Inorganic‐Rich Interphase Induced by Boric Oxide Solid Acid toward Long Cyclic Solid‐State Lithium‐Metal Batteries

Abstract Solid‐state electrolytes paired with lithium‐metal anodes is considered a next‐generation energy storage technology. However, the slow ionic transportation of the solid‐state electrolyte and the instability against the lithium‐metal anode impede their practical application. Here a cellulose separator modified with highly uniform boric oxide solid acid, contributing to a high transference number (0.75) and good ionic conductivity of 0.52 mS cm −1 due to the strengthened binding of the salt anions with this solid acid, is reported. Moreover, the boron ions with occupied interstitial sites can release free electrons to regulate the electrochemical dynamics of the electrolyte, in situ inducing the formation of Li 2 CO 3 /LiF‐rich heterostructured solid electrolyte interphase layer. The cellulose/B 2 O 3 ‐based composite electrolyte paired with LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) cathode and Li‐metal anode displays a specific capacity of 155 mAh g −1 with a capacity retention of 92% in 200 cycles. Additionally, this electrolyte paired with high‐mass‐loading NCM622 cathode (10 mg cm −2 ) in a pouch cell can be stably operated for 50 cycles with a capacity retention of over 90%.