Nanochannels regulating ionic transport for boosting electrochemical energy storage and conversion: a review

Fiber-Based Composite Separator with Stable Interface and High Ionic Transport Capacity for Lithium Batteries

Fiber-based separators are generally designed with particle composites to address the internal shorts brought by the oversized pores of the fiber layer, which typically results in high resistance due to poor interfacial compatibility and high thickness and is detrimental to the lithium battery performance. Herein, we proposed a fibrous composite separator by easily coating a nonwoven fabric with a mixture of cross-linked poly(vinyl alcohol) and silica nanoparticles. Owing to the interlaced fiber network and abundant polar groups, the prepared separator exhibits excellent electrolyte wettability, low internal resistance, and great interfacial compatibility and stability. The fiber-based separator performs better than the polyolefin separator in terms of ionic conductivity, lithium-ion transference number, and lithium dendrite inhibition. Consequently, the fiber-based separator enables the LiFePO4//Li battery to cycle steadily with a capacity retention rate of 99.1% after 100 cycles and 82.2% after 500 cycles and also helps the Li//Li symmetrical cell to cycle stably for 800 h at 0.5 mA cm-2. Therefore, the fibrous composite separator that can be scaled up might be a competitive substitute for lithium separators that are already on the market.

Robust, High-Temperature-Resistant Polyimide Separators with Vertically Aligned Uniform Nanochannels for High-Performance Lithium-Ion Batteries

Separator is an essential component of lithium-ion batteries (LIBs), playing a pivotal role in battery safety and electrochemical performance. However, conventional polyolefin separators suffer from poor thermal stability and nonuniform pore structures, hindering their effectiveness in preventing thermal shrinkage and inhibiting lithium (Li) dendrites. Herein, we present a robust, high-temperature-resistant polyimide (PI) separator with vertically aligned uniform nanochannels, fabricated via ion track-etching technology. The resultant PI track-etched membranes (PITEMs) effectively homogenize Li-ion distribution, demonstrating enhanced ionic conductivity (0.57 mS cm-1) and a high Li+ transfer number (0.61). PITEMs significantly prolong the cycle life of Li/Li cells to 1200 h at 3 mA cm-2. For Li/LiFePO4 cells, this approach enables a specific capacity of 143 mAh g-1 and retains 83.88% capacity after 300 cycles at room temperature. At 80 °C, the capacity retention remains at 85.92% after 200 cycles. Additionally, graphite/LiFePO4 pouch cells with PITEMs display enhanced cycling stability, retaining 73.25% capacity after 1000 cycles at room temperature and 78.41% after 100 cycles at 80 °C. Finally, PITEMs-based pouch cells can operate at 150 °C. This separator not only addresses the limitations of traditional separators, but also holds promise for mass production via roll-to-roll methods. We expect this work to offer insights into designing and manufacturing of functional separators for high-safety LIBs.

Microfiber/Nanofiber/Attapulgite Multilayer Separator with a Pore-Size Gradient for High-Performance and Safe Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have an extremely diverse application nowadays as an environmentally friendly and renewable new energy storage technology. The porous structure of the separator, one essential component of LIBs, provides an ion transport channel for the migration of ions and directly affects the overall performance of the battery. In this work, we fabricated a composite separator (GOP-PH-ATP) via simply laminating an electrospun polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) nanofibrous membrane coated with attapulgite (ATP) nanoparticles onto a PP nonwoven microfibrous fabric, which exhibits a unique porous structure with a pore-size gradient along the thickness direction that ranges from tens of microns to hundreds of nanometers. As a result, besides the enhanced thermal stability given by the chosen materials, the GOP-PH-ATP separator was endowed with a superhigh porosity of ~95%, strong affinity with electrolyte, and great electrolyte uptake of ~760%, thus effectively enabling an ionic conductivity of 2.38 mS cm-1 and a lithium-ion transference number of 0.62. Furthermore, the cell with the GOP-PH-ATP separator shows an excellent cycling performance with a capacity retention of 91.2% after 150 cycles at 1 C, suggesting that the composite separator with a pore-size gradient structure has great potential to be applied in LIBs.

Modulation mechanism of ionic transport through short nanopores by charged exterior surfaces

Short nanopores have various applications in biosensing, desalination, and energy conversion. Here, the modulation of ionic transport by charged exterior surfaces is investigated through simulations with sub-200 nm long nanopores under applied voltages. Detailed analysis of the ionic current, electric field strength, and fluid flow inside and outside nanopores reveals that charged exterior surfaces can increase ionic conductance by increasing both the concentration and migration speed of charge carriers. The electric double layers near charged exterior surfaces provide an ion pool and an additional passageway for counterions, which lead to enhanced exterior surface conductance and ionic concentrations at pore entrances and inside the nanopores. We also report that charges on the membrane surfaces increase the electric field strength inside nanopores. The effective width of a ring with surface charges placed at pore entrances (Lcs) is considered as well by studying the dependence of the current on Lcs. We find a linear relationship between the effective Lcs and the surface charge density and voltage, and an inverse relationship between the geometrical pore length and salt concentration. Our results elucidate the modulation mechanism of ionic transport through short nanopores by charged exterior surfaces, which is important for the design and fabrication of porous membranes.

Layer-by-Layer Nanofluidic Membranes for Promoting Blue Energy Conversion

Access to and use of energy resources are now crucial components of modern human existence thanks to the exponential growth of technology. Traditional energy sources provide significant challenges, such as pollution, scarcity, and excessive prices. As a result, there is more need than ever before to replace depleting resources with brand-new, reliable, and environmentally friendly ones. With the aid of reverse electrodialysis, the salinity gradient between rivers and seawater as a clean supply with easy and infinite availability is a viable choice for energy generation. The development of nanofluidic-based reverse electrodialysis (NRED) as a novel high-efficiency technology is attributable to the progress of nanoscience. However, understanding the predominant mechanisms of this process at the nanoscale is necessary to develop and disseminate this technology. One viable option to gain insight into these systems while saving expenses is to employ simulation tools. In this study, we looked at how a layer-by-layer (LBL) soft layer influences ion transport and energy production in charged nanochannels. We solved the steady-state Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations for three different types of nanochannels with a trumpet geometry, where the narrow part is covered with a built-up LbL soft layer and the rest is a hard wall with a surface charge density of σ = -10, 0, or +10 mC/m2. The findings show that in type (I) nanochannels, at NPEL/NA = 100 mol/m3 and pH = 7, the maximum power output rises 675-fold as the concentration ratio rises from 10 to 1000. The results of this study can aid in a better understanding of energy harvesting processes using nanofluidic-based reverse electrodialysis in order to identify optimal conditions for the design of an intelligent route with great controllability and minimal pollution.

Cellulose-Based Ionic Conductor: An Emerging Material toward Sustainable Devices

Ionic conductors (ICs) find widespread applications across different fields, such as smart electronic, ionotronic, sensor, biomedical, and energy harvesting/storage devices, and largely determine the function and performance of these devices. In the pursuit of developing ICs required for better performing and sustainable devices, cellulose appears as an attractive and promising building block due to its high abundance, renewability, striking mechanical strength, and other functional features. In this review, we provide a comprehensive summary regarding ICs fabricated from cellulose and cellulose-derived materials in terms of fundamental structural features of cellulose, the materials design and fabrication techniques for engineering, main properties and characterization, and diverse applications. Next, the potential of cellulose-based ICs to relieve the increasing concern about electronic waste within the frame of circularity and environmental sustainability and the future directions to be explored for advancing this field are discussed. Overall, we hope this review can provide a comprehensive summary and unique perspectives on the design and application of advanced cellulose-based ICs and thereby encourage the utilization of cellulosic materials toward sustainable devices.

Functional Separator Enabled by Covalent Organic Frameworks for High-Performance Li Metal Batteries

Uncontrolled ion transport and susceptible SEI films are the key factors that induce lithium dendrite growth, which hinders the development of lithium metal batteries (LMBs). Herein, a TpPa-2SO₃ H covalent organic framework (COF) nanosheet adhered cellulose nanofibers (CNF) on the polypropylene separator (COF@PP) is successfully designed as a battery separator to respond to the aforementioned issues. The COF@PP displays dual-functional characteristics with the aligned nanochannels and abundant functional groups of COFs, which can simultaneously modulate ion transport and SEI film components to build robust lithium metal anodes. The Li//COF@PP//Li symmetric cell exhibits stable cycling over 800 h with low ion diffusion activation energy and fast lithium ion transport kinetics, which effectively suppresses the dendrite growth and improves the stability of Li+ plating/stripping. Moreover, The LiFePO4//Li cells with COF@PP separator deliver a high discharge capacity of 109.6 mAh g^-1 even at a high current density of 3 C. And it exhibits excellent cycle stability and high capacity retention due to the robust LiF-rich SEI film induced by COFs. This COFs-based dual-functional separator promotes the practical application of lithium metal batteries.

Densifiable Ink Extrusion for Roll-To-Roll Fiber Lithium-Ion Batteries with Ultra-High Linear and Volumetric Energy Densities

Conventional bulky and rigid planar architecting power systems are difficult to satisfy the growing demand for wearable applications. 1D fiber batteries bearing appealing features of miniaturization, adaptability, and weavability represent a promising solution, yet challenges remain pertaining to energy density and scalability. Herein, an ingenious densifiable functional ink is invented to fabricate scalable, flexible, and high-mass-loading fiber lithium-ion batteries (LIBs) by adopting a fast ink-extrusion technology. In the formulated ink, pyrrole-modified reduced graphene oxide is elaborately introduced and exerts multiple influences; it not only assembles carbon nanotubes and poly(vinylidene fluoride-co-hexafluoropropylene) to compose a sturdy, conductive, and agglomeration-free 3D network that realizes an ultra-high content (75 wt%) of the active materials and endows the electrode excellent flexibility but also serves as a capillary densification inducer, encouraging an extremely large linear mass loading (1.01 mg cm^-1 per fiber) and packing density (782.1 mg cm^-3 ). As a result, the assembled fiber LIBs deliver impressive linear and volumetric energy densities with superb mechanical compliance, demonstrating the best performance among all the reported extruded fiber batteries. This work highlights a highly effective and facile approach to fabricate high-performance fiber energy storage devices for future practical wearable applications.

Specific Ion and Electric Field Controlled Diverse Ion Distribution and Electroosmotic Transport in a Polyelectrolyte Brush Grafted Nanochannel

Controlling ion distribution inside a charged nanochannel is central to using such channels in diverse applications. Here, we show the possibility of using a charged polyelectrolyte (PE) brush-grafted nanochannel for triggering diverse nanoscopic ion distribution and nanofluidic electroosmotic transport by controlling the valence and size of the counterions (that screen the charges of the PE brushes) and the strength of an externally applied axial electric field. We atomistically simulate separate cases of fully charged polyacrylic acid (PAA) brush functionalized nanochannels with Na⁺, Cs⁺, Ca²⁺, Ba²⁺, and Y³⁺ counterions screening the PE charges. Four key findings emerge from our simulations. First, we find that the counterions with a greater valence and a smaller size prefer to remain localized inside the brush layer. Second, for the case where there is an added chloride salt with the same cation (as the screening counterions), there are more coions (Cl^- ions) in the brush-free bulk than counterions (for counterions Na⁺, Ca²⁺, Ba²⁺, Y³⁺): this is a manifestation of the overscreening (OS) of the PE brush layer. Contrastingly, the number of Cs⁺ ions remain higher than the Cl^- ions inside the brush-free bulk, ensuring that there is no OS effect for this case. Third, large applied electric field enables a few Na⁺, Cs⁺, and Ba²⁺ counterions to leave the brush layer and to go to the bulk: this makes the OS of the PE brush layer disappear for the cases of PE brushes being screened by the Na⁺ and Ba²⁺ ions. On the other hand, no such electric-field-mediated disappearance of OS is observed for the cases of Ca²⁺ and Y³⁺ screening counterions; we attribute this to the firm attachment of these counterions to the negatively charged monomers. Free energy associated with a counterion binding to a PE chain corroborates this diversity in the counterion-specific response to the applied electric field. Finally, we demonstrate that such diverse ion distributions, along with specific electric-field-strength-dependent ion properties, lead to (1) electroosmotic (EOS) transport in nanochannels grafted with PAA brushes screened with Cs⁺ ions to be always counterion dominated, (2) EOS transport in nanochannels grafted with PAA brushes screened with Ca²⁺ and Y³⁺ ions to be always coion-dominated, and (3) EOS transport in nanochannels grafted with PAA brushes screened with Na⁺ and Ba²⁺ ions to be coion dominated for smaller electric fields and counterion dominated for larger electric fields.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Energy Harvesting: Roles of Nanochannel Geometry and Bipolar Soft Layer

Researchers are looking for new, clean, and accessible sources of energy due to rising global warming caused by the usage of fossil fuels and the irreversible harm that this does to the environment. Water salinity is one of the newest and most accessible renewable energy sources, which has sparked a lot of interest. Reverse electrodialysis (RED) has been utilized in the past to turn saline water into electricity. NRED, a reverse electrodialysis method utilizing nanofluidics, has gained popularity as nanoscale research advances. Developing and evaluating NRED systems is time-consuming and expensive due to the method's novelty; thus, modeling is required to identify the best locations for implementation and to comprehend its workings. In this work, we examined the influence of bipolar soft layer and nanochannel geometry on ion transfer and power production simultaneously. To achieve this, the two trumpet and cigarette geometries were coated with a bipolar soft layer so that both negative (type (I)) and positive (type (II)) charges could be positioned in the nanochannel's small aperture. After that, at steady state conditions, the Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations were solved concurrently. The findings revealed that altering the nanochannel coating from type (I) to type (II) alters the channel's selectivity from cations to anions. An approximately 22-fold improvement in energy conversion efficiency was achieved by raising the concentration ratio from 10 to 100 for the type (I) trumpet nanochannel. Type (I) cigarette geometry is advised for maximum power output at low and medium concentration ratios, whereas type (I) trumpet geometry is recommended for the maximum power production at high concentration ratios.

Nanoionics from Biological to Artificial Systems: An Alternative Beyond Nanoelectronics

Ion transport under nanoconfined spaces is a ubiquitous phenomenon in nature and plays an important role in the energy conversion and signal transduction processes of both biological and artificial systems. Unlike the free diffusion in continuum media, anomalous behaviors of ions are often observed in nanostructured systems, which is governed by the complex interplay between various interfacial interactions. Conventionally, nanoionics mainly refers to the study of ion transport in solid-state nanosystems. In this review, to extent this concept is proposed and a new framework to understand the phenomena, mechanism, methodology, and application associated with ion transport at the nanoscale is put forward. Specifically, here nanoionics is summarized into three categories, i.e., biological, artificial, and hybrid, and discussed the characteristics of each system. Compared with nanoelectronics, nanoionics is an emerging research field with many theoretical and practical challenges. With this forward-looking perspective, it is hoped that nanoionics can attract increasing attention and find wide range of applications as nanoelectronics.

Biomimetic Nanochannels: From Fabrication Principles to Theoretical Insights

Biological nanochannels which can regulate ionic transport across cell membranes intelligently play a significant role in physiological functions. Inspired by these nanochannels, numerous artificial nanochannels have been developed during recent years. The exploration of smart solid-state nanochannels can lay a solid foundation, not only for fundamental studies of biological systems but also practical applications in various fields. The basic fabrication principles, functional materials, and diverse applications based on artificial nanochannels are summarized in this review. In addition, theoretical insights into transport mechanisms and structure-function relationships are discussed. Meanwhile, it is believed that improvements will be made via computer-guided strategy in designing more efficient devices with upgrading accuracy. Finally, some remaining challenges and perspectives for developments in both novel conceptions and technology of this inspiring research field are stated.

Crystal Channel Engineering for Rapid Ion Transport: From Nature to Batteries

Ion transport behaviours through cell membranes are commonly identified in biological systems, which are crucial for sustaining life for organisms. Similarly, ion transport is significant for electrochemical ion storage in rechargeable batteries, which has attracted much attention in recent years. Rapid ion transport can be well achieved by crystal channels engineering, such as creating pores or tailoring interlayer spacing down to the nanometre or even sub-nanometre scale. Furthermore, some functional channels, such as ion selective channels and stimulus-responsive channels, are developed for smart ion storage applications. In this review, the typical ion transport phenomena in the biological systems, including ion channels and pumps, are first introduced, and then ion transport mechanisms in solid and liquid crystals are comprehensively reviewed, particularly for the widely studied porous inorganic/organic hybrid crystals and ultrathin inorganic materials. Subsequently, recent progress on the ion transport properties in electrodes and electrolytes is reviewed for rechargeable batteries. Finally, current challenges in the ion transport behaviours in rechargeable batteries are analysed and some potential research approaches, such as bioinspired ultrafast ion transport structures and membranes, are proposed for future studies. It is expected that this review can give a comprehensive understanding on the ion transport mechanisms within crystals and provide some novel design concepts on promoting electrochemical ion storage capability in rechargeable batteries.

Recent advances in polymeric nanostructured ion selective membranes for biomedical applications

Nano structured ion-selective membranes (ISMs) are very attractive materials for a wide range of sensing and ion separation applications. The present review focuses on the design principles of various ISMs; nanostructured and ionophore/ion acceptor doped ISMs, and their use in biomedical engineering. Applications of ISMs in the biomedical field have been well-known for more than half a century in potentiometric analysis of biological fluids and pharmaceutical products. However, the emergence of nanotechnology and sophisticated sensing methods assisted in miniaturising ion-selective electrodes to needle-like sensors that can be designed in the form of implantable or wearable devices (smartwatch, tattoo, sweatband, fabric patch) for health monitoring. This article provides a critical review of recent advances in miniaturization, sensing and construction of new devices over last decade (2011-2021). The designing of tunable ISM with biomimetic artificial ion channels offered intensive opportunities and innovative clinical analysis applications, including precise biosensing, controlled drug delivery and early disease diagnosis. This paper will also address the future perspective on potential applications and challenges in the widespread use of ISM for clinical use. Finally, this review details some recommendations and future directions to improve the accuracy and robustness of ISMs for biomedical applications.

Two-dimensional materials towards separator functionalization in advanced Li-S batteries

Functional separators have played important roles in improving the electrochemical performance of lithium-sulfur (Li-S) batteries by addressing the key issues of both the sulfur cathode and lithium anode. Compared with other materials that are used for separator functionalization, two-dimensional (2D) materials with atomic layer thickness and infinite lateral dimensions feature several advantages of ultra-thin laminate structure, remarkable physical properties and tunable surface chemistry, which show potential applications in separator functionalization towards addressing the issues of both the shuttle effect and formation of Li dendrites in Li-S batteries. In this review, the unique advantages of 2D materials for separator functionalization in Li-S batteries and their common construction methods are introduced. Then, recent progress and advances in the construction of 2D materials functional separators are summarized in detail towards inhibiting the shuttle effect of polysulfides and suppressing Li dendrite growth in Li-S batteries. Finally, some opportunities and challenges of 2D materials for constructing high-performance functional separators are proposed. We anticipate that this review will provide new insights into separator functionalization for developing advanced Li-S batteries.

Titanium Dioxide Derived Materials with Superwettability

Titanium dioxide (TiO2) is widely used in various fields both in daily life and industry owing to its excellent photoelectric properties and its induced superwettability. Over the past several decades, various methods have been reported to improve the wettability of TiO2 and plenty of practical applications have been developed. The TiO2-derived materials with different morphologies display a variety of functions including photocatalysis, self-cleaning, oil-water separation, etc. Herein, various functions and applications of TiO2 with superwettability are summarized and described in different sections. First, a brief introduction about the discovery of photoelectrodes made of TiO2 is revealed. The ultra-fast spreading behaviors on TiO2 are shown in the part of ultra-fast spreading with superwettability. The part of controllable wettability introduces the controllable wettability of TiO2-derived materials and their related applications. Recent developments of interfacial photocatalysis and photoelectrochemical reactions with TiO2 are presented in the part of interfacial photocatalysis and photoelectrochemical reactions. The part of nanochannels for ion rectification describes ion transportation in nanochannels based on TiO2-derived materials. In the final section, a brief conclusion and a future outlook based on the superwettability of TiO2 are shown.

Cation-Selective Separators for Addressing the Lithium-Sulfur Battery Challenges

Lithium-sulfur batteries (LSBs) have become one of the most promising candidates for next-generation energy storage systems owing to their high theoretical energy density, environmental friendliness, and cost effectiveness. However, real-word applications are seriously restricted by an undesirable shuttle effect and Li dendrite formation. In essence, uncontrollable anion transport is a key factor that causes both polysulfide shuttling and dendrite formation, which creates the possibility of simultaneously addressing the two critical issues in LSBs. An effective strategy to control anion transport is the construction of cation-selective separators. Significant progress has been achieved in the inhibition of the shuttle effect, whereas addressing the problem of Li dendrite formation by utilizing a cation-selective separator is still under way. From this viewpoint, this Review analyzes critical issues with regard to the shuttle effect and Li dendrite formation caused by uncontrollable anion transport, based on which roles and advantages of cation-selective separators toward high-performance LSBs are presented. According to the separator-construction principle, the latest advances and progress in cation-selective separators in inhibiting the shuttle effect and Li dendrite formation are reviewed in detail. Finally, some challenges and prospects are proposed for the future development of cation-selective separators. This Review is anticipated to provide a new perspective for simultaneously addressing the two critical issues in LSBs.

Functional separators towards the suppression of lithium dendrites for rechargeable high-energy batteries

Lithium metal battery (LMB) is considered to be one of the most promising electrochemical energy storage devices due to the high theoretical specific capacity and the lowest redox potential of metallic lithium; however, some key issues caused by lithium dendrites on the lithium metal anode seriously hinder its real-world applications. As an indispensable part of LMBs, the separator could serve as a physical barrier to prevent direct contact of the two electrodes and control ionic transport in batteries; it is an ideal platform for the suppression of lithium dendrites. In this review, the mechanism of lithium dendrite nucleation and growth are firstly discussed and then some advanced techniques are introduced for the precise characterization of lithium dendrites. On the basis of dendritic nucleation and growth principle, several feasible strategies are summarized for suppressing lithium dendrites by utilizing functional separators, including providing a mechanical barrier, promoting homogeneous lithium deposition, and regulating ionic transport. Finally, some challenges and prospects are proposed to clear the future development of functional separators. We anticipate that this paper will provide a new insight into the design and construction of functional separators for addressing the issues of lithium dendrites in high-energy batteries.