A New Solid-State Sodium-Metal Battery

Sodium batteries and solid-state electrolytes are two research directions in the effort to develop electrochemical energy storage that goes beyond the lithium ion. In this issue of Chem, Goodenough and colleagues combine a sodium-metal anode, a NASICON solid electrolyte, and a Prussian blue analog cathode to create an energy-dense, long-lived battery. Sodium batteries and solid-state electrolytes are two research directions in the effort to develop electrochemical energy storage that goes beyond the lithium ion. In this issue of Chem, Goodenough and colleagues combine a sodium-metal anode, a NASICON solid electrolyte, and a Prussian blue analog cathode to create an energy-dense, long-lived battery. Electrical energy from renewable sources, notably the sun and wind, is inherently intermittent and unpredictable. Therefore, strategies and technologies must be developed to ensure that energy supply and demand are matched on timescales ranging from seconds to days. One such strategy is using grid-scale energy storage, the critical performance metrics of which are cost, cycle life, and calendar life; conversely, energy density is key for portable electronics.1Barnhart C.J. Benson S.M. On the importance of reducing the energetic and material demands of electrical energy storage.Energy Environ. Sci. 2013; 6: 1083Crossref Scopus (180) Google Scholar Economies of scale and decades of optimization have driven down the cost of lithium-ion batteries, but there are concerns around the increasing cost of lithium. In contrast, sodium’s abundance gives it a lower cost as a raw material, and its ubiquity ensures stability of that cost over time. Moreover, a sodium-based chemistry allows for the substitution of the costly and heavy copper current collector with inexpensive and lighter aluminum.2Vaalma C. Buchholz D. Weil M. Passerini S. A cost and resource analysis of sodium-ion batteries.Nat. Rev. Mater. 2018; 3: 18013Crossref Scopus (1005) Google Scholar In addition, the transition metals used for making cathodes in current lithium-ion batteries are expensive, and the overall cycle life of these systems is likely to be inadequate for grid-scale storage. One family of candidate materials for cathodes in sodium batteries is composed of inexpensive and readily available raw materials that have been shown to operate reversibly for tens of thousands of deep discharge cycles: Prussian blue analogs (PBAs).3Pasta M. Wessells C.D. Liu N. Nelson J. McDowell M.T. Huggins R.A. Toney M.F. Cui Y. Full open-framework batteries for stationary energy storage.Nat. Commun. 2014; 5: 3007Crossref PubMed Scopus (364) Google Scholar Prussian blue is the prototype of PBAs, which share a common crystal structure but span a range of compositions. They contain two transition-metal ions, bridging cyanide ligands, and interstices accommodating inserting ions. The transition metals are ordered in a face-centered-cubic (fcc) arrangement: one is octahedrally coordinated to carbon atoms, and the other is coordinated to nitrogen atoms (Figure 1A). The general formula is AxP[R(CN)6]1−y · ▢y · wH2O, where P and R are transition metals, A is an interstitial ion species, and ▢ represents a hexacyanometallate vacancy. Vacancies and water content strongly affect the material’s stability and ionic conductivity.4Moritomo Y. Kurihara Y. Matsuda T. Kim J. Structural phase diagram of Mn-Fe cyanide against cation concentration.J. Phys. Soc. Jpn. 2011; 80: 8-11Crossref Scopus (18) Google Scholar The open-framework structure allows for the insertion of a range of alkali and alkaline earth cations with excellent kinetics and minimal change to the framework itself; this leads to the long cycle life discussed above. Hexacyanoferrates with iron at the carbon-coordinated position have redox potentials ranging from 3.0 to 3.8 V versus Na+/Na, making them ideal cathode materials. To reach its maximum theoretical specific capacity, around 170 mA h g−1, a PBA must be electrochemically active at both transition-metal centers within the relevant potential range.5Pasta M. Wang R.Y. Ruffo R. Qiao R. Lee H.-W. Shyam B. Guo M. Wang Y. Wray L.A. Yang W. et al.Manganese–cobalt hexacyanoferrate cathodes for sodium-ion batteries.J. Mater. Chem. A Mater. Energy Sustain. 2016; 4: 4211-4223Crossref Google Scholar This allows a two-electron reaction per formula unit and full utilization of interstitial ion sites in the structure (Figure 1B). In addition, to maximize the capacity, the hexacyanoferrate vacancy content must be minimized. Thorough research into hexacyanoferrates has identified three analogs in which the nitrogen-coordinated ion is electrochemically active: hexacyanoferrates of iron,6You Y. Wu X.-L. Yin Y.-X. Guo Y.-G. High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries.Energy Environ. Sci. 2014; 7: 1643-1647Crossref Google Scholar manganese,7Song J. Wang L. Lu Y. Liu J. Guo B. Xiao P. Lee J.J. Yang X.Q. Henkelman G. Goodenough J.B. Removal of interstitial H2O in hexacyanometallates for a superior cathode of a sodium-ion battery.J. Am. Chem. Soc. 2015; 137: 2658-2664Crossref PubMed Scopus (511) Google Scholar and cobalt.8Wu X. Wu C. Wei C. Hu L. Qian J. Cao Y. Ai X. Wang J. Yang H. Highly crystallized Na2CoFe(CN)6 with suppressed lattice defects as superior cathode material for sodium-ion batteries.ACS Appl. Mater. Interfaces. 2016; 8: 5393-5399Crossref PubMed Scopus (262) Google Scholar The price of cobalt is high and increasing. This leaves manganese and iron hexacyanoferrate as primary candidates for cathode materials in low-cost grid-scale batteries. Of the two, manganese hexacyanoferrate has a higher average discharge potential for a higher energy density. However, it does suffer from a limited cycle life, which is caused by three phenomena. First, on oxidation to Mn3+, a Jahn-Teller distortion of the MnN6 octahedra deforms the crystal structure and decreases reversibility.9Kurihara Y. Matsuda T. Moritomo Y. Structural properties of manganese hexacyanoferrates against Li concentration.Jpn. J. Appl. Physiol. 2013; 52Crossref Scopus (21) Google Scholar The second is the disproportionation reaction of Mn3+ into Mn2+ and Mn4+, which can accelerate the dissolution into the liquid organic electrolyte and lead to side reactions and irreversible capacity loss. Finally, manganese catalyzes the decomposition of the electrolyte, forming corrosive species that further accelerate the cathode dissolution. In this issue of Chem, Goodenough and colleagues10Gao H. Xin S. Xue L. Goodenough J.B. Stabilizing a high-energy-density rechargeable sodium battery with a solid electrolyte.Chem. 2018; 4: 833-844Abstract Full Text Full Text PDF Scopus (134) Google Scholar tackle these problems for a manganese hexacyanoferrate cathode by replacing the organic liquid electrolyte (Figure 1C) with a solid NASICON electrolyte (Figure 1D). The solid electrolyte enabled the use of a sodium-metal anode that simultaneously showed a high specific capacity (1,166 mA h g−1) and a low redox potential (−2.7 V versus standard hydrogen electrode). The authors directly compared cells containing the solid electrolyte and cells containing conventional organic electrolyte. Cells using an organic electrolyte showed very poor cycle life and considerable capacity fade over the first 200 cycles, although the Goodenough group has previously achieved significantly better performance with the same cathode in a different organic electrolyte.7Song J. Wang L. Lu Y. Liu J. Guo B. Xiao P. Lee J.J. Yang X.Q. Henkelman G. Goodenough J.B. Removal of interstitial H2O in hexacyanometallates for a superior cathode of a sodium-ion battery.J. Am. Chem. Soc. 2015; 137: 2658-2664Crossref PubMed Scopus (511) Google Scholar In contrast, cells using the solid electrolyte showed significantly slower capacity fade. As seen through ex situ X-ray diffraction (XRD), the diffraction pattern of the cathode was maintained with a solid electrolyte after cycling. The intensities of XRD reflections decreased significantly, however, after cycling with the organic electrolyte. The authors propose that decomposition of the organic electrolyte, potentially catalyzed by the PBA active material itself, produced reactive species such as hydrogen fluoride and water. These species then attacked the PBA active material to accelerate its dissolution. On the other hand, their NASICON solid electrolyte was not susceptible to oxidative decomposition. They analyzed the electrode surfaces after cycling to investigate the composition of the solid electrolyte interface and shed light on the decomposition processes. X-ray photoelectron spectroscopy of the surfaces of both electrodes cycled in organic electrolyte showed the presence of numerous chemical species, including oxides and fluorides of iron, manganese, and sodium. In cells cycled with the NASICON electrolyte, sodium oxide and organic molecules were present on the cathode surface. Only sodium oxide was detected on the anode, although some products could have been lost during sample preparation. Scanning electron microscopy images of the two surfaces showed that, compared with the organic electrolyte, the solid electrolyte inhibited the formation of sodium dendrites. To ensure adequate ionic conductivity of the solid electrolyte, Goodenough and colleagues operated the NASICON cells at 60°C. This elevated temperature affected the mechanical properties of the sodium metal and therefore its propensity to form dendrites, as well as the wettability at the interface. Additionally, this was relatively close to the melting point of sodium (97.8°C), thus limiting the feasible operating temperature range. This work introduces a full cell that incorporates a PBA cathode with a solid electrolyte. Eliminating the organic electrolyte suppresses dissolution of the active material for an improved cycle and calendar life. The solid electrolyte inhibits dendrite growth to enable a sodium-metal anode with a substantially higher specific capacity. With scalable methods of manufacturing solid-state batteries, the work presented here can lead to a low-cost, long-lived battery suitable for use in grid-scale energy storage. This publication arose from research funded by the John Fell Oxford University Press Research Fund. Stabilizing a High-Energy-Density Rechargeable Sodium Battery with a Solid ElectrolyteGao et al.ChemMarch 8, 2018In BriefDissolution of the Na2MnFe(CN)6 cathode and the growth of sodium dendrites are eliminated in an all-solid-state sodium battery with a ceramic solid electrolyte contacting the sodium-metal anode and the Na2MnFe(CN)6 cathode. Full-Text PDF Open Archive