Germanium coating boosts lithium uptake in Si nanotube battery anodes.

Si nanotubes for reversible alloying reaction with lithium are able to accommodate large volume changes and offer improved cycle retention and reliable response when incorporated into battery anodes. However, Si nanotube electrodes exhibit poor rate capability because of their inherently low electron conductivity and Li ion diffusivity. Si/Ge double-layered nanotube electrodes show promise to improve structural stability and electrochemical kinetics, as compared to homogeneous Si nanotube arrays. The mechanism explaining the enhancement in the rate capabilities is revealed here by means of electrochemical impedance methods. The Ge shell efficiently provides electrons to the active materials, which increase the semiconductor conductivity thereby assisting Li(+) ion incorporation. The charge transfer resistance which accounts for the interfacial Li(+) ion intake from the electrolyte is reduced by two orders of magnitude, indicating the key role of the Ge layer as an electron supplier. Other resistive processes hindering the electrode charge-discharge process are observed to show comparable values for Si and Si/Ge array electrodes.