Fabrication process validation and investigations of lithium-ionic conductors for all-solid Li-ion batteries
The rapid adoption of Li-ion batteries with liquid organic electrolytes created many safety issues, due to gas production and leakage of the flammable liquid organic electrolytes when operating at high voltages of ~6V and/or elevated temperatures of ~150°C. A possible solution to this problem is to use solid state electrolytes instead of liquid electrolytes. It has been demonstrated that some solid electrolytes can perform as well as their liquid electrolyte counterparts during battery operation. One such promising solid-state electrolyte is Lithium Aluminum Titanium Phosphate (LATP). Thick LATP films (several micrometers) have shown ionic conductivity of ~3×10-3 S cm-1, which is similar to that of a typical liquid electrolyte's conductivity. It has excellent long-term stability in contact with the lithium anode and has been evaluated as a solid electrolyte for Li-ion batteries, as well as for electrochromics and deep neural networks. This study is therefore aimed at in-depth exploration of the LATP electrolyte for both the lithium ion battery and neuromorphic devices applications. Our work is focused on studying the influence of sputtering deposition parameters on the composition and the ionic conductivity of LATP that is not well understood. A systematic study to optimize sputtering target power, substrate heating, sputtering vacuum pressure, annealing temperature, atmospheric composition during annealing, and sputtering atmospheric composition was performed. Compositional uniformity of LATP films were analyzed via dynamic secondary ion mass spectroscopy (D-SIMS), nuclear reaction analysis (NRA), and Rutherford backscattered electron spectroscopy (RBS). Results from the aforementioned techniques have shown that deposition of compositionally uniform LATP films can be achieved by co-sputtering of Ti, Al and Li3PO4 on a Si or Si/SiO2 substrate. However, annealing of these films at > 400°C is required to enhance their performance. Microscale batteries (~ 100 μm × 100 μm) created with the annealed LATP films show promising electrolyte behavior. Charging of the batteries with a constant current of 200pA to 4.2V displayed charging and discharging characteristics of a typical battery with no measurable leakage.