High-Performance Quasi-Solid-State Lithium Metal Batteries: Interface Engineering Using Solid–Liquid Dual Therapy by Solvated Ionic Liquid and NiO

Developing efficient solid-state lithium metal batteries (SSLMBs) remains challenging due to interfacial incompatibility and dendrite growth at the metal-electrolyte boundary. To address these issues, a dual-therapy approach was introduced, combining solid therapy of LLZO via a NiO sintering aid and liquid therapy using a solvated ionic liquid (SIL) for SSLMBs. The solid therapy treatment enhanced the grain boundary conductivity by ∼7 times and improved densification by forming a new K₂NiF₄-type lithiated phase upon introducing 0.5 wt % of NiO into the Ga-doped LLZO matrix (GN050). Meanwhile, SIL infusion in GN050 (GN050-SIL) achieves high ionic conductivity at RT (0.252 mS·cm^-1), low Li/LLZO interfacial resistance (87.4 Ω), and broader potential window (>5.5 V). This combined approach also minimized the SIL dependency for ionic conduction- a critical step toward scalable, almost solid-state batteries. GN050-SIL shows stable plating/stripping behavior for over 1000 h without any short circuit, with a critical current density (CCD) of 0.55 mA·cm^-2. In the full-cell test with a Li-metal anode and LiMn₂O₄ cathodes, GN050-SIL exhibited superior charge/discharge performance at different current densities (0.1-1.4 mA·cm^-2) compared to SIL-infused Ga-doped LLZO electrolyte (GN000-SIL). The Li/GN050-SIL/NMC111 full cell also demonstrates excellent rate capability and long-term cycling stability at higher C-rates. XPS, FESEM/EDX, and distribution of relaxation time (DRT) analyses reveal a uniformly confined SIL within the LLZO framework as well as the formation of a stable LiF/CF_<i>x</i> interphase without any structural degradation after cycling. This study thus highlights the potential of dual-therapy engineering for next-generation SSLMBs.