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Metallic germanium is a promising anode material in secondary lithium-ion batteries (LIBs) due to its high theoretical capacity (1623 mAh/g) and low operating voltage, coupled with the high lithium-ion diffusivity and electronic conductivity of lithiated Ge. Here, the lithiation mechanism of micron-sized Ge anodes has been investigated with X-ray diffraction (XRD), pair distribution function (PDF) analysis, and in-/ex-situ high-resolution 7 Li solid-state nuclear magnetic resonance (NMR), utilizing the structural information and spectroscopic fingerprints obtained by characterizing a series of relevant Li x Ge y model compounds. In contrast to previous work, which postulated the formation of Li 9 Ge 4 upon initial lithiation, we show that crystalline Ge first reacts to form a mixture of amorphous and crystalline Li 7 Ge 3 (space group P 32 1 2). Although Li 7 Ge 3 was proposed to be stable in a recent theoretical study of the Li–Ge phase diagram ( Morris, A. J.; Grey, C. P.; Pickard, C. J. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 054111 ), it had not been identified in prior experimental studies. Further lithiation results in the transformation of Li 7 Ge 3, via a series of disordered phases with related structural motifs, to form a phase that locally resembles Li 7 Ge 2, a process that involves the gradual breakage of the Ge–Ge bonds in the Ge–Ge dimers (dumbbells) on lithiation. Crystalline Li 15 Ge 4 then grows, with an overlithiated phase, Li 15+δ Ge 4, being formed at the end of discharge. This study provides comprehensive experimental evidence, by using techniques that probe short-, medium-, and long-range order, for the structural transformations that occur on electrochemical lithiation of Ge; the results are consistent with corresponding theoretical studies regarding stable lithiated Li x Ge y phases.
Jung et al. (Mon,) studied this question.