Si_1-xGe_x alloys as negative electrode for Li-ion batteries: Impact of morphology in the Li⁺ diffusion, performance and mechanism

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 was measured and shows an increase 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) cm(2) s(-1)) were obtained for BM and LP Si0.5Ge0.5. However, a big variation (10(-13) - 10(-10) cm(2) s(-1)) was found for the BM sample delithiation, which is attributed to the delithiation of c-Li-15(Si0.5Ge0.5)(4) phase into amorphous Li-x(Si0.5Ge0.5) (x<3.75). Asymmetric C-rate tests evidenced lithiation as the limiting mechanism for the Si1-xGex negative electrodes.