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Iodide and iodine comprise a promising redox couple in aqueous energy storage systems (aqua-ESSs). However, the corresponding half-redox reaction on the cathode of an aqua-ESS has most often been considered as simply I2 (or I3−)/I−. Here, we describe for the first time reversible electrochemical formation and dissolution of insoluble iodine–halide coordination networks, [(I2)n·X−] (X− = Br− and I−), in confined nanopores with microporous carbon (micro-C) serving as a positive electrode in an aqua-ESS and using I− as the redox active electrolyte during charging. In an electrochemical cell without added Br−, the main half-redox reaction changed from I2/I− to [(I2)n·I−]/I− (n = 1 and 2) as charging and discharging accelerated (i.e., as current densities increased). When Br− was added to the electrolyte with I−, [(I2)n·Br−] was formed by electro-oxidation of I−, which was stably encapsulated in nanopores of micro-C regardless of the charging/discharging rate. Our findings suggest that [(I2)n·Br−]/I− half-redox reactions can produce superior energy and power densities in an aqua-ESS with porous carbon electrodes through the addition of Br− to their electrolytes compared with electrodes with I− only.

The main drawback of organic electrode materials is their solubility in the electrolyte, leading to the shuttle effect. Using N,N'-dimethylphenazine (DMPZ) as a highly soluble cathode material, and its PF6 - and triflimide salts as models for its first oxidation state, a poor correlation was found between solubility and battery operability. Extensive electrochemical experiments suggest that the shuttle effect is unlikely to be mediated by molecular diffusion as commonly understood, but rather by electron-hopping via the electron self-exchange reaction based on spectroscopic results. These findings led to two counter-strategies to prevent the hopping process: the pre-treatment of the anode to form a solid-electrolyte interface and using DMPZ salt rather than neutral DMPZ as the active material. These strategies improved coulombic efficiency and capacity retention, demonstrating that solubility of organic materials does not necessarily exclude their applications in batteries.

The popularization of electric vehicles and the increasing use of electronic devices highlight the importance of fast charging technology. The charging process of lithium secondary battery is basically limited by a series of processes on the anode side, which include desolvation of lithium ions as well as lithium diffusion through SEI and the anode material. These series of reactions are kinetically sluggish, leading to insufficient power density. Therefore, to unravel this problem, we need to either accelerate each step or skip over some of the steps to make the whole charging process shorter. A solvated ion cointercalation into graphite has turned out to successfully exclude both desolvation of lithium ions and SEI film formation to achieve high kinetics with graphite. Herein, the solvated ion cointercalation into MoS2 demonstrated that it can help to remove desolvation of alkali ions as well as SEI formation, and thereby ultrahigh kinetics and long-term cyclability are attained by the characteristic ps...

Vanadium redox flow batteries (VRFBs) have attracted considerable attention for potential use in the development of large-scale energy storage systems. However, the commercialization of VRFBs is still challenging because of their various overpotentials, which are due to the poor reversibility and electrochemical activity of graphite felt (GF) electrodes. In this study, we fabricated a NiO-decorated GF electrode that exhibited a clear electrocatalytic effect on the V2+/V3+ and VO2+/VO2+ redox reactions. Vanadium ions preferentially attached to each NiO site because of strong electrostatic affinity to the local negatively charged O2− species. In particular, a significant amount of NiO bound to graphite by replacement of hydrogen from the hydroxyl groups with nickel ion, leading to an increase in the ratio of carboxyl groups to hydroxyl groups. The increase in the number of carboxyl groups also improved the VRFB performance, since the carboxyl functional group on GF surface acts as effective catalyst for the vanadium redox reactions. Furthermore, NiO nanoparticles enhanced the mass-transfer property of vanadium ions by the increased area and hydrophilicity of the electrode surface. To optimize the electrode structure for high electrochemical performance, the crystallinity and morphology of the NiO catalyst on GF were controlled via the operating temperature and precursor concentration. When optimized NiO/GF300 was applied to VRFBs, it exhibited high energy efficiency (74.5%) at a high current rate (125 mA cm−2), compared with GF without the catalyst (55.4%). Moreover, NiO-decorated GF exhibited durability and stability in acidic electrolyte during long-term operation for 300 cycles.

A great challenge in the application of carbon-based materials to Li–O2 batteries is to prevent the formation of carbonate-based side products, thereby extending the cycle life of Li–O2 batteries. Herein, for the first time, CNT@Ni@NiCo silicate core–shell nanocomposite is designed and used as a cathode catalyst in Li–O2 batteries. This nanocomposite shows a promising electrochemical performance with a discharge capacity of 10 046 mA h gcat−1 and a low overpotential of 1.44 V at a current density of 200 mA gcat−1, and it can sustain for more than 50 cycles within the voltage range of 2–4.7 V. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterizations prove that the formation of Li2CO3 and other side products are prevented, likely due to the encapsulation of CNTs by NiCo silicates and Ni nanoparticles, which may help decompose the side products. Finally, the synergistic effects, which are contributed by the high electrical conductivity of CNTs, high surface area, the high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of NiCo silicate, and the simple decomposition of side products by Ni nanoparticles enable outstanding performance of the CNT@Ni@NiCo silicate core–shell nanocomposite as a cathode catalyst for Li–O2 batteries.

There is increasing interest in vanadium redox flow batteries (VRFBs) for large scale-energy storage systems. Vanadium electrolytes which function as both the electrolyte and active material are highly important in terms of cost and performance. Although vanadium electrolyte technologies have notably evolved during the last few decades, they should be improved further towards higher vanadium solubility, stability and electrochemical performance for the design of energy-dense, reliable and cost-effective VRFBs. This timely review summarizes the vanadium electrolyte technologies including their synthesis, electrochemical performances, thermal stabilities, and spectroscopic characterizations and highlights the current issues in VRFB electrolyte development. The challenges that must be confronted to further develop vanadium electrolytes may stimulate more researchers to push them forward.

Irreversible phase transformation of layered structure into spinel structure is considered detrimental for most of the layered structure cathode materials. Here we report that this presumably irreversible phase transformation can be rendered to be reversible in sodium birnessite (NaxMnO2·yH2O) as a basic structural unit. This layered structure contains crystal water, which facilitates the formation of a metastable spinel-like phase and the unusual reversal back to layered structure. The mechanism of this phase reversibility was elucidated by combined soft and hard X-ray absorption spectroscopy with X-ray diffraction, corroborated by first-principle calculations and kinetics investigation. These results show that the reversibility, modulated by the crystal water content between the layered and spinel-like phases during the electrochemical reaction, could activate new cation sites, enhance ion diffusion kinetics and improve its structural stability. This work thus provides in-depth insights into the intercalating materials capable of reversible framework changes, thereby setting the precedent for alternative approaches to the development of cathode materials for next-generation rechargeable batteries., The irreversible layered-to-spinel phase transformation is detrimental for many cathode materials. Here, the authors show that reversibility can be realized in crystal water containing sodium birnessite by controlled dehydration, leading to enhanced ion diffusion kinetics and improved structural stability.

A zinc–bromine redox flow battery (ZBB) has attracted increasing attention as a potential energy-storage system because of its cost-effectiveness and high energy density. However, its aqueous zinc bromide phase and non-aqueous polybromide phase are inhomogeneously mixed in the positive electrolyte. Furthermore, various equilibrium reactions, e.g., charge-transfer reactions, polybromide formation, and complexation, simultaneously occur in the battery. Because of these complex reactions, it is difficult to systematically analyze its electrolyte, which a component crucial for the stable operation of the battery. Especially, although the state-of-charge (SoC) of an electrolyte is crucial for preventing overcharging or discharging and side reactions, its accurate estimation is difficult. As a result, there have been few studies on estimation of the SoC in ZBBs. In this study, in situ Raman spectroscopy is employed for the real-time estimation of the SoC in 25 charge–discharge cycles. To exclude errors arising from the inhomogeneous dispersion of the non-aqueous phase, SoC is monitored on the negative electrolyte. External standard solutions are measured, and the calibration curve is constructed just before in situ measurements at every cycle to minimize instrumental errors, e.g., those caused by alignment. This in situ methodology exhibits high accuracy and reproducibility.

The presence of nitrogen atoms in reduced graphene oxide (RGO) sheets considerably modulates their intrinsic physical and chemical properties to finally improve their electrochemical properties in electric double layer supercapacitors. However, this also accelerates the restacking phenomena of RGO, which results in a decreased active surface area and pore volume. To solve this problem, we fabricated para-phenylenediamine (p-PDA) functionalized nitrogen doped RGO (NRGO) to inhibit the restacking phenomenon and thus preserve the active surface area and pore volume via chemical bonding between RGO and p-PDA. Finally, we realized an impressive electrochemical performance through the synergistic effect of nitrogen doping and p-PDA functionalization.

Even though Na3V2(PO4)3 (NVP) is regarded as one of the next-generation cathode materials in sodium-ion batteries (SIBs), its undesirable rate performance due to its inherently low electrical conductivity has limited its application in demanding fields such as electric vehicles. Motivated by this fact, the present study profitably employed a conductive carbon grown in situ to obtain an NVP@C composite with a pomegranate-like structure by a simple sol–gel assisted hydrothermal technique. The as-prepared NVP@C composite consists of small carbon-coated NVP particles (∼200 nm) embedded in a conductive carbon matrix, which ensures short ion diffusion distances, percolating electron/ion conduction pathways and stable structural integrity. As a result, the pomegranate-structured NVP@C composite displayed remarkable overall electrochemical performance: a high discharge capacity (110 mA h g−1 at 1C), excellent rate capability (92 mA h g−1 even at 50C) and impressive cycling stability (capacity retention of 91.3% over 2000 cycles at 10C). Such a feasible and beneficial design provides a good strategy for other materials that require both size reduction and high electronic/ionic conductivity.

Silicon (Si) has attracted much attention as a promising anode material for Li ion battery because of its high theoretical specific capacity and low working potential. However, Si has shown poor cycling behavior and reversibility, which result from its huge volume change and the following pulverization. In this study, an electrospinning method was adopted to synthesize Pitch-incorporated into Si/Carbon nanofibers (Si/Pitch CNFs), which has high crystalline carbon network compared to other Si/Carbon nanofibers without the carbon matrix obtained from the decomposition of pitch. We demonstrated that this high crystalline carbon network in the form of nanofiber has two kinds of merits: it not only reduced the diffusion length for Li ion transport thanks to its 1D morphology but also helped to tolerate high strain of Si and maintain electron transport throughout the entire electrode. As a result, Si/Pitch CNFs showed a greatly enhanced kinetic performance, even at 10C, thus showing its feasibility as a high-power material for future applications like electric vehicles (EV) and energy storage systems (ESS).

Because of their outstanding features such as safety, long cycle life, and design flexibility, vanadium redox flow batteries (VRFBs) have attracted much attention from those involved in the development of electrical energy-storage system. However, the performance of VRFBs remains limited due to their significant polarization. Here, we report a new fabrication method for highly porous graphenated graphite felt electrode with high-performance, which enables operation of VRFBs at high current rates by alleviating polarization. The etched graphite felt (EGF) electrode is optimized by repetition of a NiO/Ni redox reaction cycle, which is a facile, scalable, and controllable etching process that produces a high surface area. The EGF also has stepped edges, which act as preferred sites for incorporating oxygen defects. The plentiful oxygen defects on the stepped edges show catalytic effect and good wettability for vanadium electrolyte, leading to substantially reduced overpotentials. VRFBs with the EGF electrode exhibit a strongly enhanced electrochemical performance with respect to energy efficiency and discharge capacity at 150 mA cm−2. Furthermore, the robustness of the graphenated structure provides stability and durability in acidic electrolyte during long-term battery operation, facilitating stable cycling performance for 200 cycles at high current rates.

Because of the rise in renewable energy use, the redox flow battery (RFB) has attracted extensive attention as an energy storage system. Thus, many studies have focused on improving the performance of the felt electrodes used in RFBs. However, existing analysis cells are unsuitable for characterizing felt electrodes because of their complex 3-dimensional structure. Analysis is also greatly affected by the measurement conditions, viz. compression ratio, contact area, and contact strength between the felt and current collector. To address the growing need for practical analytical apparatus, we report a new analysis cell for accurate electrochemical characterization of felt electrodes under various conditions, and compare it with previous ones. In this cell, the measurement conditions can be exhaustively controlled with a compression supporter. The cell showed excellent reproducibility in cyclic voltammetry analysis and the results agreed well with actual RFB charge-discharge performance.

To demonstrate an aqueous sodium-polyiodide flow battery (SIFB) for the first time, sodium titanium phosphate (NaTi2(PO4)3) was utilized as the negative electrode and I−/I3− couple was adopted as the positive redox active species. SIFB not only shows advantages of cost-effectiveness and environmental friendliness based on earth-abundant elements of sodium and iodine, but also exhibits a high energy density due to the high solubility of sodium iodide (~12.3 M at room temperature). Although NaTi2(PO4)3 has several advantages such as high Na-ion conductivity and good structural stability, its use as a negative electrode is limited because of intrinsically low electrical conductivity. In this study, NaTi2(PO4)3 nanoparticles were embedded in double carbon networks comprising internal and external carbon layers. Individual effects by the internal and external carbon networks on the electrochemical performance of NaTi2(PO4)3 were clearly distinguished. With the simultaneous formation of the two carbon networks, a SIFB exhibited outstanding electrochemical performance (86.0 mAh•g−1 at a rate of 7 C) and a long cycle life (93.5% capacity retention after 100 cycles).

This work demonstrates a simple, large scale, cost effective facile route to a well crystallized only anatase TiO2 nanotube synthesized in Schlenk flask at 120 °C for 12 h in the 8 M KOH solution (TNTK) without a Teflon line autoclaves. For the first time all the XRD peaks at 2θ degrees of 25.39, 39.92, 48.11, 54.04, 55.03, 62.73, 68.97, 70.46, 75.48 corresponding to (101), (004), (200), (105), (211), (204), (116), (220), and (215) appeared in the final products contrary to the hydrothermal method where one/two anatase phases, often accompanied by a 2θ peaks at 14.5°. The as prepared nanotubes shows an outer diameter of ∼10 nm and inner diameter of ∼4.4 nm with a micrometers of tube lengths. It possess a high BET surface area of 203.75 m2/g with band gap of 3.70 eV, whereas 8 M NaOH solution (TNSNa) at the same condition gives a scattered flowered structure with tapering petals of surface area 71.5 m2/g and a mixed crystal phase of anatase/rutile and sodium hexatitanate. When these nanotubes applied as anode materials in LIBs, it shows an excellent first discharge/charge capacity of 635/240 mAhg−1 and retains 164 mAhg−1 after 36 cycles much higher than that of titanium dioxide nanoparticles and scattered flowers. The excellent performance can be ascribed to the 1D morphology, enhanced conductivity, fewer localized states near the band edges, high surface area and robustness of tubules which could effectively withstand expansion/contraction occurring during the lithium ion insertion/extraction process. Details of the synthesized nanotubes and electrochemical properties were characterized by HR-TEM, TEM-EDS, FE-SEM, XRD, XPS, UV-vis, electrochemical, CV, impedance and BET surface area techniques.

A novel all-organic redox cell employing 4-Oxo 2,2,6,6-tetramethyl-1-piperidinyloxy (4-Oxo TEMPO) (as a catholyte) and (1S)-(+)-Camphorquinone (as an anolyte) is investigated through electrochemical measurements. From cyclic voltammetry tests, it is confirmed that electrochemically reversible redox processes of 4-Oxo TEMPO and (1S)-(+)-Camphorquinone occur at 0.48 V and −1.64 V (vs. Ag/Ag+), respectively. A non-aqueous redox battery with these redox couples can yield an estimated open circuit potential of 2.12 V at a 50% state of charge. The charge–discharge performance is assessed with an in-house designed non-flow single cell. Coulombic and energy efficiencies are 80.3% and 71.3%, respectively. Keywords: Redox flow battery, Non-aqueous electrolyte, All-organic redox couples, 4-Oxo TEMPO, Camphorquinone

Nitrogen doped reduced graphene oxide (N-RGO) is synthesized using microwave-assisted hydrothermal (MAHA) reaction. The proper configurations of nitrogen atoms in graphene sheets considerably increase the intrinsic electrical properties of N-RGO resultantly improving its capacitance and other kinetic properties in supercapacitor. Here, under the controlled MAHA reaction, we adjusted the ratio of nitrogen configurations (pyridinic-N, pyrrolic-N and quaternary-N) for the most optimum supercapacitor performances of N-RGOs in the shortest time ever reported, and clarified that its enhanced electrical conductivity and supercapacitor performances are attributed to its enlarged concentration of quaternary-N. With this MAHA reaction, we present a supercapacitor based on N-RGO, which is capable of displaying the promising electrochemical properties.

During the charging of a Zn/Br redox flow battery, cyclic voltammetry and electrochemical impedance spectroscopy measurements were carried out in-situ. As the state of charge (SOC) increased, some polybromide complex accumulated on the Br-side electrode surface and showed a positive effect on the adsorption of bromide ion as well as bromine. The deposition of polybromide complex onto the electrode surface was identified by SEM, EDS, and Raman spectroscopy. As a result, the charge transfer resistance for bromine oxidation decreased from 2.13 ohm to 1.27 ohm as the SOC increased from 0.0% to 80.0%. This may be due to the amphiphilic characteristics of the polybromide complex. While the solution resistance for catholyte was independent of the SOC, that for anolyte sharply decreased with increasing SOC. This could be explained by the increase in zinc ion mobility and the anolyte thickness reduction by growth of zinc metal dendrites.

Sodium pyrophosphate tetrabasic (SPT) is employed as an inorganic additive in the positive electrolyte of a vanadium redox flow battery (VRFB) to improve its long-term stability and electrochemical performance. The results of precipitation tests show that the long-term stability of positive electrolytes (2 MV(V) solution in 4 M total sulfates with 0.05 M SPT additive) is improved compared to the blank one. UV-vis and cyclic voltammetry (CV) measurements also suggest that the addition of SPT can effectively delay the formation of precipitation in positive electrolytes, and no new substances are formed in V(V) electrolytes with SPT. The calcined precipitates extracted from the electrolytes with and without a SPT additive are identified as V2O5 by X-ray diffraction (XRD) analysis. A VRFB single-unit cell employing positive electrolytes with an additive exhibits the high energy efficiency of 74.6% at a current density of 40 mA cm2 at the 500th cycle at 20°C, compared to 71.8% for the cell employing the electrolyte without an additive. Moreover, the cell employing the electrolyte with an additive exhibits less discharge capacity fading during cycling in comparison with the pristine one. The disassembled cell without an additive shows a large number of V2O5 precipitation particles on the felt electrode after 500 cycles. Meanwhile, the felt electrode of the cell with an additive has little precipitation. That precipitation gives rise to an imbalance between the positive and negative half-cell electrolytes, which results in a significant capacity loss. The additive has shown positive results under limited laboratory short-term and small-scale conditions.

Fluorinated activated carbon (F-AC) exhibits significantly improved electrochemical capacitive performance compared to raw activated carbon (R-AC). F-AC electrode shows a specific capacitance coming up to 19.8 F cm −3 , whereas the capacitance of R-AC electrode is 18.4 F cm −3 . Besides, the kinetic enhancement of F-AC is also memorable. This phenomenon indicates that F-AC tends to form electric double-layer ions on its surface more rapidly than does R-AC. This formation is a result of the increased electrical conductivity attributed to the semi-ionic bonding character between fluorine and activated carbon. The electrochemical improvement of F-AC proves that fluorination is a very effective method for providing greater possibilities for supercapacitor applications of AC in nonaqueous electrolytes.

Compressed carbon felt electrodes with various percentages of compression are prepared by stacking pieces of PVC gaskets; the performance of VRFB cells prepared using these electrodes is evaluated in order to better understand the influence of the compressed electrodes on the fundamental properties of VRFBs. It is found that the specific resistance and porosity of the electrodes decreases with increase of the percentage of electrode compression. In addition, as the percentage of electrode compression increases, the discharge time and maximum power of the VRFB cells gradually increase due to the increased electron transfer. The energy efficiency of the cell increases with the increase of the percentage of electrode compression up to 20%. When the percentage of electrode compression is greater than 20%, the energy efficiency decreases due to the combined effects of reduced cell resistance, poor electrolyte transport, and longer charge/discharge time. Based on our results, it can be concluded that compressed electrodes have a positive effect on cell performance; however, their inevitable reduced porosity is detrimental to electrolyte transport, thereby resulting in a decrease of energy efficiency. Consequently, it is suggested that carbon felt electrodes with an optimized percentage of compression have considerable potential for use in VRFB applications without incurring additional cost.

Capacity decay in vanadium redox flow batteries during charge–discharge cycling has become an important issue because it lowers the practical energy density of the battery. The battery capacity tends to drop rapidly within the first tens of cycles and then drops more gradually over subsequent cycles during long-term operation. This paper analyzes and discusses the reasons for this early capacity decay. The imbalanced crossover rate of vanadium species was found to remain high until the total difference in vanadium concentration between the positive and negative electrolytes reached almost 1 mol dm−3. To minimize the initial crossover imbalance, we introduced an asymmetric volume ratio between the positive and negative electrolytes during cell operation. Changing this ratio significantly reduced the capacity fading rate of the battery during the early cycles and improved its capacity retention at steady state. As an example, the practical energy density of the battery increased from 15.5 to 25.2 Wh L−1 simply after reduction of the positive volume by 25 %.

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium-oxygen (Na-O2 ) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na-O2 battery is lower than that of the lithium-oxygen (Li-O2 ) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium-oxygen and sodium-oxygen cells. An in-depth understanding of the differences and similarities between Li-O2 and Na-O2 battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal-oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium-oxygen and sodium-oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.

Fast lithium ion and electron transport inside electrode materials are essential to realize its superb electrochemical performances for lithium rechargeable batteries. Herein, a distinctive structure of cathode material is proposed, which can simultaneously satisfy these requirements. Nanosized Li3V2(PO4)3 (LVP) particles can be successfully grown up on the carbon nanofiber via electrospinning method followed by a controlled heat-treatment. Herein, LVP particles are anchored onto the surface of carbon nanofiber, and with this growing process, the size of LVP particles as well as the thickness of carbon nanofiber can be regulated together. The morphological features of this composite structure enable not only direct contact between electrolytes and LVP particles that can enhance lithium ion diffusivity, but also fast electron transport through 1D carbon network along nanofibers simultaneously. Finally, it is demonstrated that this unique structure is an ideal one to realize high electron transport and ion diffusivity together, which are essential for enhancing the electrochemical performances of electrode materials.