One important phenomenon largely absent in both the planar and porous silicon electrodes is increased polarization caused by an insulating SEI, which builds up on the nanoparticles and impedes lithium diffusion. Evidence of polarization can be seen in the rightward shift in the delithiation dQ/dVs with increasing cycles, most easily seen in Figure . Polarization is observed using both CC and CCCV cycling, but is more pronounced with the CCCV step. Cycle 75 in the 5 mV CCCV cell, Figure , has a peak at ~0.34 V characteristic of delithiation from a-LixSi, while the corresponding peak in 5