Surface modification of electrospun nitrogen-doped Ge@C fiber with highly porous NiCo₂O₄ layer as high-performance lithium-ion battery anode

Elemental germanium (Ge) is considered a high-capacity anode material for lithium-ion batteries (LIBs). However, it suffers from severe capacity degradation and inherent material instability owing to inevitable volumetric changes during the alloying/dealloying reactions with lithium. In this study, we report a hierarchical architecture comprising Ge nanoparticles in electrospun carbon fibers (Ge@C) coated with an in situ grown NiCo₂O₄ (NCO) layer to enhance the structural stability and electrochemical reversibility of Ge. The Ge@C@NCO fibers possess unique features, including well-dispersed Ge in nitrogen-doped porous carbon network that serves as a conductive volumetric buffer. This configuration allows for effective volume accommodation and improved electronic conductivity. Moreover, the porous NCO contributed to enhanced reversible capacity and rapid ionic transfer during electrochemical reactions. As a result, the Ge@C@NCO anode exhibited an ultrahigh specific capacity of 981.7 mAh g⁻¹ and excellent capacity retention over 200 cycles under a current density of 1 A g⁻¹, indicating superior lithium storage properties compared to pure Ge. Additionally, it retained approximately 80 % of initial capacity after 300 cycles even at 5 A g⁻¹, demonstrating fast charging capability. The outstanding performance of this hierarchical structure presents a new path for designing alloying-based anodes for high-energy-density LIBs.