The capacity of the silicon nanoparticle electrodes degrades most rapidly between cycles 15 and 25 (Figure ). Approximately half of the capacity degradation between cycles 10 and 60 occurs before the relative delithiation capacity below 0.35 V begins to decrease past cycle 30. From cycle 30 onwards, the degradation rate is more or less constant. Polarization begins to increase around cycle 25 as well, see Figure , coinciding with the decline in the amount of c-Li15Si4 as evidenced by the decrease in relative delithiation capacity below 0.35 V. Therefore, whether SEI build-up also intrinsically affects formation of c-Li15Si4 in silicon nanoparticles, as it did for the x = 0.30 porous Si film cycled in DME-based electrolyte, is not clear as the increased polarization may be limiting the quantity of Li insertion. For the porous film, the c-Li15Si4 peak declined very rapidly within the first 6 cycles. No increase in polarization was observed (Figure ) and the decrease in c-Li15Si4 is therefore likely caused by stressvoltage coupling. The capacity decay in the film was less than 5% over the same period, as seen in Figure . After 60 cycles, the 5 mV CCCV silicon nanoparticle cell had been reduced to 65% of its maximum capacity (Figure ), and the polarization increased (Figure ), at least partly due to SEI formation. At this point, the relative delithiation capacity below 0.35 V reaches its maximum, indicating that c-Li15Si4 is no longer formed.