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Composite polymer electrolytes (CPEs) for solid-state Li metal batteries (SSLBs) still suffer from gradually increased interface resistance and unconstrained Li dendrite growth. Herein, we addressed the challenges by designing a LiF-rich inorganic solid-electrolyte interphase (SEI) through introducing a fluoride-salt concentrated interlayer on CPE film. The rigid and flexible CPE helps accommodate the volume change of electrodes, while the polymeric high-concentrated electrolyte (PHCE) surface-layer regulates Li-ion flux due to the formation of a stable LiF-rich SEI via anion reduction. The designed CPE-PHCE presents enhanced ionic conductivity and high oxidation stability of > 5.0V (vs. Li/Li+). What’s more, it dramatically reduces the interfacial resistance and achieves a high critical current density of 4.5 mA cm-2 for dendrite-free cycling. The SSLBs, fabricated with thin CPE-PHCE membrane (< 100 μm) and Co-free LiNiO2 cathode, exhibit exceptional electrochemical performance and long cycling stability. This approach of SEI design can also be applied to other types of batteries. 

Significance Historically, battery self-heating has been viewed negatively as an undesirable attribute. However, we report that battery self-heat, if properly controlled, can smoothen dendritic features in potassium metal batteries. This could open the door to high gravimetric and volumetric energy density potassium-ion batteries that could offer a sustainable and low-cost alternative to the incumbent lithium-ion technology.