A 5-µm-thick electrolyte with a high-entropy interphase for practical solid-state lithium metal batteries

Solid-state lithium metal batteries with ultrathin and lightweight electrolytes are promising for high-energy-density storage but are prone to dendrite penetration under practical current densities. This limitation arises primarily from the uncontrolled and chemically heterogeneous solid-electrolyte interphase (SEI), inducing obvious interfacial instability and leading to uneven and sluggish Li⁺ flux at the interface. Here, we design a homogeneous interphase with high configurational entropy, with its optimized composition screened by machine learning, providing dual functions by stabilizing the SEI structure and promoting efficient Li⁺ transport. The high-entropy nature of this interphase is experimentally supported and promotes uniform and fast interfacial Li⁺ flux, enabling a 5-µm-thick electrolyte to sustain a critical current density of 5.2 mA cm^-2 and maintain stable long-term lithium plating/stripping at 3 mA cm^-2 for 850 h. Li||LiNi_0.8Co_0.1Mn_0.1O₂ pouch cells with this ultrathin and lightweight electrolyte deliver an areal capacity of 5.4 mAh cm^-2 and an energy density of 573 Wh kg^-1, showing great potential for practical solid-state lithium metal batteries.