Li reaction pathways in Ge and high-performance Ge nanocomposite anodes for Li-ion batteries

[Display omitted] •Nanocrystalline Ge exhibited higher Li reversibility than bulk Ge.•Li reaction pathways in nanocrystalline Ge were definitively established.•Ge/Al2O3/C was synthesized by one-pot mechanical solid-state reduction.•Ge/Al2O3/C exhibited good cyclic behavior, rate capability, and reversible capacity. Ge is a highly researched anode material for Li-ion batteries (LIBs) owing to its high theoretical Li storage capacity and higher electrical conductivity compared to that of Si. However, Li reaction pathways in Ge have not been definitively established due to the formation of various and complicated Li-Ge alloy phases during lithiation/delithiation. Evaluation of the Li storage characteristics of bulk and nanocrystalline Ge demonstrated that nanocrystalline Ge showed higher Li reversibility than bulk Ge. Various cutting-edge ex situ techniques were used to analyze nanocrystalline Ge, and the Li reaction pathways in Ge were thoroughly demonstrated. To enhance the electrochemical Li storage characteristics of Ge, a Ge-based nanocomposite, Ge/Al2O3/C, was synthesized via simple one-pot mechanical solid-state reduction using GeO2, Al, and C. Ge/Al2O3/C exhibited very stable cyclic behavior at a current density of 100 mA g−1 over 300 cycles (capacity retention after 300 cycles: ∼95 %). Moreover, Ge/Al2O3/C exhibited excellent rate capability with high reversible capacity and stable cyclic behavior at a high 1C-rate. The superior electrochemical performance of Ge/Al2O3/C was attributed to the nanoscale dimensions of Li-active nanocrystalline Ge (∼10 nm) and Li-inactive Al2O3 (∼12 nm) embedded uniformly in the amorphous carbon matrices. The size of nanocrystalline Ge in nanocomposite was consistently maintained during repeated cycles owing to the anti-agglomeration effect of the uniformly distributed Li-inactive Al2O3 and amorphous carbon matrices. We anticipate that the electrochemical Li reaction pathways in Ge and the resulting high-performance nanocomposite anodes will be highly useful in investigating high-performance Ge-based anodes for LIBs.