Frontiers of solid-state batteries
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Relevance and historical background The appeal of solid-state battery systems is undeniable.1 In the case of lithium-based batteries, many of the issues associated with the use of the organic liquid electrolytes can be mitigated. The removal of organic solvents can reduce flammability and the amount of combustible materials, making the system safer. A solid electrolyte layer also serves as the battery separator. This enables complete decoupling of the anode and cathode chemistries. Materials that suffer from dissolution in liquid electrolytes can be made to cycle reversibly. Most importantly, the use of Li-metal anodes becomes possible since a solid electrolyte can eliminate dendrite growth if lithium ion is the only charge carrier and the material has a high modulus. Further, inorganic electrolytes can potentially be stable in the presence of lithium metal. In contrast, batteries with liquid electrolytes have relied on the formation of a solid electrolyte interphase to maintain stability. Liquid electrolytes are limited by their freezing and boiling points, near which the electrolytes lose their conductivities. Solid electrolytes, on the other hand, can function over a wide temperature range and their conductivities can vary continuously. This advantage becomes especially enabling at high temperatures. Solid-state battery cells can be stacked in a bipolar arrangement to form a high-voltage single cell, thus yielding a simplified system architecture.
Although the history of solid-state batteries can be traced back to the 1830s, the advantages of solid-state batteries were not fully recognized until the 1960s with the discovery of beta-alumina, a sodium-ion conductor.2 This stable, highly conductive ion conductor led to the development of a commercially relevant high-temperature Na-S battery by the Ford Motor Company in the 1960s and the ZEBRA battery by the Zeolite Battery Research Africa Project group at the Council for Scientific and Industrial Research (CSIR) in Pretoria, South Africa, in the 1980s. It should be recognized that these two batteries are not literally solid-state batteries since the electrode materials were in a molten state (they are more appropriately called solid-state electrolyte batteries). In the 1970s, solid polymer electrolytes based on lithium salt-poly(ethylene oxide) complexes were discovered, which led to true all-solid-state batteries.3 The subsequent discovery in 1983 of lithium phosphorus oxynitride (LiPON), at Oak Ridge National Laboratory, resulted in the development of thin-film solid-state batteries that function at ambient temperatures with exceptional cycling stabilities.4 The ensuing decades saw the emergence of new solid-ion conductors with ever-increasing conductivities. Evolution of the Li-M-(P)-S-based system, which is rich in the variety of M elements that can be used, culminated in the discovery of Li10GeP2S12 (LGPS) in 2011 with a conductivity
Jagjit Nanda, Oak Ridge National Laboratory, USA; [email protected] Chongmin Wang, Environmental Molecular Sciences Laboratory,
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