All-solid-state lithium batteries (ASSLBs) are promising for next-generation energy storage owing to their enhanced safety and potential for high energy density. However, their practical deployment is constrained by challenges such as immature manufacturing processes and the high cost of key materials. Fe-based halides (FBHs) have recently emerged as compelling candidates for addressing these issues, benefiting from their low cost, structural tunability, and inherent multifunctionality. This review highlights the unique dual function of FBHs in ASSLBs, wherein they can serve simultaneously or separately as solid-state electrolytes with acceptable ionic conductivity and as high-capacity cathodes featuring reversible redox chemistry. We first discuss the structural characteristics and cost advantages of FBHs. Subsequently, we examine their applications as solid electrolytes and as cathodes in full-cell configurations, with emphasis on their interfacial compatibility and electrochemical performance. Finally, we outline the remaining challenges and perspectives on the practical implementation of FBHs in ASSLBs. This review is expected to enable systematic understanding of the dual-function potential of FBHs and offers significant guidance for the development of low-cost, high-energy-density ASSLBs.

Lithium metal batteries are the most promising next-generation energy storage technologies due to their high energy density. However, their practical application is impeded by serious interfacial side reactions and uncontrolled dendrite growth of lithium metal anode. Herein, copper 2,4,5-trifluorophenylacetate is designed and explored to stabilize lithium metal anode by in-situ constructing a dense and mixed-conductive interfacial protective layer. The formed passivated layer not only significantly inhibits interfacial side reactions by avoiding direct contact between lithium metal anode and electrolyte but also effectively suppresses lithium dendrite growth due to the unique inorganic-rich compositions and mixed-conductive properties. As a result, the copper 2,4,5-trifluorophenylacetate-treated lithium metal anodes show greatly improved cycle stability under both high current density and high areal deposition capacity. Notably, the assembled liquid symmetrical cells with copper 2,4,5-trifluorophenylacetate-treated lithium metal anodes can stably work for more than 3000, 5000, and 4800 h at 1.0 mA cm⁻²–1.0 mAh cm⁻², 2.0 mA cm⁻²–5.0 mAh cm⁻², and 10 mA cm⁻²–5.0 mAh cm⁻², respectively. Furthermore, the assembled liquid full cell with a high LiFePO₄ loading (~16.9 mg cm⁻²) shows a significantly enhanced cycle life of 250 cycles with stable Coulombic efficiencies (>99.1%). Moreover, the assembled all-solid-state lithium metal battery with a high LiNi_0.6Co_0.2Mn_0.2O₂ loading (~5.0 mg cm⁻²) also exhibits improved cycle stability. These findings underline that the copper 2,4,5-trifluorophenylacetate-treated lithium metal anodes show great promise for high-performance lithium metal batteries.