Figure 3 depicts the voltage versus capacity curves we measured at different C-rates (1 C, 2 C, 6 C, and 10 C), where discharging curves are represented by solid lines and charging curves by dashed lines. The voltage gap (between charging and discharging) increases with increasing C-rate: for example, at a capacity = 0.015 (A h/g), the voltage gaps are approximately 0.18 (V), 0.25 (V), 0.45 (V), and 0.68 (V) for 1 C, 2 C, 6 C, and 10 C, respectively (Fig. 3). The trend observed in these data is consistent with observations of Dreyer et al. . Further, due to a characteristic of the two-phase system for LiFePO4 materials, essentially flat voltage curves are observed for all C-rates. However, by comparing the discharging curves on a millivolt scale, we observe that the slopes of the voltage curves in this section are not identical, as shown in the inset of Fig. 3. An increasing downward slope of the curves is observed with higher C-rate, as well as what appears to be a subtle low frequency voltage fluctuation (approx. 0.02 Hz) for the 10 C sample, potentially offering additional evidence supporting previous observations that an extra overpotential is required to overcome the energy barrier during the phase transformation of LiFePO4 [28-30].