π-Backbonding Interfaces Stabilize Deep Lithium Deposition for High-Performance Anode-Free Solid-State Batteries

Aggressive chemistry of solid-state batteries (SSBs) involving anode-free cell design is a promising strategy to address energy density and safety limitations of conventional lithium-ion batteries, but their practical application is stifled by unsatisfactory Li heterogeneous nucleation on Cu current collectors with high energy barriers, dendrite growth, and side reactions, leading to continuous Li consumption, low Coulombic efficiency, and poor cycling stability. The lack of understanding on nucleation behavior and microenvironment property further impedes the rational design of reliable interfaces for anode-free solid-state lithium batteries (AFSSLBs). Herein, we introduce polynitrile complexes as plasticizers that coordinate with both solvated Li⁺ and Ag-C current collectors in 1,3-dioxolane (DOL)-based polymer electrolytes (PEs). Through data-driven screening, we identify the cyanide group bond energy (E_b) and π* orbital level as key descriptors governing interfacial Li nucleation. 1,3,6-hexanetrinitrile (HTCN) with balanced E_b-π* level, enables uniform Li⁺ flux and self-excited σ-π* backbonding, strengthening interfacial uniformity and Li-Ag alloying depth. HTCN-assisted AFSSLBs thus achieve >99% Coulombic efficiency over 600 cycles and 82.5% capacity retention after 300 cycles at 1C. A 4Ah NCM811-based pouch cell delivers 451.5 Wh kg^-1 energy density. This study provides theoretical insights and practical guidelines for designing reliable solid-state interface toward next-generation AFSSLBs.