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A strong correlation between host tortuosity and cycling reversibility of a hosted Li-metal anode is revealed for the first time. High tortuosity leads to preferential top-surface Li deposition based on locally enhanced current density and concentration gradient. This top-surface accumulated Li blocks inward ion transport and invalidates the internal electrode, further aggravating the uneven current distribution and non-uniform plating and stripping. Decreased electrode tortuosity can significantly improve the anodic Coulombic efficiency, uniformity of Li-metal stripping and plating, and cycling stability of the rGO host.

Safety issues in lithium-ion batteries have raised serious concerns due to their ubiquitous utilization and close contact with the human body. Replacing flammable liquid electrolytes, solid-state electrolytes (SSEs) is thought to address this issue as well as provide unmatched energy densities in Li-based batteries. However, among the most intensively studied SSEs, polymeric solid electrolyte and polymer/ceramic composites are usually flammable, leaving the safety issue unattended. Here, we report the first design of a fireproof, ultralightweight polymer–polymer SSE. The SSE is composed of a porous mechanic enforcer (polyimide, PI), a fire-retardant additive (decabromodiphenyl ethane, DBDPE), and a ionic conductive polymer electrolyte (poly­(ethylene oxide)/lithium bis­(trifluoromethanesulfonyl)­imide). The whole SSE is made from organic materials, with a thin, tunable thickness (10–25 μm), which endorse the energy density comparable to conventional separator/liquid electrolytes. The PI/DBDPE film is thermally stable, nonflammable, and mechanically strong, preventing Li–Li symmetrical cells from short-circuiting after more than 300 h of cycling. LiFePO4/Li half cells with our SSE show a high rate performance (131 mAh g–1 at 1 C) as well as cycling performance (300 cycles at C/2 rate) at 60 °C. Most intriguingly, pouch cells made with our polymer–polymer SSE still functioned well even under flame abuse tests.

Despite considerable efforts to prevent lithium (Li) dendrite growth, stable cycling of Li metal anodes with various structures remains extremely difficult due to the direct contact of the liquid electrolyte with Li. Rational design of solid‐electrolyte interphase (SEI) for 3D electrodes is a promising but still challenging strategy for preventing Li dendrite growth and avoiding lithium–electrolyte side reactions in Li‐metal batteries. Here, a 3D architecture is constructed with g‐C3N4/graphene/g‐C3N4 insulator–metal–insulator sandwiched nanosheets to guide uniform Li plating/stripping in the van der Waals gap between the graphene and the g‐C3N4, and the function of which can be regarded as a 3D artificial SEI. Li deposition on the surface of g‐C3N4 is suppressed due to its insulating nature. However, its uniform lithiophilic sites and nanopore channels enable homogeneous lithium plating between the graphene and the g‐C3N4, prohibiting the direct contact of the electrolyte with the Li metal. The use of the g‐C3N4‐layer‐modified 3D anode enables long‐term Li deposition with a high Coulombic efficiency and stable cycling of full cells under high cathode loading, limited Li excess, and lean electrolyte conditions. The concept of a 3D artificial SEI will shed light on developing safe and stable Li‐metal anodes.

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