Nanoscale Mixed Ion-Electron-Conducting NASICON-Type Thin Films: Lithium Titanium Phosphate via Atomic Layer Deposition

Advancements in ionic devices for energy applications (e.g., solid-state microbatteries, ionic capacitors, ion-tunable transistors, etc.) require significant development of compatible materials and fabrication processes to enable high-performance conduction and storage of ions. Atomic layer deposition (ALD) enables fabrication of solid-state devices with high energy and power densities due to the complex structures made possible by its angstrom-scale thickness control and high conformality. The ionic conductivity of thin film materials fabricated by ALD has been limited by material development and crystallinity control challenges, as suitable materials must be fabricated with both the appropriate composition and crystal structure. A mixed metal phosphate like LiTi₂(PO₄)₃ (LTP) is a prime candidate to push the boundary of ALD ionic materials due to the fast Li⁺-conducting NASICON-type crystalline phase. We developed a mixed ion-electron conducting NASICON-type thin film ALD process for LiTi₂(PO₄)₃ suitable for microbattery and pseudocapacitor applications. Compositional tunability was achieved by alternating between constituent lithium oxide (Li₂O) and titanium phosphate (TiPO) subprocesses. By adjusting the ratio between Li₂O and TiPO cycles, the Li content in LTP can be tuned between 8 and 34 atomic % Li. A NASICON-type crystalline structure is observed after postdeposition annealing of the LTP films between 650 and 850 °C. The semicrystalline LTP thin film has an ionic conductivity of 9.3 × 10^–7 S/cm at RT and 1.7 × 10^–5 S/cm at 80 °C and an electronic conductivity of 2.5 × 10^–7 S/cm at RT. In this work, we discuss the complexities of how tuning the composition of LTP influences film properties such as structure and conductivity in the mixed metal phosphate phase space.