Ion-conducting glass-ceramics for energy-storage applications
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troduction The development of efficient devices for energy storage, conversion, and transmission is one of the key priority areas in materials science today. At the focus of these activities is the development of high-energy and high-power battery technologies based on lithium-1,2 and sodium-ion transport.3 During the past two decades, a large number of crystalline and glassy cathode, anode, and electrolyte materials for potential battery applications have been developed and characterized. Most lithium- and sodium-ion batteries currently in use still rely on liquid-organic electrolytes, which restricts cyclability due to electrode corrosion. They also present design challenges to avoid leakage and shock damage, and pose safety and environmental concerns in everyday use.4 For this reason, glassy and glass-ceramic inorganic materials play an important role in current efforts to design all-solid-state batteries.4–7 Such systems offer increased safety, environmental sustainability, and simplified cell design. The fractional contribution of the migrating ionic species to the total ionic conductivity is usually near unity in such materials, eliminating complications that might arise from emerging concentration gradients during operation. In addition, the ionic transport number (i.e, the ratio of the ionic conductivity and the total ionic + electronic conductivity of the migrating ionic species) is also usually near unity, which means that only one cationic specie is moving.
The principal challenge in this field still rests with the development of optimized materials having sufficiently high ionic conductivities and appropriate electrochemical stability. The highest alkaline-ion conductivities in the solid state are generally encountered in crystalline compounds with highly concentrated and disordered cation sublattices, termed superionic crystals. Li- and Na-superionic conducting crystals and glass-ceramics have been subject to several recent reviews.4–13 All of them are characterized by open-framework structures creating periodic three-dimensional (3D) arrays of partially occupied ionic sites at close distances, facilitating 3D ionic motion and transport. Beside high-ionic conductivities, solid electrolytes for battery applications must meet further demands—they should be stable under ambient atmospheric conditions and possess sufficient oxidative and reductive stability at the interfaces with the cathode and anode compartments. For application as solid electrolytes in solid-state battery devices, fine powders of these crystals have to be well compacted to maximize interparticle contacts. Even so, ineffective ion transport across grain boundaries presents a problem and reduces device performance. Other serious transport limitations arise at the electrolyte–electrode interfaces. In principle, these problems may be avoided by use of suitable glassy solid electrolytes, which are dense and can be prepared in well-defined sizes and shapes (including thin films) by melt cooling or
Hellmut Eckert, São Carlos Institute of Physics, Univ
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