Ba, Nb Co‐Doped <scp>LLZO</scp> Reinforced <scp>PEO</scp> Composite Solid Electrolyte for High‐Performance Solid‐State Lithium Batteries

ABSTRACT Owing to its high lithium‐ion conductivity, the cubic garnet‐phase material Li 7 La 3 Zr 2 O 12 (LLZO) is a widely adopted key component in the fabrication of composite solid‐state electrolytes (CSSEs). Nevertheless, the advancement of all‐solid‐state lithium batteries (ASSLBs) is still hindered by two major challenges: the high intrinsic crystallinity of poly(ethylene oxide) (PEO) and poor interfacial stability. Herein, cubic garnet Li 6.3 La 2.7 Ba 0.3 ZrNbO 12 (LLBZNO) featuring Ba and Nb co‐doping was first prepared by a conventional solid‐state method. Following this, the material was dispersed within a PEO matrix to construct composite solid‐state electrolytes. Adding LLBZNO served to decrease PEO's crystallinity while enhancing its mechanical flexibility and bolstering its interfacial compatibility with lithium metal. The optimized membrane demonstrated superior properties, including a high Li + transference number of 0.75 (at 60°C) and an extended electrochemical stability window of up to 5.1 V (vs. Li/Li + ) under the present testing conditions. It also showed an excellent ability to suppress dendrites, facilitating stable cycling at 0.1 mA cm −2 for 1500 h without short‐circuiting. Furthermore, the LFP/LLBZNO–PEO/Li battery delivered a high discharge capacity of 101.6 mA h g −1 after 300 cycles at 1°C with nearly 99% coulombic efficiency. These findings highlight that LLBZNO‐PEO composite solid electrolytes possess superior ionic transport, electrochemical stability, and mechanical robustness, providing a viable pathway toward the advancement of next‐generation high‐performance ASSLBs.