Evidence for Fast Lithium-Ion Diffusion and Charge-Transfer Reactions in Amorphous TiO_x Nanotubes: Insights for High-Rate Electrochemical Energy Storage

Abstract

The charge-storage kinetics of amorphous TiO x nanotube electrodes formed by anodizing three-dimensional porous Ti scaffolds are reported. The resultant electrodes demonstrated not only superior storage capacities and rate capability to anatase TiO x nanotube electrodes but also improved areal capacities (324 μAh cm–2 at 50 μA cm–2 and 182 μAh cm–2 at 5 mA cm–2) and cycling stability (over 2000 cycles) over previously reported TiO x nanotube electrodes using planar current collectors. Amorphous TiO x exhibits very different electrochemical storage behavior from its anatase counterpart as the majority of its storage capacity can be attributed to capacitive-like processes with more than 74 and 95% relative contributions being attained at 0.05 and 1 mV s–1, respectively. The kinetic analysis revealed that the insertion/extraction process of Li+ in amorphous TiO x is significantly faster than in anatase structure and controlled by both solid-state diffusion and interfacial charge-transfer kinetics. It is concluded that the large capacitive contribution in amorphous TiO x originates from its highly defective and loosely packed structure and lack of long-range ordering, which facilitate not only a significantly faster Li+ diffusion process (diffusion coefficients of 2 × 10–14 to 3 × 10–13 cm2 s–1) but also more facile interfacial charge-transfer kinetics than anatase TiO x .

Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

“…The results obtained here are strongly reminiscent of what was observed with the reversible Li + insertion in TiO2-based electrodes. 28,[43][44][45][46][47][48][49][50][51][52][53] In anatase, it was established that Li + insertion is accompanied by a reversible phase transition from a lithium-poor tetragonal phase to a lithium-rich orthorhombic Li0.5TiO2 phase. 28,47 This translates into a pair of well-defined faradaic peaks in CV, separated by a significant potential hysteresis under thermodynamic equilibrium (peak-to-peak potential separation Ep > 0), 43,46 as also reported here in Figure 3B.…”

High-rate lithium ion energy storage to facilitate increased penetration of photovoltaic systems in electricity grids

“…136 Coating can be achieved by thermal oxidation, 139,140 electrodeposition, 141 casting of a slurry, 136,142,143 or surface anodization. 144,145 With processes where the active material is directly grown on the metal scaffold, the need for binder can be eliminated. Typical fluoropolymer binders are insulating and electrochemically inactive; they decrease gravimetric energy density and increase the resistive losses and hence power losses, especially under high-rate operating conditions.…”

“…146,147 Anodization of metal foams in fluoride-containing electrolytes to form nanotube arrays on the foam surface is one approach that has been successfully used to fabricate hierarchically structured electrodes. 144,145 Figure 6 depicts the growth of TiO 2−x nanotubes directly on a Ti foam current collector. The foam allows increased Li ion access to the surface structures and the 20-50 nm thickness of the TiO 2−x nanotube walls improves electronic conductivity of the electrode.…”

“…The foam allows increased Li ion access to the surface structures and the 20-50 nm thickness of the TiO 2−x nanotube walls improves electronic conductivity of the electrode. 145 A key practical concern for all structuring methods is whether a process can be practically scaled to manufacturing and whether the improvements in rate or cycle life that can be achieved result in a commercial advantage. Nanostructuring generally reduces tap density and so it is necessary to consider volumetric as well as gravimetric energy and power densities.…”

“…For many years, it has been assumed that Li ion diffusion limits charging/discharging rates 168 ; however, recent reports are now suggesting that the charge transfer kinetics may also become limiting in cases where Li ion diffusion may be rapid. 145,169 This suggests that more advanced simulation models may be required to predict ionic and electronic flow though percolating porous networks and rates of charge transfer reactions. 169,170 Integration into systems A possibly advantageous attribute of high-rate LIB electrodes is that the linearly responsive capacitive-shaped (potential versus charge) discharge curves typically observed with fast electrode reactions can enable energy storage control systems to more reliably determine the state-of-charge (SoC) of the battery.…”