Solid-State Lithium Batteries with In Situ Polymerized Electrolytes: Interphase Chemistry, Solvation Structure, and Practical Cell Design

Solid-state lithium metal batteries hold immense promise for next-generation energy storage devices because they can potentially combine the high energy density of lithium metal anodes (LMAs) and high-voltage cathodes with the improved safety of nonleaking solid electrolytes. In practice, these merits are difficult to achieve simultaneously. High energy density requires high-voltage cathode operation, high cathode loading, thin lithium, lean electrolyte, and improved practical cell formats, whereas high safety requires suppressed dendrite growth, stable electrode/electrolyte interphases, reduced volatility and flammability, and resistance to cell-level abuse. In situ polymerization has emerged as a powerful strategy to address these coupled demands by converting liquid precursors into polymer networks directly inside the cell. Early systems, largely based on homopolymerized or weakly cross-linked polymers, preserved electrode wetting but offered limited control over interphase formation, Li+ desolvation, solvation-shell composition, and high-voltage stability, thereby restricting their applications in high-energy-density and high-safety batteries. In this Account, we summarize our recent endeavors in evolving in situ polymerization from a simple liquid-to-solid processing step into a highly tunable platform for interphase chemistry, solvation-structure regulation, and practical cell design. We first show how in situ gelation and interfacial additives improve electrode wetting and stabilize the solid electrolyte interphase (SEI) on lithium metal. We then describe how three-dimensional cross-linked networks regulate solvent uptake, Li+ desolvation, and Li deposition, thereby transforming polymer topology from a mechanical descriptor into a transport and interphase parameter. These advances provide the interfacial foundation required for long-cycling lithium metal batteries. We next discuss how high-voltage stability can be encoded through progressively deeper chemical design. Fluorinated monomers, fluorine/phosphorus-containing multifunctional cross-linkers, and main-chain fluorinated polymer backbones successively improve oxidative stability while preserving lithium compatibility. These designs promote anion-involved solvation and inorganic-rich SEI/CEI formation, enabling lithium metal batteries with practical operation at 4.5-4.9 V, long cycling with Ni-rich and high-voltage cathodes, ultrahigh-loading electrodes up to 70 mg cm-2, and pouch-cell energy densities up to 405.3 Wh kg-1. We further show that polymer matrices can actively regulate local solvation structure. Localized high-concentration gel polymer electrolytes (GPEs) and silsesquioxane-based GPEs demonstrate that polymer skeletons can host and reshape anion-rich Li+ solvation sheaths, accelerate Li+ desolvation, and stabilize both cathode and anode interphases. This strategy enables 4.7 V solid-state 18650 lithium metal batteries, 4.6 V industrial cylindrical lithium-ion cells, and 6.08 Ah pouch cells delivering 511.2 Wh kg-1 under lean-electrolyte conditions. Equally important, these electrolytes translate chemical design into safety at multiple levels. Condensed polymer frameworks, nonleakage behavior, flame-retardant or nonflammable components, stable electrode-electrolyte interphase, and uniform Li deposition collectively improve intrinsic and interfacial safety. At the cell level, pouch and 18650 batteries withstand bending, cutting, punching, nail penetration, overcharge, and other abuse conditions. Taken together, these studies show that in situ polymerized electrolytes are not merely soft ion conductors, but programmable chemical environments in which network topology, solvation chemistry, interphase formation, and manufacturable cell architecture can be codesigned to realize high-energy and safe solid-state lithium batteries.