XPS and ToF-SIMS Study of Electrode Processes on Sn−Ni Alloy Anodes for Li-Ion Batteries

The characterization of electrode processes induced by lithiation/delithiation of Sn−Ni alloy films electroplated on a copper substrate is presented. Galvanostatic discharge/charge measurements were combined with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS shows the buildup of a solid electrolyte interphase (SEI) layer formed by reductive decomposition of the electrolyte at the surface of the Sn−Ni anode during the first discharge. The SEI layer is constituted of a mixture of Li 2 CO 3, ROCO 2 Li, Li 2 C 2 O 4, and/or ROLi whose balance is not markedly modified upon cycling. ToF-SIMS depth profiling evidences an incomplete initial alloying process of lithium ion with Sn and the resulting partition of the Sn−Ni layer alloy into a fully lithiated outer part and a partially lithiated inner layer during the first discharge. After the first cycle, the volume expansion/shrink associated with the alloying/dealloying reaction, also evidenced by ToF-SIMS, irreversibly cracks and divides the Sn−Ni alloy into island-like morphology with gaps filled by the SEI layer. Multicycling (tested up to 9 cycles) amplifies the division of the Sn−Ni alloy layer and the related penetration of the SEI layer as indicated by the increase of trapped lithium and chlorine but with no apparent loss of active material or drop of capacity.