Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive

Silicon-based electrodes of various areal capacities, from about 1.5 to 15 mAh.cm−2, were prepared with different conductive additives (carbon black, carbon nanofibers, and carbon nanoplatelets). The sensitivity of the cycling performance to the active mass loading is significant, with a major decrease of the capacity retention with increasing the loading in all cases. There is moreover a critical loading value above which the capacity retention abruptly drops. This critical loading depends on the conductive additive (∼1.75 mg cm−2 for carbon black, ∼2.25 mg cm−2 for carbon nanofibers and ∼3 mg cm−2 for carbon nanoplatelets). The lower capacity retention capability for thicker electrode is attributed to (i) higher mechanical stresses within the electrode films and at the interface with the current collector and to (ii) poorer cohesion of electrodes with higher active silicon loading. Better capacity retention of electrodes with carbon nanoplatelets is attributed to (i) higher initial cohesion of the electrodes and to (ii) good ability of the electrode architecture to reversibly expand/contract upon cycling as shown by in situ electrochemical dilatometry. The efficiency of carbon nanoplatelets as conductive additive allows decreasing its amount in the electrode formulation to 6 wt% without sacrificing cycling performance. Contribution of carbon additives to the mechanical properties of the electrode is as important as their contribution to the electrical properties for silicon.