Low‐Temperature Reduction Strategy Synthesized Si/Ti3C2 MXene Composite Anodes for High‐Performance Li‐Ion Batteries

Silicon is attracting enormous attention due to its theoretical capacity of 4200 mAh g^?1 as an anode for Li‐ion batteries (LIBs). It is of fundamental importance and challenge to develop low‐temperature reaction route to controllably synthesize Si/Ti₃C₂ MXene LIBs anodes. Herein, a novel and efficient strategy integrating in situ orthosilicate hydrolysis and a low‐temperature reduction process to synthesize Si/Ti₃C₂ MXene composites is reported. The hydrolysis of tetraethyl orthosilicate leads to homogenous nucleation and growth of SiO₂ nanoparticles on the surface of Ti₃C₂ MXene. Subsequently, SiO₂ nanoparticles are reduced to Si via a low‐temperature (200 °C) reduction route. Importantly, Ti₃C₂ MXene not only provides fast transfer channels for Li⁺ and electrons, but also relieves volume expansion of Si during cycling. Moreover, the characteristics of excellent pseudocapacitive performance and high conductivity of Ti₃C₂ MXene can synergistically contribute to the enhancement of energy storage performance. As expected, Ti₃C₂/Si anode exhibits an outstanding specific capacity of 1849 mAh g^?1 at 100 mA g^?1, even retaining 956 mAh g^?1 at 1 A g^?1. The low‐temperature synthetic route to Si/Ti₃C₂ MXene electrodes and involved battery‐capacitive dual‐model energy storage mechanism has potential in the design of novel high‐performance electrodes for energy storage devices.     

Xylene‐Bridged Phosphaviologen Oligomers and Polymers as High‐Performance Electrode‐Modifiers for Li‐Ion Batteries

Lithium‐ion batteries are one of the most common forms of energy storage devices used in society today. Due to the inherent limitations of conventional Li‐ion batteries, organic materials have surfaced as potentially suitable electrode alternatives with improved performance and sustainability. Viologens and phosphaviologens in particular, are strong electron‐accepting materials with excellent kinetic properties, making them suitable candidates for battery applications. In this paper, new polymeric species of the latter moieties are reported that lead to improved electrode stability and device performance. The performance of the phosphaviologen is further enhanced through the utilization of both redox steps, allowing for good performance proof‐of‐concept hybrid organic/Li‐ion batteries. This opens the potential for more sustainable and improved battery performance for use in current energy applications.     

Achieving High Pseudocapacitance of 2D Titanium Carbide (MXene) by Cation Intercalation and Surface Modification

Supercapacitors attract great interest because of the increasing and urgent demand for environment‐friendly high‐power energy sources. Ti₃C₂, a member of MXene family, is a promising electrode material for supercapacitors owing to its excellent chemical and physical properties. However, the highest gravimetric capacitance of the MXene‐based electrodes is still relatively low (245 F g^?1) and the key challenge to improve this is to exploit more pseudocapacitance by increasing the active site concentration. Here, a method to significantly improve the gravimetric capacitance of Ti₃C₂T_x MXenes by cation intercalation and surface modification is reported. After K⁺ intercalation and terminal groups (OH^?/F^?) removing , the intercalation pseudocapacitance is three times higher than the pristine MXene, and MXene sheets exhibit a significant enhancement (about 211% of the origin) in the gravimetric capacitance (517 F g^?1 at a discharge rate of 1 A g^?1). Moreover, the as‐prepared electrodes show above 99% retention over 10 000 cycles. This improved electrochemical performance is attributed to the large interlayer voids of Ti₃C₂ and lowest terminated surface group concentration. This study demonstrates a new strategy applicable to other MXenes (Ti₂CT_x , Nb₂CT_x , etc.) in maximizing their potential applications in energy storage.     

High Energy Density Aqueous Li‐Ion Flow Capacitor

High energy density Li‐ion hybrid flow capacitors are demonstrated by employing LiMn₂O₄ and activated carbon slurry electrodes. Compared to the existing aqueous flow electrochemical capacitors, the hybrid one exhibits much higher energy densities due to the introduction of high capacity Li‐insertion materials (e.g., LiMn₂O₄ in the present work) as the flowable electrode with asymmetrical cell configuration. A record energy density, i.e., 23.4 W h kg^?1 at a power of 50.0 W kg^?1 has been achieved for aqueous flow capacitors tested at static condition reported to date. A full operational Li‐ion flow capacitor tested in an intermittent‐flow mode has also been demonstrated. The Li‐ion hybrid flow capacitor shows great promise for high‐rate grid applications.     

Nanoscale Elastic Changes in 2D Ti3C2Tx (MXene) Pseudocapacitive Electrodes

Designing sustainable electrodes for next generation energy storage devices relies on the understanding of their fundamental properties at the nanoscale, including the comprehension of ions insertion into the electrode and their interactions with the active material. One consequence of ion storage is the change in the electrode volume resulting in mechanical strain and stress that can strongly affect the cycle life. Therefore, it is important to understand the changes of dimensions and mechanical properties occurring during electrochemical reactions. While the characterization of mechanical properties via macroscopic measurements is well documented, in situ characterization of their evolution has never been achieved at the nanoscale. It is reported here with in situ imaging, combined with density functional theory of the elastic changes of a 2D titanium carbide (Ti₃C₂T_x) based electrode in direction normal to the basal plane (electrode surface) during alkaline cation intercalation/extraction. 2D carbides, known as MXenes, are promising new materials for supercapacitors and various kinds of batteries, and understanding the coupling between their mechanical and electrochemical properties is therefore necessary. The results show a strong correlation between the cations content and the out‐of‐plane elastic modulus. This strategy enables identifying the preferential intercalation pathways within a single particle, which is important for understanding ionic transport in these materials.     

Boosting Photoelectrochemical Water Splitting by TENG‐Charged Li‐Ion Battery

The need for cost‐effective and sustainable power supplies has spurred a growing interest in hybrid energy harvesting systems, and the most elementary energy production process relies on intermittent solar power. Here, it is shown how the ambient mechanical energy leads to water splitting in a photoelectrochemical (PEC) cell boosted by a triboelectric nanogenerator (TENG). In this strategy, a flexible TENG collects and transforms mechanical energy into electric current, which boosts the PEC water splitting via the charged Li‐ion battery. Au nanoparticles are deposited on TiO₂ nanoarrays for extending the available light spectrum to visible part by surface plasmon resonance effect, which yields a photocurrent density of 1.32 mA cm^?2 under AM 1.5 G illumination and 0.12 mA cm^?2 under visible light with a bias of 0.5 V. The TENG‐charged battery boosts the water splitting performance through coupling electrolysis and enhanced electron–hole separation efficiency. The hybrid cell exhibits an instantaneous current more than 9 mA with a working electrode area of 0.3 cm², suggesting a simple but efficient route for simultaneously converting solar radiation and mechanical energy into hydrogen.     

Cascade‐Type Prelithiation Approach for Li‐Ion Capacitors

An industry‐relevant method for pre‐lithiation of lithium‐ion capacitors to balance the first charge irreversibility is demonstrated, which addresses the prime bottleneck for their market integration. Based on a composite positive electrode that integrates pyrene monomers and an insoluble lithiated base, Li₃PO₄, a “cascade‐type” process involving two consecutive irreversible reactions is proposed: i) oxidative electropolymerization of the pyrene moieties releases electrons and protons; ii) protons are captured by Li₃PO₄ and exchanged for a stoichiometric amount of Li⁺ into the electrolyte. (¹H, ¹⁹F, and ³¹P) NMR spectroscopy, operando X‐ray diffraction, and Raman spectroscopy support this mechanism. By decoupling the irreversible source of lithium ions from electrons, the cascade‐type pre‐lithiation allows the simultaneous enhancement of the capacity of the positive electrode, thanks to p‐doping of the resulting polymer. Remarkably, the proton scavenging properties of Li₃PO₄ also boost the polymerization process, which enables a 16% increase in capacity without detrimental effect on power properties and cyclability. Full cells integrating a cheap carbon black based negative electrode, show much‐improved capacity of 17 mAh g^‐1_electrodes (44 F g^‐1_electrodes, 3–4.4 V) and excellent stability over 2200 cycles at 1 A g^‐1. Thanks to its versatile chemistry and flexibility this approach in principle can be applied to any kind of ion‐battery.     

Vertically Aligned Sn4+ Preintercalated Ti2CTX MXene Sphere with Enhanced Zn Ion Transportation and Superior Cycle Lifespan

While 2D MXenes have been widely used in energy storage systems, surface barriers induced by restacking of nanosheets and the limited kinetics resulting from insufficient interlayer spacing are two unresolved issues. Here an Sn⁴⁺ preintercalated Ti₂CT_X with effectively enlarged interlayer spacing is synthesized. The preintercalated Ti₂CT_X is aligned on a carbon sphere to further enhance ion transportation by shortening the ion diffusion path and enhancing the reaction kinetics. As a result, when paired with a Zn anode, 12 500 cycles, which equals 2 800 h cycle time, and 5% capacity fluctuation are obtained, surpassing all reported MXene‐based aqueous electrodes. At 0.1 A g^‐1, the capacity reaches 138 mAh g^‐1, and 92 mAh g^‐1 remains even at 5 A g^‐1. In addition, the low anti‐self‐discharge rate of 0.989 mV h^‐1 associated with a high capacity retention of 80.5% over 548 h is obtained. Moreover, the fabricated quasi‐solid capacitor based on a hydrogel film electrolyte exhibits good mechanical deformation and weather resistance. This work employs both preintercalation and alignment to MXene and achieves enhanced ion diffusion kinetics in an aqueous zinc ion capacitors (ZICs) system, which may be applied to other MXene batteries for enhanced performance.     

Evidence of Enhanced Ion Transport in Li‐Rich Silicate Intercalation Materials

The silicate compounds Li₂MSiO₄ (where M = Mn, Fe, Co) have received significant attention recently as Li intercalation electrodes. Overwhelmingly they exhibit relatively poor kinetics of ion intercalation. By synthesizing Li‐rich solid solutions of the form Li_2+2x Fe_1?x SiO₄ (with 0 ≤ x ≤ 0.3), the structural requirements for fast ion transport and hence relatively fast intercalation have been identified. Specifically the presence of additional Li⁺ in interstitial sites, not normally occupied in the stoichiometric Li₂FeSiO₄ compound, enhances ion transport by more than two orders of magnitude. The results, obtained by combining electrochemical measurements, with powder X‐ray and neutron diffraction and atomistic modeling of the ion dynamics, provide valuable guidance in designing future intercalation electrodes with high Li‐ion transport and, hence, fast electrode kinetics.     

Na‐Ion Intercalation and Charge Storage Mechanism in 2D Vanadium Carbide

2D vanadium carbide MXene containing surface functional groups (denoted as V₂CT_x , where T_x are surface functional groups) is synthesized and studied as anode material for Na‐ion batteries. V₂CT_x anode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V₂CT_x material during Na⁺ intercalation/deintercalation and the redox reaction of vanadium are studied using a combination of synchrotron based X‐ray diffraction, hard X‐ray absorption near edge spectroscopy (XANES), and soft X‐ray absorption spectroscopy (sXAS). Experimental evidence of a major contribution of redox reaction of vanadium to the charge storage and the reversible capacity of V₂CT_x during sodiation/desodiation process are provided through V K ‐edge XANES and V L _2,3‐edge sXAS results. A correlation between the CO₃^2? content and the Na⁺ intercalation/deintercalation states in the V₂CT_x electrode observed from C and O K ‐edge in sXAS results implies that some additional charge storage reactions may take place between the Na⁺‐intercalated V₂CT_x and the carbonate‐based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V₂CT_x as anode material for Na‐ion batteries and capacitors.     

An Ultrafast Rechargeable Hybrid Sodium‐Based Dual‐Ion Capacitor Based on Hard Carbon Cathodes

Hybrid sodium‐based dual‐ion capacitors (NDICs), which integrate the advantages of supercapacitors and sodium‐ion batteries, have attracted tremendous attention recently. In this work, hybrid sodium‐based dual‐ion capacitors are successfully developed with nitrogen‐doped microporous hard carbon as the cathode and soft carbon as the anode. N‐doping is beneficial to the functional groups, porous structure, and electric conductivity of hard carbon. Hybrid NDICs possess a wide voltage range (0.01–4.7 V), high‐energy density of 245.7 W h kg^?1 at a power density of 1626 W kg^?1, long cycle life (1000 cycles), and outstanding rate performance.     

High‐Energy Density Li‐Ion Capacitor with Layered SnS2/Reduced Graphene Oxide Anode and BCN Nanosheet Cathode

Lithium‐ion capacitors (LICs) with capacitor‐type cathodes and battery‐type anodes are considered a promising next‐generation advanced energy storages system that meet the requirements of high energy density and power density. However, the mismatch of charge‐storage capacity and electrode kinetics between positive and negative electrodes remains a challenge. Herein, layered SnS₂/reduced graphene oxide (RGO) nanocomposites are developed for negative electrodes and a 2D B/N codoped carbon (BCN) nanosheet is designed for the positive electrode. The SnS₂/RGO derived from SnS₂‐bonded RGO of high conductivity exhibits a capacity of 1198 mA h g^?1 at 100 mA g^?1. Boron and nitrogen atoms in BCN are found to promote adsorption of anions, which enhance the pseudocapacitive contribution as well as expanding the voltage of LICs. A quantitative kinetics analysis indicates that the SnS₂/RGO electrodes with a dominating capacitive mechanism and a diminished intercalation process, benefit the kinetic balance between the two electrodes. With this particular structure, the LIC is able to operate at the highest operating voltage for these devices recorded to date (4.5 V), exhibiting an energy density of 149.5 W h kg^?1, a power density of 35 kW kg^?1, and a capacity retention ratio of 90% after 10 000 cycles.     

Biomimetic MXene Textures with Enhanced Light‐to‐Heat Conversion for Solar Steam Generation and Wearable Thermal Management

2D materials are of particular interest in light‐to‐heat conversion, yet challenges remain in developing a facile method to suppress their light reflection. Herein, inspired by the black scales of Bitis rhinoceros, a generalized approach via sequential thermal actuations to construct biomimetic 2D‐material nanocoatings, including Ti₃C₂T_x MXene, reduced graphene oxide (rGO), and molybdenum disulfide (MoS₂) is designed. The hierarchical MXene nanocoatings result in broadband light absorption (up to 93.2%), theoretically validated by optical modeling and simulations, and realize improved light‐to‐heat performance (equilibrium temperature of 65.4 °C under one‐sun illumination). With efficient light‐to‐heat conversion, the bioinspired MXene nanocoatings are next incorporated into solar steam‐generation devices and stretchable solar/electric dual‐heaters. The MXene steam‐generation devices require much lower solar‐thermal material loading (0.32 mg cm^?2) and still guarantee high steam‐generation performance (1.33 kg m^?2 h^?1) compared with other state‐of‐the‐art devices. Additionally, the mechanically deformed MXene structures enable the fabrication of stretchable and wearable heaters dual‐powered by sunlight and electricity, which are reversibly stretched and heated above 100 °C. This simple fabrication process with effective utilization of active materials promises its practical application value for multiple solar–thermal technologies.     

Interface Engineering of MXene Composite Separator for High‐Performance Li–Se and Na–Se Batteries

Selenium (Se), due to its high electronic conductivity and high energy density, has recently attracted considerable interest as a cathode material for rechargeable Li/Na batteries. However, the poor cycling stability originating from the severe shuttle effect of polyselenides hinders their practical applications. Herein, highly stable Li/Na–Se batteries are developed using ultrathin (≈270 nm, loading of 0.09 mg cm^?2) cetrimonium bromide (CTAB)/carbon nanotube (CNT)/Ti₃C₂T_x MXene hybrid modified polypropylene (PP) (CCNT/MXene/PP) separators. The hybrid separator can immobilize the polyselenides via enhanced Lewis acid–base interactions between CTAB/MXene and polyselenides, which is demonstrated by theoretical calculations and X‐ray photoelectron spectroscopy. The incorporation of CNT helps to improve the electrolyte infiltration and facilitate the ionic transport. In situ permeation experiments are conducted for the first time to visually study the behavior of polyselenides, revealing the prohibited shuttle effect and protected Li anode from corrosion with CCNT/MXene/PP separators. As a result, the Li–Se batteries with CCNT/MXene/PP separators deliver an outstanding cycling performance over 500 cycles at 1C with an extremely low capacity decay of 0.05% per cycle. Moreover, the hybrid separators also perform well in Na–Se batteries. This study develops a preferable separator–electrolyte interface and the concept can be applied in other conversion‐type battery systems.     

Rational Construction of Nitrogen‐Doped Hierarchical Dual‐Carbon for Advanced Potassium‐Ion Hybrid Capacitors

Potassium‐ion hybrid capacitors (PIHCs), elaborately integrate the advantages of high output power as well as long lifespan of supercapacitors and the high energy density of batteries, and exhibit great possibilities for the future generations of energy storage devices. The critical next step for future implementation lies in exploring a high‐rate battery‐type anode with an ultra‐stable structure to match the capacitor‐type cathode. Herein, a “dual‐carbon” is constructed, in which a three‐dimensional nitrogen‐doped microporous carbon polyhedron (NMCP) derived from metal‐organic frameworks is tightly wrapped by two‐dimensional reduced graphene oxide (NMCP@rGO). Benefiting from the synergistic effect of the inner NMCP and outer rGO, the NMCP@rGO exhibits a superior K‐ion storage capability with a high reversible capacity of 386 mAh g^?1 at 0.05 A g^?1 and ultra‐long cycle stability with a capacity of 151.4 mAh g^?1 after 6000 cycles at 5.0 A g^?1. As expected, the as‐assembled PIHCs with a working voltage as high as 4.2 V present a high energy/power density (63.6 Wh kg^?1 at 19 091 W kg^?1) and excellent capacity retention of 84.7% after 12 000 cycles. This rational construction of advanced PIHCs with excellent performance opens a new avenue for further application and development.