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10/15/2012 10/15/2012 • We explored the use of PPy as a minor additive component to define the potential of AC. • This hybrid anode combines the fast kinetics of AC and the low reaction potential of fully reduced PPy by intimately mixing AC with small amounts (10%) of reduced PPy in a single electrode structure. • The as-synthesized PPy powder is fully oxidized but can be reduced by NaBH 4 . • The increasing deployment of renewable energy sources such as solar and wind power requires a commensurate increase in energy storage capacity in order to integrate them into the electrical power grid [1]. • Combining these sources with the energy grid is especially challenging due to the rapid variability in their output. • Inexpensive energy storage that has rapid response, long cycle life, high power and high energy efficiency that can be distributed throughout the grid is needed to allow broad penetration of solar, wind, and other variable energy sources. Here, we demonstrate a new type of safe, fast, inexpensive, long-life aqueous electrolyte battery, which relies on the insertion of K + ions into a copper hexacyanoferrate (CuHCF) cathode and a hybrid activated carbon (AC)/polypyrrole (PPy) anode [2]. 1. ABSTRACT 2. COPPER HEXACYANOFERRATE POSITIVE ELECTRODE Department of Materials Science and Engineering, Stanford University, Stanford, CA, 94305, USA Mauro Pasta , Colin D. Wessells, Robert A. Huggins, Yi Cui High Rate, Long Cycle Life Aqueous Electrolyte Battery 4. PPy/AC NEGATIVE ELECTRODE • Recently, we developed a family of open framework nanoparticle materials with the Prussian Blue crystal structure [3,4]. These materials have an open framework crystal structure containing large interstitial sites that allows fast insertion and extraction of Na + and/or K + with very little crystallographic lattice strain. • Copper hexacyanoferrate (CuHCF) reacts with K + by a single-phase insertion reaction: 2K x Cu y [Fe III (CN) 6 ] + a(K + +e - ) = 2K x+a Cu y [Fe III (CN) 6 ] 1-a [Fe II (CN) 6 ] a • CuHCF electrodes are promising for grid-scale energy storage applications because of their ultra-long cycle life (83% capacity retention after 40,000 cycles), high power (67% capacity at 80C), high energy efficiency, and potentially, a very low-cost [3]. Schematic diagram of full cell device. In the PPy/AC negative electrode the reduced PPy particles fix the open circuit potential close to the lower stability limit of the electrolyte, while the charge is stored in the electrical double layer at the high surface activated carbon. The CuHCF positive electrode has the open framework Prussian Blue crystal structure. The electrolyte is an aqueous solution of 1M K 2 HPO 4 , pH=1. Structure and morphology of CuHCF. (a) CuHCF has the Prussian Blue crystal structure, in which octahedrally coordinated transition metals such as Cu and Fe are linked by CN ligands, forming a face- centred cubic structure. Each of the eight subcells of the unit cell contains a large ‘A site’ that may be occupied by hydrated alkali cations such as K + . (b) X-ray diffraction patterns of CuHCF. Bulk synthesis of CuHCF at room temperature by co-precipitation results in highly crystalline material. (c) TEM of CuHCF shows polydisperse 20 – 50 nm particles. Here we introduced a new type of aqueous electrolyte battery, specifically designed for grid-scale energy storage. 1) The CuHCF cathode reacts rapidly with very little hysteresis 2) The hybrid anode uses an electrochemically-active additive to tune its operating potential 3) The cell has a 95% round trip energy efficiency at a 5C rate, and a 79% energy efficiency at 50C 4) The cell shows zero capacity loss after 1000 deep-discharge cycles 5) Bulk quantities of the electrode materials are produced by a room temperature chemical synthesis from earth-abundant precursors 6) The cell operates in a safe and inexpensive aqueous electrolyte 6. SUMMARY 3. COPPER HEXACYANOFERRATE CHEMICAL REDUCTION (a) (b) (d) (c) Copper hexacyanoferrate chemical reduction. (a) Charge state of the CuHCF and cathode open circuit potential as a function of the molar ratio sodium thiosulfate added/CuHCF. The decrease in the lattice parameter of CuHCF with chemical reduction (b) is illustrated by the shift of the <600> diffraction peak to smaller angles (c,d). • The as synthesized CuHCF has a fractional initial charge state because fully oxidized CuHCF has a potential so high that it can be reduced by water. • A reductive titration method was developed to controllably reduce CuHCF to a desired oxidation state and open circuit potential using Na 2 S 2 O 3 : 2K x Cu y [Fe III (CN) 6 ] + 2Na 2 S 2 O 3 + 2K + = 2K 1+x Cu y [Fe II (CN) 6 ] 2 + Na 2 S 4 O 6 + 2Na + • The addition of Na 2 S 2 O 3 in a ratio of Na 2 S 2 O 3 :CuHCF of 0.8 results in full reduction. 100 nm 7. REFERENCES 1. Rastler, D. Electricity Energy Storage Technology Options. EPRI Report, 170 (2010) 2. Pasta, M., Wessells, C. D., Huggins, R. A. & Cui, Y . A high-rate and long cycle life aqueous electrolyte battery for grid-scale energy storage. Nat Commun, doi:10.1038/ncomms2139 3. Wessells, C. D., Huggins, R. A. & Cui, Y. Copper hexacyanoferrate battery electrodes with long cycle life and high power. Nat Commun 2, 550 (2011) 4. Wessells, C. D., Peddada, S. V., Huggins, R. A. & Cui, Y. Nickel Hexacyanoferrate Nanoparticle Electrodes For Aqueous Sodium and Potassium Ion Batteries. Nano Letters 11, 5421-5425, (2011) 5. FULL CELL ASSEMBLY PPy/AC Negative Electrode. (a) SEM of PPy shows particles of the order of 200-400 nm. Galvanostatic cycling of a pure AC anode (b) and a 10%PPy/AC (c) at 1C rate. PPy/AC anodes before (---) and after (—) the NaBH 4 chemical reduction step with 0.1M NaBH 4 and CuHCF cathode (···). The specific capacity values have been normalized by the mass of cathode. (a) (b) (c) (d) (a) (b) (c) (b) (c) Full cell electrochemical characterization. (a) Full cell potential profiles at different C rates (1C, 10C, 50C) (b) Energy efficiency and fractional capacity retention as a function of the C rate (c) Potential profiles of copper hexacyanoferrate (CuHCF) positive electrode, 10%PPy/AC negative electrode and full cell profile at 10C, (d) Cycling of the CuHCF-10%PPy/AC cell at a rate of 10C showed no capacity loss after 1000 cycles and a coulombic efficiency of 99.9%. • Fully-reduced CuHCF treated with Na 2 S 2 O 3 in a 0.8:1 molar ratio was used as the cathode. • The anode was 10% PPy-AC, pretreated with NaBH 4 . • The CuHCF vs. AC/PPy cells contained a 10 mg/cm 2 CuHCF cathode and a 50 mg/cm 2 , 10% PPy/AC anode. (a) 2 um (d)
