in high vacuum. The results showed that the UN0.35, UN0.75, and UN1.08 samples were mainly composed of uranium nitride (UN) and metallic uranium, while the surface microstructure of the UN1.5 sample was U2N3. The dense and uniform nitride layer with a grain size of 20 to 50 nm was obtained on the uranium surface, which acted as a barrier and prevented the further diffusion of anions into the matrix. The corrosion products of the UN0.35, UN0.75, and UN1.08 samples were mainly UO2-xNy and UO2 after reaction with the water vapor. The contents of UO2-xNy increased with increasing nitrogen contents, and the corrosion rate decreased significantly. The intermediate compounds UO2-xNy reacted slowly with the water vapor, and eventually converted to UO2. Meanwhile, the corrosion products of the UN1.5 sample were mainly U2N3+xOy and UO2-xNy after reaction with the water vapor. The percentage of U2N3+xOy and UO2-xNy remained almost stable over a long period of time, which indicated that the high contents of U2N3+xOy and UO2-xNy prolonged the time for complete conversion to UO2. It can be concluded that the U-N-O ternary compounds retarded the corrosion process and the UNx layers with high nitrogen contents showed excellent corrosion resistance.

Cation Distribution and Anion Transport in the La3Ga5-xGe1+xO14+0.5x Langasite Structure.

Exploration of compositional disorder using conventional diffraction-based techniques is challenging for systems containing isoelectronic ions possessing similar coherent neutron scattering lengths. Here, we show that a multinuclear solid-state Nuclear Magnetic Resonance (NMR) approach provides compelling insight into the Ga3+/Ge4+ cation distribution and oxygen anion transport in a family of solid electrolytes with langasite structure and La3Ga5-xGe1+xO14+0.5x composition. Ultrahigh field 71Ga Magic Angle Spinning (MAS) NMR experiments acquired at 35.2 T offer striking resolution enhancement, thereby enabling clear detection of Ga sites in different coordination environments. Three-connected GaO4, four-connected GaO4 and GaO6 polyhedra are probed for the parent La3Ga5GeO14 structure, while one additional spectral feature corresponding to the key (Ga,Ge)2O8 structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge4+-doped La3Ga3.5Ge2.5O14.75 phase. The complex spectral line shapes observed in the MAS NMR spectra are reproduced very accurately by the NMR parameters computed for a symmetry-adapted configurational ensemble that comprehensively models site disorder. This approach further reveals a Ga3+/Ge4+ distribution across all Ga/Ge sites that is controlled by a kinetically governed cation diffusion process. Variable temperature 17O MAS NMR experiments up to 700 °C importantly indicate that the presence of interstitial oxide ions triggers chemical exchange between all oxygen sites, thereby enabling atomic-scale understanding of the anion diffusion mechanism underpinning the transport properties of these materials.

A Theoretical Study of Oxygen Anion Diffusion through the A-Site Deficient SrTiO3 Lattice Structure: A Machine Learning Approach.

The performance of strontium titanate-based perovskite materials, widely employed as electrode materials for reversible solid oxide cells, is directly characterized by their efficiency and their ability to facilitate the diffusion of generated oxygen ions. A technique frequently employed for enhancing oxygen ion diffusivity involves artificially generating A-site vacancies in these structures. In this study, the molecular-level mechanism of oxygen ion diffusion for a range of A-site deficient structures is extensively investigated using combined molecular dynamics simulations and machine learning-based technique. The analysis of molecular simulation trajectories yields diffusion parameters for the bulk system. Additionally, clustering analysis of time-overlapped locations of oxygen ions provides a spatial distribution of oxygen ion dislocations. Concurrently, tracking the motion of individual oxygen ions elucidates the contribution of each ion to the overall ionic conductance. Overall, the systematic generation of A-site deficiency is found to significantly influence oxygen ion dislocations, thereby impacting the performance of these materials in terms of oxide ion conduction.

Exhaustive Study of Electrical Conductivity in the MNb2−x TixO6–0.5x (M = Mg, Ca, Zn; x = 0, 0.1, 0.2) Columbites

The CaNb2O6 and ZnNb2O6 columbites (Sp.gr. Pbcn) were studied as oxygen ion conductors both theoretically and experimentally. A theoretical approach included geometrical-topological analysis, bond valence site energy (BVSE) and density functional theory (DFT) calculations. The BVSE approach showed the possibility of pure oxygen ions diffusion with migration energies less than 0.45 eV in both compounds. However, DFT calculations indicated the possibility of diffusion of both anions and cations. The single-phases columbites were synthesized by the Pechini method for accurately determine charge carriers type and investigated by impedance spectroscopy, by the Tubandt method, which confirmed the absence of cationic conductivity, and measured the electrical conductivity as a function of oxygen partial pressures. The CaNb2O6 sample was characterized by the pure oxygen-ionic conductivity ∼2 × 10−6 S cm–1 at 800 °C (E a = 0.82 eV), while the ZnNb2O6 had a similar conductivity value due to mixed ionic-electronic contribution (E a = 0.83 eV). The electromotive force method also showed the predominance of the ionic type of conductivity in CaNb2O6, while ZnNb2O6 has a mixed conductivity with ion transport number of about 0.4. Additionally, we synthesized Ti-doped samples MNb2−x Ti x O6–0.5x (M = Mg, Ca; x = 0.1, 0.2) to study the doping effect on conducting properties.

Spatially Resolved Anion Diffusion and Tunable Waveguides in Bismuth Halide Perovskites.

Bismuth halide perovskites are widely regarded as nontoxic alternatives to lead halide perovskites for optoelectronics and solar energy harvesting applications. With a tailorable composition and intriguing optical properties, bismuth halide perovskites are also promising candidates for tunable photonic devices. However, robust control of the anion composition in bismuth halide perovskites remains elusive. Here, we established chemical vapor deposition and anion exchange protocols to synthesize bismuth halide perovskite nanoflakes with controlled dimensions and variable compositions. In particular, we demonstrated the gradient bromide distribution by controlling the anion exchange and diffusion processes, which is spatially resolved by time-of-flight secondary ion mass spectrometry. Moreover, the optical waveguiding properties of bismuth halide perovskites can be modulated by flake thicknesses and anion compositions. With a unique gradient anion distribution and controllable optical properties, bismuth halide perovskites provide new possibilities for applications in optoelectronic devices and integrated photonics.

A comparative study of corrosion mechanisms in recrystallized and stress-relieved Zircaloy-4 by 3D-FIB tomography and ACOM-TEM

The corrosion behavior of recrystallized annealed (RXA) and stress-relieved annealed (SRA) Zircaloy-4 was assessed in 360 ℃ lithiated water for 400 days. SRA Zircaloy-4 with localized corrosion sites below the oxide/metal interface exhibits a much faster out-of-reactor corrosion rate than RXA Zircaloy-4. High-density dislocations can enhance the diffusion of oxygen anions along dislocations, promoting the advancement of corrosive reactions and forming localized corrosion sites. Such a mechanism leads to a less uniform crystallography orientation of monoclinic ZrO2 grains at the oxide/metal interface of SRA Zircaloy-4 compared to its RXA counterpart, resulting in higher compressive stress and oxide cracking rates.

Thermal distribution evolution model of SEI in lithium metal anodes

Lithium metal batteries are considered an advantageous choice for high energy density batteries due to their extremely high theoretical specific capacity. However, the interface instability has always been the biggest challenge for the commercial development of lithium metal batteries. The evolution mechanism of the thermal field is a key factor affecting the cyclic life during the evolution of lithium metal interfaces. Herein, the interface evolution mechanism for the impact of thermal distribution on Li dendrites is revealed and quantified by the thermal distribution evolution model of the solid electrolyte interphase (SEI). Three influencing factors are outlined as follows: 1) the ratio of the SEI to electrolyte diffusion capability. 2) The SEI diffusion coefficients for both anionic and cationic species. 3) The anisotropy in the SEI. The results indicate that under the appropriate ratio, the temperature gradient at the tip of lithium dendrites and the SEI-electrolyte interface are lower, which is favorable for a more even distribution of heat within the SEI. Additionally, the diffusion of anions can also influence the temperature gradient during the evolution process of lithium dendrites. Furthermore, introducing anisotropy to the SEI can induce lateral growth of lithium dendrites and reduce the temperature gradient at the tip of the lithium dendrites and the SEI-electrolyte interface. This work provides valuable insights for the interface design of lithium metal batteries.

Onset of Nitriles Hydration with an Environmentally Benign Catalyst: in-Water versus on-Water Conditions.

The reaction yield of nitriles hydration using a catalyst depends on the aqueous medium. Using ab initio molecular dynamics, we probed whether "in-water" (in bulk medium) or "on-water" (at the interface with vacuum) conditions can change the onset of the reaction. Investigating a hydrogen-bond mediated mechanism, the lifetimes of the intermolecular interaction between benzonitrile and choline in the two protocols were compared, and the diffusion of the hydroxide anion around the cyano group was discussed.

Electrochemical intercalation of anions into graphite: Fundamental aspects, material synthesis, and application to the cathode of dual-ion batteries.

In this review, fundamental aspects of the electrochemical intercalation of anions into graphite have been first summarized, and then described the electrochemical preparation of covalent-type GICs and application of graphite as the cathode of dual-ion battery. Electrochemical overoxidation of anion GICs provides graphite oxide and covalent-fluorine GICs, which are key functional materials for various applications including energy storage devices. The reaction conditions to obtain fully oxidized graphite has been mentioned. Concerning the application of graphite for the cathode of dual-ion battery, it stably delivers about 110 mA h g-1 of reversible capacity in usual organic electrolyte solutions. The combination of anion and solvent as well as the concentration of the anions in the electrolyte solutions greatly affect the performance of graphite cathode such as oxidation potential, rate capability, cycling properties, etc. The interfacial phenomenon is also important, and fundamental studies of charge transfer resistance, anion diffusion coefficient, and surface film formation behavior have also been summarized. The use of smaller anions, such as AlCl4 -, Br- can increase the capacity of graphite cathode. Several efforts on the structural modification of graphite and development of electrolyte solutions in which graphite cathode delivers higher capacity were also described.

A high-current hydrogel generator with engineered mechanoionic asymmetry.

Mechanoelectrical energy conversion is a potential solution for the power supply of miniaturized wearable and implantable systems; yet it remains challenging due to limited current output when exploiting low-frequency motions with soft devices. We report a design of a hydrogel generator with mechanoionic current generation amplified by orders of magnitudes with engineered structural and chemical asymmetry. Under compressive loading, relief structures in the hydrogel intensify net ion fluxes induced by deformation gradient, which synergize with asymmetric ion adsorption characteristics of the electrodes and distinct diffusivity of cations and anions in the hydrogel matrix. This engineered mechanoionic process can yield 4 mA (5.5 A m-2) of peak current under cyclic compression of 80 kPa applied at 0.1 Hz, with the transferred charge reaching up to 916 mC m-2 per cycle. The high current output of this miniaturized hydrogel generator is beneficial for the powering of wearable devices, as exemplified by a controlled drug-releasing system for wound healing. The demonstrated mechanisms for amplifying mechanoionic effect will enable further designs for a variety of self-powered biomedical systems.