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Developing high-performance all-solid-state batteries is contingent on finding solid electrolyte materials with high ionic conductivity and ductility.Here we report new halide-rich solid solution phases in the argyrodite Li 6 PS 5 Cl family, Li 6Àx PS 5Àx Cl 1+x ,a nd combine electrochemical impedance spectroscopy, neutron diffraction, and 7 Li NMR MAS and PFG spectroscopytoshow that increasing the Cl À /S 2À ratio has as ystematic, and remarkable impact on Li-ion diffusivity in the lattice.T he phase at the limit of the solid solution regime, Li 5.5 PS 4.5 Cl 1.5 ,e xhibits ac old-pressed conductivity of 9.4 AE 0.1 mS cm À1 at 298 K( and 12.0 AE 0.2 mS cm À1 on sintering)almost four-fold greater than Li 6 PS 5 Cl under identical processing conditions and comparable to metastable superionic Li 7 P 3 S 11 .W eakened interactions between the mobile Li-ions and surrounding framework anions incurred by substitution of divalent S 2À for monovalent Cl À play amajor role in enhancing Li + -ion diffusivity,a long with increased site disorder and ahigher lithium vacancy population. Figure 5. a) 7 Li MAS NMR for Li 6Àx PS 5Àx Cl 1+x (x = 0, 0.25, 0.375, 0.5) b) correlation of the activation energies from both techniques with the 7 Li isotropic chemical shift and the Haven ratio for all values of x under study.

Li-O 2 cells with slight variations in design and material were employed in three different laboratories for this work, but all show consistent results. Figures 2, 3 and 6 were performed in one lab, Figure 1, 5, 7 and 8 in another, and Figure 4 in a third, but key experiments were repeated in all labs to confirm consistency. In order to provide specific repeatability, we will discuss specific cell materials, cathode preparation, and cell assembly pertaining to the data from Figures 2, 3 and 6. Materials.Lithium iodide was purchased from Sigma Aldrich and was dried under vacuum in a heated glove box antechamber at 150 °C for 24 hours before use. Lithium bis(trifluoromethane) sulfonamide (LiTFSI) and 1,2-dimethoxyethane (DME) were purchased from BASF and used as received. PTFE (both 60 wt% dispersion in H 2 O and 1 µm particle-size powder) was purchased from Sigma Aldrich. Vulcan XC72 was purchased from Fuel Cell Store and was filtered through a 60-mesh screen. Ketjenblack ® (KB) was received from Toyota. T316 stainless steel 120 mesh, with wire diameter 0.0026", was purchased from TWP Inc. Research-grade oxygen and argon were purchased from Praxair. 99% 18 O 2 was purchased from Sigma Aldrich. 90% H 2 18 O was purchased from Cambridge Isotopes. Water used to contaminate the electrolytes was ultrapure (18.2 MΩ cm, Millipore). All electrolyte and cell preparation was carried out in an argon glove box with < 0.1 ppm O 2 and <0.1 ppm H 2 O. Water was quickly and carefully added to electrolytes in the glovebox via micropipette. Graphene oxide was prepared via the oxidation of graphite (Alfa Aesar), and reduced it to rGO according to the procedure previously reported by Grey et al; the material was centrifuged in the initial washing steps.Cathode Preparation. The XC72 cathodes used for Figure 6 were prepared via a similar method to that described previously. 1,2 A mixture of 3:1 w:w ratio of Vulcan XC72 to PTFE binder in isopropanol (IPA) and water (4:1 water:IPA; and 15 mL total for 400 mg C) was sonicated for 30 seconds and homogenized for 6 minutes. A Badger model 250 air-sprayer was used to spray the

The dendritic growth of Li metal leads to electrode degradation and safety concerns, impeding its application in building high energy density batteries. Forming a protective layer on the Li surface that is electron-insulating, ion-conducting, and maintains an intimate interface is critical. We herein demonstrate that Li plating is stabilized by a biphasic surface layer composed of a lithium-indium alloy and a lithium halide, formed in situ by the reaction of an electrolyte additive with Li metal. This stabilization is attributed to the fast lithium migration though the alloy bulk and lithium halide surface, which is enabled by the electric field across the layer that is established owing to the electron-insulating halide phase. A greatly stabilized Li-electrolyte interface and dendrite-free plating over 400 hours in Li|Li symmetric cells using an alkyl carbonate electrolyte is demonstrated. High energy efficiency operation of the Li Ti O (LTO)|Li cell over 1000 cycles is achieved.