Superionic conducting vacancy-rich β-Li₃N electrolyte for stable cycling of all-solid-state lithium metal batteries

The advancement of all-solid-state lithium metal batteries requires breakthroughs in solid-state electrolytes (SSEs) for the suppression of lithium dendrite growth at high current densities and high capacities (>3 mAh cm^-2) and innovation of SSEs in terms of crystal structure, ionic conductivity and rigidness. Here we report a superionic conducting, highly lithium-compatible and air-stable vacancy-rich β-Li₃N SSE. This vacancy-rich β-Li₃N SSE shows a high ionic conductivity of 2.14 × 10^-3 S cm^-1 at 25 °C and surpasses almost all the reported nitride-based SSEs. A Li- and N-vacancy-mediated fast lithium-ion migration mechanism is unravelled regarding vacancy-triggered reduced activation energy and increased mobile lithium-ion population. All-solid-state lithium symmetric cells using vacancy-rich β-Li₃N achieve breakthroughs in high critical current densities up to 45 mA cm^-2 and high capacities up to 7.5 mAh cm^-2, and ultra-stable lithium stripping and plating processes over 2,000 cycles. The high lithium compatibility mechanism of vacancy-rich β-Li₃N is unveiled as intrinsic stability to lithium metal. In addition, β-Li₃N possesses excellent air stability through the formation of protection surfaces. All-solid-state lithium metal batteries using the vacancy-rich β-Li₃N as SSE interlayers and lithium cobalt oxide (LCO) and Ni-rich LiNi_0.83Co_0.11Mn_0.06O₂ (NCM83) cathodes exhibit excellent battery performance. Extremely stable cycling performance is demonstrated with high capacity retentions of 82.05% with 95.2 mAh g^-1 over 5,000 cycles at 1.0 C for LCO and 92.5% with 153.6 mAh g^-1 over 3,500 cycles at 1.0 C for NCM83. Utilizing the vacancy-rich β-Li₃N SSE and NCM83 cathodes, the all-solid-state lithium metal batteries successfully accomplished mild rapid charge and discharge rates up to 5.0 C, retaining 60.47% of the capacity. Notably, these batteries exhibited a high areal capacity, registering approximately 5.0 mAh cm^-2 for the compact pellet-type cells and around 2.2 mAh cm^-2 for the all-solid-state lithium metal pouch cells.