Lithium‐Ion and Sodium‐Ion Hybrid Capacitors: From Insertion‐Type Materials Design to Devices Construction

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with RuO2·xH2O in an acidic electrolyte. However, we recently demonstrated that a pseudocapacitive mechanism occurs when lithium ions are inserted into mesoporous and nanocrystal films of orthorhombic Nb2O5 (T-Nb2O5; refs 1,2). Here, we quantify the kinetics of charge storage in T-Nb2O5: currents that vary inversely with time, charge-storage capacity that is mostly independent of rate, and redox peaks that exhibit small voltage offsets even at high rates. We also define the structural characteristics necessary for this process, termed intercalation pseudocapacitance, which are a crystalline network that offers two-dimensional transport pathways and little structural change on intercalation. The principal benefit realized from intercalation pseudocapacitance is that high levels of charge storage are achieved within short periods of time because there are no limitations from solid-state diffusion. Thick electrodes (up to 40 μm thick) prepared with T-Nb2O5 offer the promise of exploiting intercalation pseudocapacitance to obtain high-rate charge-storage devices. Show more

Capacitive energy storage is distinguished from other types of electrochemical energy storage by short charging times and the ability to deliver significantly more power than batteries. A key limitation to this technology is its low energy density and for this reason there is considerable interest in exploring pseudocapacitive materials where faradaic mechanisms offer increased levels of energy storage. Here we show that the capacitive charge-storage properties of mesoporous films of iso-oriented alpha-MoO(3) are superior to those of either mesoporous amorphous material or non-porous crystalline MoO(3). Whereas both crystalline and amorphous mesoporous materials show redox pseudocapacitance, the iso-oriented layered crystalline domains enable lithium ions to be inserted into the van der Waals gaps of the alpha-MoO(3). We propose that this extra contribution arises from an intercalation pseudocapacitance, which occurs on the same timescale as redox pseudocapacitance. The result is increased charge-storage capacity without compromising charge/discharge kinetics in mesoporous crystalline MoO(3). Show more