Symposium EE5 : Next-Generation Electrical Energy Storage Chemistries

Flammability and high cost of lithium ion batteries limit their market adoption rate in price-sensitive applications. Therefore, rechargeable aqueous batteries have attracted renewed attention due to their environmental friendliness, the intrinsic flame resistance of aqueous electrolytes, most importantly, greatly reduced capital and operating cost.^1-3 Recently, we successfully utilized a solution-based synthesis of strongly coupled nanoFe/multiwalled carbon nanotube (MWCNT) and nanoNiO/MWCNT nanocomposite materials for use as anodes and cathodes in rechargeable alkaline Ni-Fe batteries. The produced aqueous batteries demonstrate very high discharge capacities, which exceed that of commercial Ni-Fe cells by nearly an order of magnitude at comparable current densities. The use of highly conductive MWCNT network allows for high capacity utilization due to rapid and efficient electron transport to active metal nanoparticles in oxidized (such as Fe(OH)₂ or Fe₃O₄) states. The flexible nature of MWCNTs accommodates significant volume changes taking place during phase transformation accompanying reduction-oxidation reactions in metal electrodes. The electrolyte molarity and composition have a significant impact on the capacity utilization and cycling stability. Based on our post-mortem analyses, we came to the following conclusions: (i) at high alkaline electrolyte concentration and thus high pH values the Fe dissolution and re-precipitation takes place, which reduces the rate performance and capacity utilization of the nanostructured Fe anodes; (ii) at lower pH values Fe dissolution could be mitigated, but hydrogen evolution (HE) takes place, which becomes particularly significant if high surface area nanostructured Fe anodes are used; (iii) the addition of LiOH to KOH electrolyte enhances the Fe dissolution, but reduces the anode polarization and capacity utilization; these findings correlate well with the formation of porous oxidized Fe in LiOH-comprising electrolytes.
References
[1] S. Liu, G. L. Pan, N. F. Yan, X. P. Gao, Energy Environ. Sci., 2010, 3, 1732.
[2] C. Xu, B. Li, H. Du, F. Kang, Angew. Chem. Int. Ed. 2012, 51, 933.
[3] Z. Liu, S. W. Tay, X. Li, Chem. Commun., 2011, 47, 12473.
Acknowledgement
This work was partially supported by the Advanced Research Projects Agency- Energy (grant DE-AR0000400).

The development of rechargeable battery systems have recently accelerated the various energy storage applications from portable electronic devices to hybrid electrical vehicles. Nickel-metal hydride (Ni-MH) battery has a high capacity (2-3 times than that of an equivalent size NiCd battery) at an energy density similar to that of a lithium-ion battery. In addition, its aqueous electrolyte system is safer than the Li-ion battery, where the aprotic electrolyte system causes the flammable problem especially in the hybrid electric vehicles. However, its cell voltage is low (1.32 V) because of the aqueous electrolyte system. It is important to meet both the high cell voltage and high cell capacity in the aqueous electrolyte system for practical applications.
Herein, we propose a rechargeable Ni-Na aqueous battery system with Na electrode and wrinkled Ni-based nanostructures as anode and cathode materials, respectively. Na based rechargeable battery system has recently attracted great attentions as the next-generation battery system because of the similar mechanism with the Li-based rechargeable battery system and their abundant reserves. In addition, as the anode material, 3D-wrinkled Ni-based nanostructures are directly constructed on carbon microfibers offering the binder-free electrodes. The 3D Ni-based wrinkled nanostructure shows an enhanced performance with high surface area and high capacity (280mAh/g at rate of 1mA). Importantly, the Ni-Na battery offers the high average discharge voltage (3.1V) and shows a high capacity resulting in the high energy density (837Wh/kg).

Rechargeable batteries are receiving particular attention in diverse areas of portable electronic devices, electric vehicles (EV) and other energy storage systems. We previously reported the proof-of-principle of a new concept of rechargeable batteries based on chloride shuttle, i.e., chloride ion batteries. The concept has the advantage of a broad variety of potential electrochemical couples with high theoretical energy density up to values of 2500 Wh/L, which is higher than the theoretical energy density of the current lithium ion battery. Moreover, chloride ion batteries can be built from abundant material resources and have environmentally friendly features. These attributes could make the chloride ion battery a potential alternative in the field of rechargeable batteries.
A key challenge is to suppress the dissolution of cathode materials mainly composed of transition metal chlorides, which are Lewis acid and can react with a Lewis base containing chloride ion in the electrolyte, resulting in the formation of soluble complex ions. One approach is to use metal oxychlorides as cathode materials. For the FeOCl cathode (FeOCl/Li), a reversible discharge capacity of 184 mAh g^-1 was measured, which is based on the phase transformation between FeOCl and FeO. Two stages of this phase transformation are observed for the FeOCl cathode. These results suggest that metal oxychlorides are promising cathode materials for chloride ion batteries.
An advantage of chloride ion batteries is the use abundant materials such as Mg, La, Ca and Na as anode materials. We found that Mg is promising as anode material based on our new results. For instance, the BiOCl cathode (BiOCl/Mg) showed a discharge capacity of 70 mAh g-1, i.e., 68% of theoretical capacity at the second cycle. The electrochemical performance of metal oxychloride/Mg systems was investigated including single electron or multi-electron cathode. Moreover, a new approach was tested using multi-electron vanadium oxychloride (VOCl) cathode and Mg/MgCl₂ composite anode.

Organic batteries are an extremely promising electrochemical storage technology as they promise high rate (high power) and sustainable batteries with environment-friendly inputs. Organic batteries are also promising for the development of post-lithoum batteries such as sodium ion batteries. While there is a growing body of experimental works studying various potential organic electrode materials, theoretical/computational studies are rare; they are however essential to guide rational as opposed to ad hoc design of better organic electrode materials.
Here, we present a study of the mechanism of interaction of Li with molecules which are promising for use in organic electrodes: tetracyanoethylene (TCNE) and tetracyaniquinodimethane (TCNQ). TCNE and TCNQ have previosuly been proposed as promising candidate materials for organic battery electrodes, including lithium ion as well as sodium ion batteries. Their high theoretical capacities are in particular due to the possibility to store more than one alkali atom per molecule. We present a density functional theory study of lithium attachment to TCNE and TCNQ. Trends in the Li binding strengths (which determines the electrode voltage) are presented between TCNE and TCNQ and a function of the number of attached Li atoms. We show that multiple Li attachment induces non-trivial changes in the electronic structure which involves electron donation to higher (than LUMO) unoccupied molecular orbitals as well as Li-centered orbitals. Strain effects induced by Li attachment lead to significant changes in the electronic structure including changes in orbital ordering. A new cyclic molecular structure stabilized by Li attachment to TCNE is identified. We conclude that design of organic electrode materials should consider the energies of higher (than LUMO) orbitals as well as effects of structural changes on the electronic structure.

Oxygen reduction reaction (ORR) catalytic activities have been of great scientific importance for extensive studies in energy storage technologies, specifically in metal-air batteries. Metal-air batteries are promising for future applications of especially electrical vehicles due to the utilization of oxygen from the air as one of the battery’s main components. Kinetic performance of the electrochemical reaction taking place in these batteries mainly depends on the reduction of oxygen at the cathode. Different groups of catalysts have been considered in order to facilitate ORR at the air electrodes including precious metal catalysts, metal oxides and carbons. Contrary to the ORR on metals and metal alloys in acidic environments, little is known about the influence of intrinsic properties of complex oxides on their activity toward the ORR in alkaline media. To develop novel and highly active cathode materials, a deeper understanding of the correlation between the cathode material properties and ORR activity is necessary. The crucial role of electronic structure in determining the electrochemical activity has been well recognized in the field of catalysis. First-principles computational studies, in particular density functional theory (DFT) studies, have made important contributions to such efforts, identifying fundamental correlations between catalytic activity and electronic structure for different types of catalyst materials. Therefore, ab initio methods become a useful tool to characterize catalytic properties by examining electronic structures, reaction energetics and activation energies. In this study, the effect of surface crystallography on oxygen molecule dissociation and adsorption properties on different types of transition metal perovskites of the type ABO₃ (A=La, B= Mn, Cr, Fe, O=oxygen) are presented. To analyze the oxygen–perovskite interaction, ab initio molecular dynamics method is used to simulate the behavior of O₂ at the surface. This method is expected to show whether the transition metal elements displayed an effect of catalyzer in terms of dissociation of the O₂ molecule into O atoms at the surface. A systematical study within adsorption characteristics of oxygen on La-based perovskite surfaces was performed. In addition, via ab initio molecular dynamics simulations, what kind of an effect would different planes make on oxygen behavior at the surface was studied. Ab initio molecular dynamics simulations were executed on clean low- index (001) and (111) surfaces of perovskites. Simulations were done by ab initio pseudopotential method within the generalized gradient approximation (GGA) to density functional theory (DFT).

Sulfur has been considered as a promising cathode candidate for next generation batteries due to its high theoretical capacity and energy density. However, the severe self-discharge behavior strongly limits the practical applications of lithium-sulfur (Li-S) batteries. Here, we report a sustainable and highly porous polyacrylonitrile/graphene oxide (PAN/GO) nanofiber membrane which can be performed as a novel separator for lithium-sulfur batteries to achieve high stable capacity and excellent anti-self-discharge feature. A superior low retention loss of 5% can be obtained even after a resting time of 24 h not only due to the relatively high energy binding between –C≡N and Li₂S/Li-S radicals but to the electrostatic interactions between GO and negatively charged species (S_n^2-). It is, therefore, demonstrated that this GO incorporated PAN as-spun nanofiber with highly porous structure and excellent electrolyte wettability is a promising separator candidate for high-performance Li-S batteries.

Vanadium redox flow batteries (VRFBs) have been demonstrated as energy efficient conversion devices with long service life. Moreover, VRFBs have been assumed as highly cost effective when inexpensive membranes were utilized. The state-of-art membrane Nafion™, perfluorinated copolymer, has been the most commonly used membrane although it suffers from its high cost, lower proton conductivity and higher vanadium permeability. Therefore, much efforts has been focused on replacing Nafion™. Also there is a great interest to compare and contrast VRBF performances of both fluorinated and non-fluorinated based hydrocarbon membranes. This study focuses on the effect of fluorine moeity of directly disulfonated (35 molar %) poly(arylene ether benzonitrile) copolymer (PAEB) membranes on the VRFB performance. The fluorine incorporation from 25 to 100 molar percents has been systematically varied by using hexafluoro-isopropylidene diphenol (6F) monomer during copolymerization. It was demonstrated that efficiencies of VRBF prepared from both fluorinated and nonfluorinated copolymer membranes were better than that of N212. The columbic efficiencies of copolymer membranes not bearing any fluorine content (PAEB35) and Nafion™ were about 98.7 and 87.6 percents at the current density of 20 A cm^-2, respectively. Once a 100 molar percent fluorination (6F100PAEB35) was achieved, the columbic efficiency was about 99.5, which was one of the best efficiencies reported in the literature. This trend was also observed at higher current densities. The superiority in the columbic efficiency was due to the lower vanadium permeabilities of the 6F100PAEB35 copolymer membranes (1.0 x 10^-13 m² s^-1). Among the series, 6F100PAEB35 had 4-fold lower vanadium permeability than PAEB35. On the other hand the vanadium permeabilities of 6F100PAEB35 were about 8 times lower than that of N212 (1.3 x 10^-12 m² s^-1). The proton conductivity of the PAEB 35 is 77 mS cm^-1. As the fluorine content increased from 0 to 100 molar percent the ion exchange capacity (IEC) decreased from 1.86 to 1.31 mequiv g^-1. Since, the proton conductivity and water uptake values which were function of IEC followed the same trend and decreased from 77 to 56 mS cm^-1 and from 39 to 22 wt% with increasing fluorine content, respectively. Therefore voltage efficiencies of fluorine bearing copolymer membranes were slightly lower than their non-fluorinated analogous in the series. For example, the voltage efficiencies of PAEB 35 and 6F100PAEB35 were about 92.8 to 91.8 % at 80 mA cm^-2, respectively. Moreover, overall efficiencies of both fluorinated (91.2 %) and nofluorinated copolymer membranes (91.6 %) were far better than N212 (80.5%). The selectivity (e.g., proton conductivity/permeability) of fluorinated series was about 3 and 20 times higher than non-fluorinated copolymer membranes and N212, respectively. These highly selective copolymer membranes have been utilized as promising candidates for VRFBs.

Transition metal fluorides are promising high-capacity battery cathode for large-scale applications (i.e. electric vehicles), but issues related to low voltage, large hysteresis and limited cycling reversibility remain a major hurdle to their commercial application. Here we report on the synthesis, structural and electrochemical properties of new nanostructured ternary metal fluorides, which may overcome some of these issues. By substituting Cu into the Fe lattice, forming the solid solution Cu_yFe_1-yF₂, reversible Cu and Fe redox reactions were achieved with surprisingly small hysteresis (<150 mV). This finding indicates that cation substitution may provide a new pathway for tailoring electrochemical properties of conversion electrodes [1]. The Li storage/release mechanisms and limits to cycling stability of Cu_yFe_1-yF₂ were also investigated by combining electrochemical measurement with comprehensive structural and chemical analysis using in-situ X-ray absorption spectroscopy, X-ray diffraction, and transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS). Detailed lithium reaction mechanisms, Cu-loss related issues along with possible remedy solutions in the Cu_yFe_1-yF₂ system, will be discussed. The work was supported as part of the NorthEastern Center for Chemical Energy Storage, an EFRC Center funded by the U.S. DOE-BES, under Award Number DESC0001294, and by DOE-EERE under the Advanced Battery Materials Research program, under Contract No. DE-SC0012704. [1] Wang et al., “Ternary Metal Fluorides as High-Energy Cathodes with Low Cycling Hysteresis”, Nat. Commun. 6:6668 (2015).

Introduction of well-designed micro-, meso- or macro-porous structures into functional building blocks, generally leads to significantly improved or unexpected physiochemical properties and performance. MXenes are a family of 2D transition metal carbides, which have shown great promise as electrodes in lithium-ion batteries and supercapacitors. However, the accessibility of electrolyte ions into MXenes is limited due to the compact stacking of the 2D flakes, hindering the full utilization of their energy storage performance. Herein, we successfully introduce porous structure into MXene flakes and demonstrate the significant improvement of their Li-ion storage capability.

The porous structure was introduced into MXenes by an easy and controllable chemical etching method. Typically, Ti₃C₂ colloidal solution was mixed with an aqueous solution of transition metal salts (e.g. CuSO₄, CoSO₄ or FeSO₄) at room temperature while stirring for 30 min. The suspension was washed by 5 wt.% HF aqueous solution and vacuum filtered. This process resulted in a freestanding and flexible p-Ti₃C₂ film with large quantity of pores on Ti₃C₂ flakes, which enhanced its specific surface area. The electrochemical performance of p-Ti₃C₂ was evaluated by using p-Ti₃C₂/CNT composite film as anode material in Li-ion batteries. At 0.5 C, a capacity of ≈750 mAh g^-1 was achieved, much larger compared to that of the non-porous Ti₃C₂/CNT film (≈220 mAhg^-1), along with excellent cycling stability. The p-Ti₃C₂/CNT electrode also showed impressive rate performance. A high capacity of ≈1250 mAh g^-1 was achieved at 0.1 C after a pre-cycling process, and ≈330 mAh g^-1 was retained at 10 C. This chemical etching method, which is applicable to other kinds of MXenes (V₂C and Nb₂C), provides a simple yet effective method to improve the physiochemical properties of MXenes, and possibly other kinds of 2D materials.