Elucidation of the Local and Long-Range Structural Changes that Occur in Germanium Anodes in Lithium-Ion Batteries

Abstract

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.

Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate

“…The voltage where c-Ge becomes undetectable decreases with increased C-rate, i.e., more driving force is needed to transform c-Ge. The disappearance of c-Ge from XRD indicates the complete transformation of c-Ge into a-Ge and a-Li x Ge phases as reported by Lim et al [ 8 ] and Jung et al [ 10 ] However, a large c-Ge phase fraction is found to be present even at the end of the fi rst lithiation cycle for C/3 and 2C rates. Signifi cantly close to 100% c-Ge is observed at the end of lithiation for the anode cycled at 2C.…”

“…This reported a reaction sequence involving a succession of mostly disordered phases (similar to Li 7 Ge 3 and Li 7 Ge 2 ) ending with c-Li 15 Ge 4 and over-lithiated Li 15+ δ Ge 4 at the end of lithiation (although these were not quantifi ed). [ 10 ] The differences between this reaction sequence and…”

“…This is motivated because the different C-rates affect the electrochemical reaction mechanisms and it is important to understand how these lithiation kinetics affect the reaction pathways. Different from previous work, [ 8,10 ] here we use carbon nanotubes (CNT) as the conductive additive because these provide better cycling performance, and there is no difference in the structural changes in Ge when a different conductive additive is used. [ 11 ] To the best of our knowledge, there have been no reports on utilizing operando X-ray studies on micrometer-size c-Ge particles to study and understand the dependence of the phase transformation, Li storage capacity, and cycling stability on its C-rate.…”

Structural Evolution of Electrochemically Lithiated MoS2 Nanosheets and the Role of Carbon Additive in Li-Ion Batteries

“…51 Not limited to ground-state crystal structures, AIRSS has been used to understand point defects 52 and continuous phase transitions in LIBs. 53 AIRSS searches were carried out on stoichiometries of Li x Mo y S 2 y at ratios of less than of 8:1 of Li:MoS 2 which corresponds to a theoretical capacity of up to 1688 mAh/g. To limit the number of atoms in the simulation cells, stoichiometries were constrained to x + 3 y ≤ 11.…”

Atomistic Study of Lithium Ion Dynamics in Li12Si7

“…Group IV elements, such as silicon, germanium, and tin as promising anode materials for the LIBs were extensively investigated by experiments and computational simulation methods 3,[5][6][7][8] . Among them, Si was paid considerable attention because of its higher theoretical specific capacity (~3579 mA h g -1 for Li 15 Si 4 at room temperature), low cost and abundant resources.…”