Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States.
Nano Lett. 2011 Sep 14;11(9):3991-7. doi: 10.1021/nl2024118. Epub 2011 Aug 24.
Retaining the high energy density of rechargeable lithium ion batteries depends critically on the cycle stability of microstructures in electrode materials. We report the reversible formation of nanoporosity in individual germanium nanowires during lithiation-delithiation cycling by in situ transmission electron microscopy. Upon lithium insertion, the initial crystalline Ge underwent a two-step phase transformation process: forming the intermediate amorphous Li(x)Ge and final crystalline Li(15)Ge(4) phases. Nanopores developed only during delithiation, involving the aggregation of vacancies produced by lithium extraction, similar to the formation of porous metals in dealloying. A delithiation front was observed to separate a dense nanowire segment of crystalline Li(15)Ge(4) with a porous spongelike segment composed of interconnected ligaments of amorphous Ge. This front sweeps along the wire with a logarithmic time law. Intriguingly, the porous nanowires exhibited fast lithiation/delithiation rates and excellent mechanical robustness, attributed to the high rate of lithium diffusion and the porous network structure for facile stress relaxation, respectively. These results suggest that Ge, which can develop a reversible nanoporous network structure, is a promising anode material for lithium ion batteries with superior energy capacity, rate performance, and cycle stability.
可充电锂离子电池的高能量密度取决于电极材料微观结构的循环稳定性。我们通过原位透射电子显微镜报告了在锂化-脱锂循环过程中单个锗纳米线中纳米多孔结构的可逆形成。在锂离子插入时，初始的结晶 Ge 经历了两步相转变过程：形成中间非晶 Li(x)Ge 和最终的结晶 Li(15)Ge(4)相。纳米孔仅在脱锂过程中形成，涉及由锂提取产生的空位的聚集，类似于脱合金中多孔金属的形成。观察到脱锂前沿将由结晶 Li(15)Ge(4)组成的致密纳米线段与由互连的非晶 Ge 链组成的多孔海绵状段分开。这个前沿以对数时间律沿纳米线移动。有趣的是，多孔纳米线表现出快速的锂化/脱锂速率和优异的机械鲁棒性，这归因于锂离子扩散的高速率和多孔网络结构，便于应力松弛。这些结果表明，锗可以形成可逆的纳米多孔网络结构，是具有卓越能量容量、倍率性能和循环稳定性的锂离子电池的有前途的阳极材料。
