Hybrid All-Solid-State Thin-Film Micro-supercapacitor Based on a Pseudocapacitive Amorphous TiO₂ Electrode

In this work, nanometric (6–21 nm thick) amorphous TiO₂ films have been elaborated and characterized in liquid- and solid-state electrolyte (LiPON) half-cell architectures. For all considered configurations, the volumetric capacity extracted from cyclic voltammetry and galvanostatic cycling within the 0.5–3 V potential range almost corresponds to the theoretical value expected for the Li_xTiO₂ (x ∼ 1) phase at low current density. Interestingly, TiO₂ films after LiPON deposition exhibited a thickness-independent constant initial amount of intercalated lithium ions and did not require a first activation process, in comparison to the liquid electrolyte configuration. Furthermore, the cooperative effects of high Li⁺ intercalation kinetics and low interfacial charge transfer resistance for a 6 nm TiO₂ electrode led to an outstanding surface capacity of 0.7 μAh cm^–2 at 1 μA cm^–2 and high rate performance with 60% capacity holding ratio at 1 mA cm^–2, thus highlighting the extrinsic pseudocapacitive behavior of our sub-10 nm TiO₂ electrodes. A Li_xTiO₂ 6 nm/LiPON 100 nm/Pt hybrid micro-supercapacitor has been successfully fabricated, achieving an operating voltage window of 3 V and a surface capacitance of 94 μF cm^–2 at 50 mV s^–1. In addition, the device also exhibited 97% coulombic efficiency upon cycling for 10,000 continuous charge–discharge cycles. This work proposes an approach that allows us to adjust the Li-ion storage properties of TiO₂ by nanoengineering and gives insights into the electrochemical performance enhancement by taking advantage of the pseudocapacitance-assisted lithium storage mechanism.