High-Capacity Silicon–Air Battery in Alkaline Solution
Xing Zhong,[a] Hua Zhang,[b] Yuan Liu,[b] Jingwei Bai,[b] Lei Liao,[a] Yu Huang,[b] and Xiangfeng Duan*[a] The ever-increasing demand for portable power sources has motivated considerable research efforts towards a variety of power and energy systems.[1–9] The metal–air battery, using the reduction of oxygen from the atmosphere as cathode reaction, is known for its high energy density. The zinc–air battery, being the first commercialized metal–air battery, has received significant attention since the 1960s.[11–15] More recently, there has been renewed interest in this battery for application in electric vehicles. However, the zinc–air battery can provide a practical energy density of only 470 Wh kgÀ1, from a theoretical value of 1370 Wh kgÀ1. The aluminum–air battery has a high theoretical energy density (8100 Wh kgÀ1),[18, 19] but is limited to military applications owing to its high self-discharge rate. Alloying the aluminum with tin or with other elements (in proprietary formulations) has made the battery’s electrodes less corrosive in alkaline solutions.[21, 22] As an alternative to aluminum– and zinc–air batteries, the lithium–air battery possesses a higher theoretical energy density of 13 000 Wh kgÀ1 and an expected practical value of 1700 Wh kgÀ1,[23–25] but it suffers from potential safety and cost issues.[26–28] The silicon–air battery is another interesting system, with a theoretical energy density of 8470 Wh kgÀ1. This is less than lithium–air systems but compares favorably to the zinc– and aluminum–air systems. In addition silicon, unlike lithium, is one of the most abundant elements on Earth and therefore may offer a cost-effective alternative. Recently, a silicon–air battery was reported using EMI·(HF)2.3F ionic liquid-based electrolyte.[29, 30] The battery system showed a practically unlimited shelf-life with a working potential in the range of 1.0–1.2 V. The practical application of this battery system, however, might be complicated by serious chemical safety issues, associated with the use of a fluoride-based electrolyte. Herein, we report a high capacity silicon–air battery using nanostructured silicon and alkaline solution based electrolyte that only involves environmentally friendly elements such as silicon, potassium, oxygen, and hydrogen. The silicon surface is first modified by the metal-assisted electroless chemical etching method.[31–35] The assembled battery displays a flat and stable discharge curve with a voltage ranging from 0.9 to 1.2 V (under different discharge current densities) over days. In con[a] X. Zhong,+ Dr. L. Liao, Prof. X. Duan Department of Chemistry and Biochemistry California Nanosystems Institute University of California, Los Angeles 607 Charles E. Young Drive East, Los Angeles, CA 90095 (USA) E-mail: email@example.com [b] H. Zhang,+ Y. Liu, Dr. J. Bai, Prof. Y. Huang Department of Materials Science and Engineering California Nanosystems Institute University of California, Los Angeles 410 Westwood Plaza, Los Angeles, CA 90095 (USA) [+] These authors contributed equally to this work.
trast, the unmodified silicon wafer becomes passivated quickly in the alkaline solution and therefore the potential drops rapidly after discharging for a short period of time (minutes). We propose that the formation of the porous surface structure increases the overall Si(OH)4 dissolving rate in the KOH electrolyte, which effectively removes the oxide and reactivates the silicon surface. The corrosion of the silicon in the KOH electrolyte is also carefully investigated to minimize self-discharge. Corrosion of the silicon is effectively minimized by using a lower KOH concentration (0.6 m), enabling a specific capacity as high as 1206.0 mA h gÀ1, which is about 2 times the practical value of a commercial zinc–air battery (ca. 650 mA h gÀ1, Energizer) and 3 times that of a commercial...
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