Manganese‐Based Materials for Rechargeable Batteries beyond Lithium‐Ion

Introduction Rechargeable battery systems with different transporting ions are an alternative choice to Li batteries that can expand and diversify the existing energy storage market. Magnesium batteries are the most promising technology to achieve substantially greater energy density than Li-ion batteries, due to their divalent nature (two electron redox couple). Furthermore, because of its “green” character Mg is environmentally friendly, with high natural abundance in the Earth’s crust (13.9% as compared to 0.0007 % for Li), and atmospheric stability.The current quest for new cathode materials for rechargeable Mg batteries is focused on intercalation compounds exhibiting high working potential and capacity1. When transporting ions have much larger diameter or charge than the host metal ion, segregation of transporting and host metal ions in the crystalline structure is forseable. In this case deintercalation will lead to the collapse of the crystalline structure and breaking of electric conductance. Therefore, our approach to achieving magnesium intercalation is to use nanoscale material that have adjustable d spacing and two-dimensional layered structure that can to control polarizability of Mg2+ions and accommodate large charge/volume changes.Nanostructured electrode materials represent one of the most attractive strategies to dramatically enhance battery performance. Short diffusion length associated with the nanosized dimensions effectively reduces the distance that ions and electrons must travel during cycling. Their large surface area increases the contact area between electrode and electrolyte and hence the number of active sites for electrode reactions, which in turn reduces electrode polarization loss and improves power density, energy efficiency, and usable energy.Nanostructured bilayered V2O5 is highly efficient 3 V cathode material for ambient temperature sodium ion batteries2. It shows superb performance: theoretical reversible capacity for Na2V2O5 stochiometry of 250 mAh/g, excellent rate capability and cycle life, as well as high energy and power densities. Preliminary results show that it is possible to extend the reversible ion-insertion chemistry of nanostructured layered V2O5 beyond lithium and larger in size Na+ ions to those with higher charge: Mg2+ions. Results and Discussion As–prepared nanostructured bilayered V2O5 cathode is synthesized in the charged state. To provide source enrichment, prior to assembling into full Mg-ion cell, it was necessary to pre-condition V2O5 cathode (intercalate with Mg2+ ions). Half-cells were discharged galvanostatically at 20mA down to and 0.01 V versus Mg/ Mg2+, respectively, using an automated Maccor battery tester at ambient temperature. As shown in Figure 1., upon electrochemical pre-condition (Mg-ion half cell) the X-ray edge energy for vanadium is shifted toward the lower energy as a consequence of a vanadium reduction due to Mg intercalation. Pre-edge peak is proportional to the octahedral symmetry of the vanadium sites and its decrease upon transporting ion intercalation is due to modification of vanadium local environment into more ordered, symmetric structure. Based on the main-edge peak position and comparison with Na-ion half cell we estimate 1 Mg is incorporated in the V2O5 structure (V5+ is reduced to V4+). It was previously shown that the V5+ was reduced to V4+ when discharged to 1.5V versus Na/ Na+ (Na-ion half cell)2. As a result of this electrochemical pre-condition we created Mg-rich V2O5 cathodes that were able to provide source of Mg-ions in full-cell. Furthermore, we employed HRTEM, X-Ray fluorescence (XRF) imaging and classical molecular dynamics simulations to understand the role of environment in the intercalation processes of Mg-ions into V2O5cathode. Conclusions We have developed high-voltage nanostructured V2O5 cathode for Mg-ion rechargeable batteries that can operate in commercial electrolytes. Our results emphasize the importance of tailoring nanoarchitecture of electrode materials and open up new opportunities for rechargeable Mg batteries. In the near future, we plan using the combined experimental and theoretical approach to guide our efforts to increase the Mg enrichment to reach the ultimate goal of inserting 2 Mg-ions/V2O5. Acknowledgement. This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by DOE, Office of Science, Basic Energy Science (BES). Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. References (1) On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials, E. Levi, Y. Gofer, and D. Aurbach, Chem. Mater. 2010, 22, 860–868.(2) Vanadium Oxide Electrodes for Rechargeable Sodium-Ion Batteries, Sanja Tepavcevic, Hui Xiong, Vojislav R. Stamenkovic, Xiaobing Zuo, Mahalingam Balasubramanian, Vitali B. Prakapenka, Christopher S. Johnson, and Tijana Rajh, Nanostructured Bilayered ACS Nano, 2012, 6 (1), 530–538.

Understanding of the thermodynamic and kinetic properties of electrode materials is of great importance to develop new materials for high performance rechargeable batteries. Compared with computational understanding of physical and chemical properties of electrode materials, experimental methods provide direct and convenient evaluation of these properties. Often, the information gained from experimental work can not only offer feedback for the computational methods but also provide useful insights for improving the performance of materials. However, accurate experimental quantification of some properties can still be challenging. Among them, chemical diffusion coefficient is one representative example. It is one of the most crucial parameters determining the kinetics of intercalation compounds, which are by far the dominant electrode type used in rechargeable batteries. Therefore, it is of significance to quantitatively evaluate this parameter. For this purpose, various electrochemical techniques have been invented, for example, galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). One salient advantage of these electrochemical techniques over other characterization techniques is that some implicit thermodynamic and kinetic quantities can be linked with the readily measurable electrical signals, current, and voltage, with very high precision. Nevertheless, proper application of these techniques requires not just an understanding of the structure and chemistry of the studied materials but sufficient knowledge of the physical model for ion transport within solid host materials and the analysis method to solve for chemical diffusion coefficient. Our group has been focusing on using various electrochemical techniques to investigate battery materials, as well as developing models for studying some emerging materials. In this Account, the principles of the aforementioned four electrochemical techniques and the corresponding analytical equations for calculating the chemical diffusion coefficients are first briefly summarized, followed by a discussion of the hidden assumptions for deriving these analytical equations and the resulting limitations in their implementation. To address these limitations, various corrections have been made in the literature. Nevertheless, the phase transition behavior, which is the typical feature for many intercalation materials, is barely considered. Here we retrospect our previous work on developing a two-phase model for describing the phase transition behavior of some intercalation compounds and discuss how to obtain the chemical diffusion coefficients based on the model, using LiFePO4 as an example material. After that, we have a discussion on the methodology for using electrochemical techniques to investigate new material features. It is our hope that this Account can serve as a call for more endeavors into the development of novel electrochemical tools for battery research.

The field of advanced batteries faces today two main challenges: development of power sources for electro-mobility (i.e., electrochemical propulsion) and development of storage technologies for load-leveling applications (i.e., large scale batteries). Our starting point in this presentation is state-of-the-art Li-ion batteries. Using carbonaceous anodes and lithiated transition metal cathodes with selected combinations of transition metals may offer very suitable batteries for full EVs. Ni rich Li[NiCoMn]O2, Mn and Li rich Li1+x[NiMnCo]1-xO2 cathode materials operating in the right potential windows can provide Li-ion batteries high enough energy density as needed for full electrochemical propulsion. Also, by combining Li-Ti-O anodes with LiMPO4 olivine cathodes it is possible to demonstrate very suitable battery systems for stationary large energy storage applications. We will review briefly the updated frontier of Li-ion battery technology. Despite the high capability of this battery technology, there are so many applications for power sources in modern life, what promote very strongly a search for more new battery systems. The use of sulfur cathodes in Li batteries can increase considerably their gravimetric energy density, however, the volumetric energy density of Li-sulfur battery systems is not advantageous compared to advanced Li ion batteries. The good news are that in recent years, we see a nice progress in developing durable composite sulfur cathodes. It seems that there are 3 main approaches: encapsulating sulfur in activated carbon matrices, the use of substrates to which the LiSn species formed by sulfur reduction adsorb very strongly and the use of composite cathodes with high loading of sulfur and semi-permeable membranes that avoid transport of LiSn species to the negative electrodes. There is intensive work on Li-oxygen systems. We see progress in development of electro-catalysis for these systems. We will demonstrate electro-catalytic electrodes in Li-oxygen cells, which anodic reactions (Li-peroxide oxidation and molecular oxygen evolution) occur at very low over-potential, that does not endanger the stability of the electrolyte solutions. We demonstrate also red-ox mediators in solutions that facilitate both ORR and OER. Despite the frustration from the slow progress made towards practical directions, it is important to continue investing efforts in Li-oxygen systems. The community should understand that a long-term research is still required in order to see practical horizons to Li-air battery systems. A main problem is the reactivity of all kinds of polar-aprotic solvents towards superoxide and peroxide species formed by oxygen reduction, in the presence of the super-electrophilic Li ions. The intensive work on both Li-S and Li-oxygen systems is connected to a renaissance in efforts to develop practical Li metal anodes for rechargeable batteries. However, intensive studies that were carried out during more than 3 decades, concluded more than 15 years ago that there is no way to cycle Li metal anodes in practical rechargeable Li battery systems. Even if dendrites formation is avoided, continuous detrimental reactions between the active metal and any type of relevant electrolyte solution is inevitable. We will examine briefly what are the chances to overcome the intrinsic problems in using Li metal anodes in practical rechargeable batteries. We may be able to suggest practical alternative anodes for non-aqueous batteries based on sulfur or oxygen cathodes. Discussing other options for “beyond Li-ion batteries” usually includes Na-ion and magnesium batteries. In fact, the plethora of relevant Na-ion insertion cathodes available today is a good surprise, especially since many of them demonstrate excellent kinetics. However, there is no way that Na-ion systems can rival Li-ion battery systems in terms of energy density. Hence, Na-ion systems may be relevant for large energy storage applications, that can benefit from the high abundance of Na in earth crust. However for such uses, demonstrating high stability during prolonged cycling is mandatory. Consequently, a major challenge in R&D of Na-ion batteries is to exhibit the durability required for load-leveling applications. Finally, we will mention the status of R&D of rechargeable Mg battery systems: excellent work was done so far on the anode-solutions part. Several families of electrolyte solutions with wide electrochemical windows, in which Mg anodes are fully reversible were demonstrated. We have not seen yet cathodes materials that function well in the solutions in which Mg anodes are fully reversible, except the low voltage/low capacity Chevrel phase cathode materials. Hence, these systems still require long term research work, especially towards high voltage/high capacity cathode materials. A main incentive to develop rechargeable Mg batteries may relate to advantages in safety and high volumetric energy density.

Rechargeable batteries are playing an increasingly prominent role in our lives due to the ongoing transition from fossil energy sources to green energy. The purpose of this study was to investigate variables that impact the effectiveness of rechargeable batteries. Alkaline (non-rechargeable) and rechargeable batteries share common features that are critical for the operation of a battery. The positive and negative electrodes, also known as the cathode and anode, are where the energy of the battery is stored. The electrolyte is what facilitates the transfer of cations and anions in a battery to generate electricity. Due to the importance of these components, we felt that a systematic investigation examining the surface area of the cathode and anode as well the impact of electrolytes with different properties on battery performance was justified. Utilizing a copper cathode and aluminum anode coupled with a water in salt electrolyte, a model rechargeable battery system was developed to test two hypotheses: a) increasing the contact area between the electrodes and electrolyte would improve battery capacity, and b) more soluble salt-based electrolytes would improve battery capacity. After soaking in an electrolyte solution, the battery was charged and the capacity, starting voltage, and ending voltage of each battery were measured. The results of this study supported our hypothesis that larger anode/cathodes surface areas and more ionic electrolytes such as sodium chloride, potassium chloride and potassium sulfate resulted in superior battery capacity. Incorporating these findings can help maximize the efficiency of commercial rechargeable batteries.

Scalable Room-Temperature Synthesis of Multi-shelled Na3(VOPO4)2F Microsphere Cathodes

Innovative rechargeable batteries beyond the present lithium-ion batteries (LIB) have been investigated for the environmental and energy problem. Among the batteries, rechargeable magnesium metal batteries have attracted much attention, since magnesium metal as negative electrode can bring high volumetric energy density owing to the two-electron reaction, low electrode potential, and lightness. Until now, the electrolyte solutions used for rechargeable magnesium metal batteries are tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME) based electrolyte solutions. The boiling points of THF and DME are 66°C and 85°C, respectively. In order to eliminate the water in the rechargeable magnesium metal battery system, at least 100°C is required for the working temperature. However, THF or DME based electrolyte solutions do not meet the criteria. Therefore, we focused on the triethylene glycol dimethyl ether (triglyme) as a solvent since the boiling point of triglyme is 216°C. As a magnesium salt, magnesium(II) bis(trifluoromethane sulfonyl) amide (Mg(TFSA)2) was selected since TFSA- is a large anion and Mg(TFSA)2 is expected to be highly dissociative in triglyme. By using this system, we can increase the working temperature more than 100°C for rechargeable magnesium metal battery system[1]. As a proof-of-concept, Orikasa et al. reported that a rechargeable magnesium metal battery composed of insertion-type MgFeSiO4 positive electrode can work at 100°C in triglyme-based electrolyte solution [2]. This result indicated that we can fabricate high safety rechargeable magnesium metal batteries at ambient temperature by selecting appropriate positive electrodes. Then, the electrochemical behavior of electrolyte solution at ambient temperature is also important. In this conference, we reports the reversible magnesium metal deposition and dissolution in the Mg(TFSA)2/glymes solution at ambient temperature. In addition, we report new magnesium salt in triglyme. [1] T. Fukutsuka, K. Asaka, A. Inoo, R. Yasui, K. Miyazaki, T. Abe, K. Nishio, and Y. Uchimoto, Chemistry Letters, 11, 1788-1790 (2014).[2] Y. Orikasa, T. Masese, Y. Koyama, T. Mori, M. Hattori, K. Yamamoto, T. Okado, Z. D. Huang, T. Minato, C. Tassel, J. Kim, Y. Kobayashi, T. Abe, H. Kageyama, and Y. Uchimoto, Sci. Rep. 2014, doi: 10.1038/srep05622.

Rechargeable Li batteries as electrochemical energy storage and conversion devices are continuously changing human life. In order to meet the increasing demand for energy and power density, it is essential and urgent to exploit the electrode materials with high capacity and fast charge transfer (for Li-ion and Li-S batteries) and electrocatalysts with high activity (for rechargeable Li-O2 batteries). The high capacity is attributed to high electron transfer number and low molecular weight of the electrode materials. Combined with proper nanostructure design, the electronic transfer and ionic conductivity will be improved. This review summarizes recent efforts to apply electrode materials for Li-ion batteries with multi-electron reaction, Li-S batteries, and efficient electrocatalysts for Li-O2 batteries. The methods to enhance the cycling and rate performance have been discussed in detail. Advanced rechargeable Li batteries with multi-electron reaction will become the research emphasis in the future.

Rechargeable sodium batteries, as the replacement of rechargeable lithium batteries, are attracting much attention for large-scale applications such as electric vehicles and stationary energy storage due to the abundant resource of sodium compared to lithium. Up to now, there are only a few materials which have reasonable performances as negative electrodes of rechargeable sodium batteries, such as alloy materials [1], hard carbon [2], and sodium titanium oxides [3]. Recently, TiO2, an attractive alternative negative electrode material to graphite for rechargeable lithium batteries, was also investigated for rechargeable sodium batteries [4, 5]. However, the reported reversible capacities of TiO2 electrodes are less than 200 mAh g-1 and the Na insertion/extraction mechanism is still unclear. In the previous study, carbon-coated anatase TiO2 (TiO2/C) nanopowders were investigated as negative electrode material for rechargeable sodium battery with Na[FSA]-[C3C1pyrrr][FSA] (FSA = bis(fluorosulfonyl)amide; C3C1pyrr = N-methyl-N-propylpyrrolidinium) ionic liquid electrolyte [6]. In the present study, the electrochemical performance of TiO2/C electrode was further investigated in Na[FSA]-[C3C1pyrrr][FSA] ionic liquid electrolyte, and the Na insertion/extraction mechanism was explored. The TiO2/C electrode shows a high reversible discharge capacity of 275 mAh g-1 at 10 mA g-1 at 363 K. The observed capacity is higher than that reported for anatase TiO2 nanorods (193 mAh g-1 at 10 mA g-1) in organic electrolytes at room temperature [5]. Moreover, this capacity is comparable to that of a hard carbon negative electrode (260 mAh g-1) in Na[FSA]-[C3C1pyrrr][FSA] electrolyte [7]. Since the density of anatase TiO2 (ca. 3.9 g cm-3) is much higher than that of hard carbon (ca. 1.52 g cm-3), the TiO2/C nanopowder electrode is a promising negative electrode material for the rechargeable sodium batteries with high volumetric energy densities. The Na insertion/extraction mechanism was investigated by means of ex-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and electron diffraction. Ex-situ XRD analysis of the TiO2/C electrodes in the different charged/discharged states show that during the charge process, the anatase TiO2 crystal phase disappears and a new crystal phase is formed. During the discharge process, the newly formed crystal phase disappears and no anatase TiO2 phase appears again. According to the TEM, EDX and electron diffraction studies, the fully discharged TiO2/C electrode is suggested to consist of amorphous TiO2, amorphous Na2TiO3 and cubic TiO phases, and the crystal phase in the fully charged TiO2/C electrode is possibly a solid solution of cubic Na2TiO3 and cubic TiO phases. Thus, amorphous TiO2, amorphous Na2TiO3 and cubic TiO are regarded as active electrode materials, and reversibly react with Na ions to form Na2TiO3•TiO solid solution. The TiO2/C negative electrode also exhibits a high rate capability and good cycling performance. When the charge-discharge rate is increased, the shapes of the charge-discharge curves remain unchanged. The discharge capacities decrease gradually with an increase in the current rate. The TiO2/C electrode shows reversible discharge capacities of 174 and 93 mAh g-1 at current rates of 500 and 2000 mA g-1, respectively, corresponding to about 69 and 37% of the capacity at 20 mA g -1. The discharge capacity is 174 mAh g-1 after 400 cycles at a current rate of 200 mA g-1, exhibiting only 10% drop in capacity. After 1000 cycles, the capacity decreases by approximately 21%. Acknowledgements This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA, No. 3428) of the Japan Science and Technology Agency (JST) and the “Elements Strategy Initiative to Form Core Research Center" program of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).

1. Introduction Conventional Li rechargeable battery system contains transition metal based electrodes. These conventional transition metal based electrodes have severe problems such as scarcity of resources, potentially high cost, and toxicity. With respect to the high sustainability for the next-generation energy storage system, another candidate of electrode materials is needed. Due to high sustainability and chemical tunability, organic battery system holds a great potential in next generation energy storage system. In order to develop these new battery system, understanding factors affecting a performance should have priority than others. It is well-known that the LUMO level of organic compounds has a linear correlation with redox voltage in rechargeable batteries (Figure 2). 6 However, it has been recently reported additional factors affecting redox voltage of organic electrode materials. 4,5 This research is focusing on categorization of factors affecting a voltage, especially, on cation-substituents interaction. With many references and our experiment results, it is convinced that voltage of organic materials is determined by several factors. Additional substituent effect and cation species effect should be considered for predicting an exact voltage behavior with molecular tuning. 2. Results and Discussion The target species based on terephthalate are shown in Figure 3. Terephthalate is one of the simplest form in organic electrode materials. Therefore, another effect can be reduced to study cation-substituent effect. The plot of LUMO level vs. discharge voltage of target species is shown in Figure 4. Four types of cathode materials have same redox center based on terephthalate already reported by J.-M. Tarascon group.7 Tendency of almost species follows conventional rules which of correlation between LUMO level and discharge voltage, but some species having special substituent shows different behavior. Table 1. LUMO level vs. Experimental voltage Species Na2H2TP Na2TP Na2DMTP Na2MeO2TP Li cell (vs. Li/Li+) 0.93 0.84 0.81 0.9 Na cell (vs. Na/Na+) 0.55 0.59 0.54 0.38 LUMO (eV) -1.58 -1.41 -1.4 -1.34 3. Concluding remarks Firstly, some special substituent gives a big effect to determine a voltage. Methoxy group (-OCH3) is an electron-donating substituent. With LUMO level, voltage should be lower than others. But, in Li cell system, it does not follow. Interaction between lithium and oxygen from methoxy group make a stable discharge form. On the other hand, in Na cell system, sodium ion has a bigger size than lithium ion. So, interaction between oxygen cannot be occurred because of steric strain. Secondly, with LUMO level, voltage of Na2H­2TP should be higher than Na2TP. But, in Na cell system, it is not. Steric strain affect to the voltage of Na2H2TP. In our conclusion, lithium ion size is negligible affecting to the voltage but sodium ion size starts to affect the lowering of voltage in Na2H2TP electrode material. It is expected that competition between factors affecting a voltage of organic electrode materials. In summary, cation-substituent interaction should be considered to predict an exact voltage. It is important for tuning an organic electrode material to make an appropriate voltage in our use. More additional factors may exist and it is needed to categorize them. The more understanding of factors affecting to a voltage gathered, then more accurate prediction and molecular tuning can be possible. 4.

<div class="htmlview paragraph">At the request of the FAA, a committee was formed by the RTCA organization to prepare a Minimum Operating Performance Standard (MOPS) for Rechargeable Aircraft Lithium Battery Systems. This committee has being working for the last 2 years and completed the final draft during their 6<sup>th</sup> meeting in February 2008.</div> <div class="htmlview paragraph">Lithium Battery approval will be different than Nickel-Cadmium and Lead-Acid in that they will incorporate additional safety and documentation requirements.</div> <div class="htmlview paragraph">The MOPS, to be used by the FAA as the basis for a Technical Standard Order (TSO), includes the testing and evaluation criteria required for rechargeable lithium battery systems airworthiness demonstration.</div>

Graphene is commonly used to improve the electrochemical performance of electrode materials in rechargeable batteries by forming graphene-based heterostructures. Two-dimensional graphitic carbon nitride (C3N4) is an analogue of graphene, and it is often used to form 1D/2D and 2D/2D C3N4/graphene heterostructures. However, a theoretical understanding of the heterointerface in these heterostructures and how this affects their electrochemical performance is lacking. In this work we study the heterointerface of 1D/2D and 2D/2D C3N4/graphene heterostructures and how the different dimensions influence the lithium ion battery performance of the heterostructure. Our density functional theory (DFT) study showed that the common problem of C-N bond breakage experienced in 2D/2D C3N4/graphene heterostructure does not occur in the 1D/2D heterostructure. Furthermore, the 1D/2D heterostructure showed superior conductivity in comparison to that of the 2D/2D heterostructure of C3N4/graphene. The 1D/2D C3N4/graphene heterostructure also recorded a high theoretical capacity and rapid charge transfer. These results suggest that the properties of a heterostructure are influenced by the dimension of materials at the interface. These discoveries on the relationship between material dimension in heterostructure electrodes and their electrochemical performance will motivate the design of advanced electrode materials for rechargeable batteries.

In recent years, due to their structural diversity, adjustability, versatility, and excellent electrochemical properties, organic compounds with nitrogen‐containing groups (OCNs) have become some of the most promising organic electrode materials. The nitrogen‐containing groups acting as electrochemical active sites include carbon–nitrogen groups, nitrogen–nitrogen groups, nitrogen–oxygen groups in OCNs, and nitrogen‐containing groups in covalent organic frameworks. The molecular structure regulation of OCNs with nitrogen‐containing groups acting as electrochemical active centers can suppress dissolution in electrolytes, increase electronic conductivity, and improve the kinetics of redox reactions. The kinetics behavior and electrochemical characteristics of OCN electrode materials in alkali metal rechargeable batteries with organic electrolytes are reviewed, and the related relationships between the structure and electrochemical properties of OCNs are the core of this review. Herein, the electrochemical reaction mechanisms and the strategies to improve the electrochemical activity of nitrogen‐containing groups in OCNs are clarified, and the conjugate molecular structure of OCNs is shown to be an important direction for improvement. These results will have implications for research on electrode materials and provide more choices for rechargeable batteries. Moreover, this work will guide the study of more efficient OCNs that can be used as electrode materials.

The rapid worldwide energy consumption is necessitating more energy generation from environmentally benign sources like solar, wind, or tide. However, the lack of efficient energy storage devices is the major limitation for the practical harvest of these clean energies. Rechargeable batteries are the promising solution for the efficient energy storage. We have witnessed the great success of secondary lithium-ion batteries (LIBs) in portable electronic devices in the last two decades. However, the cost and safety barriers make traditional LIBs not desirable to meet the requirements for large power applications, such as the electric or plug-in hybrid vehicles. In contrast, the multivalent ion rechargeable batteries such as Mg1-3 and Ca4 , 5 ion batteries are more promising as larger and safer energy storage devices due to their advantages in safety, cost and capacity. For instance, compared to lithium, magnesium metal is non-toxic, more air resistant and free from the dangerous dendrite formation. The higher natural abundance of magnesium makes it more than 20 times cheaper than lithium in cost. In addition, the divalent characteristic of magnesium ion (Mg2+) enables magnesium to have a nearly doubled volumetric capacity as an anode than the lithium (3832 mA cm-3 vs 2062 mA cm-3). All these merits have brought significant attention on magnesium-ion batteries as the new generation rechargeable energy storage device. Inspired by the pioneer work by Aurbach group for the development of the famous dichloro-complex magnesium electrolyte (2EtAlCl2-MgBu2),6 a lot of efforts have been devoted to the development of high voltage electrolytes for rechargeable magnesium-ion batteries. In contrast to the rapid development of effective magnesium electrolytes capable of reversibly plating/stripping magnesium with wide electrochemical windows, the design of high potential cathode material for magnesium ion batteries is far behind. To date, the Chevrel phase Mo6S8 is still the only cathode material with excellent cycling reversibility and reasonable rate capability in rechargeable magnesium-ion batteries. However, Mo6S8 could only operate at a relatively low voltage in magnesium-ion batteries (< 1.3 V) with a typical below 100 mAh g-1 capacity. Thus, development of high operating voltage cathode material deserves great attention in nowadays. Herein, we would like to present our recent progress on the development of high voltage and high performance rechargeable magnesium-ion batteries using redox active organic molecules as cathode material. As shown in Figure 1(a), when the 0.3M Mg(HMDS)2-4MgCl2 in THF solution was used as the electrolyte,7 excellent reversible redox processes have been observed in the cyclic voltammetry measurement when the redox active organic molecule was used as the cathode material. The redox potential was detected at around 1.7 V (vs Mg/Mg2+), which is much higher than the well-known Chevral phase Mo6S8. The compatibility and reversibility of the organic cathode material with the electrolyte and magnesium metal anode was further demonstrated in the batteries upon cycling. As shown in Figure 1(b), excellent cycling performance has been observed with around 120 mAh g-1 initial specific discharge capacity at the current rate of 0.5C, and still about 102 mAh g-1specific discharge capacity was obtained in 100 cycles (around 84% capacity retention). The results further suggest the great success of using redox active organic molecules as cathode material for secondary multivalent-ion batteries. (1) Muldoon, J.; Bucur, C. B.; Gregory, T. Chem Rev 2014, 114, 11683. (2) Saha, P.; Datta, M. K.; Velikokhatnyi, O. I.; Manivannan, A.; Alman, D.; Kumta, P. N. Prog Mater Sci 2014, 66, 1. (3) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Energ Environ Sci 2013, 6, 2265. (4) Padigi, P.; Goncher, G.; Evans, D.; Solanki, R. J Power Sources 2015, 273, 460. (5) Lipson, A. L.; Pan, B.; Lapidus, S. H.; Liao, C.; Vaughey, J. T.; Ingram, B. J. Chem Mater 2015, ASAP. (6) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724. (7) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J. J.; Hu, L. B.; Pan, B. F.; Zhang, Z. C. Journal of Materials Chemistry A 2015, 3, 6082. Figure 1

The rechargeable batteries like redox flow which are not aged by frequent charging/discharging, have a quick response equivalent to SMES and outstanding capability of overload. The response time and the maximum overload rate of redox flow battery are verified as 350 /spl mu/s and 4.6 times. These systems are advantageous for power control under wide frequency band which covers from spinning reserve to economic load dispatching, and also used for leveling load and maintaining power quality. The instantaneous controls using these systems are effective for dynamic stability, flicker, voltage dip and charging/discharging regenerative power of electric railways. The authors clarify the chemical reactive characteristics of the redox flow battery physically and the effect of secondary control utilizing characteristics of quick response and overload capability of the redox flow battery by dynamic simulation. The battery efficiency increases when the cycle period of charging/discharging becomes shorter. The comprehension about these performances can be used to determine the optimum rechargeable battery system configuration for each use of end users.

The development of new rechargeable battery systems could fuel various energy applications, from personal electronics to grid storage. Rechargeable aluminium-based batteries offer the possibilities of low cost and low flammability, together with three-electron-redox properties leading to high capacity. However, research efforts over the past 30years have encountered numerous problems, such as cathode material disintegration, low cell discharge voltage (about 0.55volts; ref. 5), capacitive behaviour without discharge voltage plateaus (1.1-0.2volts or 1.8-0.8volts) and insufficient cycle life (less than 100 cycles) with rapid capacity decay (by 26-85 per cent over 100 cycles). Here we present a rechargeable aluminium battery with high-rate capability that uses an aluminium metal anode and a three-dimensional graphitic-foam cathode. The battery operates through the electrochemical deposition and dissolution of aluminium at the anode, and intercalation/de-intercalation of chloroaluminate anions in the graphite, using a non-flammable ionic liquid electrolyte. The cell exhibits well-defined discharge voltage plateaus near 2volts, a specific capacity of about 70mAhg(-1) and a Coulombic efficiency of approximately 98 per cent. The cathode was found to enable fast anion diffusion and intercalation, affording charging times of around one minute with a current density of ~4,000mAg(-1) (equivalent to ~3,000Wkg(-1)), and to withstand more than 7,500 cycles without capacity decay.