<p>Solid polymer electrolytes hold significant promise for high-energy-density and safety batteries, particularly given their compatibility with scalable manufacturing. However, their practical development is currently impeded by limitations including insufficient mechanical robustness, excessive electrolyte thickness, and interfacial instability. Herein, an ultrathin (18 µm), all-in one, free-standing single-ion conducting polymer electrolyte (PBFG) is designed using a dual Lewis-acid anion-binding strategy and a new designed fluorinated tetraglyme plasticizer. Incorporating boron nitride and borate ester units, the dual Lewis-acid polymer framework effectively immobilizes anions to enhance lithium salt dissociation, resulting in a high Li⁺ transference number (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(t_{\rm Li^{+}} = 0.86\)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <msub> <mi>t</mi> <mrow> <mi mathvariant="normal">L</mi> <msup> <mi mathvariant="normal">i</mi> <mrow> <mo>+</mo> </mrow> </msup> </mrow> </msub> <mo>=</mo> <mn>0.86</mn> </math></EquationSource> </InlineEquation>). Concurrently, the fluorinated tetraglyme plasticizer promotes the formation of a stable, LiF-rich solid electrolyte interphase. This synergistic design imparts the PBFG with a high ionic conductivity of 1.0 × 10⁻³ S cm⁻¹ at 25 °C. Impressively, by replacing the conventional separator with this ultrathin all-in-one electrolyte, we achieve 0.46 Ah Li-metal pouch cells that deliver exceptional gravimetric and volumetric energy densities of 403 Wh kg⁻¹ and 1190 Wh L⁻¹, respectively, while passing industry-standard nail penetration tests. These results underscore the tremendous potential of replacing separators with ultrathin all-in-one electrolytes for practical, high-energy lithium metal batteries.</p>