An All-Solid-State Battery with a Tailored Electrode–Electrolyte Interface Using Surface Chemistry and Interlayer-Based Approaches

Major challenges in the development of solid-state batteries using garnet-type solid-state electrolytes (SSEs) include suppressing dendrite growth, improving moisture stability, and reducing interfacial resistance. Prior attempts to remove surface impurities of SSEs through dry polishing caused high interfacial resistance that proves this method to be unviable. Further, several efforts on depositing thin-film protective layers on SSEs without understanding surface chemistry failed to demonstrate improved electrochemical performance. Here, we report the simultaneous removal of the surface impurities and protection of the SSE against air and moisture by regulating its surface chemistry. In situ X-ray photoelectron spectroscopy (XPS) studies revealed that primary surface contaminants such as lithium carbonate (Li2CO3) and lithium hydroxide on the SSE, Li6.5La3Zr1.5Ta0.5O12 (LLZT), could be removed by either argon-ion sputtering at 227 °C or annealing at 777 °C in ultrahigh vacuum conditions. To protect the cleaned LLZT surface from further ambient contamination, in situ atomic layer deposition was used to deposit ∼3 nm-thick h-BN using tris(dimethylamino)borane and ammonia precursors at 450 °C. Intermittent XPS analysis confirmed the absence of Li2CO3 formation and the stability of h-BN-coated LLZT pellets for over 2 months of exposure to atmospheric air and moisture. Electrochemical impedance spectroscopy studies revealed that an ultrathin layer of ∼3 nm h-BN drastically reduced the interfacial resistance from 1145 to 18 Ω cm2 (∼65× reduction). Li plating/stripping studies revealed a constant polarization of 27 mV at a 0.5 mA cm–2 current density over prolonged cycling and a high critical current density of 0.9 mA cm–2. An all-solid-state battery using a LiFePO4 cathode exhibited a stable capacity of 130 mAh g–1 for over 100 cycles and a negligible capacity fade-off of 0.11 mAh g–1 per cycle at an average Coulombic efficiency of 98.4%.