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Journal of the American Chemical Society 143(43), 18216 - 18232 (2021). doi:10.1021/jacs.1c07874, Extended anionic frameworks based on condensation of polyhedral main group non-metal anions offer a wide range of structure types. Despite the widespread chemistry and earth abundance of phosphates and silicates, there are no reports of extended ultraphosphate anions with lithium. We describe the lithium ultraphosphates Li$_3$P$_5$O$_{14}$ and Li$_4$P$_6$O$_{17}$ based on extended layers and chains of phosphate, respectively. Li$_3$P$_5$O$_{14}$ presents a complex structure containing infinite ultraphosphate layers with 12-membered rings that are stacked alternately with lithium polyhedral layers. Two distinct vacant tetrahedral sites were identified at the end of two distinct finite Li$_6$O_{16}$^{26���}$ chains. Li$_4$P$_6$O$_{17}$ features a new type of loop-branched chain defined by six PO43��� tetrahedra. The ionic conductivities and electrochemical properties of Li$_3$P$_5$O$_{14}$ were examined by impedance spectroscopy combined with DC polarization, NMR spectroscopy, and galvanostatic plating/stripping measurements. The structure of Li$_3$P$_5$O$_{14}$ enables three-dimensional lithium migration that affords the highest ionic conductivity (8.5(5) �� 10$^{���7}$ S cm$^{���1}$ at room temperature for bulk), comparable to that of commercialized LiPON glass thin film electrolytes, and lowest activation energy (0.43(7) eV) among all reported ternary Li���P���O phases. Both new lithium ultraphosphates are predicted to have high thermodynamic stability against oxidation, especially Li$_3$P$_5$O$_{14}$ which is predicted to be stable to 4.8 V, significantly higher than that of LiPON and other solid electrolytes. The condensed phosphate units defining these ultraphosphate structures offer a new route to optimize the interplay of conductivity and electrochemical stability required, for example, in cathode coatings for lithium ion batteries., Published by American Chemical Society, Washington, DC