Nanometric Amorphous TiO₂ As an Extrinsic Pseudo-Capacitive Electrode for All-Solid-State Asymmetric Hybrid Micro-Supercapacitors: Effects on the Electrochemical Performance

On-chip solid-state micro-supercapacitors (MSCs) have gained widespread attention owing to their versatile benefits, including high power density and long cycle life1,2. Recently asymmetric hybrid microsupercapacitors (AsHMSCs) introducing a charge storage mechanism based on both faradaic reaction and electric double-layer phenomena have been considered to increase the energy density. Within this context, the selection of electrode materials and their interaction at the interface with the electrolyte is a key issue to optimize the AsHMSC electrochemical performance. In this work, nanometric (5-20 nm thick) amorphous TiO2 films have been elaborated and characterized in liquid and solid-state electrolyte (LiPON) half-cell configurations. Interestingly, TiO2 films after LiPON deposition exhibited a constant initial amount of intercalated lithium ions for all considered thicknesses and did not require a first activation process, in comparison to the liquid electrolyte configuration. For all considered configurations, the specific capacity extracted from cyclic voltammetry and galvanostatic cycling within [0.5-3V] potential range correspond to the theoretical value expected for the Lix=1TiO2 phase at low current density. Furthermore, the cooperative effects of high Li ion intercalation kinetics and Low interfacial charge transfer resistance for 5nm TiO2 electrode (extracted from electrochemical impedance spectroscopy analysis) exhibit an outstanding specific capacity of 2mC.cm-2 at 1µA.cm-2, and high rate performance with 60% capacity holding ratio at 100µA.cm-2, thus highlighting the extrinsic pseudo-capacitive behavior of the sub 10nm electrodes. A TiO2 5nm/LiPON 100nm/Pt AsHMSC is successfully fabricated, which achieves an operating voltage window of 3V and a specific capacity of 0.3mC.cm-2 at 1mA.cm-2.In addition, the device also exhibited a 100% coulombic efficiency even after cycling for 6000 continuous charge-discharge cycles. This work offers an approach to tune the Li ion storage properties of TiO2 by nano-engineering and gives insights into the enhancement of pseudocapacitance-assisted lithium-storage capacity. References Kyeremateng et al., Nature Nanotech 12, 7–15 (2017). https://doi.org/10.1038/nnano.2016.196 Sallaz et al., J. Power Sources 451, 227786 (2020), doi: 10.1016/j.jpowsour.2020.227786. Figure 1