Functionalized germanane/SWCNT hybrid films as flexible anodes for lithium-ion batteries

Abstract

Germanium, with a high theoretical capacity based on alloyed lithium and germanium (1384 mA h g⁻¹ Li₁₅Ge₄), has stimulated tremendous research as a promising candidate anode material for lithium-ion batteries (LIBs). However, due to the alloying reaction of Li/Ge, the problems of inferior cycle life and massive volume expansion of germanium are equally obvious. Among all Ge-based materials, the unique layered 2D germanane (GeH and GeCH₃) with a graphene-like structure, obtained by a chemical etching process from the Zintl phase CaGe₂, could enable storage of large quantities of lithium between their interlayers. Besides, the layered structure has the merit of buffering the volume expansion due to the tunable interlayer spacing. In this work, the beyond theoretical capacities of 1637 mA h g⁻¹ for GeH and 2048 mA h g⁻¹ for GeCH₃ were achieved in the initial lithiation reaction. Unfortunately, the dreadful capacity fading and electrode fracture happened during the subsequent electrochemical process. A solution, i.e. introducing single-wall carbon nanotubes (SWCNTs) into the structure of the electrodes, was found and further confirmed to improve their electrochemical performance. More noteworthy is the GeH/SWCNT flexible electrode, which exhibits a capacity of 1032.0 mA h g⁻¹ at a high current density of 2000 mA g⁻¹ and a remaining capacity of 653.6 mA h g⁻¹ after 100 cycles at 500 mA g⁻¹. After 100 cycles, the hybrid germanane/SWCNT electrodes maintained good integrity without visible fractures. These results indicate that introducing SWCNTs into germanane effectively improves the electrochemical performance and maintains the integrity of the electrodes for LIBs.