Elucidation of the local and long-range structural changes that occur in germanium anodes in lithium-ion batteries

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 ⁷Li solid-state nuclear magnetic resonance (NMR), utilizing the structural information and spectroscopic fingerprints obtained by characterizing a series of relevant Li_xGe_y model compounds. In contrast to previous work, which postulated the formation of Li₉Ge₄ upon initial lithiation, we show that crystalline Ge first reacts to form a mixture of amorphous and crystalline Li₇Ge₃ (space group P32₁2). Although Li₇Ge₃ 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 Li₇Ge₃, via a series of disordered phases with related structural motifs, to form a phase that locally resembles Li₇Ge₂, a process that involves the gradual breakage of the Ge-Ge bonds in the Ge-Ge dimers (dumbbells) on lithiation. Crystalline Li₁₅Ge₄ then grows, with an overlithiated phase, Li₁₅₊Ge₄, 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_xGe_y phases.