Ensemble Design of Electrode–Electrolyte Interfaces: Toward High-Performance Thin-Film All-Solid-State Li–Metal Batteries

In accordance with the fourth industrial revolution (4IR), thin-film all-solid-state batteries (TF-ASSBs) are being revived as the most promising energy source to power small electronic devices. However, current TF-ASSBs still suffer from the perpetual necessity of high-performance battery components. While every component, a series of a TF solid electrolyte (i.e., lithium phosphorus oxynitride (LiPON)) and electrodes (cathode and Li metal anode), has been considered vital, the lack of understanding of and ability to ameliorate the cathode (or anode)-electrolyte interface (CEI) (or AEI) has impeded the development of TF-ASSBs. In this work, we suggest an ensemble design of TF-ASSBs using LiPON (500 nm), an amorphous TF-V₂O_5-x cathode with oxygen vacancies (O_vacancy), a thin evaporated Li anode (evp-Li) with a thickness of 1 μm, and an artificial ultrathin Al₂O₃ layer between evp-Li and LiPON. Well-defined O_vacancy sites, such as O(II)_vacancy and O(III)_vacancy, in amorphous TF-V₂O_5-x not only allow isotropic Li⁺ diffusion at the CEI but also enhance both the ionic and electronic conductivities. For the AEI, we employed protective Al₂O₃, which was specially sputtered using the facing target sputtering (FTS) method to form a homogeneous layer without damage from plasma. In regard to the contact with evp-Li, interfacial stability, electrochemical impedance, and battery performance, the nanometric Al₂O₃ layers (1 nm) were optimized at different temperatures (40, 60, and 80 °C). The TF-ASSB cell containing Al₂O₃ (1 nm) delivers a high specific capacity of 474.01 mAh cm^-3 under 60 °C at 2 C for the 400th cycle, and it achieves a long lifespan as well as ultrafast rate capability levels, even at 100 C; these results were comparable to those of TF Li-ion battery cells using a liquid electrolyte. We demonstrated the reaction mechanism at the AEI utilizing time-of-flight secondary ion mass spectrometry (TOF-SIMS) and molecular dynamics (MD) simulations for a better understanding. Our design provides a signpost for future research on the rational structure of TF-LIBs.