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Layered sodium transition metal (TM) oxides exhibit great potential as high energy density cathode materials for sodium-ion batteries (SIBs). The large Na ions, nevertheless, adopts various coordination environments that are dependent of the sodium concentration, giving rise to cyclical gliding of TM layers and P-O phase transitions upon Na extraction/insertion process. The detrimental interlayer-gliding induced phase transformations lead to deteriorated round-trip energy efficiency, rate capability and cycling stability of electrodes. Herein, we demonstrate a shear-resistant interface via the supersaturation of lithium to overcome the interlayer-gliding behavior and inhibit the multiple P-O phase transitions in P2-type Na0.67Mn0.67Ni0.33O2. The results indicate that the nanoscale interface is composed of lithium-enriched O3 nanodomains in the P2 phase matrix, resulting in smooth charge/discharge profiles and superior cycling stability of P2-type Na0.67Mn0.67Ni0.33O2 cathode under a high cut-off voltage of 4.5V. This work highlights the concept of modulating the interfacial shear stress for improving the long-term cycling stability of high-voltage layered cathode materials that suffer from severe phase transformations.

Metal phosphates, such as LiFePO4 (LFP), have been attracting extensive attention as electrode materials for next-generation rechargeable metal-ion batteries (MIBs), due to their high theoretical capacity, good chemical stability, long lifespan, and natural abundance. In this review, the recent progress of the design and engineering of metal phosphate-based electrode materials for MIBs is summarized. Specifically, the survey will focus on three types of phosphates, monometallic phosphates, bimetallic phosphates, and multi-metal phosphates, within the context of their intrinsic structure and corresponding electrochemical performance. A range of experimental variables will be carefully analyzed, such as sample synthesis, crystal structure, and electrochemical reaction mechanism, in conjunction with theoretical calculations. The applications of these materials as MIB electrodes are then featured for diverse MIBs, such as lithium-ion battery, sodium-ion batteries, potassium-ion batteries, calcium-ion batteries, and magnesium-ion batteries. We conclude the review with a perspective where the promises and challenges of phosphate-based electrodes for MIBs are highlighted, along with future research directions.

Na superionic conductor (NASICON)-type Na

Zinc ion batteries (ZIBs) have attracted increasing attention due to their low cost, high safety and environmental friendliness. However, the simultaneous achievement of long cycle life and high energy density of ZIBs remains an open challenge. In this work, edge-rich cathodes with vertically orientated graphene (VG) nanosheets templating V2O5 nanosheets are designed for high-performance ZIBs. In such a hybrid structure, free-standing VGs on carbon cloth (CC) with non-aggregated characters, open networks and sharp edges as nanotemplates provide fast electron-transfer channels and large accessible surface area to electrolyte. Moreover, edge-rich Zn nanosheets are deposited on CC as anodes to match with edge-rich V2O5/VG/CC cathode. As a result, the assembled aqueous ZIB shows a high capacity of 370 mAh g−1 (at a current density of 0.2 A g−1), high rate capability (capacity retention of over 50% at a current density up to 50 A g−1) and excellent cycle life (showing a high capacity retention of 85% over 5000 cycles).

Two-dimensional (2D) titanium (Ti)-based nanomaterials have been extensively investigated due to their small strain expansion, abundant ion diffusion paths, high safety, and low cost. Nonetheless, such Ti-based nanosheets typically exhibit only poor electrical conductivity and can easily restack. Great efforts have been contributed to alleviating these issues based on nanofabrication and nanoengineering strategies. In this review, we survey the recent progress of the preparation and applications of 2D Ti-based nanostructures in electrochemical energy storage, such as supercapacitors, lithium-ion batteries, sodium-ion batteries, and other multivalent metal-ion batteries, with a focus on the crystal structure, electrochemical energy storage mechanism, and advanced in situ characterization techniques. Finally, a perspective is included to highlight the challenges and promises in the exploitation of 2D Ti-based materials for further enhancement of the performance of energy storage devices.

All-solid-state lithium batteries (ASSLBs) based on sulfide solid-state electrolytes and high voltage layered oxide cathode are regarded as one of the most promising candidates for energy storage systems with high energy density and high safety. However, they usually suffer poor cathode/electrolyte interfacial stability, severely limiting their practical applications. In this work, a core-shell cathode with uniformly nanosized Li0.5La0.5TiO3 (LLTO) electrolyte coating on LiNi0.5Co0.3Mn0.2O2 (NCM532) is designed to improve the cathode/electrolyte interface stability. Nanosized LLTO coating layer not only significantly boosts interfacial migration of lithium ions, but also efficiently alleviates space-charge layer and inhibits the electrochemical decomposition of electrolyte. As a result, the assembled ASSLBs with high mass loading (9 mg cm−2) LLTO coated NCM532 (LLTO@NCM532) cathode exhibit high initial capacity (135 mAh g−1) and excellent cycling performance with high capacity retention (80% after 200 cycles) at 0.1 C and 25 °C. This nanosized LLTO coating layer design provides a facile and effective strategy for constructing high performance ASSLBs with superior interfacial stability.

In this study, highly pure Mo2CTx MXene was successfully synthesized from Mo2Ga2C by etching in hydrothermal conditions for 24 h at 140–180°C. The etching solutions were lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF) or ammonium fluoride (NH4F) mixed with hydrochloric acid (HCl). The temperature to obtain pure Mo2CTx MXene in NH4F + HCl solution was 140°C, lower than that in other etching solutions. The thickness of as-prepared monolayer Mo2CTx nanosheet was measured to be 1.5 ± 0.3 nm by atomic force microscope. From thermogravimetry and differential thermal analysis, Mo2CTx MXene was found to be stable in argon atmosphere at a temperature of up to 500°C. As the temperature increased, Mo2CTx MXene gradually transformed, and β-Mo2C/MoC/Mo3C2 was eventually formed to coexist with Mo2CTx MXene at 700°C. In air atmosphere, Mo2CTx MXene was stable at 200°C. At 590°C, Mo2CTx MXene was completely oxidized to MoO3.

Though chemical vapor deposition (CVD) methods have been widely used in the growth of two-dimensional transition-metal dichalcogenides (2D TMDCs), the controllable fabrication of 2D TMDCs is yet hard to achieve because of the great challenge of concisely controlling the release of precursors vapor, one of the most critical growth kinetic factors. To solve this important issue, here we report the utilization of oxide inhibitors covering Mo source during CVD reactions to manipulate the release of Mo vapor. In contrast to the lack of capability of conventional CVD methods, 2D molybdenum dichalcogenide (MoX2, X = S, Se, Te) monolayers were successfully fabricated through the proposed CVD protocol with the oxide-inhibitor-assisted growth (OIAG) strategy. In this way, despite the fact that only separated MoTe2 flakes were prepared, both MoS2 (continuous and clean) and MoSe2 (continuous but dotted) monolayer films at the scale of centimeter were obtained. The presented OIAG method enables a comprehensive understanding and precise control of the reaction kinetics for improved growth of 2D MoX2.

Conventional lithium-ion batteries with liquid organic electrolytes generally suffer potential security risks concerning volatilization, flammability and explosion. Nonflammable and thin solid-state electrolytes particularly composite solid electrolytes (CSEs) that integrate the merits of different electrolyte systems have attracted increasing attention for advanced lithium batteries with improved energy density and high safety. In this work, a three-dimensional (3D) fiber-network-reinforced CSE, which consists of a mechanically robust, porous polyimide (PI) film as a host, Li6.75La3Zr1.75Ta0.25O12 (LLZTO) nanoparticles and poly (vinylidene fluoride) (PVDF) polymer matrix with bis-trifluoromethanesulfonimide lithium salt as electrolyte filler, is designed and fabricated. Such unique 3D CSE films with PI fiber network holding for uniform dispersion of LLZTO in PVDF show continuous lithium ion transfer pathways and effective prevention for lithium dendrite growth, consequently exhibiting improved mechanical property (high tensile strength of 11.5 ​MPa) and high cyclic stability (more than 1000 ​h of cycling for Li symmetric batteries). Furthermore, solid-state LiNi0.5Co0.2Mn0.3O2/Li pouch cells with the PI-PVDF/LLZTO CSE exhibit excellent cyclic stability (152.6 ​mA ​h ​g−1 with capacity retention of 94.9% at 0.1C after 80 cycles) at room temperature, and high functionality and safety (withstand harsh environments such as folding, cutting and nail penetration) in practical applications.

Aqueous zinc ion batteries (ZIBs) have attracted intensive attention due to their low cost, environmental friendliness and high safety. However, exploring suitable cathode materials and a deep understanding of their energy storage mechanisms remain a daunting challenge. In this work, a three-dimensional (3D) nest-like V6O13 structure is grown on carbon cloth (CC) as a free-standing ZIB cathode. Such interconnected V6O13 nanoneedles forming a 3D nest structure provide a large accessible surface area to the electrolyte and rapid channels for Zn2+ diffusion, and V6O13 with a V5+-rich state allows for more possibilities of multielectron reaction upon Zn (de)intercalation. As a result, an aqueous ZIB based on the V6O13/CC cathode and low-cost ZnSO4 electrolyte shows an ultrahigh capacity of 520 mA h g−1 (at a current density of 0.5 A g−1), desirable rate capability and cycle life (showing a stable capacity of 335 mA h g−1 over 1000 cycles). Moreover, the Zn2+ storage mechanism is deeply investigated based on experimental data and density functional theory simulation, and flexible solid-state ZIBs are established based on such a V6O13/CC cathode to reveal its high functional flexibility and excellent electrochemical performance, showing great potential applications in high-performance and safe electronic devices.

Micro-supercapacitors provide high peak power, long cycle life, and high charge/discharge rates for practical applications in microsystems. However, current micro-supercapacitors generally suffer from low energy density and complicated fabrication process. Here, we report carbon nanosheet (CN) selectively grown on carbon nanotube (CNT) patterns as three-dimensional hybrid electrodes for high-performance micro-supercapacitor applications. The growth mechanism is revealed that CNs prefers to grow on CNT surface rather than on SiO2 substrate due to the high binding energy of carbon atom absorbed on CNT surface. The symmetric all-carbon CN/CNT micro-supercapacitors exhibit ultrahigh areal capacitance of approximately 110 mF cm−2 at a current density of 0.3 mA cm−2, outstanding long-term cyclic stability (≈7% loss of initial capacitance after 10,000 cycles). Furthermore, flexible microsupercapacitors are achieved by peeling off microelectrodes using H3PO4/PVA electrolyte and showing high functional flexibility (capacitance loss less than 9% after 100-cyclic bending tests). The outstanding electrochemical performance and functional flexibility of present micro-supercapacitors show great potential applications in smart and miniaturized electronic devices.

Zinc-ion battery (ZIB) has been attracting extensive attention due to its high theoretical capacity, high safety, and low cost. However, the exploration of suitable cathode materials to host Zn ions remains a challenge, owing to the strong electrostatic interactions and large steric hindrance effects between Zn ion and host materials. Great efforts have been devoted to the optimization of the structure and electrochemical property of the cathode materials. In this review article, we summarize recent cathode-based strategies to address the issues of performance degradation and structural collapse of electrode materials within the context of microstructure design and ion/charge transport mechanism, and include a perspective to highlight the challenges and promises in the exploitation of cathode materials for further enhancement of the performance of ZIBs.

Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the low contact resistance (down to 90 Ωµm in MoS2, towards the quantum limit), the high field-effect mobility (up to 358,000 cm2V−1s−1 in WSe2), and the prominent transport characteristics at cryogenic temperatures. This method also offers possibilities of the local manipulation of atomic structures and electronic properties for TMDSC device design.

Designing electrode materials with engineered exposed facets provides a novel strategy to improve their electrochemical properties. However, the controllability of the exposed facet remains a daunting challenge, and a deep understanding of the correlation between exposed facet and Li+-transfer behavior has been rarely reported. In this work, single-crystal α-Fe2O3 hexagonal nanosheets with an exposed (001) facet are prepared with the assistance of aluminum ions through a one-step hydrothermal process, and structural characterizations reveal an Al3+-concentration-dependent-growth mechanism for the α-Fe2O3 nanosheets. Furthermore, such α-Fe2O3 nanosheets, when used as lithium-ion battery anodes, exhibit high specific capacity (1261.3 mAh g-1 at 200 mA g-1), high rate capability (with a reversible capacity of approximately 605 mAh g-1 at 10 A g-1), and excellent cyclic stability (with a capacity of over 900 mAh g-1 during 500 cycles). The superior electrochemical performance of α-Fe2O3 nanosheets is attributed to the pseudocapacitive behavior, Al-doping in the α-Fe2O3 structure, and improved Li+-transfer property across the (001) facet, as elucidated by first-principles calculations based on density functional theory. These results reveal the underlying mechanism of Li+ transfer across different facets and thus provide insights into the understanding of the excellent electrochemical performance.

Layered transition-metal oxides are one kind of the most promising cathode materials for sodium-ion batteries. In this study, we propose a strategy to enhance the electrochemical properties of P3-type manganese-based layered oxide cathode by introducing a small amount of layered P2 and Li-O'3 phases. Powder X-ray diffraction (PXRD) structural refinement and aberration-corrected scanning transmission electron microscopy (STEM) are performed to confirm the microstructures of different samples. PXRD refinement results show that the elevated annealing temperature leads to a partial conversion of the P3 phase to the P2 phase and the addition of lithium results in the formation of a new O'3 phase in the P3/P2-layered matrix. STEM results identify the intergrowth of P3/P2 and P3/P2/O'3 in biphasic and triphasic materials, respectively. Electron energy loss spectroscopy verifies that the alkali metal layer in the O'3 phase is occupied by the lithium ion. The intergrowth of biphasic and triphasic materials in these layered P3/P2 and P3/P2/O'3 hybrid structures brings forth a positive effect on the electrochemical properties. In particular, the formation of P3/P2/O'3-intergrown hybrid structures greatly improves the cycling stability of the P3 phase that the capacity retention of P3/P2/O'3 hybrid structures remains 78%, while capacity retention of the pure P3 phase is only 54.1% after 50 cycles at a rate of 0.2 C, and the rate performance of the P3 phase has also been enhanced.

Layered O3-type NaMn0.5Ni0.5O2 has been widely investigated as cathode material for sodium-ion batteries (SIBs). However, it usually suffers from detrimental phase transformation upon high voltage (>4.1 V) and sluggish Na+ migration kinetics, leading to rapid capacity decay and limited rate capability. Herein, guided by the first principles calculations, a structure modulation strategy to construct mechanically robust transition metal oxides (TMO2) layers for O3-NaMn0.5Ni0.5O2 is realized through Mg/Ti co-substitution. After Mg/Ti co-substitution, the capability of the TMO2 layers framework of O3 phase has been significantly improved to go against and tolerant strains and distortions, and thus remarkably enhanced the electrochemical performance of O3 phase upon high voltage. The as-prepared O3-type NaMn0.45Ni0.45Mg0.05Ti0.05O2 exhibits an initial discharge capacity of 177.7 mAh g−1 at a current density of 0.1 C in a voltage range of 2.0–4.2 V, and the detrimental P3-O1 phase transition upon 4.0 V can be effectively suppressed as well as the Na+ diffusion kinetics is enhanced under high voltage, subsequently leading to improved high voltage cycling-stability and rate-capability.

Sulfide electrolytes have attracted increasing attention due to their ultrahigh ionic conductivity and intrinsically soft properties. However, current solid-state lithium metal batteries (SSLMBs) based on sulfide electrolyte generally suffer from poor electrochemical performance due to the thick electrolyte layer and its terrible compatibility with Li metal anode. Herein, an ultrathin (∼60 μm) sulfide-based electrolyte film is prepared by a scalable slurry-casting method using Li10GeP2S12, polyethylene oxide, (3-chloropropyl)trimethoxysilane and nylon mesh (as a scaffold). Such film exhibits high ambient-temperature ionic conductivity (2.4 × 10−4 S cm−1), wide electrochemical window (4.7 V versus Li/Li+), outstanding heat-resistance and high mechanical strength (13.8 MPa). Furthermore, the compatibility of anode interface has been tremendously enhanced by the interface engineering of fluorine-rich gel protective layer on Li anode (Gel-Li) via in-situ polymerization, which can effectively prevent the lithium dendrite growth and endow outstanding long-term cyclic stability of the Li symmetric battery. As a result, the assembled MWCNT@S/Gel-Li and LiNbO3@LiNi0.6Mn0.2Co0.2O2/Gel-Li batteries based on such electrolyte film display desirable discharge specific capacities and outstanding cycling stability at ambient-temperature. In addition, the integrated pouch cell can operate well even under harsh environments of bending, cutting, punching situations, demonstrating that such thin electrolyte and its interface engineering applied in SSLMBs with high safety and durability.

To address the safety and low energy density of conventional liquid electrolyte lithium-ion batteries, ultrathin solid-state electrolyte with high mechanical modulus and ionic conductivity has been widely concerned. Here, we report an ultrathin, mechanically robust and flexible composite solid-state electrolyte, composed of three-dimensional network host (polyimide (PI) film), ionic conductive inorganic particle (Li6.75La3Zr1.75Ta0.25O12), polymer binder (polyvinylidene fluoride), and lithium salt-ionic liquid solution. PI film guarantees the flexibility and thin thickness (∼25 μm) of the composite electrolyte. The introduction of high-content ceramic filler and ionic liquid enhance mechanical strength and room-temperature ionic conductivity (3.7 × 10−4 S cm−1) of composite electrolyte while reducing the interfacial impedance between the electrolyte and the electrode. The capacity retention (153 mAh g−1) of the LiNi0.5Co0.2Mn0.3O2|Li battery based on the prepared electrolyte is 88% after 100 cycles at 0.1C, which has high safety in practical applications.

High-performance pseudocapacitive electrode with vertically oriented graphene nanosheet (GN) templating Ni–Co hydroxide (NCH) is fabricated through a facial electrodeposition way. The structural evolution of electrodes after cyclic voltammetry treatment at different scan rates (from 5 to 500 mV s−1) and its impact on the electrodes’ electrochemical performance are for the first time systematically investigated. The results reveal that the NCH/GN hybrid electrode exhibits ultrahigh electrochemical stability that the electrochemical properties of hybrid electrodes are not affected by the different rate-scanning treatments. GN grown on carbon cloth (CC) as nanotemplate for NCH pseudocapacitive material creates a rapid channel for ion/charge transfer between the electrolyte and electrode, and the robust interfacial bonding of hybrid structure is prerequisite for the high performance stability. Furthermore, asymmetric supercapacitor devices consisting of NCH/GN/CC as positive electrode and GN/CC as negative electrode were fabricated and electrochemically treated as well, further demonstrating the ultrahigh electrochemical stability of such NCH/GN/CC hybrid structures at various scan rates.

A three-dimensional graphene-based nanostructure with graphene aerogel templating graphene nanosheets (GA-GNs) has been fabricated via a modified hydrothermal method, followed by a microwave plasma chemical vapor deposition process. The mechanical properties of such hybrid nanostructures, for the first time, were quantitatively measured to demonstrate their superior mechanical robustness with stress and Young's modulus up to 96 kPa and 181.25 kPa, respectively. Finite-element modeling further reveals the mechanical strengthening mechanism with GNs as stiffening ribs layered over thin cellular walls within the GA. Moreover, GA-GNs present excellent electrical conductivity as high as similar to 1000 S/m, showing great promise as high-performance supercapacitor electrodes. The as-prepared free-standing and binder-free GA-GN electrode exhibits a high specific capacitance of 245 F g(-1) (based on the entire electrode mass) which corresponds to a high areal capacitance of 1.1 F cm(-2), desirable rate capability and outstanding cyclic stability with a capacitance retention of 92% over 10000 cycles. To assess their practical functionality, a two-terminal symmetric all-solid-state supercapacitor based on such all-carbon electrodes was fabricated. Such supercapacitor devices exhibited desirable areal capacitance, low internal resistance and outstanding cyclic stability. (C) 2017 Elsevier Ltd. All rights reserved.

Sulfide solid electrolytes have attracted much attention for next generation lithium batteries due to their high ionic conductivity and low grain boundary impedance. However, they generally suffer poor processability into a thin membrane applied in high energy–density batteries and usually unstable with some organic solvents and moistures. Furthermore, the huge interfacial resistance, originated from the side reactions at electrode/electrolyte interfaces, deteriorates the electrochemical performance of batteries. Here, we develop a thin (~100 μm) Li10GeP2S12 (LGPS) membrane composed by trace polytetrafluoroethylene (PTFE) binder with LGPS powders through all-dry grinding method and reinforced with a nylon mesh. Thin LGPS can not only reduce the cost, but also improve the energy density of the battery. Moreover, the interface stability between the LGPS electrolyte and electrodes has been significantly enhanced by employing an in-situ gel polymerized electrolyte (GPE) on lithium metal anode, and coating LiNbO3 on LiNi0.5Mn0.3Co0.2O2 (LNO@NCM532) separately. As a result, the prepared LNO@NCM532|C-LGPS|GPE|Li batteries reveal satisfying discharge specific capacities and good cyclic stabilities at room temperature.

Controllable layered MoS2 on 3D graphene aerogel (MoS2/GA) was synthesized via a facial hydrothermal process. As-prepared MoS2/graphene hybrid aerogels with desirable sizes serve directly as free-standing and binder-free anodes for lithium-ion batteries (LIB) without the need for mechanical cutting or mixing. The mechanical properties of MoS2/GA samples were quantitatively investigated to demonstrate their high mechanical robustness with Young’s modulus values up to 59.2 kPa and recoverable deformability of 90% elastic strain, indicating their ability to resist large-scale deformation or fatigue stress during LIB applications. Moreover, the effects of the MoS2 mass loading on both microstructure and electrochemical performance of MoS2/GA were systematically investigated. Hierarchical nanostructures of MoS2/GA with a mass ratio of MoS2 to graphene oxide as 1:2.5 exhibit a high specific capacity of 1360.5 mAh g−1 at a current density of 100 mAh g−1. These materials also have desirable rate capability and good cyclic stability, demonstrating their utility in energy storage devices.

A fluoroalkyl-silane (FS)-modified three-dimensional (3D) graphene foam (GF) composite (FS-GF) with a hierarchical porous microstructure was fabricated via a nickel foam template chemical vapor deposition and followed modification. The 3D GF, as an electrically conductive framework and hydrophobic porous substrate, was grafted with FS to further improve the hydrophobicity due to its long non-polar molecular chains and numerous fluoric/silicon functional groups. Porous FS-GF with high electrical conductivity (~650 S/m) is functionalized as a Joule-heating mesh for airflow, which synchronously demonstrates rapid heating rates, high conversion efficiencies, and uniform temperature distributions. Moreover, FS-GF exhibits self-cleaning and anti-icing properties that are superior to metallic and inorganic coatings. The outstanding performances of FS-GF suggest the potential for functionalizing 3D graphene composites using hierarchical structures at the multi-scale and chemical modifications of functional molecular groups.

Wire supercapacitors have recently elicited attention due to their potential to be woven into textiles as flexible power supplies for wearable electronic devices. However, contemporary wire supercapacitors generally suffer from low energy density and complicated fabrication and assembly processes. Here, we report a unique design of asymmetric wire supercapacitors in a wrap-twist architecture, with graphene nanopetals grown on carbon fiber tow as negative electrodes and MnO 2 nanosheets electrodeposited on carbon nanotube paper as positive electrodes. The wrap-twist structure integrates both positive and negative electrodes with edge-enriched nanostructures, and a new assembly procedure greatly increases the contact area between the two electrodes, leading to significantly improved electrochemical performance. Such asymmetric wire supercapacitors exhibit ultrahigh capacitances of over 40 mF cm −1 , outstanding energy and power densities, with energy densities up to 12 µW h cm −1 (approx. 50 µW h cm −2 on an area basis) and power densities up to approx. 3.7 mW cm −1 (approx. 15.4 mW cm −2 ), good cyclic stability and excellent flexibility. Moreover, the thermal performance of such wire supercapacitors is assessed by characterizing temperature effects on electrochemical performance over a temperature window ranging from 2 to 80 °C. The outstanding electrochemical and thermal performance of the wire supercapacitors demonstrates their promising potential as flexible power sources for wearable electronic devices.

Ni–Co–Mn multi-component metal oxides with various morphologies for high-performance supercapacitor electrodes were controllably synthesized on carbon cloth through a facile hydrothermal method followed by a subsequent annealing process. The crystalline structure, morphology and electrochemical property of the metal oxides can be manipulated by simply adjusting the Ni/Co/Mn ratio in the solution. The metal oxide prepared in the solution containing a Ni/Co/Mn ratio of 1/1/2 presents a structure consisting of nanosheets and nanoneedles, and exhibits the highest specific capacitance of 1434.2 F g−1 at a current density of 2 mA cm−2, desirable rate capability and excellent cyclic stability with a capacitance retention of 94% over 3000 cycles. To demonstrate practical application, flexible asymmetric supercapacitors based on such metal oxides were prepared and show high capacitance, low internal resistance, excellent cyclic stability and outstanding flexibility, indicating great potential as high-performance energy storage devices.

Designing electrode materials with engineered exposed facets provides a novel strategy to improve their electrochemical properties. However, the controllability of the exposed facet remains a daunting challenge, and a deep understanding of the correlation between exposed facet and Li

Rechargeable zinc-ion batteries (ZIBs) hold great potential for energy storage applications due to their cost-effectiveness, high safety, and high theoretical capacity. However, divalent zinc ions suffer from strong electrostatic interaction with their host materials during the charge/discharge process, resulting in the sluggish reaction kinetics. Improving the reaction kinetics of ZIBs is thereby of great significance to boost the electrochemical performance. Herein, we provide a comprehensive review on the energy storage mechanisms in ZIBs within the context of reaction kinetics including ion-diffusion coefficients, pseudocapacitive behaviors, theoretically computational Zn-ion migration paths and energy barriers, as well as ion transference in electrolyte and anode. Moreover, the advanced electrochemical measurements and theoretical calculation techniques for characterizing the kinetic parameters are also introduced. This review elucidates the correlations between and among electrode/electrolyte characters, reaction kinetics, reaction mechanisms, and electrochemical performance of ZIBs, and includes a perspective to highlight the challenges and promises in improving the performance of battery materials toward enhancing the reaction process kinetics, which can offer deep insights into the charge-storage process of ZIBs and provide valuable guides for the future design of fast-kinetics ZIBs.

A hierarchical structure consisting of Ni–Co hydroxide nanopetals (NCHPs) grown on a thin free-standing graphene petal foam (GPF) has been designed and fabricated by a two-step process for pseudocapacitive electrode applications. The mechanical behavior of GPFs has been, for the first time to our knowledge, quantitatively measured from in situ scanning electron microscope characterization of the petal foams during in-plane compression and bending processes. The Young's modulus of a typical GPF is 3.42 GPa, indicating its outstanding mechanical robustness as a nanotemplate. The GPF/NCHP electrodes exhibit volumetric capacitances as high as 765 F cm−3, equivalent to an areal capacitance of 15.3 F cm−2 and high rate capability. To assess practical functionality, two-terminal asymmetric solid-state supercapacitors with 3D GPF/NCHPs as positive electrodes are fabricated and shown to exhibit outstanding energy and power densities, with maximum average energy density of ≈10 mWh cm−3 and maximum power density of ≈3 W cm−3, high rate capability (a capacitance retention of ≈60% at 100 mA cm−2), and excellent long-term cyclic stability (full capacitance retention over 15 000 cycles).

All-solid-state lithium-sulfur batteries show increasing potential for practical applications in portable electronic devices and electric vehicles due to their high energy density, low cost, and environmental friendliness. However, the large interface impedance of solid-state lithium-sulfur batteries leads to low capacity and short cycle life. In this study, a hierarchical design of sandwiched NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid-state electrolyte with surface modification by a solid polymer electrolyte (SPE) is proposed for high-performance lithium-sulfur batteries. Due to the unique hierarchical structure of the solid electrolyte in which SPE (PEO: LiTFSI = 8:1) layer is coated on NASICON solid electrolyte surface, the direct contact between lithium metal anode and LATP solid electrolyte can be avoided, and the improved interfacial property between the electrolyte and electrodes can be achieved. As a result, the as-prepared solid-state Li|SPE-LATP-SPE|Li symmetrical cell exhibits a stable cycling performance. Moreover, solid-state lithium-sulfurized polyacrylonitrile (SPAN) battery based on such electrolyte delivers an ultrahigh initial discharge capacity of 1793 mAh g−1 at 75 °C with high coulombic efficiencies and stable cycling performance. The present work provides a creative design of battery configuration for manufacturing high-performance and safe solid-state lithium-sulfur batteries.

Designing electrodes in a highly ordered structure simultaneously with appropriate orientation, outstanding mechanical robustness, and high electrical conductivity to achieve excellent electrochemical performance remains a daunting challenge. Inspired by the phenomenon in nature that leaves significantly increase exposed tree surface area to absorb carbon dioxide (like ions) from the environments (like electrolyte) for photosynthesis, we report a design of micro-conduits in a bioinspired leaves-on-branchlet structure consisting of carbon nanotube arrays serving as branchlets and graphene petals as leaves for such electrodes. The hierarchical all-carbon micro-conduit electrodes with hollow channels exhibit high areal capacitance of 2.35 F cm−2 (~500 F g−1 based on active material mass), high rate capability and outstanding cyclic stability (capacitance retention of ~95% over 10,000 cycles). Furthermore, Nernst–Planck–Poisson calculations elucidate the underlying mechanism of charge transfer and storage governed by sharp graphene petal edges, and thus provides insights into their outstanding electrochemical performance., One way to improve the performance of supercapacitors is to use hybrid carbon nanomaterials. Here the authors show a bioinspired electrode design with graphene petals and carbon nanotube arrays serving as leaves and branchlets, respectively. The structure affords excellent electrochemical characteristics.

A hierarchical structure consisting of Ni–Co hydroxide nanosheets (NCHN) electrodeposited on vertically-oriented graphene nanosheets (GN) on carbon cloth (CC) was fabricated for high-performance pseudocapacitive electrodes. NCHN was uniformly distributed on GN, forming a sheet-on-sheet hierarchical structure. Such NCHN/GN/CC hybrid electrodes exhibit high capacitance and ultrahigh electrochemical-stability that structure and electrochemical properties of hybrid electrodes are not affected by the cyclic low-rate scanning (at 5 mV s−1 even over 1000 cycles). GN vertically grown on CC is used as nano-bridge between NCHN active materials and CC current collector, which effectively facilitates ion/charge transfer between the electrolyte and electrode, consequently leading to the ultrahigh electrochemical-stability of hybrid electrodes. To assess functional behavior, two-terminal flexible asymmetric supercapacitor devices with NCHN/GN/CC as positive electrode and GN/CC as negative electrode were assembled and electrochemically treated to demonstrate the ultrahigh electrochemical stability.

Polygonal carbon nanofibers (PCNFs) were prepared on a large scale by chemical vapor deposition using Ni3Sn2 intermetallic compound as a catalyst. The PCNFs feature polygonal cross sections with side lengths ranging from 200 nm to 400 nm, as primarily determined by the orthorhombic structure of the Ni3Sn2 compound. The PCNFs were subsequently activated by KOH with different concentrations, denoted as a-PCNFs, for supercapacitor electrode applications. The PCNFs were significantly etched during the activation process under a high KOH concentration, forming a unique thin ribbon-like nanostructure with large specific surface area and high content of oxygen-containing functional groups. The electrochemical measurements reveal that a-PCNFs, activated by KOH at a KOH : C weight ratio of 4 : 1 under 800 °C, exhibit favorable electrochemical properties with a specific capacitance of 186 F g−1 at a current density of 3 A g−1 in 1 M Na2SO4, good rate capability, low internal resistance, and reasonably stable cycle life. These promising electrochemical properties indicate significant potential for use as scalable supercapacitor electrodes.

Ni–Co–Mn triple hydroxide (NCMTH) nanoneedles were coated on plasma-grown graphitic petals (GPs) by a facile one-step hydrothermal method for high-rate and long-cycle-life pseudocapacitive electrodes. Structural and compositional characteristics of NCMTHs indicate that the multi-component metal elements distribute homogeneously within the NCMTHs. Comparison of the electrochemical performance of the three-dimensional NCMTH electrodes to Ni–Co double hydroxides reveals that a synergistic effect of the hierarchical structure of GPs and NCMTHs enables their high rate capability and long cycle life. The NCMTH electrode maintains over 95% of its capacitance at a high charge/discharge rate of 100 mA cm−2 relative to its low-current (1 mA cm−2) capacitance; and it exhibits very high specific capacitance of approximately 1400 F g−1 (based on the mass of NCMTH), high specific energy density (≈30 W h kg−1) and power density (≈39 kW kg−1) at a high current density of 100 mA cm−2, and excellent long-term cyclic stability (full capacitance retention over 3000 cycles). To assess functional behavior, two-terminal asymmetric supercapacitor devices with NCMTHs on graphitic petals as positive electrodes were assembled and tested to reveal ultrafast charge/discharge rates up to 5000 mV s−1 (approx. two orders of magnitude faster than conventional asymmetric devices based on metal hydroxides) with high rate capabilities, and excellent long-term cyclic stability (full capacitance retention over 10 000 cycles).