Current Trends in Solid-State Electrochemical Energy Conversion and Storage Devices

Published as part of ACS Energy Letters special issue “The Evolving Landscape of Energy Research: Views from the Editorial Team”. Figure 1. (a) Schematic representation showing the operating principle of a solid oxide fuel cell (O-SOFC) and ceramic proton conducting H-SOFC. Unlike SOFC, there is no fuel dilution in proton conducting fuel cells since water is produced at the oxygen electrode (cathode). (b) Schematic representation showing a thermodynamically stable cell with proposed/desired energy levels of oxidant and reductant in an electrochemical cell. (c) Schematic representation showing typical polarization curves of “ideal”, high, moderate, and poor ceramic fuel cell performance. High oxide ion conductivity (about 10^–2 S/cm) in the intermediate temperature regime (500–750 °C) without structural phase transition. Electrochemical stability window of at least 1.2 V against oxygen at operating temperature. Chemical stability in the working environment including reactions at electrode/electrolyte and reactant/electrolyte interfaces during the preparation and operation. Thermodynamic stability can be achieved only by placing the bottom of the electrolyte conduction band above the highest occupied molecular orbital (HOMO) of the cathode and the top of the electrolyte valence band below the lowest unoccupied molecular orbital (LUMO) of the anode (Figure 1b). (11) Negligible electronic conductivity (due to electrons and holes) over the entire range of oxygen partial pressures and temperatures. Capable of forming dense, gastight, pore-free thin-films (about 10–20 μm) with good adhesion to anode and cathode materials, with matching thermal expansion coefficients. Low interfacial and charge transfer resistances (0.1 Ω·cm²) between the electrolyte and electrodes (cathode and anode) and low ohmic overpotential (Figure 1c). Low-cost, nontoxic, easily prepared, and chemically stable under ambient conditions, particularly in the presence of moisture and atmospheric CO₂. Figure 2. Idealized crystal structures of (a) fluorite-type (CeO₂/Y-doped ZrO₂) (oxide ion); (b) pyrochlore (Gd₂Ti₂O₇) (oxide ion); (c) perovskite-type (LaGaO₃) (oxide, proton, electrons); (d) brownmillerite (Ba₂In₂O₅) (oxide ion); (e) ordered double perovskite (Ba₃CaNb₂O₉) (proton, electrons), and (f) layered perovskite (Sr₂TiO₄) (oxide ion, electrons). Unit cell axis orientation is shown for noncubic structures only. Figure 3. Schematic diagram showing ion conduction pathways (vacancy and interstitial migrations) in typical solid-state electrolytes. Panels a and b show the position of ions and vacancies before and after ion migration, respectively. The dashed circle represents a vacant position after ion migration. Filled blue circles represent dopants. Filled yellow and red circles represent mobile ions. Total (bulk + grain-boundary) ionic conductivity on the order of 10^–3 S/cm with negligible electronic conductivity at high and low lithium activities. (28) Stability against chemical reaction with metallic or alloy anodes and high voltage cathodes. Ability to prepare dense thin films (<20 μm) using current ceramic processing technologies such as tape casting and thin-film deposition methods. Excellent wettability of elemental anodes with low ion charge transfer resistance. Wide electrochemical potential stability window, cost-effective method of preparation, and environmentally benign. Ability to work with high-capacity conversion electrodes such as sulfur and oxygen. Free from dendrite formation at higher critical current densities and matching chemomechanical properties with electrode materials. Figure 4. Idealized crystal structures for lithium and sodium ion conduction. (a) Li₄SiO₄ (lithium ion); (b) Li₆PS₅Cl (lithium ion); (c) garnet-type Li₇La₃Zr₂O₁₂ (lithium ion); (d) Na₅SmSi₄O₁₂ (sodium ion); (e) Na₃Zr₂Si₂PO₁₂ (monoclinic; sodium ion); (f) Na₃Zr₂Si₂PO₁₂ (rhombohedral; Na ion); (g) Na₃PS₄ (sodium ion), and (h) perovskite-type La_0.67–xLi_3x□_0.33–2xTiO₃ (□ = A-site vacancy) (lithium ion). Unit cell axis orientation is shown for noncubic structures only. Figure 5. Estimated single cell level energy density comparison for conventional Li ion batteries, Li metal liquid electrolyte, Li metal solid-state electrolyte batteries, and anode-free solid-state batteries (adopted from ref (21)). V. T. would like to sincerely thank his Ph.D. supervisors, Profs. Jagannatha Gopalakrishnan (late) and Ashok Kumar Shukla, at Solid-state and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India. V. T. thanks Profs. Werner Weppner and Robert (Bob) Huggins for their mentorship and encouragement during his PDF at the University of Kiel, Germany. V.T. thanks his family and friends, all his students, and colleagues at University of Kiel, University of Calgary, and University of St. Andrews for their support. This article references 41 other publications. This document has been updated Click for further information. This article has not yet been cited by other publications.