Electrochemical Phase Engineering of Li-Ti-P-O Nanocomposite Thin Films

Abstract

Nanoscale materials processing advancements have enabled on-chip ionic devices like microbatteries, super capacitors, ion-gated transistors, etc. using standard semiconductor processes (such as atomic layer deposition - ALD) to develop electrochemically active thin films. However, challenges remain in understanding and controlling ionic and electronic transport in nanoscale systems. Nanoscale ionic systems that are tunable at multiple scales (chemical composition, phase distribution, and crystal structure) are critical in understanding how these parameters affect transport and materials properties. In this work we study a system with these qualities and introduce a lever - electrochemical phase engineering - to remove an electrochemically active crystalline phase (LiTi2(PO4)3 - LTP) that allows us to understand the effects of individual phases on ionic transport in a quaternary ALD nanocomposite. We use the Li-Ti-P-O nanocomposite developed by ALD, which consists of crystalline LTP, anatase TiO2 and an amorphous LTP matrix. Each phase contributes to Li+ ion storage at different redox potentials, and the crystalline LTP phases are structurally unstable below 0.5 V vs Li+/Li, allowing the electrochemical removal of the crystalline LTP phase to study its effects on ionic and electronic transport in the nanocomposite system. We demonstrate the Li-Ti-P-O materials system to be able to switch from high power (60 % capacity retention at an ultrafast charging rate of 200 C) to high capacity (1302 mAh/g at 1 C), making this an interesting material for thin film ionic devices, especially in applications where phase-selectivity provides an advantage.