Satisfactory Electrochemical Performance Research Articles

Zincophile Zn2+ Conductor Regulation by Ultrathin Nano MoO3 Coating for Dendrite-Free Zn Anode.

Aqueous battery with nonflammable and instinctive safe properties has received great attention. However, issues related to Zn anode such as side reactions and rampant dendrite growth hinder the long-term circulation of AZMBs. Herein, an ultrathin(35nm) MoO3 coating is deposited on the Zn anode by means of vacuum vapor deposition for the first time. Due to the peculiar layer structure of MoO3, insertion of Zn2+ in ZnxMoO3 acts as Zn2+ ion conductor, which regulates Zn2+ deposition in an ordered manner. Additionally, the MoO3 coating can also inhibit the hydrogen evolution and corrosion reactions at the interface. Therefore, both Zn//MoO3@Cu asymmetric battery and Zn symmetric battery cells manage to deliver satisfactory electrochemical performances. The symmetric cell assembled with MoO3@Zn shows a significant long cycle life of more than 1600h at a current density of 2mA cm-2. Meanwhile, the MoO3@Zn//Cu asymmetric cell exhibits an ultrahigh Zn deposition/stripping efficiency of 99.82% after a stable cycling of 650h at 2mA cm-2. This study proposes a concept of "zincophile Zn2+ conductor regulation" to dictate Zn electrodeposition and broadens novel design of vacuum evaporation for nano MoO3 modified Zn anodes.

Reviving Sodium Tunnel Oxide Cathodes Based on Structural Modulation and Sodium Compensation Strategy Toward Practical Sodium-Ion Cylindrical Battery.

As a typical tunnel oxide, Na0.44MnO2 features excellent electrochemical performance and outstanding structural stability, making it a promising cathode for sodium-ion batteries (SIBs). However, it suffers from undesirable challenges such as surface residual alkali, multiple voltage plateaus, and low initial charge specific capacity. Herein, an internal and external synergistic modulation strategy is adopted by replacing part of the Mn with Ti to optimize the bulk phase and construct a Ti-containing epitaxial stabilization layer, resulting in reduced surface residual alkali, excellent Na+ transport kinetics and improved water/air stability. Specifically, the Na0.44Mn0.85Ti0.15O2 using water-soluble carboxymethyl cellulose as a binder can realize a capacity retention rate of 94.30% after 1,000 cycles at 2C, and excellent stability is further verified in kilogram large-up applications. In addition, taking advantage of the rich Na content in Prussian blue analog (PBA), PBA-Na0.44Mn1-xTixO2 composites are designed to compensate for the insufficient Na in the tunnel oxide and are matched with hard carbon to achieve the preparation of coin full cell and 18650 cylindrical battery with satisfactory electrochemical performance. This work enables the application of tunnel oxides cathode for SIBs in 18650 cylindrical batteries for the first time and promotes the commercialization of SIBs.

Sulfonic group modifying cobalt (II) phthalocyanine molecule to boost the polysulfide reaction kinetic for high-performance lithium sulfur battery

Metal phthalocyanine have been regarded as an effective additive to restrain the LiPS diffusion process in sulfur cathode to achieve high-performance Li-S battery. Herein, molecular engineering has been used to boost the LiPS adsorption and catalysis abilities of metal phthalocyanine distinctly through a simple sulfonic group grafting strategy. The derived CNT/s-CoPc/S cathode could deliver the initial discharge capacity of 679.8 mAh/g at 1.0 C and no obvious capacity fading even after 500 cycles. The satisfactory electrochemical performance indicates the promising application of CNT/s-CoPc composite as an efficient sulfur carrier. The research presents a successful modification method for the improvement of metal phthalocyanine based additive, contributing to their advancements in Li-S battery furtherly.

Phenolic resin-based nanoflower porous carbon synthesized by self-assembly of MgO hydrolysis for boosting supercapacitor performance

Although electrode materials with flower-like structures have showing promising achievement for battery energy storage for stable structure and rich specific surface area, there are still some challenges in precisely controlling their microstructure. Inspired by the hydrolysis and self-assembly of MgO into petal-like magnesium hydroxide crystals under alkaline critical micelle conditions, we have proposed to synthesize a unique phenolic resin with a 3D flower-like structure as a carbon precursor. After a series of treatments, the optimal sample consists of nanosheets (thickness ≈27 nm) extending in all directions from the core, with a porous structure, significant specific surface area of 2028 m2/g and pore volume of 1.79 m3/g. Electrochemical experiments confirmed that the sample exhibits satisfactory electrochemical performance as the electrode of supercapacitor. It shows an amazing unique capacitance of 320.1 F/g, and the charge–discharge capacity retention rate exceeds 94.6 % after 5000 cycles. Moreover, a high energy density of 9.44 Wh/kg at 126 W/kg is shown for the symmetrical supercapacitors. We believe that the 3D flower-like morphology formed by the unique lamellar structure can provide more active sites, promote ion transports and increase the stability of structure. These findings suggest a new strategy to control the morphology of phenolic resin-based carbon materials.

Aqueous cerium ion battery: A novel aqueous rechargeable battery

Aqueous rechargeable batteries (ARBs) are a potential energy storage system with lowcost aqueous electrolyte and high safety. Multivalent host carriers in electrolyte can offer a wider variety of valence states and redox properties leading to high capacity. In this work, an aqueous cerium ion aqueous (ACIBs) battery has been reported with a satisfactory electrochemical performance for the first time. By utilizing the advantages of stable structure and three-dimensional (3D) open framework of Prussian blue analogues (PBAs), the problem of cerium ions insertion/de-insertion into host materials due to high charge and large size can been successfully solved. Herein, the copper hexacyanoferrate (CuPBA) is prepared through a facile coprecipitation strategy as cathode material of ACIBs. The CuPBA cathode deliver a satisfactory capacity (79 mAh g−1 at 50 mA g−1) and an excellent cycling stability after long-time cycles (86.8 % capacity retention after 5000 cycles at 1000 mA g−1). As a result, the cathode material in ACIBs system displays excellent rate capability and cycling performance. The successful assemble of the full battery also verifies the feasibility of ACIBs. In addition, the energy storage mechanism of Ce3+ ion insertion/de-insertion process is explained by ex situ X-ray diffraction and X-ray photoelectron spectroscopy measurements. This work opens up a new direction and provides a foundation for development of novel ARBs.

In situ synthesis of cross-linking gel polymer electrolyte for lithium metal batteries

The room temperature ionic conductivity of polyethylene oxide (PEO)-based polymer electrolytes is low, the electrochemical window is narrow, and the mechanical strength is relatively poor. In this work, a cross-linking gel-based solid composite polymer electrolyte (CG-SCPE) was synthesized by introducing electrochemically stable carbonate-based functional groups. The synthesized CG-SCPE presents excellent tensile strength (26 MPa) and a wide electrochemical stability window (> 4.5 V vs. Li/Li+). Meanwhile, the in situ polymerization method induced by thermal heating resulted in good compatibility between electrodes/electrolytes. In addition, the assembled LiNi0.5Co0.2Mn0.3O2/CG-SCPE/Li battery exhibited satisfactory electrochemical performance. Therefore, the gel polymer electrolyte CG-SCPE with cross-linking network provides the possibility for future application of safe and high-performance solid-state lithium metal batteries. The results indicate that the introduction of cross-linking framework can simultaneously improve the mechanical strength, thermal stability, and electrochemical performance of solid polymer electrolyte.

Restricted and epitaxial growth of MnO2-x nano-flowers in/out carbon nanofibers for long-term cycling stability supercapacitor electrodes

Carbon nanofibers (CFs) have been widely applied as electrodes for energy storage devices owing to the features of increased contact area between electrodes and electrolyte, and shortened transmission route of electrons. However, the poor electrochemical activity and severe waste of space hinder their further application as supercapacitors electrodes. In this work, MnO2−x nanoflowers restricted and epitaxial growth in/out carbon nanofibers (MnO2/MnO@CF) were prepared as excellent electrode materials for supercapacitors. With the synergistic effect of uniquely designed structure and the introduction of MnO and MnO2 nanoflowers, the prepared interconnected MnO2/MnO@CF electrodes demonstrated satisfactory electrochemical performance. Furthermore, the MnO2/MnO@CF//activated carbon (AC) asymmetric supercapacitor offered an outstanding long-term cycle stability. Besides, kinetic analysis of MnO2/MnO@CF-90 was conducted and the diffusion-dominated storage mechanism was well-revealed. This concept of “internal and external simultaneous decoration” with different valence states of manganese oxides was proven to improve the electrochemical performance of carbon nanofibers, which could be generalized to the preparation and performance improvement of other fiber-based electrodes.

Unlocking the potential of ionic liquid-functionalized aqueous electrolytes for aqueous ammonium-bromine/ion batteries

Aqueous ammonium-ion batteries are promising candidates for grid-scale energy storage owing to their non-flammability, eco-friendliness, and low-cost. Nevertheless, their further development suffers from electrolyte/electrode instability and limited energy density. Herein, we develop an aqueous ammonium-bromine/ion battery composed of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) organic materials as the anode, Br2/Br− redox couple as the cathode, and ionic liquid-functionalized aqueous solution as the electrolyte. The experimental investigation and theoretical calculation results reveal that two organic cations (TPA+ and BMIM+) in the functionalized electrolyte can insert into the PTCDA anode with different intercalation chemistry, which enhances the reversibility of NH4+ storage. Furthermore, organic cations not only construct a hydrophobic interfacial layer to expel water molecules from the anode surface, but also stabilize the active species at the cathode side by forming a solid complexation phase. Consequently, the as-developed aqueous ammonium-bromine/ion batteries present satisfactory electrochemical performance at both room temperature and low temperature, which promotes its potential application in grid-scale energy storage.

3D-printed topological-structured electrodes with exceptional mechanical properties for high-performance flexible Li-ion batteries

A vital aspect in advancing flexible batteries is the development of flexible electrodes capable of enduring repeated stretching while upholding satisfactory electrochemical performance. Thus, adopting a systematic and efficient approach to structural design and fabrication becomes imperative. In this study, we introduce an optimal structural design achieved through topology optimization and fabricate flexible electrodes via 3D printing, representing a departure from traditional design and manufacture methodologies in the development of flexible electrodes for batteries. Our research underscores the impressive mechanical strength of these topologically-structured electrodes (TSEs), validated through rigorous finite element analysis (FEA) and tensile strength testing. The results of both the stretch deformation and twist deformation analysis on the TSEs and the conventional mesh-structured electrodes (MSEs) show that the peak strain and stress of TSEs are much lower than those of MSEs. Notably, even under 50 % stretching, the TSEs maintain structural integrity, contrasting sharply with conventional mesh-structured electrodes (MSEs) and flat film electrodes which often crack under similar conditions. Moreover, after enduring 50 cycles of stretching, the TSEs retain an outstanding 98 % of their original capacity, surpassing MSEs which retain only 80 % of their capacity. These findings highlight the significant potential of topologically designed flexible electrodes, offering promising avenues for the development of stretchable and flexible energy storage devices such as wearable tech and bio-integrated electronics.

Controlling the graphite-like microcrystalline structure of lignin-based ultrafine carbon fibers via the design of condensed structures

Obtaining lignin-based graphite-like microcrystallites at a relatively low carbonization temperature is still very challenging. In this work, we report a new method based on condensed structures, for regulating graphite-like microcrystalline structures via the incorporation of 4,4′-diphenylmethane diisocyanate (MDI) into the main structure of lignin. The effects of MDI on the thermal properties of lignin and the graphite-like microcrystalline structure of lignin-based ultrafine carbon fibers were extensively studied and investigated. The incorporation of MDI decreased the thermal stability of lignin, increased the carbon yield and enhanced the formation of graphite-like microcrystallites, which are beneficial for reducing energy consumption during the preparation of lignin-based carbon fibers. The modified lignin-based ultrafine carbon fibers (M-LCFs) demonstrated satisfactory electrochemical performance, including high specific capacitance, low charge transfer resistance, and good cycle performance. The M-LCFs-3/2 electrode had a specific capacitance of 241.3 F g−1 at a current density of 0.5 A g−1, and a residual ratio of 90.2 % after 2000 charge and discharge cycles. This study provides a new approach to control the graphite-like microcrystalline structure and electrochemical performance while also optimizing the temperature.

MIL-101(Fe)-derivatized cathode as an efficient oxygen electrocatalyst for rechargeable Zn-air battery

The exploration of multifunctional electrocatalysts with cost-effective and high kinetic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for the development of advanced energy conversion and storage equipment. Herein, a novel hierarchical mesoporous/macropores MIL-101(Fe) derivative carbon catalyst material was prepared by a simple molten ZnCl2-assisted synthesis route. Specifically, 1H-benzotriazole (BTA) organic ligands were intentionally introduced as nitrogen sources in order to induce the formation of charge-rich regions through an electronegative nitrogen doping control strategy, and most importantly, the lone pair electron-rich nature of element N could facilitate the separation and anchoring of iron species enchanted the utilization rate of active sites. Because of these properties, the as-prepared catalyst (denoted as FeSACs/NxC) possesses unrivalled bifunction electrocatalytic activity and durability for the ORR and OER. The FeSACs/N1.25C as working electrode exhibits a quite satisfactory electrochemical performance for the ORR (half-wave potential of 0.82 V) and OER (a small overpotential of 302 mV at 10 mA cm−2) in the classic three-electrode configuration. Moreover, the FeSACs/N1.25C-based air cathode imparts encouraging performance in a rechargeable Zn–air battery prototype with an open-circuit voltage of 1.45 V, a specific capacity of 792.16 mAh g−1, an energy density of 871.38 Wh kg−1, and excellent stability for 120 h. This work has opened the way for the development of low-cost, fast kinetic and stable non-noble metal multifunctional catalysts.

3D hierarchical mesoporous carbon with P and N dual-doped as electrocatalyst host for high-performance Li-S batteries

Lithium-sulfur (Li-S) batteries are thought to be the most likely to replace traditional intercalation-based lithium-ion batteries for energy storage devices because of its high theoretical mass energy density. However, the poor cycling stability resulting from dissolution of polysulfide intermediates (Li2Sx, 3≤ x ≤8) greatly hampers the commercial applications. Herein, a 3D hierarchical mesoporous carbon with P/N dual-doped (3D-PNMC) as a catalytic host for sulfur electrode is successfully prepared. The as-prepared Li-S batteries with S@3D-PNMC cathode deliver a specific capacity of 1326 mAh g−1 at 0.1 C and the corresponding area capacity of up to 5.57 mAh cm−2. And also, the calculative capacity decay rate of S@3D-PNMC cathode is only 0.05% per cycle, which significantly lower than that of S@C electrode (0.21%). The satisfactory electrochemical performance can be ascribed to the 3D host not only provides space for the active sulfur loading, but also accelerates the electron transfer inside the electrode. Especially, electroactive sites created by phosphorus and nitrogen doping can effectively improve the electrochemical reaction kinetics to achieving long cycling stability.

Fluorine-doped porous carbon coating on Sn/SnOx as anode materials for high-performance potassium ion batteries

Potassium-ion batteries (PIBs) are considered alternatives to lithium-ion batteries because of their comparatively cheaper manufacturing costs and lower potential (2.936 V compared to standard hydrogen electrodes). However, their large ionic radii and unstable structures hindered their satisfactory electrochemical performance. In this study, F–Sn/SnOx@C, a novel porous layered composite material, was prepared using NaCl as a template for in situ etching to obtain a promising negative electrode material for PIBs. The significant specific surface area of the composite material and the uniform embedding of tin nanoparticles within the layered porous carbon shell, contributed to its excellent potassium intercalation/deintercalation performance. Fluorine doping enhanced electron transfer, prevented nanoparticle aggregation, and improved electrolyte penetration and potassium ion migration. The PIBs prepared from this electrode showed high potassium storage capacity, outstanding cycle stability (retaining 263 mAh g−1 after 200 cycles at 100 mA g−1), and remarkable rate capability (505.6, 288.4, 193.5, 134.1, and 83.3 mAh g−1 at 0.1, 0.2, 0.3, 0.5, and 1.0 A g−1, respectively). These results provide a straightforward solution for improving the electrochemical performance of PIBs.

Synchronous organic-inorganic co-intercalated ammonium vanadate cathode for advanced aqueous zinc-ion batteries

Vanadium-based cathode materials are attractive for aqueous zinc-ion batteries (AZIBs) owing to the high capacity from their open frameworks and multiple valences. However, the cycle stability and rate capability are still restricted by the low electrical conductivity and trapped diffusion kinetics. Here, we propose an organic–inorganic co-intercalation strategy to regulate the structure of ammonium vanadate (NH4V4O10, NVO). The introduction of Al3+ and polyaniline (PANI) induces the optimized layered structure and generation of urchin-like hierarchical construction (AP-NVO), based on heterogeneous nucleation and dissolution–recrystallization growth mechanism. Owing to these favorable features, the AP-NVO electrode delivers a desirable discharge capacity of 386 mA h g−1 at 1.0 A g−1, high-rate capability of 263 mA h g−1 at 5.0 A g−1 and excellent cycling stability with 80.4% capacity retention over 2000 cycles at 5.0 A g−1. Such satisfactory electrochemical performance is believed to result from the enhanced reaction kinetics provided by the stable layered structure and a high intercalation pseudo-capacitance reaction. These results could provide enlightening insights into the design of layered vanadium oxide cathode materials.

Tailoring Interfacial Electric Field by Gold Nanoparticles Enable Electrocatalytic Lithium Polysulfides Conversion for Lithium-Sulfur Batteries.

Although lithium-sulfur batteries (LSBs) are considered as the promising next rechargeable storage system ascribing to their decent specific capacity of inorganic sulfur, the development is partially impeded by inferior electronic conductivity, severe shuttle effect, and large volume variation. To tackle the issues above, a great deal of effort is made on sulfur-containing polymer (SCP) that shows better electrochemical performance. Nevertheless, sluggish conversion of lithium polysulfides (LiPSs) obstructs battery performance yet. Herein, electrocatalytic LiPSs with full conversion by tailoring the interfacial electric field are discovered based on gold nanoparticles (AuNPs) anchored on sulfurized polyaniline (SPANI). A downhill path of Gibbs free energy from organosulfur polymer to intermediate product means more spontaneously and favorable for full conversion, as the significant enhancement of electron density of state in the vicinity of the HOMO level for the AuNPs increase the electron transition probability rate. This composite delivers satisfactory electrochemical performance, especially increased rate capacity of >300 mAh g-1. Furthermore, catalyst mechanism on molecule level is proposed that AuNPsdominate chemical enhancement and higher electron delocalizablility betweenAuNPs and LiPSs molecules. These results can erect a promising strategy for enhancing lithium polysulfides full conversion.

Establishing Pinhole Deposition Mode of Zn via Scalable Monolayer Graphene Film.

The uneven texture evolution of Zn during electrodeposition would adversely impact upon the lifespan of aqueous Zn metal batteries. To address this issue, tremendous endeavors are made to induce Zn(002) orientational deposition employing graphene and its derivatives. Nevertheless, the effect of prototype graphene film over Zn deposition behavior has garnered less attention. Here, it is attempted to solve such a puzzle via utilizing transferred high-quality graphene film with controllable layer numbers in a scalable manner on a Zn foil. The multilayer graphene fails to facilitate a Zn epitaxial deposition, whereas the monolayer film with slight breakages steers a unique pinhole deposition mode. In-depth electrochemical measurements and theoretical simulations discover that the transferred graphene film not only acts as an armor to inhibit side reactions but also serves as a buffer layer to homogenize initial Zn nucleation and decrease Zn migration barrier, accordingly enabling a smooth deposition layer with closely stacked polycrystalline domains. As a result, both assembled symmetric and full cells manage to deliver satisfactory electrochemical performances. This study proposes a concept of "pinhole deposition" to dictate Zn electrodeposition and broadens the horizons of graphene-modified Zn anodes.

Fused Functional Organic Material with the Alternating Conjugation of Quinone-Pyrazine as Cathode for Aqueous Zinc Ion Batteries.

Organic cathode materials for aqueous rechargeable zinc batteries (ARZBs) are rapidly gaining prominence, while the exploration of compounds with affordable synthesis, satisfactory electrochemical performance, and understandable mechanisms still remains challenging. In this study, 6,8,15,17-tetraaza-heptacene-5,7,9,14,16,18-hexaone (TAHQ) as an easily synthesized organic cathode material with novel quinone/pyrazine alternately conjugated molecule structure is presented. This organic electrode exhibits good capacity with highly reversible redox reactions, and the influence of multi-active structures on the Zn2+/H+ loading behavior is systematically investigated by ex situ spectroscopy, electrochemical tests, and computation. Both experimental and theoretical studies effectively address the Zn2+/H+ intercalation/deintercalation kinetics. Benefitting from the fused active functionalities, the assembled Zn//TAHQ battery displays a maximum discharge specific capacity of 254.3mAhg-1 at 0.5Ag-1, and it maintains remarkable cycle performance with 71% capacity retention after 1000 cycles under 5Ag-1.

Interface Engineering for 3D Printed Energy Storage Materials and Devices

Abstract3D printed energy storage materials and devices (3DP‐ESMDs) have become an emerging and cutting‐edge research branch in advanced energy fields. To achieve satisfactory electrochemical performance, energy storage interfaces play a decisive role in burgeoning ESMD‐based 3D printing. Hence, it is imperative to develop effective interface engineering routes toward desirable 3DP‐ESMDs. In this tutorial review, recent advances in interface engineering for 3DP‐ESMDs are comprehensively provided and in‐depth discussion is offered. To begin with, basic interface engineering principles are introduced. Critical interface engineering strategies including 3D printing‐enabled structural design, composition modification, protective layer design, and 3D printed device optimization are then summarized and illustrated, accompanied by a discussion of pioneering work on 3D printed rechargeable batteries and electrochemical capacitors that has been made through significant interface engineering strategies. Finally, perspectives on interface engineering approaches in future 3DP‐ESMDs are presented.

Synergetic impact of oxygen and vanadium defects endows NH4V4O10 cathode with superior performances for aqueous zinc-ion battery

Owing to multi-electron redox reactions and versatile cation storage capabilities, laminated structured metallic vanadate of NH4V4O10 (NHVO) has been regarded as a kind of promising cathode materials for aqueous Zn-ion batteries with satisfactory electrochemical performance. Nevertheless, the NHVO cathode is still limited by the sluggish electrochemical kinetics and structural instability. Here, defect engineering was conducted to introduce the vanadium (V) and oxygen (O) dual defects into NHVO cathode for improving electrochemical performance. As revealed experimentally and theoretically, the migration of Zn-ion inside V/O-defected NHVO cathode can be effectively fostered by increasing active sites, lowering the migration barrier and reducing Gibbs free energy, which are rooted in the synergy impact of the presence of O defect and V defect. As the consequence, the outstanding electrochemical performance has been achieved, which delivers higher capacity of 489 mAh g−1 at 0.5 A g−1 with the capacity retention of 98 % after 100 cycles and exhibits singular cyclic stability at 15 A g−1, maintaining reversible capacity of 198 mAh g−1 after 8000 cycles. This synergistic strategy paves an avenue for accelerating the application of HNVO-based cathode for aqueous Zn-ion battery with satisfactory electrochemical performance and lifespan, advancing the approach of “carbon neutrality” society.

High-performance ε-VOPO4 cathode materials for sodium ion battery applications

The nanoscale ε-VOPO4 materials were successfully prepared by hydrothermal synthesis of H2VOPO4 precursors and further calcination. XRD and AFM characterization showed that the prepared nano-ε-VOPO4 belonged to monoclinic crystalline system with high purity of physical phase and an average particle size of 150 nm. FT-IR spectroscopy showed that the characteristic peak of the H2VOPO4 precursor (at 797 cm−1) disappeared after calcination and a characteristic peak representing ε-VOPO4 was captured at 914 cm−1. Furthermore, the XPS results showed that the V in the prepared ε-VOPO4 was mainly +5 valence. In order to further evaluate the potential of ε-VOPO4 as an energy storage material, it was applied as cathode material in sodium ion battery system. Electrochemical tests have demonstrated that ε-VOPO4 has satisfactory electrochemical performance as cathode material for sodium ion batteries. The initial capacities of ε-VOPO4 electrode materials reached 162.3, 135.1, 86.7, 51.4, and 29.7 mAh g−1 at 0.05, 0.2, 1, 2, and 5 C, respectively, and the long-cycle performance results showed that the electrochemical stability was still superior after 200 cycles at a current density of 0.5 C. Meanwhile, the density functional theory calculation shows that the optimized lattice constants (a = 7.279 Å, b = 6.896 Å and c = 7.268 Å) are in good agreement with the experimental data. Notably, the experimental ε-VOPO4 material has a higher voltage plateau than the theoretical calculation, which shows the potential advantage of ε-VOPO4 as a new generation electrode material for high energy storage systems.