Résumé
Si1-xGex alloys present interesting cycling performance as Li-ion anodes, owing to the synergetic effect from silicon’s high capacity and germanium’s electrical conductivity and Li+ diffusion. Various morphologies of Si1-xGex powders were obtained, micron-sized by ball-Milling (BM) and nano-sized by laser pyrolysis (LP) to study the effect of the morphology and composition on the electrochemical behavior. The electrical conductivity increased with the Ge content, from 4.58E-03 for Si to 0.11 S m-1 for Si0.5Ge0.5. Half-cells were cycled at C/5, LP samples showed a better capacity retention than their BM counterparts (88% vs 72% at the 35th cycle for Si0.5Ge0.5). To rationalize these trends, Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) were used to determine the apparent Li+ Diffusion in the Si1-xGex series. It was shown that the apparent Li+ Diffusion is dependent to the state of charge. Moreover, it gave insight about phase transformations during cycling. During lithiation, similar values (10-11 cm2 s-1) were obtained for BM and LP Si0.5Ge0.5. However, a big variation (10-13 – 10-10 cm2 s-1) was found for the BM sample delithiation, which is attributed to the conversion of c-Li15(Si0.5Ge0.5)4 phase into amorphous Lix(Si0.5Ge0.5) (x<3.75). Asymmetric C-rate tests evidenced lithiation as the limiting mechanism for the Si1-xGex negative electrodes. Volume expansion within cycling was investigated in full-cell pouch configuration (NMC622 // Si0.5Ge0.5). The drastic capacity decrease in the BM material was linked to the loss of active material, with the swelling reducing from 55% in the 1st cycle to 4% in the 50th. In the other hand, the LP alloy showed a much better capacity retention with a reversible swelling of 20% from the 2nd cycle. This study highlights how important is the morphology of Si based alloys to ensure a better electrochemical performance.