Reversible Nanopore Formation in Ge Nanowires during Lithiation–Delithiation Cycling: An In Situ Transmission Electron Microscopy Study

TLDR

Battery energy density depends on the cycle stability of electrode microstructures. The study reports reversible nanoporosity formation in individual germanium nanowires during lithiation–delithiation cycling observed by in situ transmission electron microscopy. Lithium insertion drives crystalline Ge first to amorphous LixGe and then to crystalline Li15Ge4, while nanopores form during delithiation as vacancies aggregate, analogous to porous metal dealloying. A delithiation front separates dense Li15Ge4 from porous amorphous Ge, moving logarithmically; the porous nanowires exhibit rapid lithiation/delithiation and mechanical robustness due to fast Li diffusion and a stress‑relaxing network, underscoring Ge’s potential as a high‑capacity, rate‑capable, cycle‑stable anode.

Abstract

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 LixGe and final crystalline Li15Ge4 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 Li15Ge4 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.