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Owing to the advantages of high operating voltage, environmental benignity, and low cost, potassium-based dual-ion batteries (KDIBs) have been considered as a potential candidate for large-scale energy storage. However, KDIBs generally suffer from poor cycling performance and unsatisfied capacity, and inactive components of conductive agents, binders, and current collector further lower their overall capacity. Herein, we prepare coral-like carbon nanowires (CCNWs) doped with nitrogen as a binder-free anode material for K+-ion storage, in which the unique coral-like porous nanostructure and amorphous/short-range-ordered composite feature are conducive to enhancing the structural stability, to facilitating the ion transfer and to boosting the full utilization of active sites during potassiation/de-potassiation process. As a result, the CCNW anode possesses a hybrid K+-storage mechanism of diffusive behavior and capacitive adsorption, and stably delivers a high capacity of 276 mAh g-1 at 50 mA g-1, good rate capability up to 2 A g-1, and long-term cycling stability with 93 % capacity retention after 2000 cycles at 1 A g-1. Further, assembling this CCNW anode with an environmentally benign expanded graphite (EG) cathode yields a proof-of-concept KDIB, which shows a high specific capacity of 134.4 mAh g-1 at 100 mA g-1, excellent rate capability of 106.5 mAh g-1 at 1 A g-1, and long-term cycling stability over 1000 cycles without negligible capacity loss. This study provides a feasible approach to developing high-performance anodes for potassium-based energy storage devices.

High-energy-density lithium-sulfur batteries has attracted substantial attention as competitive candidates for large-scale energy storage technologies. Still, the adverse “shuttle effect” and sluggish sulfur conversion reaction kinetics immensely obstruct their commercial viability. Herein, a two-dimensional metallic 1T phase WS2 (1T-WS2) nanosheets modified functional separator is developed to improve the electrochemical performance. Meanwhile, the semiconducting bulk-WS2 crystals, and 2H phase WS2 (2H-WS2) nanosheets with more basal-plane S-vacancy defects are also prepared to probe the contributions of the crystal structure (phase), S-vacancy defects, and edges to the Li–S batteries performance experimentally and theoretically. In merits of the synergistic effect of high ion and electron conductivity, enhanced binding ability to lithium polysulfides (LiPSs), and sufficient electrocatalytic active sites, the 1T-WS2 shows highly efficient electrocatalysis of LiPSs conversion and further improves Li–S battery performance. As expected, thus-fabricated cells with 1T-WS2 nanosheets present superior cycle stability that maintain capacity decline of 0.039% per cycle after 1000 cycles at 1.0 C. The strategy presented here offers a viable approach to reveal the critical factors for LiPSs catalytic conversion, which is beneficial to developing advanced Li–S batteries with enhanced properties.

Lithium metal batteries are promising devices for the next-generation energy storage due to their ultrahigh theoretical specific capacity and extremely low electrochemical potential. Their inherent problem is the formation of lithium dendrites in cycling, which has induced safety concerns for almost half a century. After understanding the formation mechanism of branching structures, we propose to suppress lithium dendrites by adopting external magnetic fields to induce diffusion enhancement at the interface of the anode, thus attenuating concentration gradient there and reducing the driving force for the formation of dendritic structures. The diffusion coefficient of lithium ions is dependent on the strength of magnetic fields, confirming the effectiveness of magnetic fields in improving Li+ diffusion. After employing the magnetic field of 0.8 T, the concentration gradients at the growth front becomes nearly half of the control case, which leads to a dendrite-free lithium deposition up to the high current density of 10 mA cm−2. Both the CuLiCoO2 batteries and the symmetric LiLi coin cells show a long-term stable cycling at high current densities under the assistance of magnetic field. This diffusion enhanced technique promises a facile and general approach to suppress dendritic structures in secondary batteries, which may help to develop quick charging strategies.

To explore the natural resources as sustainable precursors offers a family of green materials. The use of bio-waste precursors especially the remaining from food processing is a scalable, highly abundant, and cost-effective strategy. Exploring waste materials is highly important especially for new materials discovery in emerging energy storage technologies such as lithium sulfur batteries (LSBs). Herein, waste milk powder is carbonized and constructed as the sulfur host with the hollow micro-/mesoporous framework, and the resulting carbonized milk powder and sulfur (CMP/S) composites are employed as cathodes for LSBs. It is revealed that the hollow micro-/mesoporous CMP/S framework can not only accommodate the volume expansion but also endow smooth pathways for the fast diffusion of electrons and Li-ions, leading to both high capacity and long cycling stability. The CMP/S composite electrode with 56 wt.% loaded sulfur exhibits a remarkable initial capacity of 1596 mAh g−1 at 0.1 C, corresponding to 95% of the theoretical capacity. Even at a rate of 1 C, it maintains a high capacity of 730 mAh g−1 with a capacity retention of 72.6% after 500 cycles, demonstrating a very low capacity fading of only 0.05% per cycle. Importantly, the Coulombic efficiency is always higher than 96% during all the cycles. The only used source material is expired waste milk powders in our proposal. We believe that this “trash to treasure” approach will open up a new way for the utilization of waste material as environmentally safe and high performance electrodes for advanced LSBs.

Rational design of advanced structure for transition metal oxides (TMOs) is attractive for achieving high-performance supercapacitors. However, it is hampered by sluggish reaction kinetics, low mass loading, and volume change upon cycling. Herein, hierarchical NiCo2O4 architectures with 2D-nanosheets-shell and 3D-nanocages-core (2D/3D h-NCO) are directly assembled on nickel foam via a facile one-step way. The 2D nanosheets are in-situ generated from the self-evolution of initial NCO nanospheres. This 2D/3D hierarchical structures ensure fast ion/electron transport and maintain the structural integrity to buffer the volume expansion. The 2D/3D h-NCO electrode with an ultrahigh mass loading (30 mg cm−2) achieves a high areal capacity of 4.65 C cm−2 (equivalent to 1.29 mAh cm−2) at a current density of 4 mA cm−2, and retains 3.7 C cm−2 even at 50 mA cm−2. Furthermore, the assembled solid-state hybrid supercapacitor yields a high volumetric energy density of 4.25 mWh cm−3 at a power density of 39.3 mW cm−3, with a high capacity retention of 92.4% after 5000 cycles. Therefore, this work provides a new insight to constuct hierarchical electrodes for energy storage application.

To effectively alleviate the ever-increasing energy crisis and environmental issues, clean and sustainable energy-related materials as well as the corresponding storage/conversion devices are in urgent demand. Silicon (Si) with the second most elemental abundance on the crust in the form of silicate or silica (SiO2) minerals, is an advanced emerging material showing high performance in energy-related fields (e.g. batteries, photocatalytic hydrogen evolution). For the improved performance in industry-scale applications, Si materials with delicate nanostructures and ideal compositions in a massive production are highly cherished. On account of the reserve, low cost and diverse micro-nanostructures, silicate minerals are proposed as promising raw materials. In the article, crystal structures and the reduction approaches for silicate minerals, as well as recent progress on the as-reduced Si products for clean energy storage/conversion, are presented systematically. Moreover, some cutting-edge fields involving Si materials are discussed, which may offer deep insights into the rational design of advanced Si nanostructures for extended energy-related fields.

Exploring platinum group metal-free electrocatalysts with superior catalytic performance and favorable durability for oxygen reduction reaction is a remaining bottleneck in process of developing sustainable techniques in energy storage and conversion. Herein, a hierarchical porous single atomic Fe electrocatalyst (Fe/Z8-E-C) is rationally designed and synthesized via acid etching, calcination, adsorption of Fe precursor and recalcination processes. This unique electrocatalyst Fe/Z8-E-C shows excellent oxygen reduction performance with a half-wave potential of 0.89 V in 0.1 mol/L KOH, 30 mV superior to that of commercial Pt/C (0.86 V), which is also significantly higher than that of typical Fe-doped ZIF-8 derived carbon nanoparticles (Fe/Z8-C) with a half-wave potential of 0.84 V. Furthermore, Fe/Z8-E-C-based Zn-air battery exhibits greatly enhanced peak power density and specific capacity than those of original Fe/Z8-C, verifying the remarkable performance and practicability of this specially designed hierarchical structure due to its efficient utilization of the active sites and rapid mass transfer. This present work proposes a new method to rationally synthesize single atom electrocatalysts loaded on hierarchical porous frame materials for catalysis and energy conversion.

Transition metal oxides (TMO) bring a novel direction for the development of energy store materials due to their excellent stability. They not only have high capacity and good cycle performance, but also are cheap and easily available. Zinc oxide (ZnO) as an important part of TMO have gradually attracted attention in the research of electrochemistry. ZnO, as a metal semiconductor with the advantages of wide band gap, possesses high ion migration rate, good chemical stability, simple preparation and low cost, and is widely used in various fields. However, poor conductivity, low permittivity and quick capacity decays quickly impede the commercial application of these electrodes. In recent years, in order to improve the structural stability, ion diffusion and conductivity of zinc oxides-based anodes, various strategies have been raised, such as structural design, surface modification and composition control. In this paper, the recent advances of zinc oxides-based materials for batteries and hybrid supercapacitors (SCs) were introduced. We comprehensively reviewed the prepared process, reaction mechanism and electrochemical performance and discussed the shortcoming of zinc oxides-based nanomaterials. In particular, several insights toward the future research development, practical applications and commercialization of energy storage devices are also proposed for improving the performance of zinc oxides-based materials.

The growing demand on energy storage devices and environmental issues has enhanced the curiosity for metal oxide/porous carbon composite owing to its intriguing properties. Nano metal oxides undergo aggregation at high temperature. To overcome the issues, the studies on nano metal oxides incorporated into the porous carbon matrix are carried out. This review article discuss about the various synthetic strategies of metal oxide/porous carbon composite and their applications towards the environmental, energy devices, catalyst, sensors, etc. In particular the porosity of the carbon with respect to the physiochemical properties of metal oxide/porous carbon composite are discussed and reported. The future trends and development on the nano metal oxide/porous carbon composite and its applications in various fields are discussed.

Reduced graphene oxide (rGO)-based fibers with high electrochemical performance have recently showed great potential in the field of flexible energy storage devices. However, they still suffer from low capacitance due to the severe stacking of graphene sheets. Hybrids with nanofillers are an efficient way to improve the electrochemical performance of rGO fibers. Nevertheless, controlling the distribution of nanoparticles in the matrix is still an enormous challenge due to the strong attraction among these nanoparticles which results into agglomeration. Here, we continually prepared rGO hybrid fibers via non-liquid-crystal spinning, accompanied by chemical reduction. Manganic oxide (MnOX) nanoparticles remained well-dispersed in GO dispersion during the continuous spinning of rGO/MnOX hybrid fibers. Results showed that rGO/MnOX-20 hybrid fibers possessed the best capacitance of 123.3 F g−1 (87.6 F cm−3) and 97.1 F g−1 (68.9 F cm−3) at the current density of 0.2 A g−1, and 0.5 A g−1 respectively. Furthermore, a fiber-shaped all-solid-state supercapacitor assembly from the optimized hybrid fibers demonstrated an energy density of 2.67 mWh cm−3 (3.76 mWh g−1) at the power density of 24.76 mWh cm−3 (34.89 mWh g−1). These fiber-based devices show great potential for application in the fields of wearable electronics and energy storage devices.

Hierarchical superstructures assembled by nanosheets can effectively prevent aggregation of nanosheets and improve performance in energy storage. Therefore, we proposed a facile hydrothermal method to obtain three-dimensional (3D) superstructure assembled by nanosheets. We found that the ratio of Co2+/HMTA affected the morphology of the samples, and the 3D hierarchical structures of are obtained while the ratio of Co2+/HMTA is 12:25. The hierarchical structures with sufficient interior space preserves the original sheet-like dimensional components and results in sufficient active sites and efficient mass diffusion. Hence, the 3D Co2V2O7·nH2O hierarchical structure exhibits good rate capability and high stability while as electrode materials. Meanwhile, when power density is 745.13 W/kg, the assembled CVO-2//AC shows an energy density of 47.7 Wh/kg. The work displays a facile method for fabrication of 3D superstructure assembled by 2D nanosheets that can be applied in energy storage.

A highly green fluorescent carbon quantum dots (CQDs) synthesized from food waste via one step green synthesis. As-synthesized colloidal CQDs are highly stable due to their surface functionality with homogeneous distribution. Structural characterization is examined using XRD analysis. The optical and physicochemical properties of CQDs exemplify the green-emission under an excitation wavelength of 390 nm in UV–Vis spectra, it showed two (π → π ∗ transition of C = C bonds and n → π ∗ transition of C = O bonds) peaks around 278 and 368 nm. Synthesized CQDs are to be used for next generation energy storage and bioengineering application.

Thermal energy storage systems utilising phase change materials have the potential to overcome the intermittency issues associated with most renewable energy sources, significantly contributing to the decarbonisation of the energy sector. While the concept of storing energy in the latent heat of a phase transition is not new, large scale systems employing this concept have not yet realised their full potential. This is in large part due to the shortcomings of conventionally used materials – whether it be issues with their flammability, chemical and thermal instability, corrosivity, poor cycling stability, or a combination of these. Ionic liquids offer a suite of inherent “green” properties that translate well into the field of phase change materials, namely low volatility, low flammability, and good thermal and chemical stability, alongside the potential to tune their chemical and phase properties. In this Perspective, we discuss the evolution and promise of the emerging field of ionic liquids for renewable thermal energy storage. Systems are considered from a holistic, sustainable point of view, demonstrating the importance of assessing material origins and synthetic pathways as well as system performance through lifecycle assessment. We elucidate the emerging design rules for optimising thermal properties, and in doing so attempt to provide an overview of promising emerging systems and future directions.

Composites are a class of materials which provide a scope of tailoring properties that are not possible amongst its constituents. Magnetoelectric composite materials exhibit product property arising because of mutually connected electric and magnetic phases. The systematic and comprehensive study on structural, tunable dielectric and flexible energy storage study of NdMnO3 (ND) manganite-based composites has been reported. The phase purity is estimated X-ray diffraction studies and confirm composite formation. The dielectric studies display a step-like decrease and reveal Maxwell-Wagner polarization with relaxation processes. The effect of magnetic field on dielectric constant reveals strong variation due to modification of spatial charge distribution. The qualitative evidence of tunable energy storage studies in ND based composites is observed by measuring the ferroelectric P-E hysteresis loop in variable magnetic field and is strain originated from coupling amongst individual component phases.

Li–O2 batteries gain widespread attention as a candidate for next-generation energy storage devices due to their extraordinary theoretic specific energy. The semi-open structure of Li–O2 batteries causes many parasitic reactions, especially related to water. Water is a double-edged sword, which destroys Li anode and simultaneously triggers a solution-based pathway of the discharge product. In this work, hexamethyldisilazane (HMDS) is introduced into the electrolyte of an aprotic Li–O2 battery. HMDS has a strong ability to combine with a trace of water to generate a hydrophobic hexamethyldisiloxane (MM), which eliminates water from the electrolyte decomposition and then prevents the Li anode from producing the insulating LiOH with water. In this case, the hydrophobic MM disperses in the ether-based electrolyte, forming a dispersion hydrophobic electrolyte. This electrolyte can anchor water from the environment on the cathode side, which triggers a solution-based pathway and regulates the growth morphology of the discharge product and consequently increases the discharge capacity. Compared with the Li–O2 battery without the HMDS, the HMDS-containing Li–O2 battery contributes an about 13-fold increase of cyclability (400 cycles, 1800 h) in the extreme environment of saturated water vapor. This work opens a new approach for directly operating aprotic Li–O2 batteries in ambient air.

As the fundamental energy storage components in electronic systems, dielectric capacitors with high power densities were demanded. In this work, the anti-ferroelectric Pb0.89La0.06Sr0.05(Zr0.95Ti0.05)O3 (PLSZT) ceramics and thin film capacitor were successfully fabricated by a solid-state reaction route and pulsed laser deposition method, respectively. The ferroelectric, dielectric, energy-storage properties, and temperature stability of anti-ferroelectric PLSZT capacitor were investigated in detail. By compared with the PLSZT ceramic (energy storage density is 1.29 J/cm3 with an efficiency of 78.7% under 75 kV/cm), the anti-ferroelectric PLSZT thin film capacitors exhibited the enhanced energy storage density of 52.6 J/cm3 with efficiency of 67.7% under an electric field as high as 2068.9 kV/cm, and the enhanced energy-storage temperature stabilities from room temperature (RT) to more than 200 °C were demonstrated, and the oxygen defects mechanism and size effect were discussed. Moreover, the fast charging (∼0.05 μs) and discharging (∼0.15 μs) time were certified for the anti-ferroelectric PLSZT film capacitor. These findings broaden the horizon for PLSZT anti-ferroelectrics in high energy storage properties and show promising for manufacturing pulse power capacitor.

The ever-increasing environmental/energy crisis as well as the rapid upgrading of mobile devices had stimulated intensive research attention on promising alternative energy storage and conversion devices. Among these devices, alkali metal ion batteries, such as lithium-ion batteries (LIBs) had attracted increasing research attention due to its several advantages including, environmental friendliness, high power density, long cycle life and excellent reversibility. It had been widely used in consumer electronics, electric vehicles, and large power grids et ac. Silicon-based (silicon and their oxides, carbides) anodes had been widely studied. Its several advantages including low cost, high theoretical capacity, natural abundance, and environmental friendliness, which shows great potential as anodes of LIBs. In this review, we summarized the recently progress in the synthetic method of silicon matrix composites. The empirical method for prelithiation of silicon-based materials were also provided. Further, we also reviewed some novel characterization methods. Finally, the new design, preparation methods and properties of these nano materials were reviewed and compared. We hoped that this review can provide a general overview of recent progress and we briefly highlighted the current challenges and prospects, and will clarify the future trend of silicon anode LIBs research.

Recently, Coordination Polymers (CPs) have been widely utilized as energy storage materials for reversible Lithium-Ion Batteries (LIBs) benefiting from their tunable building blocks and adjusted electrochemical properties. However, the unsatisfied electrochemical behavior of CPs with poor conductivity and sluggish ion transport kinetics is still a bottle-neck for their large-scale energy storage applications in LIBs. Herein, we display the rational fabrication of a conductive Sn-based coordination polymer (Sn-DHTPA) via judiciously choosing suitable building units. The Sn-DHTPA is employed as anode for LIBs, exhibiting superior reversible storage capacity of 1142.6 mA h g−1 at 0.1 A g−1 after 100 cycles and impressive rate storage capability of 287.7 mA h g−1 at 20 A g−1. More importantly, a robust cycling performance of 205.5 mA h g−1 at an extra-high current density of 20 A g−1 are observed without remarkable capacity-fading up to 1000 cycles. The behavior superiority of Sn-DHTPA is related to its advanced architecture with abundant lithium storage sites, high electrical conductivity and rapid lithium transport. A series of ex-situ characterizations reveal that the impressive lithium storage capacity is contributed by the redox active sites of both the aromatic linker and metal center related to in-situ generated metallic nanoparticles dispersed in the skeleton.

Wind and solar green energy sources have increasingly been widely used by businesses and researchers worldwide. In the literature, these energy sources have also been represented. For these developments to be universal, these energy sources need more construction costs, more maintenance costs, and more operating space. An energy storage system is to be built to solve these issues, allowing us to store energy during lean hours and supply energy at a low cost later. These technologies, such as the battery, cell, and supercapacitor, can very easily store energy and energy distribution speeds. This energy sources have a high current for a limited period. We need electrode material and electrolyte material mentioned in previous research papers to make these energy sources. Due to its excellent electrical conductivity, excellent stability, and high specific surface area, electrode materials such as carbon and carbon-based materials are generally used. In these energy sources, these materials serve as dielectric materials. Carbon and carbon-based materials have recently been derived from bio-waste materials. These bio-waste products are bio-waste for agriculture, bio-waste for tree leaves and bio-waste from fruits. The use of these bio-waste products aids in the disposal of waste and turns waste into a vital resource. These products for bio-waste are readily available at no cost. The approaches used by researchers to obtain from bio-waste are explored in this study research paper on the availability of different bio-waste materials.

Mixed metal oxides gain potential interest as the conversion of energy and electrode material for storage. Some notable attributes like supremacy on power and energy density, long cyclic stability, and quick reaction to form supercapacitors are essential, like energy storage devices. Contemplating those views of supercapacitors, in the present investigation, the new manganese vanadium oxide has been synthesized by microwave synthesis and followed by a hydrothermal process with an open-faced hollow sphere-like structure. It contains a high surface that gives more electrochemically active sites for energy storage and multivalence response. The sample has been characterized using X-ray diffraction and Raman spectroscopy for confirming phase, characterized by field emission scanning electron microscopy for morphological features. Electrochemical analyses of as-synthesized samples are investigated thoroughly to anticipate the view of that system's approach in an energy storage device. The specific capacitance is analyzed to be 340F/g in 0.5A/g current density and 1900F/g at 2 mV/s scan rate. This mixed metal oxide depicts higher stability and up to 1,000 cycles, which is 91% at a current density of 10A/g. It displays a high energy density of 13Wh/kg at the power density of 3500 W/kg in the current density of 8A/g. The potential value-specific capacitance, rated capacity, and long cycle stability make it a prominent candidate to fulfill the urge to engineering new advanced materials for energy storage applications.

Recently, NaNbO3-based ceramics with excellent energy storage performances (ESPs) and fast charge-discharge characteristics have attracted great attention. In this case, BaTiO3 modified 0.85NaNbO3-0.15Bi(Zn2/3Nb1/3)O3 (0.85NN-0.15BZN-yBT) ceramics were designed and successfully fabricated using solid state synthesis. The discharge storage density of the ceramics increases from 1.22 J/cm3 at y = 0–1.84 J/cm3 at y = 8%, which is 1.5 times higher than pure 0.85NN-0.15BZN ceramics. This ceramic reveals excellent temperature stability up to 150 °C, good discharge cycling after 105 cycles and high discharge speed of 2.0 μs, broadening the scope of NaNbO3-based ceramics for energy storage applications.

Supercapacitors are favored by researchers because of their high power density, especially with the acceleration of people's life rhythm. However, their energy density, especially from the point of view of the whole energy storage device, is far lower than that of commercial batteries. In this work, a kind of customizable full paper-based supercapacitor device with excellent self-healing ability is fabricated by simple and low-cost screen printing, electropolymerization and dip coating methods. The resultant separator-free supercapacitor device exhibits both ultrahigh gravimetric and areal specific energy (power) densities of 39 Wh kg−1 (69 kW kg−1) and 692 μWh cm−2 (236 mW cm−2), achieving excellent supercapacitor performance. Notably, the addition of vitrimers endows the whole device with outstanding self-healing properties, which is helpful for enhancing the adaptability of the device to the environment. In addition, this kind of paper-based device also displays good photothermal and electromagnetic shielding performances. These striking features make paper matrix composites attractive in the fields of supercapacitors, medical photothermal treatment and electromagnetic shielding.

g-C3N4 emerges as a star 2D photocatalyst due to its unique layered structure, suitable band structure and low cost. However, its photocatalytic application is limited by the fast charge recombination and low photoabsorption. Rationally designing g-C3N4-based heterojunction is promising for improving photocatalytic activity. Besides, g-C3N4 exhibits great potentials in electrochemical energy storage. In view of the excellent performance of typical transition metal oxides (TMOs) in photocatalysis and energy storage, this review summarized the advances of TMOs/g-C3N4 heterojunctions in the above two areas. Firstly, we introduce several typical TMOs based on their crystal structures and band structures. Then, we summarize different kinds of TMOs/g-C3N4 heterojunctions, including type I/II heterojunction, Z-scheme, p-n junction and Schottky junction, with diverse photocatalytic applications (pollutant degradation, water splitting, CO2 reduction and N2 fixation) and supercapacitive energy storage. Finally, some promising strategies for improving the performance of TMOs/g-C3N4 were proposed. Particularly, the exploration of photocatalysis-assisted supercapacitors was discussed.

Graphene oxide/reduced Graphene oxide based hybrid nano composites have been a hot topic of research in the past few decades. Their Remarkable optical, electrical, mechanical and thermal properties brought them fore front among other 2-D materials. Graphene is a single 2-D sheet of graphite. So it is known to be mother of other carbon materials like carbon nanotube, fullerene, and their derivatives graphene oxide (GO), reduced Graphene Oxide (rGO). Graphene derivatives (GO, rGO) and their hybrids with metals (M) and Metal Oxides (MO) have proven to be an effective materials for sensors, solar cells, water purification and so on. This review article comprises the low cost synthesis techniques most commonly used to produce these graphene derivatives, discuss how synthesis affects their key material properties and highlight some examples of hybrid metal/metal oxide Nano composites with unique and impressive properties and its applications. Specifically, this review highlights their performance in spectroscopy based optical sensors, energy storage and water purifications. Finally, this review discusses the outlook and remaining challenges in the field of practical industrial-scale production.

Constructing high-performance electrodes with both wide potential window (e.g. ≥ 2 V in aqueous electrolyte) and excellent mechanical flexibility represents a great challenge for supercapacitors. Because of the outstanding conductivity and flexibility, carbon cloth (CC) has shown unlimited prospects for constructing flexible electrodes, but is rarely used directly as electrode material due to its electrochemical inertness and small specific surface area. To tackle these two critical limitations, we design a novel redox-etching strategy to synthesize CC-based electrode with 3D interconnecting pore structure. The sponge-like highly porous CC was further activated by strong oxidant to form abundant oxygenic groups, which occupy the interior and surface of current collector to render substantial pseudocapacitance. The as-synthesized CC electrode yielded an impressive capacitance of 4035 mF cm−2 at 3 mA cm−2 and satisfying cycling durability in a wide potential range of −1 – 1 V vs. SCE, which surpass the majority of reported CC-based electrodes. A symmetric supercapacitor with stable voltage of 2 V is assembled and delivers remarkable energy density of 6.57 mWh cm−3. Significantly, the device demonstrates an unparalleled flexibility with no capacitive decay after 100 bending cycles. This facile chemical etching and post-treatment processes are designed for large-scale manufacturing of the CC electrodes by providing high surface area and abundant electrochemically active sites, promising for industry application. The innovative synthetic strategy opens up new opportunities for high-performance flexible energy storage.

Charging the electric vehicles through the use of solar PV systems is a major hurdle in today's era. In the present work, a system is designed for charging Electric bikes at workplaces like schools, colleges, offices, etc. To ensure a reliable charging system, a standalone solar PV system with a battery bank based energy storage unit is employed. It can be seen from the present work that the electric bike at different voltage configurations i.e., 12 V, 24 V, and 36 V, can be charged efficiently. The irradiance level is varied in a way to show a real-life effect that occurs during a summer day. To depict this, irradiance level is assumed to rises from 300 W/m2 to 1000 W/m2 and decline again to 300 W/m2 during the process. The DC bus voltage is successfully maintained constant at a level of 48 V in all three cases. The contribution of the study is the application of a robust solar PV based off-grid design for charging electric bike at the workplaces.

Potassium-ions batteries (PIBs) are attracting increasing attention as up-and-coming youngster in large-scale grid-level energy storage benefiting from its low-cost and high energy density. Nevertheless, enough researches regarding indispensable cathode materials for PIBs are badly absent. Herein, we synthesize K-deficient layered manganese-based oxides (P2-K0.21MnO2 and P3-K0.23MnO2) and investigate them as cathode of PIBs for the first time. As the newcomer of potassium-containing layered manganese-based oxides (KxMnO2) group, P2-K0.21MnO2 delivers high discharge capacity of 99.3 mAh g−1 and P3-K0.23MnO2 exhibits remarkable capacity retention rate of 75.5%. Besides, in-situ XRD and ex-situ XRD measurements reveal the reversible phase transition of P2-K0.21MnO2 and P3-K0.23MnO2 with the potassium-ions extraction and reinsertion, respectively. This work contributes to a better understanding for the potassium storage in K-deficient layered KxMnO2 (x ≤ 0.23), possessing an important basic scientific significance for the exploitation and application of layered KxMnO2 in PIBs.

Supercapacitors proved to be significant power storage devices. Their high storage density, long shelf-life and reduced wear/cycle ratio promise efficient battery technologies soon. Recent advances in material science demonstrate that carbon-based composites enable enhanced performance of supercapacitors. The abundant availability of carbon composites has led a spike in the interest of researchers to take a deep look at their underlying properties that help bring up commercially viable supercapacitor technologies. In addition, the exceptional charge retention capacity and conductivity of the carbon based materials (CNTs and grapheme) trigged the researchers to use these materials in supercapacitors fabrication. In this detailed review, synthesis of carbon-based anode materials and their performance have been considered and relative examination is done. Various carbon-based composites have been analyzed based on existing research for understanding supercapacitors performance in terms of cyclic stability, shelf-life energy storage capacity and applicable to various applications. This detailed review also acknowledges the challenges and constraints in material synthesis and device fabrication.

Advanced carbon-based electrode materials derived from wastes are essential to high-performance supercapacitors due to their abundance and sustainability. In this work, we fabricate novel cathodes and anodes based on discarded surgical mask-derived carbon (DSM-C). Discarded surgical masks are good candidates for carbon-based electrode materials due to their unique fibrous structure and simple composition compared to conventional biomass sources. Benefiting from the excellent electrical conductivity of DSM-C and abundant redox reactions from nickel oxide (NiO), the electrochemical performances of NiO/DSM-C composites have been greatly improved. Specifically, the DSM-C and NiO/DSM-C electrodes show high specific capacitances of 240 F g−1 and 496 F g−1 at 1 A g−1 respectively, and excellent rate capability. Moreover, asymmetric supercapacitors (ASCs) are assembled using DSM-C and NiO/DSM-C as anodes and cathodes, respectively. They deliver a high energy density of 57 Wh kg−1 at a power density of 702 W kg−1, accompanied by superior cycling stability (98.5% capacitance retention after 10,000 cycles). This work shows prospective applications of DSM-C as an electrode material for energy storage systems.

Although transition metal hydroxides are promising candidates as advanced supercapattery materials, they suffer from poor electrical conductivity. In this regard, previous studies have typically analyzed separately the impacts of defect engineering at the atomic level and the conversion of hydroxides to phosphides on conductivity and the overall electrochemical performance. Meanwhile, this paper uniquely studies the aforementioned methodologies simultaneously inside an all-in-one simple plasma treatment for nickel cobalt carbonate hydroxide, examines the effect of altering the nickel-to-cobalt ratio in the binder-free defect-engineered bimetallic Ni-Co system, and estimates the respective quantum capacitance. Results show that the concurrent defect-engineering and phosphidation of nickel cobalt carbonate hydroxide boost the amount of effective redox and adsorption sites and increase the conductivity and the operating potential window. The electrodes exhibit ultra-high-capacity of 1462 C g−1, which is among the highest reported for a nickel–cobalt phosphide/phosphate system. Besides, a hybrid supercapacitor device was fabricated that can deliver an energy density of 48 Wh kg−1 at a power density of 800 W kg−1, along with an outstanding cycling performance, using the best performing electrode as the positive electrode and graphene hydrogel as the negative electrode. These results outperform most Ni-Co-based materials, demonstrating that plasma-assisted defect-engineered Ni-Co-P/POx is a promising material for use to assemble efficient energy storage devices.

Supercapacitors (SCs) with high power density and long cycling span life are demanding energy storage devices that will be an attractive power solution to modern electronic and electrical applications. Numerous theoretical and experimental works have been devoted to exploring various possibilities to increase the functionality and the specific capacitance of electrodes for SCs. Non-carbon two-dimensional (2D) materials have been considered as encouraging electrode candidates for their chemical and physical advantages such as tunable surface chemistry, high electronic conductivity, large mechanical strength, more active sites, and dual non-faradaic and faradaic electrochemical performances. Besides, these 2D materials also play particular roles in constructing highway channels for fast ion diffusion. This concise review summarizes cutting-edge progress of some representative 2D non-carbon materials for the aqueous electrolyte-based SCs, including transition metal oxides (TMOs), transition metal hydroxides (TMHs), transition metal chalcogenides (TMCs), MXenes, metal-organic frameworks (MOFs) and some emerging materials. Different synthetic methods, effective structural designs and corresponding electrochemical performances are reviewed in detail. And we finally present a detailed discussion of the current intractable challenges and technical bottlenecks, and highlight future directions and opportunities for the development of next-generation high-performance energy storage devices.

The performance and safety of lithium-ion batteries are plagued by several diverse, nonlinear aging mechanisms influenced by the electrochemical thermal interactions at the electrodes, usage history, and operating conditions. Understanding and deconvoluting the fundamental reaction mechanisms responsible for electrode degradation are key for developing technologies in Li-ion battery diagnostics and prognostics. Hence, there exists a need for high-precision operando techniques to investigate and characterize distinct electrode degradation modes over a gamut of operational variability. Cells embedded with a stable, nonpolarizable reference electrode offer an in situ and operando tool to decouple the complex electrochemical interplay between the electrode pair by measuring individual electrode responses simultaneously with the cell response in the time and frequency domains. This perspective comprehensively looks at 3-electrode (3ε) analytics as a versatile toolbox, highlighting recent techniques and parameters developed with an emphasis on degradation diagnostics and control strategies that is expected to drive the futuristic design of battery management systems.

The bio-nanotechnological fabrication of high-surface-area carbons has attracted widespread interest in supercapacitor applications by using readily-available natural products as raw materials or bio-templates, and is expected to refine on pore accessibility for compact energy storage. Here, a renovated design strategy of semi-biomass interpenetrating polymer network (IPN) derived carbon is demonstrated through physically knitting the biomacromolecule (sodium alginate, SA) polymeric chains into the highly crosslinked resorcinol-formaldehyde (RF) network and subsequent thermochemical conversion. Molecule-level interlacing forces in such IPN efficiently relieve the RF skeleton shrinkage when producing carbon, while the other SA network addresses the macrophase separation issue to sacrifice as an in-knitted porogen and a morphology-directing agent. As a result, porous carbon globules are equipped with moss-like surfaces and interconnected pore architecture for high accessible electrode surface (1013 m2/g), and efficient electrochemical responses are reached with the specific capacitance of 312 F/g at 1 A/g. Taking the advantage of 9 mol/kg NaClO4 complex-solvent electrolyte, the voltage window is extended to 2.4 V, endowing the two-electrode device with the high energy delivery of 32.3 Wh/kg at 240 W/kg.

Aqueous zinc-ion batteries (ZIBs) have attracted great attention as the candidates for large-scale energy storage system, recently, because of their low cost, environment-friendly, high safety, and high theoretical energy densities. Among the numerous cathode materials, layered structure vanadium based polyanionic compounds, such as VOPO4, exhibit high specific capacity for Zn ion storage. However, the low Zn ion diffusion coefficient and limited interlayer spacing make the cathodes low reversible capacity and inferior cycling stability. Herein, K ions were pre-intercalated into the VOPO4 layers via ions exchange adopting VOPO4•2H2O as the precursor. When evaluated as the cathode for ZIBs, an excellent cycle stability of 400 cycles under a current density of 500 mA g−1 was achieved by the obtained KVOPO4 electrode, verifying the positive effect of intercalation engineering. Furtherly, a solid-solution reaction Zn ion storage mechanism was confirmed. This study provides a new insight to explore high performance cathode materials for ZIBs.

It is critical to develop carbon material anodes with high initial Coulombic efficiency and energy density for sodium ion batteries. Herein, a novel mushroom spore with chitin as carbon precursor is first reported for energy storage, and its special porous spherical structure, fine structure and oxygen functional groups can be accurately controlled by carbonization temperature. The hollow porous carbon spheres obtained from mushroom spore at 1400 °C have appropriate porous structure, d002 spacing (0.364 nm), 7.12% oxygen content and ultra-low specific surface area of 5.5 m2 g−1. It could obtain 81.2% initial Coulombic efficiency and has reversible discharge capacity of 411.1 mA h g−1, wherein about 75% (308 mA h g−1) of its total capacity is derived from low-potential plateau (below 0.1 V Na+/Na), and the capacity is 384.5 mA h g−1 after 50 cycles. Furthermore, Density functional theory calculation showed that the residual oxygen functional groups (C O) in carbon materials are beneficial to sodium into graphite-like layers, and graphite-like layers spacing is smaller than the reported unadulterated carbon with 0.37 nm. Therefore, the excellent electrochemical performance and low-cost of natural mushroom spore derived hollow porous carbon spheres provide advantages for sodium ion batteries in large-scale storage devices.

Abstract An effective technique for improving electrochemical efficiency is to rationally design hierarchical nanostructures that completely optimize the advantages of single components and establish an interfacial effect between structures. In this study, core–shell NiMoO4@Ni9S8/MoS2 hetero-structured nanorods are prepared via a facile hydrothermal process followed by a direct sulfurization. The resulting hierarchical architecture with outer Ni9S8/MoS2 nanoflakes shell on the inner NiMoO4 core offers plentiful active sites and ample charge transfer pathways in continuous heterointerfaces. Ascribing to the porous core–shell configuration and synergistic effect of bimetal sulfides, the obtained NiMoO4@Ni9S8/MoS2 as electrode material presents an unsurpassed specific capacity of 373.4 F g−1 at 10 A g−1 and remarkable cycling performance in the 6 M KOH electrolyte. This work delivers a rational method for designing highly efficient electrodes for supercapacitors, enlightening the road of exploring low-cost materials in the energy storage domain. Graphical Abstract

Borophene, a monolayer of boron, has risen as a new exciting two-dimensional (2D) material having extraordinary properties, including anisotropic metallic behavior and flexible (orientation-dependent) mechanical and optical properties. This review summarizes the current progress in the synthesis of borophene on various metal substrates, including Ag(110), Ag(100), Au(111), Ir(111), Al(111), and Cu(111), as well as heterostructuring of borophene. In addition, it discusses the mechanical, thermal, magnetic, electronic, optical, and superconducting properties of borophene and the effects of elemental doping, defects, and applied mechanical strains on these properties. Furthermore, the promising potential applications of borophene for gas sensing, energy storage and conversion, gas capture and storage applications, and possible tuning of the material performance in these applications through doping, formation of defects, and heterostructures are illustrated based on available theoretical studies. Finally, research and application challenges and the outlook of the whole borophene's field are given.

The development of high-performance anode materials for potassium-based energy storage devices with long-term cyclability requires combined innovations from rational material design to electrolyte optimization. A three-dimensional K+ -pre-intercalated Ti3 C2 Tx MXene with enlarged interlayer distance was constructed for efficient electrochemical potassium-ion storage. We found that the optimized solvation structure of the concentrated ether-based electrolyte leads to the formation of a thin and inorganic-rich solid electrolyte interphase (SEI) on the K+ -pre-intercalated Ti3 C2 Tx electrode, which is beneficial for interfacial stability and reaction kinetics. As a proof of concept, 3D K+ -Ti3 C2 Tx //activated carbon (AC) potassium-ion hybrid capacitors (PIHCs) were assembled, which exhibited promising electrochemical performances. These results highlight the significant roles of both rational structure design and electrolyte optimization for highly reactive MXene-based anode materials in energy storage devices.

Air batteries are promising energy storage technologies that have gained continuous attraction due to their high energy densities. At present, investigations on anodes of air batteries are usually focused on various metals such as Li, Zn, Al. In contrast, the semiconductor anodes like Si and Ge are less investigated. Si-air battery possesses a high theoretical energy density and Ge-air battery has a high actual power density and ideal safety. Besides anodes, air cathodes where oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are also the key components in air batteries. To further promote the discharging performance and facilitate energy conversion/storage, semiconductor materials have been introduced in electrochemical cells like Li-O2 and Zn-air batteries. This review briefly summarizes semiconductor materials utilized in various air batteries, including the progress of Si-air and Ge-air batteries and recent advances in semiconductor cathodes catalysts. Finally, the remaining challenges and further perspective are discussed.

The trade-off between the electrochemical performance and mechanical strength is still a challenge for Ti3C2Tx free-standing electrode. Herein, a facile approach was proposed to fabricate a Microfibrillated cellulose@Ti3C2Tx (MFC@Ti3C2Tx) self-assembled microgel film by means of hydrogen bonding linkage. Benefiting from the rich hydroxyl groups on the MFC, the Ti3C2Tx nanosheets coated on the MFC in a time scale of minutes (within 1 min) instead of hours. The ultralong 1D frame of MFC effectively mitigated the re-aggregation of Ti3C2Tx nanosheet. The fluffy MFC@Ti3C2Tx film structure and the constructed 1D/2D conducting Ti3C2Tx pathways in horizontal and vertical directions endowed the fast ion transport of the electrolytes and the improved accessibility to the Ti3C2Tx surface. As a result, the freestanding MFC@Ti3C2Tx microgel film delivered a high specific capacitance of 451F/g. And the rate performance was increased to 71% from the 64% of that of pristine Ti3C2Tx film. Furthermore, the tensile strength of MFC@Ti3C2Tx film was also promoted to 46.3 MPa, 3 folds of that of the pristine Ti3C2Tx film, due to the high strength of MFC and the hydrogen bonding effect.

Multi-dimensional metal oxides have become a promising alternative electrode material for supercapacitors due to their inherent large surface area. Herein, P-doped NiCo2O4/NiMoO4 multi-dimensional nanostructures are synthesized on carbon clothes (CC) with a continuous multistep strategy. Especially, P has the best synergistic effect with transition metals, such as optimal deprotonation energy and OH– adsorption energy, which can further enhance electrochemical reaction activity. For the above reasons, the P-NiCo2O4/NiMoO4@CC electrode exhibits an ultra-high specific capacitance of 2334.0 F g−1 at 1 A g−1. After 1500 cycles at a current density of 10 A g−1, its specific capacity still maintains 93.7%. Besides, a P-NiCo2O4/NiMoO4@CC//activated carbon device (hybrid supercapacitor or device) was also prepared with a maximum energy density of 45.1 Wh kg−1 at a power density of 800 W kg−1. In particular, the capacity retention rate is still 89.97% after 8000 cycles due to its excellent structural stability. Our work demonstrates the vast potential of multi-dimensional metal oxides in energy storage.

Solid-state sodium-ion batteries with sodium metal anodes possess high safety and reliability, which are considered as a promising candidate for the next generation of energy storage technology. However, poor electronic and ionic conductivities at the interface between electrodes and solid-state electrolytes restrict its practical application. Herein, we demonstrate a β″-Al2O3 electrolyte with a vertically porous-dense bilayer structure to solve this problem. The carbon-coated vertically porous layer serves as a high mass-loading host for Na3V2(PO4)3 cathode and provides fast electronically and ionically conductive pathways. In addition, the dense layer is produced to prevent sodium dendrite growth and improve mechanical strength of β″-Al2O3 electrolyte. Experimental results show that the cathode loading in vertically porous layer can reach to 8 mg cm−2, and the porous-dense bilayer β″-Al2O3 electrolyte-based battery exhibits a reversible specific capacity of 87 mAh g−1 and a capacity retention of 95.5% over 100 cycles at a current density of 0.1 C, which is superior to that of the traditional dense β″-Al2O3 electrolyte-based battery. This work based on electrolyte structure design represents an efficient strategy for the development of solid-state sodium-ion batteries with high mass-loading cathode.

It is challenging to create cation vacancies in electrode materials for enhancing the performance of rechargeable lithium ion batteries (LIBs). Herein, we utilized a strong alkaline etching method to successfully create Co vacancies at the interface of atomically thin Co3−xO4/graphene@CNT heterostructure for high-energy/power lithium storage. The creation of Co-vacancies in the sample was confirmed by high-resolution scanning transmission electron microscope (HRSTEM), X-ray photoelectron spectroscopy (XPS) and electron energy loss near-edge structures (ELNES). The obtained Co3−xO4/graphene@CNT delivers an ultra-high capacity of 1688.2 mAh g−1 at 0.2 C, excellent rate capability of 83.7% capacity retention at 1 C, and an ultralong life up to 1500 cycles with a reversible capacity of 1066.3 mAh g−1. Reaction kinetic study suggests a significant contribution from pseudocapacitive storage induced by the Co-vacancies at the Co3−xO4/graphene@CNT interface. Density functional theory confirms that the Co-vacancies could dramatically enhance the Li adsorption and provide an additional pathway with a lower energy barrier for Li diffusion, which results in an intercalation pseudocapacitive behavior and high-capacity/rate energy storage.

Wearable energy storage devices are desirable to boost the rapid development of flexible and stretchable electronics. Two-dimensional (2D) materials, e.g., graphene, transition metal dichalcogenides and oxides, and MXenes, have attracted intensive attention for flexible energy storage applications because of their ultrathin 2D structures, high surface-to-volume ratio, and unique physical/chemical properties. To achieve commercialization of 2D material-based wearable energy storage devices (2DM-WESDs), scalable and cost-efficient manufacturing is a critical challenge. Among existing manufacturing technologies, solution-based assembly strategies show strong potential to achieve low-cost and scalable production. A timely review of the recent progress in solution-based assembly strategies and the resultant 2DM-WESDs will be meaningful to guide the future development of 2DM-WESDs. In this review, first, a brief introduction of exfoliation and solution preparation of 2D material species from bulk materials is discussed. Then, the solution-based assembly strategies are summarized, and the advantages and disadvantages of each method are compared. After that, two major categories of 2DM-WESDs, supercapacitor and battery, are discussed, emphasizing their state-of-the-art energy storage performances and flexibilities. Finally, insights and perspectives on current challenges and future opportunities regarding the solution assembly of 2DM-WESDs are discussed.

Lithium-sulfur (Li-S) batteries are considered as one of the promising next-generation energy storage systems because of their high energy density. While the low utilization of sulfur and sluggish reaction kinetics would lead to degradation of electrochemical performance and thus hinder the practical application of Li-S batteries. Herein, a double-shelled TiO2-graphene heterostructure (H-TiO2/rGO) with abundant oxygen vacancies (OVs) and highly exposed active plane as advanced host material in Li-S batteries is designed. This rational structure not only provides sufficient active sites and lower bandgap for lithium polysulfides (LiPSs), but also builds smooth adsorption-diffusion-conversion of LiPSs on catalyst, which greatly reduces interfacial energy barrier and promotes the utilization of sulfur through suppressing the devastating shuttling effect. Combining the synergetic merits of strong anchoring ability and catalyzing the of LiPSs, the electrode (S-TiO2/rGO-1) exhibits superior rate performance and long lifespan (1000 cycles at 1C, 0.023% capacity loss per cycle) with high columbic efficiency. This work paves an alternative way to establish smooth adsorption-diffusion-conversion of polysulfides on catalyst in Li-S batteries and provides a new sight to understand catalyst design in energy storage devices.

The energy crisis and environmental pollution require the advancement of large-scale energy storage techniques. Among the various commercialized technologies, batteries have attracted enormous attention due to their relatively high energy density and long cycle life. Nevertheless, the limited supply and uneven distribution of lithium minerals, as well as their high cost, has greatly hindered the application of lithium-ion batteries in large-scale energy storage. Therefore, building next-generation alternative rechargeable batteries that feature low cost, long service life, and high safety is of the utmost importance. Over the past several years, we have witnessed many successful research outcomes for sodium and potassium ion batteries (SIBs and PIBs) in regard to electrode and electrolyte materials. In this Perspective, we summarize the current developments on SIBs/PIBs and their challenges when facing practical applications, including their cost, energy density, ion diffusivity in solids/electrolytes/interphases, cycle life, and safety concerns. Furthermore, we provide an overview of strategies to overcome the remaining challenges for achieving better electrochemical performance. Finally, we outline several possible directions for the future development of these two battery chemistries, with the hope of aiding the transition from the laboratory to next-generation practical applications for SIBs/PIBs batteries in grid-scale energy storage.

Recently, carbon nanotubes (CNT)-based interconnected architectures exhibit promising prospects in supercapacitors due to their flexibility and high electrical conductivity. Herein, a three-dimensional (3D) interconnected network structure combined with conductive carbon nanotubes interpenetrating MOFs-derived Co-Ni-S composite spheres (Co-Ni-S/CNTs) was synthesized. Such 3D interconnected architecture significantly leads to a favorable electronic structure, fast charge-transfer capacity, and more pseudocapacitive. The Co-Ni-S/CNTs-based hybrid electrode exhibits an extraordinary specific capacitance of 540.6C g−1 at 1 A g−1 and competitive rate performance (capacity retention rate of 69.9% when the current density increases to 10 times). Subsequently, a hybrid supercapacitor is assembled using Co-Ni-S/CNTs as the positive electrode and commercial activated carbon as negative electrode. The device delivers a high energy density of 63.5 W h kg−1 at 800 W kg−1 and keeps 83.0% initial capacitance retention after 10,000 cycles. The encouraging performances demonstrate the significant contribution of the 3D interconnected architecture for the future energy storage.

Hybrid supercapacitors have the advantages of fast charging and discharging and long service life, which are an efficient and practical energy storage device. Therefore, the design of hybrid supercapacitors is the focus of current research. In this paper, the silver modified spinel NiCo2S4 nanorods (Ag2S-NiCo2S4/CF) are synthesized by an efficient and economical method, which has excellent electrochemical performance. The Ag2S-NiCo2S4/CF shows a high specific capacity of 179.7 mAh g−1 at current density of 1 A g−1, and excellent rate capability (capacitance retention of ~87% at 20 A g−1). The corresponding Ag2S-NiCo2S4/CF//AC/CF hybrid supercapacitor is assembled by Ag2S-NiCo2S4/CF as the positive electrode, which can provide an energy density of 35.978 Wh kg−1 at a high-power density of 800 W kg−1 and has significant cyclic stability (~80% of the initial capacitor after ~9600 cycles). Therefore, Ag2S-NiCo2S4/CF material is a promising electrode material that can be applied to hybrid supercapacitors.

Potassium-ion batteries (KIBs) have become the most promising alternative to lithium-ion batteries for large-scale energy storage system due to their abundance and low cost. However, previous reports focused on the intercalation-type cathode materials usually showed an inferior capacity, together with a poor cyclic life caused by the repetitive intercalation of large-size K-ions, which hinders their practical application. Here, we combine the strategies of carbon coating, template etching and hydrothermal selenization to prepare yolk-shelled FeSe2@N-doped carbon nanoboxes (FeSe2@C NBs), where the inner highly-crystalline FeSe2 clusters are completely surrounded by the self-supported carbon shell. The integrated and highly conductive carbon shell not only provides a fast electron/ion diffusion channel, but also prevents the agglomeration of FeSe2 clusters. When evaluated as a conversion-type cathode material for KIBs, the FeSe2@C NBs electrode delivers a relatively high specific capacity of 257 mAh/g at 100 mA/g and potential platform of about 1.6 V, which endow a high energy density of about 411 Wh/kg. Most importantly, by designing a robust host with large internal void space to accommodate the volumetric variation of the inner FeSe2 clusters, the battery based on FeSe2@C NBs exhibits ultra-long cycle stability. Specifically, even after 700 cycles at 100 mA/g, a capacity of 221 mAh/g along with an average fading rate of only 0.02% can be retained, which achieves the optimal balance of high specific capacity and long-cycle stability.

MXenes are a family of two-dimensional (2D) transition metal carbides, carbonitrides/nitrides with superior physical and chemical properties, which have attracted extensive attention since the discovery in 2011. The impressive electrochemical activity of MXene makes it one of the most potential electrode materials in rechargeable batteries and supercapacitors. However, single-component MXene electrodes are difficult to achieve high specific capacity, efficient ion/electron transport, and high stability compatibility in an electrochemical environment. Studies have shown that it is an effective method to introduce nanomaterials between MXene layers to construct heterostructures and to improve the electrochemical performance through the synergistic effect among the components in the heterostructures. The introduction of nanomaterials can effectively suppress the restacking of MXene nanosheets, shorten the diffusion path of ions and promote the electrolyte transport, which is beneficial to enhance the rate performance of MXene; moreover, the excellent mechanical flexibility of MXene can reduce the volume expansion of nanomaterials during charge/discharge, thereby effectively protecting the integrity of the electrode structure and improving the cycling stability. Therefore, in this review, combined with theoretical calculations, we summarize the recent advances of MXene heterostructures in terms of synthesis strategies and energy storage applications, including supercapacitors, metal-ions batteries (Li, Na, K, Mg, Zn, Al) and metal anode protection. Furthermore, potential challenges and application perspectives for MXene heterostructures are also outlined.