The future of renewable energy, due to its inherent variability, will rest on the capacity of the electrical grid to reconcile power production and consumption. Electrical energy storage can take many forms, one of which is the redox flow battery (RFB), which boasts some unique advantages over other storage technologies. Non-aqueous RFBs, while far less developed than aqueous chemistries, may possess benefits over aqueous RFBs including higher voltage and greater energy density. The electrochemistry of a series of anion radicals for possible use in flow batteries is reported. The solvent utilized in this work is tetra(ethyleneglycol) dimethylether (TEGDME) and was chosen due to its very large voltage window (~4 V vs. lithium) and capability of dissolving large concentrations of aromatic compounds. An appropriate triflate salt (sodium or lithium) was added to the TEGDME to improve conductivity, facilitating electrochemical measurements. After removing as much of the impurities as possible, TEGDME with triflate salt served as a stable electrolyte with nearly no observable electrochemical reactivity. Anion radicals were formed by simply adding sodium or lithium metal to various aromatics dissolved in TEGDME and triflate salt. The aromatics considered in this work covered a range of complexity, from biphenyl and naphthalene up to large compounds such as coronene. Our initial screening found that the redox potentials of the aromatic-anion radical couples also spanned a large redox potential range. Some were highly reversible and some were quite unsuitable for a RFB due to irreversible redox behavior. Sodium- and lithium-generated anion radicals behaved similarly, with the sodium-generated radicals exhibiting generally more favorable redox behavior (larger current and better reversibility). In addition, many compounds also formed dianions, allowing for greater potential range and energy density for that material. Ultimately, the goal is to develop a non-aqueous RFB. Initial tests to screen materials and improve cell design using the biphenyl-anion radical for the negative side are in progress. Capitalizing on the benefits of this non-aqueous RFB chemistry could result in lower overall system cost for electrical energy storage compared to other technologies. Acknowledgement: We gratefully acknowledge the support of Dr. Imre Gyuk and the US DoE Office of Electricity for support of this work. CNS and JN acknowledge Laboratory Direct Research and Development (LDRD) seed funding for this work.
Sodium-based batteries are promising for grid-storage applications because of significantly lower cost compared to lithium-based systems. The advancement of solid-state and redox-flow sodium-ion batteries requires sodium-ion exchange membranes with high conductivity, electrochemical stability, and mechanical robustness. This study demonstrates that membranes based on poly(ethylene oxide) (PEO) can meet these requirements. Membranes plasticized with tetraethylene glycol dimethyl ether (TEGDME) achieve high ionic conductivity. Plasticized PEO membranes containing sodium triflate salt (NaTFS) show about 2 orders of magnitude higher conductivity compared to nonplasticized PEO membranes. Results from vibrational spectroscopy and differential scanning calorimetry describe the coordination chemistry in these multiphase materials and explain the mechanisms behind the increased conductivity. The mechanical properties of the membranes improve by addition of 5 wt % sodium carboxymethyl cellulose (CMC) without compromising the conductivity or electrochemical stability against sodium metal. The optimized membrane is an excellent candidate for low-cost energy storage systems that operate over a wide voltage window near ambient temperature.
Orthorhombic Li2NiO2, Li2CuO2, and solid solutions thereof have been studied as potential cathode materials for lithium-ion batteries due to their high theoretical capacity and relatively low cost. While neither endmember shows good cycling stability, the intermediate composition, Li2Cu0.5Ni0.5O2, yields reasonably high reversible capacities. A new synthetic approach and detailed characterization of this phase and the parent Li2CuO2 are presented. The cycle life of Li2Cu0.5Ni0.5O2 is shown to depend critically on the voltage window. The formation of Cu1+ at low voltage and oxygen evolution at high voltage limit the electrochemical reversibility. In situ X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and gas evolution measurements are used to follow the chemical and structural changes that occur as a function of cell voltage.
Redox flow batteries (RFBs) are promising energy storage devices for grid-level applications due to their long cycle life and the ability to independently scale their energy and power densities. The energy density of conventional RFBs is dictated by their capacity (which is directly related to the solubility of the redox species in the electrolyte) and operating potential. Aqueous RFBs generally have low operating potentials ca. 1.5 V, resulting in poor energy densities (25 – 30 Wh/kg for an all vanadium RFB), whereas systems containing organic electrolytes with wider electrochemical windows have moderately higher energy densities. Our team recently demonstrated proof-of-concept for a revolutionary approach which uses mediated electrochemical reactions in a RFB configuration to drive reversible Na storage in a red P anode. Extremely high capacities ~1,000 mAh/g P have been demonstrated using this method. In this configuration, the anion radical mediators are recycled several times throughout the cell stack during a single charge/discharge cycle, effectively decoupling the RFB’s energy density from the redox species’ solubility in the electrolyte. By pairing this mediated red P anode with a sulfide-based cathode, energy densities up to 200 Wh/kg (~10x that of conventional RFBs) can potentially be achieved. This presentation will describe our recent progress developing polymer membranes and high energy density cathodes/catholytes for RFBs. The preparation and characterization of ionically conductive, mechanically robust poly(ethylene oxide) (PEO)-based membranes which are chemically resistant to and prevent crossover of the radical mediators will be discussed. The synthesis and electrochemical properties of a new class of high energy density sodium thiophosphate cathodes for RFBs will also be provided. Acknowledgements This research is supported by Dr. Imre Gyuk, Manager, Energy Storage Program, Office of Electricity Delivery and Reliability, U.S. Department of Energy and the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.
