As described above, a typical hybrid EC consists of two electrodes that use different charge storage mechanisms: a porous-carbon double-layer electrode and a pseudocapacitive electrode (Fig. 3a,c)2129303132. In 2001 Amatucci et al.29 demonstrated that these hybrid ECs can deliver a higher power density than Li+ batteries and a higher energy density than a conventional EDLC. Commercially available Li-ion hybrid ECs have gravimetric energy densities of about 20–30 Wh kg−1, about 3–5 times higher than that of conventional EDLCs using activated carbon (AC) at both electrodes. The high energy density is due to the added redox capacitance and a wider voltage window21. The Li4Ti5O12/AC hybrid EC (Fig. 3a,c), has demonstrated remarkable energy and power densities with extended cycling at extremely high (1,200 C) rates21. Another benchmark example is a commercialized hybrid device technology that represents a significant step towards environmentally friendly energy storage devices. The ‘Aquion battery' is, in fact, a spinel-MnO2/AC hybrid EC that uses sodium sulfate solution as electrolyte. This technology provides excellent cycling performance (no degradation after 10,000+ cycles) and ∼20 Wh kg−1 specific cell capacity due to the use of pseudocapacitive λ-MnO2 and operation in the 1.6 V window in the aqueous electrolyte3233. Hybrids also have successfully been applied to the next generation of lead-acid batteries. By replacing one of the lead plates with an activated carbon electrode, power density was improved by a factor of two and cycle life was extended by more than a factor of three34.