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The high volatility of the price of cobalt and the geopolitical limitations of cobalt mining have made the elimination of Co a pressing need for the automotive industry

Solid-state lithium-metal (Li

Metal anodes (e.g., lithium, sodium and zinc metal anodes) based on a unique plating/stripping mechanism have been well recognized as the most promising anodes for next-generation high-energy metal batteries owing to their superior theoretical specific capacities and low redox potentials. However, realizing full utilization and the theoretical capacity of metal anodes remains challenging because of their high reactivity, poor reversibility, and nonplanar metal evolution patterns, which lead to irreversible loss of active metals and the electrolyte. To minimize the above issues, excess metal sources and flooded electrolytes are generally used for laboratory-based studies. Despite the superior cycling performance achieved for these cells, the metal-anode-excess design deviates from practical applications due to the low anode utilization, highly inflated coulombic efficiency, and undesirable volumetric capacity. In contrast, anode-free configurations can overcome these drawbacks while reducing fabrication costs and improving cell safety. In this review, the significance of anode-free configurations is elaborated, and different types of anode-free cells are introduced, including reported designs and proposed feasible yet unexplored concepts. The optimization strategies for anode-free lithium, sodium, zinc, and aluminum metal batteries are summarized. Most importantly, the remaining challenges for extending the cycle life of anode-free cells are discussed, and the requirements for anode-free cells to reach practical applications are highlighted. This comprehensive review is expected to draw more attention to anode-free configurations and bring new inspiration to the design of high-energy metal batteries. Anode-free metal batteries can deliver higher energy densities than traditional anode-excess metal batteries and metal-ion batteries. Yet the cycle life of anode-free cells is limited by the non-planar growth and low coulombic efficiency of the metal anodes. In this review, we not only systematically elaborate the working/failure mechanisms and achieved progress for the reported anode-free Li/Na/Zn/Al battery systems, but also propose a series of conceptually-feasible yet unexplored anode-free systems.

The rechargeability of aqueous zinc metal batteries is plagued by parasitic reactions of the zinc metal anode and detrimental morphologies such as dendritic or dead zinc. To improve the zinc metal reversibility, hereby we report a new solution structure of aqueous electrolyte with hydroxyl-ion scavengers and hydrophobicity localized in solvent clusters. We show that although hydrophobicity sounds counterintuitive for an aqueous system, hydrophilic pockets may be encapsulated inside a hydrophobic outer layer, and a hydrophobic anode-electrolyte interface can be generated through the addition of a cation-philic, strongly anion-phobic, and OH

Poor stability of nanostructured electrocatalysts at rigorous industrial conditions significantly inhibits their performances in practical electrolyzers. Although many substrate-supported nanostructured electrocatalysts present attractive performance at small currents, they cannot sustain industry-level high current densities for long-term operation. Herein, by chemically organizing nanoscale electrocatalysts into a macroscopic substrate-free metallic alloy aerogel, this NiFe-based nano-catalyst achieves 1000-h durability at industrial-level current densities, with exceptionally high activities of 500 mA at the overpotential of only 281 mV. This NiFe alloy aerogel is constructed by a magnetic-field assisted growth and assembly of ferromagnetic NiFe nanoparticles, in which nanowires are loosely crosslinked by metallic joints. This alloy aerogel shows a high electric conductivity of 500 S m

Exploring electrocatalysts with satisfactory activity and durability has remained a long-lasting target for electrolyzing water, which is particularly significant for sustainable hydrogen fuel production. Here, we report a quaternary B/P-codoped transition metal Co-Mo hybrid as an efficient alternative catalyst for overall water splitting. The Co-Mo-B-P/CF dual nanowafers were deposited on a copper foam by double-pulse electrodeposition, which is favorable for achieving a nanocrystalline structure. The Co-Mo-B-P/CF catalyst shows a high catalytic activity along with good long-term stability in 1.0 M KOH solutions for both the hydrogen and oxygen evolution reactions, requiring 48 and 275 mV to reach 10 mA cm-2, respectively. The synergetic effect between Co-Mo and doped B and P elements is mainly attributed to the excellent bifunctional catalysis performance, while the dual-nanowafer structure endows Co-Mo-B-P with numerous catalytical active sites enhancing the utilization efficiency of atoms. Moreover, the catalytic capability of Co-Mo-B-P/CF as a bifunctional electrocatalyst for the overall water splitting is proved, with the current density of 10 mA cm-2 accomplished at 1.59 V. After the stability test for overall water splitting at 1.59 V for 24 h, the activity almost remains unchanged. The features of excellent electrocatalytic activity, simple preparation, and inexpensive raw materials for Co-Mo-B-P/CF as a bifunctional catalyst hold great potentials for overall water splitting.

Lithium (Li) metal, a typical alkaline metal, has been hailed as the "holy grail" anode material for next generation batteries owing to its high theoretical capacity and low redox reaction potential. However, the uncontrolled Li plating/stripping issue of Li metal anodes, associated with polymorphous Li formation, "dead Li" accumulation, poor Coulombic efficiency, inferior cyclic stability, and hazardous safety risks (such as explosion), remains as one major roadblock for their practical applications. In principle, polymorphous Li deposits on Li metal anodes includes smooth Li (film-like Li) and a group of irregularly patterned Li (e.g., whisker-like Li (Li whiskers), moss-like Li (Li mosses), tree-like Li (Li dendrites), and their combinations). The nucleation and growth of these Li polymorphs are dominantly dependent on multiphysical fields, involving the ionic concentration field, electric field, stress field, and temperature field, etc. This review provides a clear picture and in-depth discussion on the classification and initiation/growth mechanisms of polymorphous Li from the new perspective of multiphysical fields, particularly for irregular Li patterns. Specifically, we discuss the impact of multiphysical fields' distribution and intensity on Li plating behavior as well as their connection with the electrochemical and metallurgical properties of Li metal and some other factors (e.g., electrolyte composition, solid electrolyte interphase (SEI) layer, and initial nuclei states). Accordingly, the studies on the progress for delaying/suppressing/redirecting irregular Li evolution to enhance the stability and safety performance of Li metal batteries are reviewed, which are also categorized based on the multiphysical fields. Finally, an overview of the existing challenges and the future development directions of metal anodes are summarized and prospected.

It is highly desirable to develop all-pH-compatible hydrogen evolution reaction (HER) electrocatalysts that can be used universally in a variety of different electrolyzers and operated in different...

Conventional acidic water electrolysis for large-scale hydrogen production needs to involve a noble metal catalyst for the anode to resist electrochemical oxidation, while alkaline electrolysis can provide better anode protection, but hydrogen ions become a minority species, which leads to sluggish hydrogen evolution reaction (HER) kinetics. Herein, by developing a unique nano-thorn-like Pt–Ni nanowire electrode as a superior HER catalyst, we enable a local “pseudo-acidic” environment near the cathode surface in an alkaline electrolyzer. In such a situation, we observed dramatic enhancement of selective H+ adsorption versus K+, leading to an extremely high HER performance towards real applications, with low overpotentials (ηgeo-surface area) of 23 mV and 71 mV at current densities of 10 mA cm−2 and 200 mA cm−2, respectively. This result is exceptionally better than the state-of-the-art Pt-based catalysts in an alkaline electrolyte at large current densities (≥200 mA cm−2). The simulation result suggests that a strong local electric field around a nano-thorn structure can exponentially increase the diffusion rate of H+ towards the electrode surface as compared with K+, which promotes faster mass transfer and reaction kinetics for the HER in an alkaline medium.

Boosting the activity of catalysts by constructing abundant low-coordinated sites is of considerable interest for maximizing the efficiency of catalysts. Equally importantly, three-dimensional imaging of these low-coordinated sites is crucial to the fundamental understanding of the relationship between the coordination environment and catalytic performance. Herein, we fabricate a novel class of hierarchical nanoporous gold which is rich in low-coordinated sites through solid-state plasma dealloying. For the first time, by using atomic-resolution electron tomography, we map out the coordination environment of the hierarchical nanoporous gold in three dimensions at the single-atom level. Quantitative coordination analysis reveals that owing to the introduction of substantial low-coordinated active sites with coordination numbers from 5 to 7, the catalytic performance of the hierarchical nanoporous gold in CO oxidation is improved. Our work provides in-depth insights into the relationship between the catalyst's coordination environment and catalytic performance, and the proof-of-concept demonstrated in this study is expected to be generally applicable to the design and study of a broad range of catalysts.

Pseudocapacitive metal oxide active materials are promising for use in high performance supercapacitors. However, their poor electrical conductivity greatly hinders their practical application. We formed NiO and Co3O4 active materials on the surface of a highly conductive NiCo nanowire membrane current collector by a simple air oxidation method to fabricate a self-supported flexible NiO/Co3O4@NiCo electrode. This significantly improves electron transport at the interface between the current collector and the active NiO/Co3O4. Furthermore, the reticular nanowire network facilitates ion transportation and releases strain caused during charge and discharge. Due to this unique structural characteristic, the NiO/Co3O4@NiCo electrode delivers a high specific capacitance of 1.36 F cm−2 at a current density of 5 mA cm−2, and excellent cycling stability with a capacitance retention of 96.95% after 10 000 cycles. An asymmetric supercapacitor was assembled using the NiO/Co3O4@NiCo as the cathode and an activated carbon electrode as the anode, which delivered an energy density of 0.32 mWh cm−2 at a power density of 8 mW cm−2. Even at a high power density of 40 mW cm−2, an energy density of 0.17 mWh cm−2 was achieved, suggesting its promising use as an efficient electrode for high performance supercapacitors.

Despite significant advances in cathode materials, developing high-performance anodes remains a key challenge for future pseudocapacitors. Fe2O3 has been considered as a promising anode candidate due to its high theoretical capacitance, environmental benignity, and earth-abundant characteristics. However, the low electronic conductivity and poor cyclability of Fe2O3 significantly limit its practical application. In this work, a 3D nickel-metalized carbon nanofiber network was developed to deposit an Fe2O3 nanosheet anode. The nickel layer not only improved the electronic conductivity and the wettability of the 3D carbon substrate but also benefit the stability of the Fe2O3/carbon interfaces and the stress-release upon cycling. As a result, the newly designed Fe2O3 anode composite exhibited a high areal capacitance of 1.80 F cm−2 at a high mass loading of 4.2 mg cm−1 and ultra-high capacitance retention of 85.1% after successive 100 000 cycles, outperformed most of the reported Fe2O3-based anode materials. Extended the interface metallization method to a MnO2 cathode, excellent capacitance retention of 108.2% was reached after 26 000 cycles, suggesting a potentially broad application of such an interface-management method in elevating the stability of metal oxide materials in various pseudocapacitive energy storage devices.

Lithium (Li) metal is one of the most promising anode materials for future high-energy-density rechargeable batteries. However, the uncontrollable growth of dendrites and the related safety issue hindered its practical application. Involving three-dimensional (3D) frameworks for hosting Li metal can provide large electrochemically active surface area and thus endow more homogeneous deposition and delay dendrite formation. But a more ideal situation for ultimately safe and high-performance Li metal based battery is to enable a gradient Li deposition/dissolution process, with the merit of effective utilization of spatial dimension to stabilize Li metal anode to achieve superior electrochemical performance. Here, we report a conductive-dielectric gradient framework which can guide a “bottom-up” Li deposition and “top-down” Li dissolution within this structure, rendering controllable and stable Li metal deposition/dissolution process. As a result, in a symmetric cell such anode can deliver stable Li metal deposition/dissolution for 780 h with a low overpotential (

Water electrolysis represents a promising sustainable hydrogen production technology. However, in practical application which requires extremely large current densities (>500 mA cm−2), the oxygen evolution reaction (OER) becomes unstable and kinetically sluggish, which is a major hurdle to large-scale hydrogen production. Herein, we report an exceptionally active and binder-free NiFe nanowire array based OER electrode that allows durable water splitting at current densities up to 1000 mA cm−2 up to 120 hours. Specifically, NiFe oxyhydroxide (shell)–anchored NiFe alloy nanowire (core) arrays are prepared via a magnetic-field-assisted chemical deposition method. The ultrathin (1–5 nm) and amorphous NiFe oxyhydroxide is in situ formed on the NiFe alloy nanowire surface, which is identified as an intrinsically highly active phase for the OER. Additionally, the fine geometry of the hierarchical electrode can substantially improve charge and mass (reactants and oxygen bubbles) transfer. In an alkaline electrolyte, this OER electrode can yield current densities of 500 and 1000 mA cm−2 stably over 120 hours at overpotentials of only 248 mV and 258 mV respectively, which are dramatically lower than any recently reported overpotentials. Notably, the integrated alkaline electrolyzer (with pure Ni nanowires as HER electrode) is demonstrated to reach the current density of 1000 mA cm−2 with super low voltage of 1.76 V, outperforming the state-of-the-art industrial catalysts. Our result may represent a critical step towards an industrial electrolyzer for large-scale hydrogen production by water splitting.

Manganese dioxide (MnO2) with a conversion mechanism is regarded as a promising anode material for lithium-ion batteries (LIBs) owing to its high theoretical capacity (∼1223 mA h g-1) and environmental benignity as well as low cost. However, it suffers from insufficient rate capability and poor cyclic stability. To circumvent this obstacle, semiconducting polypyrrole coated-δ-MnO2 nanosheet arrays on nickel foam (denoted as MnO2@PPy/NF) are prepared via hydrothermal growth of MnO2 followed by the electrodeposition of PPy on the anode in LIBs. The electrode with ∼50 nm thick PPy coating exhibits an outstanding overall electrochemical performance. Specifically, a high rate capability is obtained with ∼430 mA h g-1 of discharge capacity at a high current density of 2.67 A g-1 and more than 95% capacity is retained after over 120 cycles at a current rate of 0.86 A g-1. These high electrochemical performances are attributed to the special structure which shortens the ion diffusion pathway, accelerates charge transfer, and alleviates volume change in the charging/discharging process, suggesting a promising route for designing a conversion-type anode material for LIBs.

Aqueous zinc batteries are of great interest as a rechargeable energy storage system, particularly owing to the low cost and high safety of aqueous electrolytes, as well as the high capacity of zinc anodes. Unfortunately, the wide commercialization of aqueous zinc batteries is impeded by the irreversible water reduction and irregular zinc evolution issues on the anode side. Hereby, a hydrophobic and ultrathin polystyrene molecule brush layer is tethered onto the surface of zinc metal anodes to tackle the above limitations. Experimental investigations reveal that the waterproof artificial layer can sustain fast interfacial ionic transportation, minimize hydrogen evolution, and smoothen Zn deposition, thus conferring enhanced electrochemical performance to the as-protected Zn anode in both symmetric Zn//Zn cells and Zn//LiV

Designing stable Li metal and supporting solid structures (SSS) is of fundamental importance in rechargeable Li-metal batteries. Yet, the stripping kinetics of Li metal and its mechanical effect on the supporting solids (including solid electrolyte interface) remain mysterious to date. Here, through nanoscale in situ observations of a solid-state Li-metal battery in an electron microscope, two distinct cavitation-mediated Li stripping modes controlled by the ratio of the SSS thickness (t) to the Li deposit's radius (r) are discovered. A quantitative criterion is established to understand the damage tolerance of SSS on the Li-metal stripping pathways. For mechanically unstable SSS (t/r 0.21), the stripping proceeds via tension-induced multisite cavitation accompanied by severe SSS buckling and necking, ultimately leading to Li "trapping" or "dead Li" formation; for mechanically stable SSS (t/r 0.21), the Li metal undergoes nearly planar stripping from the root via single cavitation, showing negligible buckling. This work proves the existence of an electronically conductive precursor film coated on the interior of solid electrolytes that however can be mechanically damaged, and it is of potential importance to the design of delicate Li-metal supporting structures to high-performance solid-state Li-metal batteries.

Developing uniform and interconnected conductive networks is critically important for future electrochemical electrodes. Reticular holey metallic nanotube networks can be a competitive candidate for this purpose, taking the advantages of a large surface area (both inner and outer surface can host active material) and excellent mechanical flexibility. However, due to the limited available fabrication techniques, there have not been any report regarding the controllable and large-scale fabrication of such a structure. Herein, we report an industry-compatible fabrication method for highly interconnected and conductive network of holey nickel nanotube membrane (HNNM) for electrode conductive scaffold, which is highly conducive for mass and charge transfer and shows superior mechanical flexibility. The as-obtained HNNM presents a high surface area of 8.67 m2 g−1 and conductivity (∼10-1 Ω/□ with the thickness of 15 μm), which is superior than those of nickel foam, carbon cloth and stainless-steel mesh etc. By electrodepositing a thin layer of MnO2 on HNNM (HNNM@MnO2) as cathode, and Zn metal on HNNM (HNNM@Zn) as anode, an aqueous flexible rechargeable Zn/MnO2 battery was successfully assembled with a capacity of 275.5 mA h g−1 at 0.2 A g−1 and a working voltage of 1.38 V. Moreover, it achieves an energy density of 380.19 Wh kg−1 with a maximum power density of 1.38 kW kg−1 and a high capacity retention after 500 cycles, which represents a critical step forward toward environmentally benign, low cost, and high performance electrochemical energy storage for practical applications.

The employment of three-dimensional (3D) conductive scaffolds can improve safety level of metallic anodes at high rates, since they can lower down the average current density per electrode area, and thus suppress/delay the formation of metal dendrites. However, metal dendrites would still preferentially grow in the anode areas which are near to cathode, due to the more concentrated electric field strength. Here, we demonstrate that in-situ forming Li2O nano-layer on the surface of 3D Ni nanowire scaffold can not only facilitate Li+ transportation, but also render more homogenous electric field strength distribution throughout the entire anode. We observed a very low voltage hysteresis (~55 mV during 200 cycles) with enhanced cycling stability of more than 150 cycles at 3 mA cm−2. When coupling with LiFePO4 cathode, the as-assembled full cell can stably run for over 300 cycles with minimal capacity degradation and an average Columbic efficiency of 99.9% at 1 C, showing significant improvement of battery cycling stability. This work may represent a key step towards scalable production of highly safe metal anodes using electric field theory.

Doped LiNiO2 has recently become one of the most promising cathode materials for its high specific energy, long cycle life, and reduced cobalt content. Despite this, the degradation mechanism of LiNiO2 and its derivatives still remains elusive. Here, by combining in situ electron microscopy and first-principles calculations, we elucidate the atomic-level chemomechanical degradation pathway of LiNiO2-derived cathodes. We uncover that the O1 phase formed at high voltages acts as a preferential site for rock-salt transformation via a two-step pathway involving cation mixing and shear along (003) planes. Moreover, electron tomography reveals that planar cracks nucleated simultaneously from particle interior and surface propagate along the [100] direction on (003) planes, accompanied by concurrent structural degradation in a discrete manner. Our results provide an in-depth understanding of the degradation mechanism of LiNiO2-derived cathodes, pointing out the concept that suppressing the O1 phase and oxygen loss is key to stabilizing LiNiO2 for developing next-generation high-energy cathode materials. U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable EnergyUnited States Department of Energy (DOE) [DE-EE0008444]; H.L.X.'s startup fund; U.S. DOE Office of Science Facility, at Brookhaven National LaboratoryUnited States Department of Energy (DOE) [DE-SC0012704]; DOE Office of Science by Argonne National LaboratoryUnited States Department of Energy (DOE) [DE-AC02-06CH11357]; Early Career Research Program, Materials Science and Engineering Divisions, Office of Basic Energy Sciences of the U.S. DOEUnited States Department of Energy (DOE) [DE-SC0021204]; National Science Foundation through the UC Irvine Materials Research Science and Engineering Center [DMR-2011967] Published version Thismaterial is based upon work supported by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy under the award number DEEE0008444. R.Z.'s and C.H.'s work done for this study was funded by H.L.X.'s startup funding. We thank Dr. Jianli Cheng for illuminating discussions. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under contract no. DE-SC0012704. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. This research made use of the AtomSegNet software42 developed and maintained under the Early Career Research Program, Materials Science and Engineering Divisions, Office of Basic Energy Sciences of the U.S. DOE, under award no. DE-SC0021204. This work used the facilities and instrumentation at the UC Irvine Materials Research Institute (IMRI), which is supported in part by the National Science Foundation through the UC Irvine Materials Research Science and Engineering Center (DMR-2011967).

The slow kinetics of oxygen evolution reaction (OER) causes high power consumption for electrochemical water splitting. Various strategies have been attempted to accelerate the OER rate, but there are few studies on regulating the transport of reactants especially under large current densities when the mass transfer factor dominates the evolution reactions. Herein, Nix Fe1- x alloy nanocones arrays (with ≈2 nm surface NiO/NiFe(OH)2 layer) are adopted to boost the transport of reactants. Finite element analysis suggests that the high-curvature tips can enhance the local electric field, which induces an order of magnitude higher concentration of hydroxide ions (OH- ) at the active sites and promotes intrinsic OER activity by 67% at 1.5 V. Experimental results show that a fabricated NiFe nanocone array electrode, with optimized alloy composition, has a small overpotential of 190 mV at 10 mA cm-2 and 255 mV at 500 mA cm-2 . When calibrated by electrochemical surface area, the nanocones electrode outperforms the state-of-the-art OER electrocatalysts. The positive effect of the tip-enhanced local electric field in promoting mass transfer is also confirmed by comparing samples with different tip curvature radii. It is suggested that this local field enhanced OER kinetics is a generic effect to other OER catalysts. Accepted version

Metallic sodium (Na) is an appealing anode material for high-energy Na batteries. However, Na metal suffers from low coulombic efficiencies and severe dendrite growth during plating/stripping cycles, causing short circuits. As an effective strategy to improve the deposition behavior of Na metal, a 3D carbon foam is developed that is sputter-coated with gold nanoparticles (Au/CF), forming a functional gradient through its thickness. The highly porous Au/CF host is proven to have gradually varying sodiophilicity, which in turn facilitates initially preferential Na deposition on the gold-rich, sodiophilic region in a "bottom-up growth" mode, leading to uniform plating over the entire Au/CF host. This finding contrasts with dendrite formation in the pristine CF host, as proven by in situ microscopy. The Na-predeposited Au/CF (Na@Au/CF) composite anode operates steadily for 1000 h at a low overpotential of ≈20 mV at 2 mA cm-2 in a symmetric cell. When the composite anode is coupled with a Na3 V2 (PO4 )2 F3 cathode, the full cell has a high capacity of 102.1 mAh g-1 after 500 cycles at 2 C. The sodiophilicity gradient design that is explored in this study offers new insight into developing porous Na metal hosts with highly stable plating/stripping performance for next-generation Na batteries.

Manganese dioxide (MnO

Micro-supercapacitors with excellent electrochemical performance and aesthetic property are realized using the carbon nanotubes/manganese dioxide nanosheets (CNTs/δ-MnO2) composite as electrodes. This CNTs/δ-MnO2 nanocomposite is excellently compatible with the slurry dispensing process for electrode fabrication, and thus is conducive for preparing thick electrode films, which exhibits a specific capacitance of 257 F/g with an electrode thickness of 13 μm. By involving laser-scribing technique, the electrode film can be patterned with a high resolution and fabricated into a planar micro-supercapacitor, showing the maximum energy density of 6.83 mWh/cm3 at the power density of 154.3 mW/cm3, and maintained a value of 2.71 mWh/cm3 at the maximum power density of 2557.5 mW/cm3. Considering the versatility of the laser-scribing technical platform, the micro-supercapacitors fabricated in this way exhibit excellent aesthetic property and can cater to various miniaturized wearable electronic applications. This technology opens up opportunities for facile and scalable fabrication of high performance energy devices with shape diversity and a meaning of art.

Well-ordered, hierarchical and nanostructured composite electrodes have gained tremendous research attention for energy storage applications, because of their highly efficient electron and ion transport channels and abundant electrochemically active sites. However, it still remains a great challenge to prepare such architectures on a large scale with high active material mass loadings for making better use of the whole electrode area. Herein, needle-like NiCo2S4 nanowhiskers are radially grown on a uniform nickel nanowire array (NNA), forming a unique densely packed test-tube-brush-like nanostructure. Since these NiCo2S4 nanowhiskers are intrinsically highly electrically conductive, the hierarchical electrode structure can drastically elevate the charge transport ability in the whole electrode region; additionally, it can greatly help to release stresses at micro- and nano-scales, leading to robust mechanical flexibility and superior energy storage capability. Experimental results show that this composite array electrode exhibits an ultrahigh specific capacitance of 1523 F g−1 at a mass loading of 4.03 mg cm−2 at a current density of 1 A g−1, and a rate capability of 61.8% from 1 to 40 A g−1, together with a superior cycle stability with 92.4% capacitance retention after 20 000 cycles at a current density as high as 10 A g−1. An asymmetric supercapacitor consisting of a NNA@NiCo2S4 (NNANCS) cathode and activated carbon (AC) anode delivers a maximum energy density of 47.29 W h kg−1 at a power density of 793.5 W kg−1 and still delivers an energy density of 29.50 W h kg−1 at a maximum power density of 27.64 kW kg−1. This work may inspire new ideas for constructing high-performance electrodes for energy storage.

The recent discovery of a fast-charging vanadium-based disordered rock-salt anode for lithium-ion batteries (Nature, 2020, 585, 63−67) has rekindled great interest to screen possible anode candidates from the existing cathodes by studying their underexplored low-voltage ion storage behavior, particularly for vanadium-based compounds. Among them, layered NH4V4O10 (NVO) material is typically known as an intercalation-type cathode material, with large interlayer spacing for facile ion intercalation; however, to date, there is no investigation of its utilization as a potential anode material. Here, the vanadium redox and structural evolution of NVO nanobelts (NBs) under an anode voltage window (0.01−3.0 V vs. Na+/Na) are carefully studied by in-situ transmission electron microscopy (TEM) and electrochemical measurements. By in-situ TEM tracking the full sodiation process in real-time, a stepwise Na-storage reaction mechanism is revealed, initiating with the interlaminar intercalation of Na ions accompanied with the appearance of NaxNVO phase and ending with the conversion reaction with the final formation of V2O3 phase. While upon desodiation, the V2O3 phase can only be oxidized to VO2 phase, rather than the original NVO phase. Afterward, a reversible conversion reaction between VO2 and V2O3 phases is established upon the subsequent (de)sodiation cycles, which delivers a reversible cycling capacity of 148 mAh g–1 at 1 C, as verified by electrochemical measurements. The in-situ observation also witnesses the emergence of nanopores in NBs that may alleviate significant structural strain and contribute to the long-term cycling stability during the following (de)sodiation cycles. This work has validated for the first time the practicability of NVO as an anode material in sodium-ion batteries and afforded a paradigm of revisiting existing cathodes to explore their possible anode utilization.

Developing supercapacitor electrodes with an ultra-long cycle life and a high specific capacitance is critical to the future energy storage devices. Herein, we report a scalable synthesis technology of mixed-valence manganese oxide nanoparticles anchored to reduced graphene oxide (rGO/MnOx) as the high-performance supercapacitor electrodes. First, 2-dimensional (2D) δ-MnO2 nanosheets are formed on a graphene oxide (GO) template, which is then in situ reduced by hydrazine vapour to mixed-valence manganese oxide nanoparticles evenly distributed on a rGO conductive network. The obtained rGO/MnOx electrode material exhibits a high specific capacitance of 202 F g−1 (mass loading of 2 mg cm−2), a large areal specific capacitance of 2.5 F cm−2 (mass loading of up to 19 mg cm−2), and a super-long-life stability of 106% capacitance retention after 115 000 charge/discharge cycles. By using an ionic liquid electrolyte and an activated carbon anode, asymmetric supercapacitors (AScs) are also constructed and can be packaged into a high performance miniaturized energy storage component in either a tailorable or surface mountable configuration. Our ASc shows superior performance characteristics, with typical figures of merit including maximum energy densities of 47.9 W h kg−1 at 270 W kg−1 and 19.1 W h kg−1 at the maximum power density of 20.8 kW kg−1. The capacitance retention of the ASc is 96% after 80 000 charge/discharge cycles, which is the most excellent stability performance in an ionic liquid electrolyte as compared with the recently reported pseudo-supercapacitors. This technology may find vast applications in future miniaturized portable and wearable electronics.

Despite significant advances in cathode materials, developing high-performance anodes remains a key challenge for future pseudocapacitors. Fe

Supercapacitors represent a promising energy storage technology for flexible, miniaturized, and wearable electronic devices in the future. At present, to achieve high mechanical flexibility, many available flexible supercapacitors are constructed into thin electronic devices. However, the thin thickness of the whole supercapacitor electrode will greatly limit the mass loading of active materials and lower the energy density, which as a result, substantially hindering the application of supercapacitors for wearable electronic devices. Herein, we report a magnetic field induced fabrication of threedimensional nonwoven cloth consists of cross-linked ultra-long NiCo sulfide nanowires as a high-performance supercapacitor cathode. Due to the stress release by slippage between the cross- linked nanowires under external force, the NiCo sulfide nonwoven cloth electrode with the thickness up to 0.5 mm can achieve ultra-high flexibility, even foldability and arbitrarily deformable properties. Most importantly, the thick threedimensional architecture enables the high mass loading of active materials, which is crucial for high energy density flexible supercapacitors. The NiCo sulfide nonwoven cloth electrode demonstrates an ultra-high areal capacitance over 4.91 F cm-2, with negligible capacitance decay after 10000 cycles, outperforming most of reported flexible supercapacitors and revealing its great potential for practical application in wearable electronic device.

Arbitrary-shaped batteries with high energy densities are strongly desired for weight- and power-sensitive applications to meet various layout and function demands. Conventional battery electrode fabrication protocol requires a relatively thick metal foil (9–15 μm) to provide sufficient mechanical strength for slurry coating, leading to high occupancy of inactive components. Besides, conventional battery assembly methods result in limited battery layouts, which is difficult for well-fitting complex-shaped electronics. To overcome these limitations, developing novel battery fabrication technologies is critically urgent and important. Herein, we report a scalable method to fabricate arbitrary-shaped lithium-ion batteries with ultra-thin current collectors. Built on a commercial polypropylene separator, an all-in-one structured lithium ion battery is fabricated by integrating active material layers and ultra-thin metal film current collectors (optimally 2-μm-thick) through stencil printing and magnetron sputtering methods. By reducing the thickness of current collectors down to 2 μm, the energy density of a LiFePO4//Li4Ti5O12 full cell can be increased by 16%–64% based on the designed areal capacities between 1 mAh cm−2 to 3 mAh cm−2. Such battery fabrication method can effectively realize the shape diversity of electrodes, thereupon enabling the seamless integration of arbitrary-shaped batteries into versatile potable electronics to improve their space utilization.

With the bloom of portable and wearable electronics, electrochemical storage devices featured with high performance, low-cost, safe, environmental-friendly, lightweight, thin and flexible features become more important than ever. Here, we construct a rechargeable Ni/Zn battery with a Co-doped Ni(OH)2 (CNH) and Zn materials on nickel nanowire arrays (NNA) for electrodes. The CNH cathodic material can be electrochemically deposited onto the NNA, which can deliver a high capacity of 346 mA h g−1 at current density of 5 A g−1. Co doping can effectively stabilize Ni(OH)2 with only ~10% capacity loss over 5 000 charge/discharge cycles at 30 A g−1. For anode, the design of Zn on NNA considerably lowers the risk of corrosion and dendrite form ation. As a result, ultrafast rechargeable Ni/Zn batteries are obtained, exhibiting a cell voltage of ~1.75 V, energy density of 148.54 Wh kg−1 (4.05 Wh L−1) and power density of 1.725 kW kg−1 (based on the mass of active materials) with a charging time of

With the development of wearable/flexible electronics, a formidable challenge is to integrate electronic components which were large in their original size into a flexible, thin, and arbitrary layout. As an indispensible component in electronics, commercial micro-supercapacitors are disadvantageous in their clumsy cuboid geometry and limited capacity, and are not promising for future applications. In comparison, film-like micro-supercapacitors are superior in miniaturized system integration since they can be folded to fit in restricted spaces while maintaining a high level of volumetric energy density. Here, we carried out a benchmark study of a state-of-the-art well-packaged thin film micro-supercapacitor toward commercial micro-supercapacitor and aluminum electrolyte capacitor. The micro-planar supercapacitor not only exhibits 3.75 times of a commercial micro-supercapacitor and 8785 times of an aluminum electrolytic capacitor in volumetric energy density under 1000 mV s −1 scan rate, but can also be tailored into diversified shapes, rolled up, and plugged into tiny interstitial spaces inside a device. Such ultrathin (18 µm) micro-supercapacitor component with high volumetric energy density (0.98 mWh cm −3 in LiCl-PVA gel, 5.7 mWh cm −3 in ionic liquid), can be integrated into an electronic device system and shows a series of superior performance characteristics over current commercial benchmarks, which may find vast applications.