NASICON-type oxide Li 1+x Al x Ti 2−x (PO 4 ) 3 (LATP) is expected to be a promising solid electrolyte (SE) for all-solidstate batteries (ASSBs) owing to its high ion conductivity and chemical stability. However, its interface properties with electrodes on the atomic scale remain unclear, but it is crucial for rational control of the ASSBs performance. Herein, we focused on the LATP SE with x = 0.17 and investigated the electron and ion transfer behaviors at the interfaces with the Li metal negative electrode and the LiCoO 2 (LCO) positive electrode via explicit interface models and density functional theory calculations. Ti reduction was found at the LATP/Li interface. For the LATP/ LCO interface, the results indicated the Li-ion transfer from LCO to LATP upon contact until a certain electric double layer is formed under equilibrium, in which LCO is partially reduced. Co−Ti exchange was also found to be favorable where the Li ion moves with Co 3+ to LATP. We also explored the possible interfacial processes during annealing by simulating the oxygen removal effect and found that oxygen vacancy can be more easily formed in the LCO at the interface. It implies that partial Li ions move back to LCO for the local charge neutrality. We also demonstrated higher Li chemical potential around the LATP/LCO interfaces, leading to the dynamical Li-ion depletion upon charging. The calculation results and the deduced mechanisms well explain the experimental results so far and provide insights into the interfacial electron and ion transfer upon contact, during annealing, and charging.

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All-solid-state batteries (ASSBs) are promising candidates for application as next-generation high-power supply and storage devices in electric vehicles. ASSBs offer excellent safety and a high energy density; however, the high interfacial resistance between the positive electrode and solid electrolyte due to solid−solid contact reactions at elevated temperatures limits their applications. To address these issues, the effect of thermal annealing on the interfacial structure between a sodium super ionic conductor (NASICON)-type Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) solid electrolyte and a LiCoO 2 (LCO) cathode in an ASSB fabricated by aerosol deposition was investigated experimentally. Specifically, spectrum imaging was conducted by combining scanning transmission electron microscopy and electron energy loss spectroscopy. Metastable degraded low-density transition layers were formed between LATP and LCO in the as-deposited sample. A significant reduction in interfacial resistance was achieved after thermal annealing at 250−300 °C, which was mainly attributed to structural recovery in this temperature range. However, thermal annealing at 400 °C resulted in increased interfacial resistance due to the formation of a Co 3 O 4 -like spinel blocking layer at the LATP/LCO interface. These findings provided valuable insights into the electronic properties of the ASSB composite under investigation and were consistent with theoretical predictions of Li and O transfer between the layers due to thermal annealing.

Garnet-type Li 7 La 3 Zr 2 O 12 (LLZ) has advantages of stability with Li metal and high Li + ionic conductivity, achieving 1 × 10 −3 S cm −1 , but it is prone to react with electrode active materials during the sintering process. LISICON-type Li 3.5 Ge 0.5 V 0.5 O 4 (LGVO) has the advantage of less reactivity with the electrode active material during the sintering process, but its ionic conductivity is on the order of 10 −5 S cm −1 . In this study, these two solid electrolytes are combined as a multilayer solid electrolyte sheet, where 2 μm thick LGVO films are coated on LLZ sheets to utilize the advantages of these two solid electrolytes. These two solid electrolytes adhere well through Ge diffusion without significant interfacial resistance. The LLZ−LGVO multilayer is combined with a LiCoO 2 positive electrode and a lithium metal anode through annealing at 700 °C. The resultant all-solid-state battery can undergo repeated charge−discharge reactions for over 100 cycles at 25 or 60 °C. The LGVO coating suppresses the increases in the resistance from the solid electrolyte and interfacial resistance induced by annealing by ca. 1/40. As with sulfide-based all-solid-state batteries, function sharing of solid electrolytes will be a promising method for developing advanced oxide-based all-solid-state batteries through a sintering process.

Interfacial resistance at electrode‐high Li+ conductive solid electrolytes must be reduced well to develop high‐power all‐solid‐state batteries using oxide‐based solid electrolytes (Ox‐SSBs). Herein, crystalline electrode films of LiCoO2 (LCO) are formed on a high Li+ conductive crystalline‐glass solid electrolyte sheet, Li1.3Al0.3Ti2(PO4)3 (LATP) (σ25 °C = 1 × 10−4 S cm−1), at room temperature by aerosol deposition (AD), and the effects of the annealing temperature on the interfacial resistivities (Rint) at the LCO/LATP are investigated. The Rint visibly increases by annealing over 500 °C with the growth of Co3O4 as a reactant. In contrast, Rint is reduced to ≈100 Ω cm2 by low‐temperature annealing at 250–350 °C due to superior contact through the structural rearrangement of an artificial metastable interface formed by the AD. These results are applied to bulk‐type Ox‐SSB, Li/Li7La3Zr2O12(LLZ)/LCO–LATP, and our best Ox‐SSB delivers a discharge capacity of 100 mA cm−2 at 100 °C.