Jeremy Come

High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance

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.

Electrochemical Kinetic Study of LiFePO4 Using Cavity Microelectrode

Lithium cation insertion and extraction in LiFePO4 were electrochemically studied with a cavity microelectrode (CME). Cyclic voltammetry measurements were used to characterize the kinetics of the material. LiFePO4 was successfully cycled from 0.1 mV s–1 up to 1 V s–1 and is therefore a suitable material to be used in high power applications, such as asymmetric hybrid supercapacitors. Several kinetic behaviors were observed depending on the sweep rate. The LiFePO4 was found to follow different kinetics behaviors depending of the sweep rate. The charge storage mechanisms were investigated for Liþ extraction/insertion.

Role of Electrical Double Layer Structure in Ionic Liquid Gated Devices

Ionic liquid gating of transition metal oxides has enabled new states (magnetic, electronic, metal-insulator), providing fundamental insights into the physics of strongly correlated oxides. However, despite much research activity, little is known about the correlation of the structure of the liquids in contact with the transition metal oxide surface, its evolution with the applied electric potential, and its correlation with the measured electronic properties of the oxide. Here, we investigate the structure of an ionic liquid at a semiconducting oxide interface during the operation of a thin film transistor where the electrical double layer gates the device using experiment and theory. We show that the transition between the ON and OFF states of the amorphous indium gallium zinc oxide transistor is accompanied by a densification and preferential spatial orientation of counterions at the oxide channel surface. This process occurs in three distinct steps, corresponding to ion orientations, and consequently, regimes of different electrical conductivity. The reason for this can be found in the surface charge densities on the oxide surface when different ion arrangements are present. Overall, the field-effect gating process is elucidated in terms of the interfacial ionic liquid structure, and this provides unprecedented insight into the working of a liquid gated transistor linking the nanoscopic structure to the functional properties. This knowledge will enable both new ionic liquid design as well as advanced device concepts.