Safe, Inexpensive, and Very High Power Batteries for Use to Reduce Short Term Transients on the Electric Grid

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1 Safe, Inexpensive, and Very High Power Batteries for Use to Reduce Short Term Transients on the Electric Grid GCEP Final Progress Report Investigators Yi Cui, Professor, Materials Science & Engineering, Stanford University; Mauro Pasta, Hyun-Wook Lee, Post-doctoral Researchers, Stanford University; Richard Y. Wang, Graduate Researchers, Stanford University. Abstract During the time my research group has been supported by GCEP, we developed a deep scientific understanding of the outstanding electrochemical properties of Prussian Blue Analogue (PBA) materials and their application for grid-scale energy storage and other related technologies. We pursued a number of different research directions, including: cathode and anode materials for grid-scale aqueous batteries, performance of PBAs in organic electrolyte batteries, and insertion of multivalent ions in PBAs for promoting future development of multivalent-ion batteries. Nickel hexacyanoferrate nanoparticles, which can be synthesized by a simple coprecipitation method, demonstrate remarkable electrochemical properties that make them suitable as cathode materials in low-cost batteries. They show insertion/extraction of sodium and potassium ions in a low-strain nickel hexacyanoferrate electrode material for at least five thousand deep cycles at high current densities in inexpensive aqueous electrolytes. The reaction potential of copper nickel alloy hexacyanoferrate nanoparticles for cathode materials may be tuned by controlling the ratio of copper to nickel in these materials.the ability to precisely tune the reaction potential of copper nickel hexacyanoferrate without sacrificing cycle life will allow the development of full cells that utilize the entire electrochemical stability window of aqueous sodium and potassium ion electrolytes. We also showed the effect of the insertion species on PBAs and demonstrated that copper hexacyanoferrate and nickel hexacyanoferrate can reversibly intercalate lithium, sodium, potassium, and ammonium ions at high rates. These materials show excellent stability during the cycling of sodium and potassium, with minimal capacity loss after 500 cycles. Using our knowledge of the material system, we developed a new type of safe, fast, inexpensive, long-life aqueous electrolyte battery, which relies on the insertion of potassium ions into a copper hexacyanoferrate cathode and a novel hybrid activated carbon/polypyrrole anode. The cathode reacts rapidly with very little hysteresis. The hybrid anode uses an electrochemically-active additive to tune its operating potential, and this concept may be generalized to many battery and ultracapacitor electrodes. Furthermore, we developed a new PBA anode, manganese hexacyanomanganate, which has the same characteristic open framework crystal structure of previously reported PBA cathode materials. By combining it with a copper hexacyanoferrate cathode we

2 introduced a new type of safe, fast, inexpensive, long-cycle life aqueous electrolyte battery which involves the insertion of sodium ions. These properties, together with the outstanding cycle life and low active materials cost, make this battery attractive for gridrelated applications, including for the smoothing of intermittent variations in power production associated with the integration of renewable energies on the grid. We explored the use of PBAs as cathode materials in sodium-ion batteries with nonaqueous, organic electrolytes. Potential applications of sodium-ion batteries in grid-scale energy storage, portable electronics, and electric vehicles have revitalized research interest in these batteries. However, the performance of sodium-ion electrode materials has not been competitive with that of lithium-ion electrode materials. We explored the effect of different alkali ions on the insertion electrochemistry of NiHCFe in aqueous and propylene carbonate-based electrolytes. The large channel diameter of the structure offers fast solid-state diffusion of Li +, Na +, and K + ions in aqueous electrolytes. However, all alkali ions in organic electrolytes and Rb + and Cs + in aqueous electrolytes show a quasi-reversible electrochemical behavior that results in poor galvanostatic cycling performance. We developed sodium manganese hexacyanomanganate (Na 2 Mn II [Mn II (CN) 6 ]), an open framework crystal structure material, as a viable positive electrode for sodium-ion batteries. We demonstrated a high discharge capacity of 209 mah g -1 at C/5 (40 ma g -1 ) and excellent capacity retention at high rates in a propylene carbonate electrolyte. These results represent a major step forward in the development of sodium-ion batteries. The reversible electrochemical insertion of multivalent ions into materials has promising applications in many fields, including batteries, seawater desalination, element purification, and wastewater treatment. However, finding materials that allow for the insertion of multivalent ions with fast kinetics and stable cycling has proven difficult because of strong electrostatic interactions between the highly charged insertion ions and atoms in the host framework. We found that nanomaterials in the Prussian Blue family of open framework materials, such as nickel hexacyanoferrate, allow for the reversible insertion of aqueous alkaline earth divalent ions, including Mg 2+, Ca 2+, Sr 2+, and Ba 2+. We show unprecedented long cycle life and high rate performance for divalent ion insertion. Furthermore, we found that copper hexacyanoferrate allows for the reversible insertion of a wide variety of monovalent, divalent, and trivalent ions (such as Rb +, Pb 2+, Al 3+, and Y 3+ ) in aqueous solution beyond that achieved in previous studies. Synchrotron X-ray diffraction experiments point towards a novel vacancy-mediated ion insertion mechanism that reduces electrostatic repulsion and helps to facilitate the observed rapid ion insertion. The results in this study suggest a new approach to multivalent ion insertion that help to advance the understanding of this complex phenomenon. Introduction

3 New renewable energy sources such as solar and wind power are fundamentally different from conventional energy generation from fossil fuels because of their inherent intermittency 1. The power output of solar and wind farms is limited both by diurnal cycles and short-term seconds-to-minutes volatility due to rapid changes in cloud cover and wind conditions 2. The scientific community, industry advocates, and policy makers have repeatedly called for fast-acting energy storage in support of volatile renewable energy sources 1 4. Energy storage is needed both for diurnal load balancing and the smoothing of intermittent spikes or drops in power production 5. Energy storage systems used for this application must be deployable across the grid, have extraordinarily long cycle life, be capable of high power charge and discharge in minutes, have very high energy efficiency, and above all, have low capital and lifetime costs. A new kind of energy storage technology is needed for short-term grid storage applications, as existing technology struggle to meet the needs of these applications at a reasonable price 1,3 5. The open framework (OF) structure of Prussian Blue analogues is fundamentally different from other insertion electrode materials because of its large channels and interstices. This structure is composed of a face-centered cubic framework of transition metal cations where each cation is octahedrally coordinated to hexacyanometallate groups. For instance, in the copper hexacyanoferrate cathode, six-fold carbon-coordinated iron and six-fold nitrogen-coordinated copper are linked by CN ligands. Large interstitial A Sites within the structure can accommodate zeolitic water and hydrated alkali ions. This results in a general chemical formula of A x PR(CN) 6 nh 2 O, where A is an alkali cation such as K + or Na +, P is a transition metal cation such as Cu 2+, Ni 2+ or Fe 3+, and R(CN) 6 is a hexacyanometallate anion such as Fe(CN) 6 3-, Mn(CN) 6 3-, or Cr(CN) Both the P-site transition metal cation and the R(CN) 6 3- hexacyanometallate anion can be electrochemically active in this structure, so in this work, we adopt a notation of the general form P I/J N C R K/L, where I, J, K, and L are valence states of the P and R cations. The Prussian Blue framework structure has wide channels between the A sites, allowing rapid insertion and removal of Na +, K +, and other ions from aqueous solutions. In addition, there is little lattice strain during cycling because the A sites are larger than the hydrated ions that are inserted and removed from them. The result is an extremely stable electrode: over 40,000 deep discharge cycles were demonstrated in the case of the Cu II N C Fe III/II cathode 6. The unique crystal structure of PBAs enables remarkable electrochemical properties that make the material system suitable for a wide variety of battery applications and research. We pursued a number of different research directions, including: cathode and anode materials for grid-scale aqueous batteries, performance of PBAs in organic electrolyte batteries, and insertion of multivalent ions in PBAs for exploring future development of multivalent-ion batteries. Background

4 Pumped hydroelectric power is the current industry standard for grid energy storage, but new facilities are location-dependent and have very high capital costs. Both mechanical flywheels and ultracapacitors provide the long cycle life, high power, and high energy efficiency needed to smooth volatile renewable energy sources. However, their extraordinarily high costs on a Watt-hour basis have limited their use on the grid thus far. Two battery technologies that are promising for low-rate grid storage applications are sodium sulfur (NaS) and flow batteries. Unfortunately, neither of these types of batteries can operate at high rates, precluding their use for transient applications. Of the existing energy storage technologies, lead acid and lithium-ion batteries are more attractive for transient grid applications, such as short-term smoothing of solar and wind, and both of these technologies have demonstrated some success in grid-scale applications 5. Barnhart et al. recently introduced a new metric to evaluate the energetic performance of different storage technologies on the grid 7. According to their analysis, the two primary factors that are preventing present forms of electrochemical energy storage from being used extensively on the grid are their limited cycle life and high embodied energy (i.e. the energy required to build a unit of stored energy). The reason is that previous technologies were never specifically designed to meet the demands of the grid but were instead adapted from other applications in which volumetric/specific energy and power are the key requirements. Results PBA cathode materials The electrical power grid faces a growing need for large-scale energy storage over a wide range of time scales due to costly short-term transients, frequency regulation, and load balancing. The durability, high power, energy efficiency, and low cost needed for gridscale storage pose substantial challenges for conventional battery technology. We demonstrates insertion/extraction of sodium and potassium ions in a low-strain nickel hexacyanoferrate electrode material for at least five thousand deep cycles at high current densities in inexpensive aqueous electrolytes (Figure 1). Figure 1. Nickel Hexacyanoferrate cathode material. (a) NiHCFe has the Prussian Blue crystal structure in which transition metal cations such as Fe and Ni are bound by bridging CN ligands, forming a facecentered cubic structure. In the case of NiHCFe, Fe is 6-fold carbon coordinated, while Ni is 6-fold nitrogen coordinated. The resulting framework has large channels oriented in the <100> directions, through which hydrated alkaline cations such as K + and Na + may diffuse. These alkaline cations occupy the

5 interstitial A sites at the center of each of the eight subcells of the unit cell. Full occupancy of the A sites is achieved upon full reduction of the material to A 2 NiFe 2 + (CN) 6. Zeolitic water is also present in the structure but is omitted here for clarity. (b) Scanning electron microscopy revealed that the as-synthesized NiHCFe powder is composed of a porous collection of nm grains. (c) NiHCFe shows no capacity loss after 5000 cycles of Na+ insertion at a 8.3C rate. However, during K + cycling, NiHCFe is stable for only about 1000 cycles, after which its capacity decays at an approximate rate of 1.75%/1000 cycles. Its open-framework structure allows retention of 66% of the initial capacity even at a very high (41.7C) rate. At low current densities, its round trip energy efficiency reaches 99%. This low-cost material is readily synthesized in bulk quantities. The long cycle life, high power, good energy efficiency, safety, and inexpensive production method make nickel hexacyanoferrate an attractive candidate for use in large-scale batteries to support the electrical grid. Based on this study, a research article titled Nickel Hexacyanoferrate Nanoparticle Electrodes For Aqueous Sodium and Potassium Ion Batteries was published on Nano Letters 8. The reaction potential of copper nickel alloy hexacyanoferrate nanoparticles may be tuned by controlling the ratio of copper to nickel in these materials. X-ray diffraction, TEM energy dispersive X-ray spectroscopy, and galvanostatic electrochemical cycling of copper nickel hexacyanoferrate reveal that copper and nickel form a fully miscible solution at particular sites in the framework without perturbing the structure. This allows copper nickel hexacyanoferrate to reversibly intercalate sodium and potassium ions for over 2000 cycles with capacity retentions of 100% and 91%, respectively (Figure 2). Figure 2. Copper and Nickel hexacyanoferrate Hybrids. (a) Potential profiles of CuNiHCFe during galvanostatic cycling in 1 M KNO 3. (b) Potential profiles of CuNiHCFe during galvanostatic cycling in 1 M NaNO 3. (c) The reaction potential of CuNiHCFe decreases with increasing Ni content in both sodium and potassium electrolytes. CuNiHCFe reacts with potassium at higher potentials than it does with sodium. (d) CuHCFe and NiHCFe show no capacity loss after 2000 cycles at 500 ma/g in 1 M KNO 3, while Cu 0.56 Ni0.44HCFe shows a capacity loss of 9%. (e) NiHCFe and Cu 0.56 Ni 0.44 HCFe show no capacity loss after 2000 cycles at 500 ma/g in 1 M NaNO 3, while CuHCFe loses 25% of its capacity.

6 The ability to precisely tune the reaction potential of copper nickel hexacyanoferrate without sacrificing cycle life will allow the development of full cells that utilize the entire electrochemical stability window of aqueous sodium and potassium ion electrolytes. Based on this study, a research article titled Tunable Reaction Potentials in Open Framework Nanoparticle Battery Electrodes for Grid-Scale Energy Storage was published on ACS Nano 9. We have demonstrated that two open framework materials, copper hexacyanoferrate and nickel hexacyanoferrate, can reversibly intercalate lithium, sodium, potassium, and ammonium ions at high rates (Figure 3). Figure 3. Rate capability, cycle life, and effect of insertion ion size on CuHCFe and NiHCFe. (a) and (b) The capacity retention of CuHCFe and NiHCFe at various densities. (c) and (d) The cycle life of CuHCFe and NiHCFe during cycling of Li +, Na +, K +, and NH 4 +, and (e) The reaction potentials CuHCFe and NiHCFe as functions of the Stokes radius of the insertion ion. These materials can achieve capacities of up to 60 mah/g. The porous, nanoparticulate morphology of these materials, synthesized by the use of simple and inexpensive methods, results in remarkable rate capabilities: e.g. copper hexacyanoferrate retains 84% of its maximum capacity during potassium cycling at a very high (41.7C) rate, while nickel hexacyanoferrate retains 66% of its maximum capacity while cycling either sodium or potassium at this same rate. These materials show excellent stability during the cycling of sodium and potassium, with minimal capacity loss after 500 cycles. Based on this study, a research article titled The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes was published on The Journal of the Electrochemical Society 10. Here, we demonstrate a new type of safe, fast, inexpensive, long-life aqueous electrolyte battery, which relies on the insertion of potassium ions into a copper hexacyanoferrate cathode and a novel hybrid activated carbon/polypyrrole anode. The cathode reacts

7 rapidly with very little hysteresis. The hybrid anode uses an electrochemically-active additive to tune its operating potential, and this concept may be generalized to many battery and ultracapacitor electrodes. This high rate, high efficiency cell has a 95% round trip energy efficiency when cycled at a 5C rate, and a 79% energy efficiency at 50C. It has shown zero capacity loss after 1000 deep-discharge cycles (Figure 3). Figure 3. (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 CuHCF positive electrode, 10%PPy/AC anode 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%. Bulk quantities of the electrode materials are produced by a room temperature chemical synthesis from earth-abundant precursors, and the cell operates in a safe and inexpensive aqueous electrolyte. Based on this study, a research article titled A High Rate, Long Cycle Life Aqueous Electrolyte Battery for Grid Scale Energy Storage was published on Nature Communications 11. PBA anode material We first developed a new PBA anode, manganese hexacyanomanganate, which has the same characteristic open framework crystal structure of previously reported PBA cathode materials. By combining it with a copper hexacyanoferrate cathode we introduced a new

8 type of safe, fast, inexpensive, long-cycle life aqueous electrolyte battery which involves the insertion of sodium ions (Figure 4). Figure 4. Symmetric open framework cell schematic. This new type of safe, fast, inexpensive, longcycle life aqueous electrolyte battery relies on the insertion of sodium ions into the copper hexacyanoferrate (Cu II N C Fe III/II ) cathode and a newly developed manganese hexacyanomanganate (Mn II N C Mn III/II ) anode, each of which have the same open framework crystal structure. This novel, symmetric open framework electrode battery delivers a maximum specific energy of 27 Wh/kg at a 1C rate on a basis of the masses of the active materials. Furthermore, this battery has a specific energy of 15 Wh/kg, a specific power of 693 W/kg, and an 84.2% energy efficiency when cycled at a 50C rate. These properties, together with the outstanding cycle life and low active materials cost, make this battery attractive for grid-related applications, including for the smoothing of intermittent variations in power production associated with the integration of renewable energies on the grid. Based on this study, a research article titled Full open-framework batteries for stationary energy storage was published on Nature Communication 12. Properties of PBAs in organic electrolytes Nickel hexacyanoferrate (NiHCFe) is an attractive cathode material in both aqueous and organic electrolytes due to a low-cost synthesis using earth-abundant precursors and also due to its open framework, Prussian Blue-like crystal structure that enables ultra-long cycle life, high energy efficiency, and high power capability. We explored the effect of different alkali ions on the insertion electrochemistry of NiHCFe in aqueous and propylene carbonate-based electrolytes. The large channel diameter of the structure offers fast solid-state diffusion of Li +, Na +, and K + ions in aqueous electrolytes. However, all

9 alkali ions in organic electrolytes and Rb + and Cs + in aqueous electrolytes show a quasireversible electrochemical behavior that results in poor galvanostatic cycling performance (Figure 5). Kinetic regimes in aqueous electrolyte were also determined, highlighting the effect of the size of the alkali ion on the electrochemical properties. Figure 5. Comparison of hysteresis in galvanostatic potential profiles at 1C between aqueous and PC electrolytes. (a) Li +, (b) Na +, and (c) K + ion systems. Typical charge and discharge potential profiles of NiHCFe electrodes at different C rates (0.2, 0.5, 1, 2, and 5 C) with (d) Li +, (e) Na +, and (f) K + ions in PC. Based on this study, a research article titled Effect of the Alkali Insertion Ion on the Electrochemical Properties of Nickel Hexacyanoferrate Electrodes has been published on Faraday Discussions 13. High-capacity cathode material for Na-ion batteries Potential applications of sodium-ion batteries in grid-scale energy storage, portable electronics, and electric vehicles have revitalized research interest in these batteries. However, the performance of sodium-ion electrode materials has not been competitive with that of lithium-ion electrode materials. We developed sodium manganese hexacyanomanganate (Na 2 Mn II [Mn II (CN) 6 ]), an open framework crystal structure

10 material, as a viable positive electrode for sodium-ion batteries. We demonstrated a high discharge capacity of 209 mah g -1 at C/5 (40 ma g -1 ) and excellent capacity retention at high rates in a propylene carbonate electrolyte (Figure 6). We provided chemical and structural evidence for the unprecedented storage of 50% more sodium cations than previously thought possible during electrochemical cycling. These results represent a step forward in the development of sodium-ion batteries. Figure 6. Electrochemical properties and Na + ion concentration in MnHCMn. a, Galvanostatic charge and discharge curves at C/5. The samples were prepared by initially discharging and then charging to the endpoints of each reaction plateau. b,c, EDS (b) and XPS (c) data show consistently increasing Na peak intensities as MnHCMn is reduced, which corresponds to the electrochemical insertion of Na + ions throughout the full voltage range of cycling. d, The schematic illustrates the nominal compositions of MnHCMn at the endpoints of each Na + ion insertion reaction plateau. e,f, Charge and discharge curves (e) and rate capability (f) as a function of different C rates are shown. g, MnHCMn exhibits promising capacity retention over 100 cycles at 2C. Based on this study, a research article titled Manganese Hexacyanomanganate Open Framework as a High-Capacity Positive Electrode Material for Sodium-Ion Batteries has been published on Nature Communications 14. Multivalent ion insertion The reversible insertion of monovalent ions such as lithium into electrode materials has enabled the development of rechargeable batteries with high energy density. Reversible insertion of divalent ions such as magnesium would allow the creation of new battery chemistries that are potentially safer and cheaper than lithium-based batteries. Here we report that nanomaterials in the Prussian Blue family of open framework materials, such as nickel hexacyanoferrate, allow for the reversible insertion of aqueous alkaline earth divalent ions, including Mg 2+, Ca 2+, Sr 2+, and Ba 2+ (Figure 7). We show unprecedented long cycle life and high rate performance for divalent ion insertion. Our results represent a step forward and pave the way for future development in divalent batteries.

11 Figure 7. (A D) The specific capacity (black line) at 5C decays at different rates for Mg 2+, Ca 2+, Sr 2+, and Ba 2+ electrolytes, respectively. In all cases, the capacity stabilizes after the electrode stops dissolving as the electrolyte becomes saturated with dissolved Ni 2+. Coulombic efficiency (red dotted line) ranges from 99.4 to 99.8%. (E G) When 20 mm Ni 2+ is added to the electrolyte before cycling, specific capacity retention improves significantly for Mg 2+, Ca 2+, and Sr 2+, respectively. (H) Energy efficiency improves when adding Ni 2+ to the electrolyte. The thicker lines indicate cells with Ni 2+. Based on this study, a research article titled Highly Reversible Open Framework Nanoscale Electrodes for Divalent Ion Batteries has been published on Nano Letters 15. The reversible electrochemical insertion of multivalent ions into materials has promising applications in many fields, including batteries, seawater desalination, element purification, and wastewater treatment. However, finding materials that allow for the insertion of multivalent ions with fast kinetics and stable cycling has proven difficult because of strong electrostatic interactions between the highly charged insertion ions and atoms in the host framework. This work presents an open framework nanomaterial, copper hexacyanoferrate, in the Prussian Blue family that allows for the reversible insertion of a wide variety of monovalent, divalent, and trivalent ions (such as Rb +, Pb 2+, Al 3+, and Y 3+ ) in aqueous solution beyond that achieved in previous studies. Electrochemical measurements demonstrate the unprecedented kinetics of multivalent ion insertion associated with this material. Synchrotron X-ray diffraction experiments point towards a novel vacancy-mediated ion insertion mechanism that reduces electrostatic repulsion and helps to facilitate the observed rapid ion insertion. The results in this study suggest a new approach to multivalent ion insertion that help to advance the understanding of this complex phenomenon.

12 a c b Figure x. Multivalent ion intercalation in PBAs. a) The so-called soluble Prussian Blue structure has no vacancies, and insertion ions can only reside in A sites in the center of each cubic sub-cell. The nominal chemical formula in the oxidized state is KCuFe(CN)6*xH2O. b) The so-called insoluble Prussian Blue structure includes Fe(CN)6 vacancies on ¼ of the iron sites. Water molecules (not shown) coordinated to the copper atoms shield charge, and insertion ions can potentially insert into both A and B sites. The B sites can be thermodynamically favorable for certain insertion ions because coordinated water molecules shield charge from the insertion ion. The nominal chemical formula in the oxidized state is K0.25Cu(Fe(CN)6)0.75*xH2O. c) The scanning electron micrograph shows polydisperse nanoparticles of CuHCFe ranging from 20 to 90 nm (scale bar = 200 nm). Based on this study, a research article titled Reversible multivalent (monovalent, divalent, trivalent) ion insertion in open framework materials has been published on Advanced Energy Materials 16. Conclusions Prussian Blue Analogue (PBA) materials have demonstrated exceptional promise in a wide variety of energy storage applications. These materials have been successfully developed as promising electrode materials for low-cost aqueous batteries with long cycle life and rapid charge/discharge capability. Such batteries would be suitable for gridscale energy storage, which are necessary for stabilizing the electrical grid as more renewable energy sources are integrated into the industry. PBAs have also shown significant potential as electrode materials in batteries with alternative chemistries beyond those of lithium-ion. Sodium manganese hexacyanomanganate demonstrates the highest capacity ever observed for a sodium-ion cathode in organic electrolyte and with high rate capability compared to competing sodium-ion cathode candidates. Nickel and copper hexacyanoferrates have exhibited the novel ability to insert a wide variety of divalent and trivalent ions in aqueous solution. This electrochemical behavior provides directions on ways to develop multivalent-ion batteries with high rate capability. These non-lithium-ion electrode materials provide

13 possible alternatives to existing lithium-ion battery technology in a future when lithium may itself become a scarce resource. References 1. Yang, Z. et al. Electrochemical Energy Storage for Green Grid. Chem. Rev. (Washington, DC, United States) 111, (2011). 2. Rastler, D. D. Electricity energy storage technology options: a white paper primer on applications, costs and benefits. EPRI 170 (2010). at < _11_EPRIStorageReport_Rastler.pdf> 3. Rastler, D. & Kamath, H. Energy storage: Enabling grid-ready solutions. EPRI J (2010). at < 4. University of California, B. S. of L., University of California, L. A. & University of California, S. D STRATEGIC ANALYSIS OF ENERGY STORAGE IN CALIFORNIA. (2011). at < 5. Rastler, D. Electricity Energy Storage Technology Options. EPRI Rep. 170 (2010). 6. Wessells, C. D., Huggins, R. A. & Cui, Y. Copper hexacyanoferrate battery electrodes with long cycle life and high power. Nat. Commun. 2, 550 (2011). 7. Barnhart, C. & Benson, S. On the importance of reducing the energetic and material demands of electrical energy storage. Energy Environ. Sci. 6, (2013). 8. Wessells, C. D., Peddada, S. V, Huggins, R. A. & Cui, Y. Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Lett. 11, (2011). 9. Wessells, C. D. et al. Tunable reaction potentials in open framework nanoparticle battery electrodes for grid-scale energy storage. ACS Nano 6, (2012). 10. Wessells, C. D., Peddada, S. V., McDowell, M. T., Huggins, R. a. & Cui, Y. The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes. J. Electrochem. Soc. 159, A98 (2012). 11. 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. 3, 1149 (2012). 12. Pasta, M. et al. Full open-framework batteries for stationary energy storage. Nat. Commun. 5, 3007 (2014). 13. Lee, H.-W., Pasta, M., Wang, R. Y., Ruffo, R. & Cui, Y. Effect of the Alkali Insertion Ion on the Electrochemical Properties of Nickel Hexacyanoferrate Electrodes. Faraday Discuss (2014). doi: /c4fd00147h 14. Lee, H.-W. et al. Manganese hexacyanomanganate open framework as a high-capacity positive electrode material for sodium-ion batteries. Nat. Commun. 5, 5280 (2014).

14 15. Wang, R. Y., Wessells, C. D., Huggins, R. A. & Cui, Y. Highly reversible open framework nanoscale electrodes for divalent ion batteries. Nano Lett. 13, (2013). 16. Wang, R. Y. et al. Reversible Multivalent (Monovalent, Divalent, Trivalent) Ion Insertion in Open Framework Materials. Adv. Energy Mater. n/a n/a (2015). doi: /aenm Publications 1. Wessells, Colin D; Huggins Robert A, Cui, Yi, Tunable reaction potentials in open framework nanoparticle battery electrodes for grid-scale energy storage, ACS Nano (2011). 2. Wessells, Colin D; Peddada, Sandeep V; McDowell Mattew T; Huggins Robert A, Cui, Yi, The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes, JES (2012) 3. Wessells, Colin D; Peddada, Sandeep V; Huggins Robert A, Cui, Yi, Nickel Hexacyanoferrate Nanoparticle Electrodes For Aqueous Sodium and Potassium Ion Batteries, Nano Letters (2012) 4. Wessells, Colin D; McDowell Mattew T; Peddada, Sandeep V; Pasta, Mauro; Huggins Robert A, Cui, Yi, Tunable reaction potentials in open framework nanoparticle battery electrodes for grid-scale energy storage, ACS Nano (2012), 6(2), Pasta, M.; Wessells, Colin D; Huggins, Robert A; Cui, Yi, A high rate, long cycle life aqueous electrolyte battery for grid scale energy storage, Nature Communications (2012) 3, Wang, R. Y.; Wessells, C. D.; Huggins, R. A; Cui, Y, Highly reversible open framework nanoscale electrodes for divalent ion batteries, Nano Lett. 2013, 13, Pasta, M.; Wessells, Colin D; Liu, Nian; Nelson, Johanna; McDowell, Matthew T.; Huggins, Robert A.; Toney, Michael F.; Cui, Yi, Full open-framework batteries for stationary energy storage, Nature Communications, (2014), 5, Lee, H.W.; Wang, R. Y.; Pasta, M.; Woo Lee, S.; Liu, N.; Cui, Y. Manganese hexacyanomanganate open framework as a high-capacity positive electrode material for sodium-ion batteries, Nat. Commun. (2014), 5, Lee, H.W.; Pasta, M.; Wang, R. Y.; Ruffo, R.; Cui, Y. Effect of the alkali insertion ion on the electrochemical properties of nickel hexacyanoferrate electrodes, Faraday Discuss. (2014). 10. Wang, R.Y.; Shyam, B.; Stone, K.H.; Weker, J.N.; Pasta, M.; Lee, H.-W; Toney, M.F.; Cui, Y. Reversible multivalent (monovalent, divalent, trivalent) ion insertion in open framework materials, Advanced Energy Materials (2015).

15 Patents Lee, H.W.; Wang, R. Y.; Pasta, M.; Cui, Y. Docket # S "Manganese Hexacyanomanganate: a High-Capacity Positive Electrode for Rechargeable Sodium Ion Batteries". Contacts Yi Cui: yicui@stanford.edu