The growing popularity of smart electronics in wearables, the Internet of Things (IoT), soft robotics, and biomedical implants simultaneously demands more reliable and durable power sources. However, limitations on battery life continue to compromise reliability, prompting the search for sustainable solutions for flexible, self-powered systems. In this work, stretchable self-powered piezoelectric nanogenerators have been designed from functionalized piezoelectric nanofibers with a bioinspired coiled helical microstructure. Composed of two-dimensional (2D) Ti₃C₂T_x MXene and silver nanoparticles (AgNPs) embedded in a poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) matrix, the coiled structure achieves a mechanoelectrical energy conversion efficiency of 17%, and a power output of 6.6 mW cm⁻³ at 50% strain, twice the performance of similarly coiled structures. These improvements were attributed to the threefold increase in the piezoelectric coefficient through the addition of 1 wt% AgNPs to the P(VDF-TrFE)/MXene (0.1 wt%) and the coiled structure further enhancing β-phase formation reaching up to 70%. An electrospun mat sensor with dimensions of 2 × 3 cm generated 3 V at 1 Hz under an applied pressure of 7 kPa. The coil compact and lightweight design enables seamless integration into miniaturized electronics and wearable biomedical devices, promising a sustainable, battery-free power solution.

Nanoconfined water exhibits unique properties compared to bulk water due to limited quantities, frustrated hydrogen bonding, and surface interactions, which are fundamental for energy storage and transport applications. We integrate machine learning–accelerated ab initio molecular dynamics with x-ray diffraction (XRD) and inelastic neutron scattering (INS) to systematically analyze the thermodynamic and dynamic behavior of water confined between functionalized (-F, -O, and -OH) two-dimensional (2D) Ti₃C₂T_x MXene layers. As water intercalates between layers, the interlayer spacing exhibits layer-dependent staging characteristics. The water polarization can be flipped by the count and morphology of intercalated molecules interacting with MXene surface groups, resulting in varying electrostatic potential profiles. On the basis of interfacial electrostatic potential, hydrogen bond lifetime, and molecular orientation, we establish a linear combination of exponential model describing water diffusivity. These computational insights align well with experimental x-ray and neutron measurements, suggesting strategies for tuning water morphology and transport by tailoring MXene surface chemistry and water content for electrochemical energy storage and nanofluidic applications.

All-solid-state batteries have great potential to outperform conventional lithium-ion batteries in both safety and energy density, as the solid electrolyte can potentially accommodate high-energy-density anodes such as metallic lithium or silicon more safely. However, the high-valence cations present in most highly conductive solid electrolytes facilitate reductive decomposition at low potentials, leading to significant irreversible lithium inventory loss. Preventing this requires the development of solid electrolytes that are thermodynamically stable at low operating potentials while providing high ionic conductivity and sufficient oxidative stability. To realize this, we explored a new family of Li-rich antifluorite irreducible solid electrolytes, Li2.65S0.35NxP0.65–x, the first reported nitrido-phosphido-sulfide, and investigated their application in all-solid-state batteries. The optimized composition Li2.65S0.35N0.15P0.5 possesses a remarkably high ionic conductivity of 1.05 mS cm–1, as well as a relatively high oxidative stability of 1.15 V vs Li+/Li for this class of materials. Ab initio molecular dynamics and density functional theory simulations reveal that enhanced Li diffusion is the result of enlarged diffusion bottleneck sizes. These are a consequence of (i) substitution with smaller anions or (ii) increased electrostatic repulsion from the substitution with high-valence anions. Importantly, the oxidative stability makes Li2.65S0.35N0.15P0.5 exhibit good compatibility with Si anodes, and in conjunction with the high ionic conductivity, this enables a high initial Coulombic efficiency of 94.2% as well as a stable cycle life of a full cell with a micron silicon–Li2.65S0.35N0.15P0.5 anode and a LiCoO2–Li3InCl6 cathode. This work highlights the potential of irreducible solid electrolytes for the design of all-solid-state batteries with low-potential and high-energy-density anodes.

MXenes exhibit considerable potential for developing high-performance electromagnetic (EM) shielding and absorption materials operating across microwave and terahertz frequencies, due to their tunable surface chemistry and exceptional charge carrier transport properties. Nevertheless, a profound understanding and precise manipulation of their broadband attenuation mechanisms remain challenging. In this review, we first examine Ti₃C₂T_x MXene as a representative system to explore EM attenuation mechanisms through polarization and conductive loss models across microwave and terahertz bands. We then discuss tuning strategies, including component tailoring, interlayer regulation, film architecture, and dynamic modulation, which are supported by both classic and emerging studies, and evaluate their impact on attenuation performance. Finally, we outline future research priorities and development directions for MXene-based EM attenuation materials. By synthesizing recent advances, this review aims to establish the structure–property relationships in MXenes and to provide forward-looking insights for the field.

MXenes, a thriving class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, demonstrate considerable potential in diverse electrochemical energy storage applications. To leverage MXenes for high-performance sulfur-based batteries, researchers have employed various strategies to modify their properties, aiming to tackle challenges such as the notorious shuttle effect induced by soluble polysulfides, sluggish redox reaction kinetics, and substantial volume expansion during the lithiation process. This review article offers an overview of MXene modification strategies, emphasizing their significant potential in adjusting the composition, surface chemistry, and morphology to address one or more challenges specially in sulfur cathodes. We first discuss internal regulation methods of MXene, including surface group engineering, heteroatom doping, and high-entropy MXene synthesis, which have been demonstrated to enhance MXene-polysulfide interactions and facilitate polysulfide conversion. Subsequently, we provide a summary of the recent design methods and advancements made in MXene-derived and MXene-based composites, with a particular emphasis on electronic structure reconstruction at the heterointerface and their synergistic roles in Li-S batteries. Following this, we outline the utilization of MXenes to address the challenges encountered in metal-sulfur batteries beyond Li-S batteries. Concluding the review, we offer prospects for the future development of utilizing MXenes in practical sulfur-based batteries.

Despite the development of various pseudocapacitive materials, full-cell pseudocapacitors have yet to surpass the power density of conventional electric double layer capacitors, primarily due to the lack of high-rate positive pseudocapacitive materials. This work reports a solid-state conjugated polyelectrolyte that achieves high-rate charge storage as a positive electrode, facilitated by a co-ion desorption mechanism. The conjugated polyelectrolyte retains 70% of its capacitance at 100 A g⁻¹ with a mass loading of 2.8 mg cm⁻² and exhibits a long cycling life of 100,000 cycles in a Swagelok cell configuration. Increasing the electrode thickness fourfold has minimal impact on ion diffusivity and accessibility, yielding a high areal capacitance of 915 mF cm⁻². When paired with a high-rate negative pseudocapacitive electrode Ti₃C₂T_x, the device leverages the redox-active potentials of both materials, achieving a device voltage of 1.5 V and supports operation rates up to 10 V s⁻¹ or 50 A g⁻¹. This configuration enables the pseudocapacitor to deliver an areal power of 160 mW cm⁻², while significantly increasing the areal energy (up to 71 μWh cm⁻²). The high areal performance, combined with the additive-free and water-based fabrication process, makes pseudocapacitors promising for on-chip and wearable energy storage applications.

Anode-free aqueous zinc metal batteries (AZMBs) offer significant potential for energy storage due to their low cost and environmental benefits. Ti₃C₂T_x MXene provides several advantages over traditional metallic current collectors like Cu and Ti, including better Zn plating affinity, lightweight, and flexibility. However, self-freestanding MXene current collectors in AZMBs remain underexplored, likely due to challenges with Zn deposition reversibility. This study investigates the combination of a Ti₃C₂T_x self-freestanding film with advanced electrolyte engineering, specifically examining the effects of Li-salt and propylene carbonate (PC) as additives on Zn plating reversibility. While using Li⁺ ions as an additive alone facilitates uniform Zn deposition on bulk metals through the electrostatic shielding effect, the addition of Li-salt negatively impacts Zn plating uniformity on Ti₃C₂T_x. Meanwhile, using PC additive alone forms an organic SEI layer on Ti₃C₂T_x and causes Zn agglomeration. The use of both additives together results in a ZnF₂-containing hybrid SEI layer with improved interfacial kinetics, promoting more uniform Zn deposition. This approach achieves an average Coulombic efficiency (CE) of 96.8% over 150 cycles (a maximum CE of 97.8%). The study highlights the strategic difference in electrolyte design, emphasizing the need for tailored approaches to optimize Zn deposition on MXenes, contrasting with traditional metallic current collectors.

The growing demand for safe, cost-efficient, high-energy and high-power electrochemical energy storage devices has stimulated the development of aqueous-based supercapacitors with high capacitance, high rate capability, and high voltage. 2D titanium carbide MXene-based electrodes have shown excellent rate capability in various dilute aqueous electrolytes, yet their potential window is usually narrower than 1.2 V. In this study, we show that the potential window of Ti₃C₂T _x MXene can be efficiently widened to 1.5 V in a cost-effective and environmentally benign polyethylene glycol (PEG) containing molecular crowding electrolyte. Additionally, a pair of redox peaks at −0.25 V/−0.05 V vs. Ag (cathodic/anodic) emerged in cyclic voltammetry after the addition of PEG, yielding an additional 25% capacitance. Interestingly, we observed the co-insertion of the molecular crowding agent PEG-400 during the Li⁺ intercalation process based on in-situ x-ray diffraction analysis. As a result, Ti₃C₂T _x electrodes presented an interlayer space change of 4.7 Å during a complete charge/discharge cycle, which is the largest reversible interlayer space change reported so far for MXene-based electrodes. This work demonstrates the potential of adding molecular crowding agents to improve the performance of MXene electrodes in aqueous electrolytes and to enlarge the change of the interlayer spacing.

Titanium carbide MXene, Ti₃C₂T_x, exhibits ultrahigh capacitance in acidic electrolytes at negative potentials yet poor stability at positive potentials, resulting in low-energy densities for Ti₃C₂T_x-based symmetric supercapacitors. Utilizing “water-in-salt” electrolytes has successfully expanded the stable operation potential window of MXenes. However, this advancement comes at the cost of sacrificing their high capacitance in acidic electrolytes. In this work, we report an acidic “water-in-salt” (AWIS) electrolyte composed of sulfuric acid and saturated lithium halide, which effectively doubled the energy density of the Ti₃C₂T_x-based symmetric supercapacitor compared to those with bare acidic electrolytes. Specifically, the AWIS electrolyte successfully expanded the voltage window of the symmetric device to 1.1 V. A high specific capacitance of 112.34 F g^-1 (at 10 mV s^-1) was obtained due to the presence of proton redox. As a result, the symmetric device achieved a high-energy density of 19.1 Wh kg^-1 and a high capacitance retention of 96.3% after 10,000 cycles. This work demonstrates the importance of designing stable and redox-active electrolytes for high-energy MXene-based symmetric supercapacitors.

Activated carbon has long been recognized as a promising electrode material for energy storage devices. The extraordinarily high specific area makes it challenging to replace in supercapacitors since electrical double-layer capacitors need such surfaces but also porous networks to enable electrolyte penetration. As a raw material for synthesizing activated carbon, sawdust offers key benefits, such as its renewability, abundance, favorable physical attributes for energy storage, and a more environmentally friendly synthesis process compared to mined alternative sources. In this work, electrochemical characterization is carried out which highlights the critical role of pelletization in enhancing the capacitive performance of sawdust-derived activated carbon, in addition to the implicit handling and logistical benefits. Subsequently, a Li-ion capacitor is assembled with an organic solvent-based electrolyte, sawdust-derived activated carbon serving as the positive electrode, and an Al-based foil negative electrode, potentially combining high energy and power density materials into a hybrid device. Despite commendable electrochemical performance and the use of a sustainable waste-derived positive electrode with a commoditized negative electrode, challenges remain regarding the ability to mitigate the role of surface functional groups that are stabilized by bio-carbon thermal treatments. Nevertheless, this distinctive architecture holds promise as an alternative high-power energy storage technology for a future filled with renewable energy, electric vehicles, and portable electronic devices.

Achieving both high redox activity and rapid ion transport is a critical and pervasive challenge in electrochemical energy storage applications. This challenge is significantly magnified when using large-sized charge carriers, such as the sustainable ammonium ion (NH₄⁺). A self-assembled MXene/n-type conjugated polyelectrolyte (CPE) superlattice-like heterostructure that enables redox-active, fast, and reversible ammonium storage is reported. The superlattice-like structure persists as the CPE:MXene ratio increases, accompanied by a linear increase in the interlayer spacing of MXene flakes and a greater overlap of CPEs. Concurrently, the redox activity per unit of CPE unexpectedly intensifies, a phenomenon that can be explained by the enhanced de-solvation of ammonium due to the increased volume of 3 Å-sized pores, as indicated by molecular dynamic simulations. At the maximum CPE mass loading (MXene:CPE ratio = 2:1), the heterostructure demonstrates the strongest polymeric redox activity with a high ammonium storage capacity of 126.1 C g⁻¹ and a superior rate capability at 10 A g⁻¹. This work unveils an effective strategy for designing tunable superlattice-like heterostructures to enhance redox activity and achieve rapid charge transfer for ions beyond lithium.

In alignment with widespread interest in carbon neutralization and sustainable practices, we disclose that conjugated polyelectrolyte (CPE) hydrogels are a type of recyclable, electrochemically stable, and environmentally friendly pseudocapacitive material for energy storage applications. By leveraging ionic-electronic coupling in a relatively fluid medium, one finds that hydrogels prepared using a fresh batch of an anionic CPE, namely, Pris-CPE-K, exhibit a specific capacitance of 32.6 ± 6.6 F g^-1 in 2 M NaCl and are capable of 80% (26.1 ± 6.5 F g^-1) capacitance retention after 100,000 galvanostatic charge-discharge (GCD) cycles at a current density (J) of 10 A g^-1. We note that equilibration under a constant potential prior to GCD analysis leads to the K⁺ counterions in the CPE exchanging with Na⁺ and, thus, the relevant active material Pris-CPE-Na. It is possible to remove the CPE material from the electrochemical cell via extraction with water and to carry out a simple purification through dialysis to produce a recycled material, namely Re-CPE-Na. The recycling workup has no significant detrimental impact on the electrochemical performance. Specifically, Re-CPE-Na hydrogels display an initial specific capacitance of 26.3 ± 1.2 F g^-1 (at 10 A g^-1) and retain 77% of the capacitance after a subsequent 100,000 GCD cycles. Characterization by NMR, FTIR, and Raman spectroscopies, together with XPS and GPC measurements, revealed no change in the structure of the backbone or side chains. However, rheological measurements gave evidence of a slight loss in G′ and G′′. Overall, that CPE hydrogels display recyclability argues in favor of considering them as a novel materials platform for energy storage applications within an economically viable circular recycling strategy.

Understanding energy storage mechanisms in electrochemical energy storage devices lays the foundations for improving their energy and power density. Here we introduce in situ ultraviolet–visible (UV–Vis) spectroscopy method to distinguish battery-type, pseudocapacitive and electrical double-layer charge storage processes. On the basis of Ti₃C₂T_x MXene in aqueous acidic and neutral electrolytes, and lithium titanium oxide in an organic electrolyte, we found a correlation between the evolution of UV–Vis spectra and the charge storage mechanism. The electron transfer number for Ti₃C₂T_x in an acidic electrolyte was calculated using quantitative analysis, which was close to previous measurements using X-ray absorption spectroscopy. Further, we tested the methodology to distinguish the non-Faradaic process in Ti₃C₂T_x MXene in a water-in-salt electrolyte, despite well-defined peaks in cyclic voltammograms. In situ UV–Vis spectroscopy is a fast and cost-effective technique that effectively supplements electrochemical characterization to track changes in oxidation state and materials chemistry and determine the charge storage mechanism.

A subclass of organic semiconductors known as conjugated polyelectrolytes (CPEs) is characterized by a conjugated backbone with ionic pendant groups. The water solubility of CPEs typically hinders applications of thin films in aqueous media. Herein, it is reported that films of an anionic CPE, namely CPE-K, drop cast from water produces single-component solid-state pseudocapacitive electrodes that are insoluble in aqueous electrolyte. That X-ray diffraction experiments reveal a more structurally ordered film, relative to the as-obtained powder from chemical synthesis, and dynamic light scattering measurements show an increase in aggregate particle size with increasing [KCl] indicate that CPE-K films are insoluble because of tight interchain contacts and electrostatic screening by the electrolyte. CPE-K film electrodes can maintain 85% of their original capacitance (84 F g⁻¹) at 500 A g⁻¹ and exhibit excellent cycling stability, where a capacitance retention of 93% after 100 000 cycles at a current density of 35 A g⁻¹. These findings demonstrate that it is possible to use initially water soluble ionic-organic materials in aqueous electrolytes, by increasing the electrolyte concentration. This strategy can be applied to the application of conjugated polyelectrolytes in batteries, organic electrochemical transistors, and electrochemical sensors, where fast electron and ion transport are required.

Conjugated polymer hydrogels (CPHs) are emerging pseudocapacitive materials capable of forming redox-active hydrogels. Current efforts focus on increasing their areal capacitance (C_Areal) and cycling stabilities by using binders tolerant to H₂SO₄-based electrolytes, while alternatives in more environmentally friendly electrolytes underperform due to low-capacity values. Herein, we demonstrate that it is possible to use conjugated polyelectrolyte (CPE), namely CPE-K, to create a single-component binder-free pseudocapacitive gel in environmentally friendly electrolytes (2 M: NaCl, MgCl₂, and MgSO₄), with C_Areal 1.9 times larger than those reported for single-component binder-free CPHs. The resulting pseudocapacitive gel exhibited C_Areal (523 mF cm⁻² at 0.25 mA cm⁻²) scalable with its thickness in NaCl electrolytes, providing an attractive solution to improve the capacitance of devices while maintaining a minimal charge-collecting electrode surface footprint. In addition, the CPE-K gel demonstrates 86% capacitance retention after 100 000 cycles at 10 mA cm⁻², which is higher than those reported for conventional state-of-the-art conjugated polymers. Electrochemical characterization revealed that C_Areal at all cycling rates tested is proportional to d_Thk up to 750 μm, primarily due to facile ionic diffusion within the 3D conductive network of the gel. Thicker electrodes (d_Thk = 1250 μm) can be operated at a rate of 15 mA cm⁻² with minimal capacity loss. These results demonstrate the potential applications of self-doped CPE gels in designing the next generation of multi-functional electrochemical energy storage and conversion technologies for targeting high energy and power density applications.

The discovery of the Ti₃C₂T_x compounds (MXenes) a decade ago opened new research directions and valuable opportunities for high-rate energy storage applications. The unique ability of the MXenes to host various mono- and multivalent cations and their high stability in different electrolyte environments including aqueous, organic, and ionic liquid solutions, promoted the rapid development of advanced MXene-based electrodes for a large variety of applications. Unlike the vast majority of typical intercalation compounds, the electrochemical performance of MXene electrodes is strongly influenced by the presence of co-inserted solvent molecules, which cannot be detected by conventional current/potential electrochemical measurements. Furthermore, the electrochemical insertion of ions into MXene interspaces results in strong coupling with the intercalation-induced structural, dimensional, and viscoelastic changes in the polarized MXene electrodes. To shed light on the charging mechanisms of MXene systems and their associated phenomena, the use of a large variety of real-time monitoring techniques has been proposed in recent years. This review summarizes the most essential findings related to the charging mechanism of Ti₃C₂T_x electrodes and their potential induced structural and mechanical phenomena obtained by in situ investigations.