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Rechargeable aqueous Zn‐VOx batteries are attracting attention in large scale energy storage applications. Yet, the sluggish Zn2+ diffusion kinetics and ambiguous structure–property relationship are always challenging to fulfil the great potential of the batteries. Here we electrodeposit vanadium oxide nanobelts (VO‐E) with highly disordered structure. The electrode achieves high capacities (e.g. ≈5 mAh cm−2, 516 mAh g−1), good rate and cycling performances. Detailed structure analysis indicates VO‐E is composed of integrated amorphous‐crystalline nanoscale domains, forming an efficient heterointerface network in the bulk electrode, which accounts for the good electrochemical properties. Theoretical calculations indicate that the amorphous‐crystalline heterostructure exhibits the favorable cation adsorption and lower ion diffusion energy barriers compared to the amorphous and crystalline counterparts, thus accelerating charge carrier mobility and electrochemical activity of the electrode. [ABSTRACT FROM AUTHOR]

The limited choice of anode materials always challenges the development of high performance aqueous ammonium-ion batteries (AAIBs). Herein, we fabricate amorphous molybdenum oxide (MoOx) materials and study the NH4+ storage performances. The results indicate that the optimized electrode exhibits high gravimetric/areal capacities of 175 mA h g−1/1.30 mA h cm−2, outperforming state-of-the-art anode materials for AAIBs. Our findings indicate that the valence state of Mo and the Mo–O–H content in MoOx synergistically control the NH4+ storage performances, offering new understanding for rational design of MoOx materials for energy storage applications. [ABSTRACT FROM AUTHOR]

The Mn2+ dissolution of MnO2 cathode materials causes rapid capacity decay in aqueous zinc batteries. We herein show that the dissolved Mn2+ can be deposited back to the cathode with the aid of a suitable conductive agent. The active material is thus retained for energy storage, and this MnO2/Mn2+ redox process also provides capacity. In the Mn2+ free ZnSO4 electrolyte, MnO2 delivers 325 mA h g−1 capacity at 0.1 A g−1, and 90.4% capacity retention is achieved after 3000 cycles at 5 A g−1. Our work demonstrates an effective strategy to realize stable cycling of MnO2 cathodes in aqueous zinc batteries without Mn2+ additives. [ABSTRACT FROM AUTHOR]

Mn‐based materials for aqueous energy storage are reaching the capacity ceiling due to the limited Mn4+/Mn3+ redox. The disproportionation of Mn3+ often occurs, forming soluble Mn2+ and thus leading to severer capacity decays. Here, an amorphous manganese phosphate material [AMP, Na1.8Mn4O1.4(PO4)3] is fabricated using an electrochemical method for the first time. Benefitting from the open framework and the insoluble nature of Mn2+ in AMP, the Mn3+/Mn2+ and Mn4+/Mn3+ redox couples can participate in the charge storage processes. The AMP electrode shows a high capacity of 253.4 mAh g−1 (912.4 F g−1 or 912.4 C g−1) at the current density 1 A g−1 and good rate capability. Experimental results indicate AMP experiences a mixed charge storage mechanism (i.e., cation intercalation and conversion reactions) in Na2SO4 electrolyte. Besides, electrolyte engineering can effectively prevent the decomposition of AMP during cycling test, achieving capacity retention of 97% in 5000 cycles. Importantly, AMP can accommodate different cations (e.g., Mg2+, Ca2+, etc.), exhibiting great potential for aqueous energy storage. [ABSTRACT FROM AUTHOR]

The energy storage behavior of the Li3V2(PO4)3 cathode in zinc batteries is evaluated. The dissolution or decomposition into vanadium oxide in aqueous electrolytes is revealed. Using the optimal combination of water and acetonitrile solvents in electrolyte, those processes are effectively prevented without sacrificing the Zn2+ de/insertion kinetics. Further investigation demonstrates a water induced phase transformation into a VOPO4 type structure, which is still a polyanion material and preserves the high voltage. It delivers 128 mA h g−1 capacity at 1C with 1.45 V discharge voltage, and 87 mA h g−1 capacity is retained at 10C. A stable cycling is obtained for 1000 cycles. [ABSTRACT FROM AUTHOR]

NH4+ ions as charge carriers show potential for aqueous rechargeable batteries. Studied here for the first time is the NH4+‐storage chemistry using electrodeposited manganese oxide (MnOx). MnOx experiences morphology and phase transformations during charge/discharge in dilute ammonium acetate (NH4Ac) electrolyte. The NH4Ac concentration plays an important role in NH4+ storage for MnOx. The transformed MnOx with a layered structure delivers a high specific capacity (176 mAh g−1) at a current density of 0.5 A g−1, and exhibits good cycling stability over 10 000 cycles in 0.5 M NH4Ac, outperforming the state‐of‐the‐art NH4+ hosting materials. Experimental results suggest a solid‐solution behavior associated with NH4+ migration in layered MnOx. Spectroscopy studies and theoretical calculations show that the reversible NH4+ insertion/deinsertion is accompanied by hydrogen‐bond formation/breaking between NH4+ and the MnOx layers. These findings provide a new prototype (i.e. layered MnOx) for NH4+‐based energy storage and contributes to the fundamental understanding of the NH4+‐storage mechanism for metal oxides. [ABSTRACT FROM AUTHOR]

Conducting polymers (CPs) have been widely studied for electrochemical energy storage. However, the dopants in CPs are often electrochemically inactive, introducing "dead‐weight" to the materials. Moreover, commercial‐level electrode materials with high mass loadings (e.g., >10 mg cm−2) often encounter the problems of inferior electrical and ionic conductivity. Here, a redox‐active poly‐counterion doping concept is proposed to improve the electrochemical performance of CPs with ultra‐high mass loadings. As a study prototype, heptamolybdate anion (Mo7O246−) doped polypyrrole (PPy) is synthesized by electro‐polymerization. A 2 mm thick PPy electrode with mass loading of ≈192 mg cm−2 reaches a record‐high areal capacitance of ≈47 F cm−2, competitive gravimetric capacitance of 235 F g−1, and volumetric capacitance of 235 F cm−3. With poly‐counterion doping, the dopants also undergo redox reactions during charge/discharge processes, providing additional capacitance to the electrode. The interaction between polymer chains and the poly‐counterions enhances the electrical conductivity of CPs. Besides, the poly‐counterions with large steric hindrance could act as structural pillars and endow CPs with open structures for facile ion transport. The concept proposed in this work enriches the electrochemistry of CPs and promotes their practical applications. [ABSTRACT FROM AUTHOR]

Various cathode materials have been proposed for high‐performance rechargeable batteries. Vanadyl phosphate is an important member of the polyanion cathode family. VOPO4 has seven known crystal polymorphs with tunneled or layered frameworks, which allow facile cation (de)intercalations. Two‐electron transfer per formula unit can be realized by using VV/VIV and VIV/VIII redox couples. The electrochemical performance is closely related to the structures of VOPO4 and the types of inserted cations. This Review outlines the crystal structures of VOPO4 polymorphs and their lithiated phases. The research progress of vanadyl phosphate cathode materials for different energy storage systems, including lithium‐ion batteries, sodium‐ion batteries, potassium‐ion batteries, multivalent batteries, and supercapacitors, as well as the related mechanism investigations are summarized. It is hoped that this Review will help with future directions of using vanadyl phosphate materials for energy storage. [ABSTRACT FROM AUTHOR]

Abstract: Electrode materials with low cost, large theoretical capacity, and fast ion/electron transport rate are highly desirable for capacitive applications. Pseudocapacitive iron oxide materials (e. g., Fe2O3, Fe3O4) satisfy the first two aspects, but are electrically/ionically insulating, hindering their practical applications in energy storage. Here, hybrid mesoporous iron oxide (Fe2O3/Fe3O4) materials are integrated with a three‐dimensional electrochemically exfoliated graphite substrate (EG) using an electrodeposition coupled with a post‐chemical transition strategy. The hybrid Fe2O3/Fe3O4 materials on EG exhibit an ultrahigh specific capacitance of 1530 F g−1 at current density of 1 A g−1. This material with such high capacitance ranks on the top of the reported iron oxide materials for capacitive applications. In addition, the assembled device with EG@Fe2O3/Fe3O4 as anode can deliver a high gravimetric energy density of 21.6 Wh kg−1 at a power density of 225 W kg−1, based on the total mass of the two electrodes (including the mass of the current collectors). This system can power various electronic devices, suggesting its great potential for energy storage applications. [ABSTRACT FROM AUTHOR]

Interconnected H 0.12 WO 3 ⋅H 2 O nanosheets with high electrochemical performances are fabricated on partial exfoliated graphite substrate (Ex-GF) by potential-limited pulse galvanostatic method (PLPG). The dead volume problem of bulk pesudocapacitive materials is addressed by the novel interconnected nanosheets structure, enabling a large specific capacitance of 5.95 F cm −2 (495.8 F g −1 ) at 2 mA cm −2 . Merited from the fluent electrolyte penetration channels established by the plenty voids among nanosheets, as well as fast electron transportation in the electronic conductive tungsten bronze which is directly grown from graphite substrate, the obtained WO 3 /Ex-GF demonstrates excellent rate capability. The material can maintain 60.0% of its capacitance when the discharge current density increases from 2 to 100 mA cm −2 . Moreover, WO 3 /Ex-GF doesn't show capacitance decay after 5000 galvanostatic charge-discharge cycles, displaying its super stability. Furthermore, a high performance asymmetric supercapacitor assembled by using WO 3 /Ex-GF and electrochemical fabricated MnO 2 /Ex-GF as negative and positive electrodes, respectively displays a high energy density of 2.88 mWh cm −3  at the power density of 11.1 mW cm −3 , demonstrating its potential application for energy storage. [ABSTRACT FROM AUTHOR]

Keywords: cathode materials; polyanions; rechargeable batteries; supercapacitors; vanadyl phosphate EN cathode materials polyanions rechargeable batteries supercapacitors vanadyl phosphate 1 1 1 07/04/20 20200702 NES 200702 B The vanadyl phosphate materials b have various crystal structures, which allow reversible cation (de)intercalation. Cathode materials, polyanions, rechargeable batteries, supercapacitors, vanadyl phosphate. [Extracted from the article]

Highlights: Recent progress of active materials in supercapacitors synthesized by electrochemical techniques is reviewed. Electrochemically synthesized nanostructures of various dimensions, compositions, and electrochemical properties are discussed. The advantages and challenges of electrochemical technologies in preparing nano-/microstructured materials for electrochemical energy storage devices are summarized. The article reviews the recent progress of electrochemical techniques on synthesizing nano-/microstructures as supercapacitor electrodes. With a history of more than a century, electrochemical techniques have evolved from metal plating since their inception to versatile synthesis tools for electrochemically active materials of diverse morphologies, compositions, and functions. The review begins with tutorials on the operating mechanisms of five commonly used electrochemical techniques, including cyclic voltammetry, potentiostatic deposition, galvanostatic deposition, pulse deposition, and electrophoretic deposition, followed by thorough surveys of the nano-/microstructured materials synthesized electrochemically. Specifically, representative synthesis mechanisms and the state-of-the-art electrochemical performances of exfoliated graphene, conducting polymers, metal oxides, metal sulfides, and their composites are surveyed. The article concludes with summaries of the unique merits, potential challenges, and associated opportunities of electrochemical synthesis techniques for electrode materials in supercapacitors. [ABSTRACT FROM AUTHOR]

Commercial electrodes need high mass loadings to realize superior energy and power densities. However, good electrochemical properties are usually achieved in the electrodes with ultrathin active materials (e.g., <1 mg cm−2). Good performance and high mass loading are often mutually exclusive characteristics. Herein, a unique 3D exfoliated carbon paper (EC) is demonstrated using a facile electrochemical method to support high mass loading MnO2 materials. The 3D‐interconnected graphene/graphite network, highly porous structure, as well as the strong interaction between the active materials and the substrate, allow efficient charge transport in the composite electrode, addressing the traditional limitations in high mass loading electrodes. The deposited MnO2 (mass loading: 9.5–10 mg cm−2) achieves a remarkable capacitive performance with high areal and gravimetric capacitances of 5.1 F cm−2 and 537 F g−1, respectively. A Zn–MnO2 battery is also assembled using MnO2/EC as the cathode. An excellent specific capacity of 368 mAh g−1 is also delivered. This MnO2/EC outperforms most of the reported MnO2‐based electrodes with similar loadings for capacitive and Zn‐ion battery applications, highlighting the great application potential of the 3D carbon paper support in electrochemical energy storage. [ABSTRACT FROM AUTHOR]

In-situ growing of energy storage materials on graphene-based substrates/current collectors with low defect is a good way to boost electron transport and so enhance rate capability for the obtained electrode. Herein, high-quality graphene-like nanopetals are partially exfoliated from graphite foil (GF) through a facile and fast cathodic process. Three-dimensional porous structure is established for the afforded cathodically-exfoliated graphite foil (CEG), with many graphene-like nanopetals vertically anchoring on the graphite substrate. A hierarchical structure is constructed by the following electrochemical growth of Co−Ni double hydroxide nanopetals on the graphene atop CEG. The double hydroxide in the obtained electrode with the optimized Co 2+ /Ni 2+ molar ratio, Co 0.75 Ni 0.25 (OH) 2 -CEG, displays much improved rate capability and so can deliver a high specific capacitance of 1460 F g −1 at an ultra-high current density of 100 A g −1 . An asymmetric device is assembled by using Co 0.75 Ni 0.25 (OH) 2 -CEG as cathode, which demonstrates a high energy density of 31.6 Wh kg −1 at an ultra-high power density of 21.5 kW kg −1 , showing the potential of the hierarchical composite electrode for high power application. The device also displays good stability, it can retain more than 90% of its capacitance after 10000 galvanostatic charge-discharge cycles. [ABSTRACT FROM AUTHOR]

[Display omitted] • An artificial protection layer of carrageenan is applied on Zn metal anode. • It allows preferential transport of Zn2+ and suppresses sulfate diffusion. • Zn is deposited uniformly under the protection layer. • Stable Zn plating/stripping is realized for 1500 h, and CE reaches 99.4 %. • A full cell with N/P = 1 achieves excellent stability. Aqueous rechargeable zinc batteries are promising candidates for grid-level energy storage systems. However, critical challenges of dendrite growth and side reactions occur on Zn metal anode, which leads to unsatisfactory electrochemical performance. Herein, we fabricate an artificial protection layer of carrageenan on Zn electrode. The sulfonic acid groups on carrageenan act as active sites for interacting with Zn2+, which facilitates Zn2+ transport and enhances Zn2+ desolvation. Carrageenan also blocks the diffusion of sulfate anions from the electrolyte. It ensures enlarged Zn2+ transference number and reduced cation concentration gradient, so that uniform Zn deposition under the protection layer is realized. Meanwhile, the carrageenan layer also inhibits side reactions with the electrolyte. As a result, the protected Zn electrode presents stable cycle life of over 1500 h (750 cycles) in symmetric cells and realizes 99.4 % stripping/plating coulombic efficiency. More importantly, the full cell assembled with protected Zn anode and MnO 2 cathode at N/P = 1 condition (based on theoretical capacities of anode and cathode) delivers 80.9 % capacity retention after 100 cycles, superior to 26.0 % with bare Zn. Our work presents an effective protection path for Zn metal anode toward practical aqueous zinc batteries. [ABSTRACT FROM AUTHOR]

Research on anode materials with high capacitive performance is lagging behind that of cathode materials, which has severely hindered the development of high-efficient energy storage devices. Compared with other anode materials, Fe 3 O 4 exhibits highly desirable advantages due to its low cost, high theoretical capacity and preferable electronic conductivity of ∼10 2 S cm −1 . Herein, hierarchical honeycomb Fe 3 O 4 is integrated on functionalized exfoliated graphite through electrochemical deposition and the following chemical conversion. The hierarchical honeycomb Fe 3 O 4 is constructed by the oxide nanorods, which are assembled by a number of nanoparticles. This unique porous structure not only ensures fast ion diffusion in the electrode, but also provides large amount of active sites for electrochemical reactions. The exfoliated graphene atop the graphite base can act as 3D conductive scaffold to facilitate the electron transport of the electrode. Therefore, FEG/Fe 3 O 4 exhibits large specific capacitances of 327 F g −1 @1 A g −1 and 275 F g −1 @10 A g −1 . Good cycling stability is also achieved due to the flexibility of the graphene substrate. The assembled asymmetric device using FEG/Fe 3 O 4 as anode can deliver a high energy density of 54 Wh kg −1 . [ABSTRACT FROM AUTHOR]

[Display omitted] • Aqueous Ca2+-storage properties of molybdenum oxide (MoO x) was studied for the first time. • MoO x electrode exhibited high capacity of 106 mAh g−1 and excellent cycle life of 30 000 cycles. • The electron structure and crystal water content of MoO x affect the Ca2+-storage performance. • The assembled aqueous Ca-ion battery delivered a high areal energy density of 0.96 mWh cm−2. Aqueous Ca-ion batteries (ACIBs) show great potential for grid-scale energy storage applications due to their low cost, high safety, and environmental sustainability. However, their development is limited by the lack of proper Ca2+-storage materials. Herein, the electrochemical Ca2+ storage chemistry of an electrodeposited molybdenum oxide (MoO x) material is studied for the first time. The d-electron concentration of Mo and the content of structural water in MoO x play important roles in Ca2+ storage. The Mo O H components in MoO x could suppress material dissolution in the weakly alkaline CaCl 2 electrolyte. As expected, the MoO x electrode delivers a good discharge capacity of 106 mAh g−1 at 0.1 A g−1 (mass loading: 10 mg cm−2), and ultra-long cycle life of 30 000 cycles at 3.0 A g−1, outperforming the state-of-the-art Ca2+ host materials. In depth mechanism study illustrates that MoO x experiences a pure Ca2+ insertion/deinsertion mechanism during charge/discharge processes. Finally, an aqueous Ca-ion full cell is assembled using MoO x anode and copper hexacyanoferrate (CuHCF) cathode, exhibiting a high areal energy density of 0.96 mWh cm−2 (42.7 Wh kg−1) and long cycle life. Importantly, the Ca2+ storage in MoO x points to a new paradigm of Ca2+-based aqueous energy storage. [ABSTRACT FROM AUTHOR]

[Display omitted] • A zinc-iodine battery with high coulombic efficiency and stability is proposed. • The polyiodide is confined at cathode by doping on polyaniline chains. • The complex in electrolyte is regulated to eliminate free iodide anions. • It delivers 99.2% coulombic efficiency with 2 mAh cm−2 capacity at 6 mA cm−2. • A stable capacity retention of 99.9% is achieved after 1000 cycles. Aqueous zinc-iodine batteries are promising electrochemical energy storage systems due to the high safety and low cost. The application of zinc halide solution as the electrolyte allows the dual-plating mechanism on both electrodes, i.e. the redox reactions of Zn2+/Zn and I 2 /I- at the anode and cathode, respectively. These solid–liquid conversion processes guarantee excellent reaction kinetics. However, soluble polyiodide (I 3 -, I 5 -, etc.) are formed at the cathode either during the oxidation of I- or from the reaction between I- and I 2. The dissolution of polyiodide in electrolytes causes rapid loss of charged products, leading to poor coulombic efficiency and fast self-discharge. Herein, we apply the synergistic regulation of electrode and electrolyte to confine the charged products. The conducting polymer polyaniline (PANI) is used as the polyiodide binder. It contains positively charged nitrogen sites, allowing the doping and effective binding of polyiodide anions through electrostatic attraction. At the same time, the complex in zinc halide electrolytes is regulated to eliminate free iodide anions and prevent the reaction with I 2 to form more polyiodide. The optimized zinc-iodine aqueous battery delivers excellent rate capability thanks to the facile solid–liquid reactions as well as the high electrical conductivity of PANI. More importantly, it achieves a high coulombic efficiency of 99.2% with the capacity of 2 mAh cm−2 at 6 mA cm−2, and an excellent capacity retention of 99.9% after 1000 cycles is realized upon long-term cycling. The work proposes a potential pathway to realize stable energy storage in aqueous zinc-halogen batteries. [ABSTRACT FROM AUTHOR]

[Display omitted] • The leucoemeraldine-emeraldine-pernigraniline reactions of PANI are realized by Nernst shift. • The pernigraniline hydrolysis is inhibited with low water activity in electrolyte. • The PANI cathode delivers 221 mAh g−1 capacity at 0.2 A g−1 in aqueous Zn batteries. • A stable capacity retention of 96.6% is achieved after 1000 cycles at 3 A g−1. Polyaniline (PANI) is a promising cathode material for aqueous zinc batteries. However, only half available capacity based on the leucoemeraldine-emeraldine redox reaction can be used. Further oxidation to pernigraniline requires potential exceeding the top voltage limit of aqueous solutions, and the hydrolysis of pernigraniline leads to PANI degradation. Herein, we show that the redox potential of emeraldine-pernigraniline reaction is effectively shifted within electrolyte voltage window by tuning ion activities in electrolyte based on Nernst shift, so that both redox processes of PANI are realized for energy storage in aqueous zinc cells. Meanwhile, the pernigraniline hydrolysis is inhibited with limited water activity in the concentrated electrolyte, and the long-term stability in ensured. The PANI electrode delivers a high capacity of 221 mAh g−1 at 0.2 A g−1 and maintains 118 mAh g−1 at 5 A g−1 in aqueous zinc batteries. An excellent capacity retention of 96.6% is achieved after 1000 cycles with 99.7% coulombic efficiency. The energy storage processes as well as the active dopants on PANI are identified by a series of characterizations. The work demonstrates an effective strategy to realize energy storage with redox couples outside the voltage window of conventional electrolytes. [ABSTRACT FROM AUTHOR]

[Display omitted] • A conversion-type CuO cathode is proposed for aqueous zinc batteries. • Two-electron transfers based on Cu2+-Cu redox reaction are realized. • The Cu2+ dissolution is suppressed by shifting the dissolution equilibrium. • Stable retentions of both capacity and voltage are achieved. • The electrode delivers 180 mAh g−1 capacity at 100 mA g−1. Cathode materials with conversion reaction would offer higher theoretical capacities than intercalation-type materials. Copper-based materials can undergo two electron transfers from the redox couples of Cu2+/Cu+ and Cu+/Cu, which at the same time provides desirable redox potentials as cathode materials for aqueous zinc batteries. However, most Cu2+ components are soluble in water, leading to the continuous loss of active material and capacity decay upon cycling. Herein, we prevent Cu2+ dissolution with fundamental equilibrium principles. The CuO cathode experiences rapid capacity decay within few cycles in conventional zinc electrolytes upon the formation of soluble charged products. The dissolution equilibrium is effectively shifted to the left by increasing the concentration of the associated anion TFSI- in the electrolyte of 21 m LiTFSI + 1 m Zn(TFSI) 2 , and Cu2+ is confined in the cathode in the forms of Cu(TFSI) 2 and CuO. A stable capacity retention of 91.2% realized after 300 cycles, and the stable evolution of differential capacity curves furthermore demonstrates the well-preserved energy storage process. A good capacity of 180 mAh g−1 is achieved at 100 mA g−1. This work provides a feasible strategy to suppress active material dissolution and highlights the potential utilization of copper-based conversion cathode materials for aqueous zinc batteries. [ABSTRACT FROM AUTHOR]

[Display omitted] • Quinone sites are introduced to PANI by a facile electrochemical method. • Polymer chains transform into expanded coil conformation at the same time. • Quinone provides extra redox sites, and expanded polymer enhances conductivity. • The electrode delivers 186 mAh g−1 capacity with quinone at 1.4 V and PANI at 1.1 V. • A stable capacity retention of 88.0% is achieved after 1500 cycles. Rechargeable aqueous batteries with zinc metal anodes are promising energy storage systems owing to their high safety and low lost. A suitable cathode material is in urgent demand. Organic materials offer adjustable electrochemical activity, and conducting polymers provide good electrical conductivity. Herein, we combine those advantages in one cathode material by the controlled introduction of quinone-type active sites onto polyaniline (PANI) nanorod-arrays through a facile electrochemical treatment. The process simultaneously transforms the polymer with a compact coil conformation into an expanded coil structure, resulting in facilitated electron transportations and charge transfer processes. In comparison to the original PANI, the electrochemically treated Q-PANI electrode exhibits extra electrochemical activity from quinone sites as well as enhanced capacity from PANI itself. Q-PANI delivers a high capacity of 186 mAh g−1 at 0.2 A g−1. Upon long-term cycling, 88.0% capacity retention is achieved after 1500 cycles at 2 A g−1. Detailed mechanism studies demonstrate the redox reaction on quinone sites at 1.4 V and nitrogen centers in PANI at 1.1 V. The work proposes a potential pathway towards high performance electrode materials for zinc batteries. [ABSTRACT FROM AUTHOR]

• Fe 3 O 4 /FeOOH heterostructure is constructed during electrochemical activation. • A built-in electric field is formed near the Fe 3 O 4 /FeOOH interface. • The activated electrode shows an enhanced capacity of 634 mAh g−1. • The assembled Ni-Fe cell shows a high energy density of 161.3 Wh kg−1 at 5.7 kW kg−1. Aqueous Ni-Fe batteries show promise for grid level energy storage due to their high safety and low cost. However, high capacities of Fe-based anodes can only be achieved under slow discharging rates. Moreover, an activation process is often required, the mechanism of which has not been fully understood. Herein, we present a facile and controllable method to uniformly deposit Fe 3 O 4 nanoparticles on a 3D graphite substrate. Post-mortem analysis demonstrates the partial conversion of Fe 3 O 4 to FeOOH during the subsequent in-situ electrochemical activation process, forming a Fe 3 O 4 /FeOOH heterostructure. Density functional theory calculations suggest that a built-in electric field is formed near the Fe 3 O 4 /FeOOH interface, which facilitates the charge transfers and lowers the adsorption energy of OH−. The above modification on the active material significantly improves its electrochemical activity. The activated electrode delivers a high capacity of 509 mAh g−1 at the ultra-high current density of 100 A g−1. A Ni-Fe cell assembled with activated Fe 3 O 4 /FeOOH anode and Ni-Co double hydroxide cathode provides a high energy density of 161.3 Wh kg−1 and a maximum power density of 43 kW kg−1, making it a good candidate for high safety, low cost, and environmental friendliness energy storage systems. [ABSTRACT FROM AUTHOR]