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
x
Ge
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 (MorrisA. J.GreyC. P.PickardC. J.
Morris, A. J.
Grey, C. P.
Pickard, C. J.
054111Phys. Rev. B: Condens.
Matter Mater. Phys.201490), 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+δGe4, 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.