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Sulfide solid-state electrolyte (SE) possesses high room-temperature ionic conductivity. However, fabrication of the free-standing, sheet-type thin sulfide SE film electrolyte to enable all-solid-state batteries to deliver high energy and power density remains challenging. Herein we show that argyrodite sulfide (Li₆PS₅Cl) SE can be slurry cast to form free-standing films with low (≤5 wt%) loadings of poly(isobutylene) (PIB) binder. Two factors contribute to a lower areal specific resistance (ASR) of the thin film SEs benchmarked to the pristine powder pellet SSE counterparts: i) 1–2 orders reduced thickness and ii) reasonably comparable ionic conductivity at room temperature after the isostatic pressing process. Nevertheless, an increasing polymer binder loading inevitably introduced voids in the thin film SEs, compromising anode/electrolyte interfacial ion transport. Our findings highlight that electrolyte/electrode interfacial stability, as well as the selection of slurry components, including sulfide SE, binder, and solvent, play essential roles in thin film sulfide electrolyte development. 

Abstract This is the first report of molybdenum carbide‐based electrocatalyst for sulfur‐based sodium‐metal batteries. MoC/Mo₂C is in situ grown on nitrogen‐doped carbon nanotubes in parallel with formation of extensive nanoporosity. Sulfur impregnation (50 wt% S) results in unique triphasic architecture termed molybdenum carbide–porous carbon nanotubes host (MoC/Mo₂C@PCNT–S). Quasi‐solid‐state phase transformation to Na₂S is promoted in carbonate electrolyte, with in situ time‐resolved Raman, X‐ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo₂C@PCNT–S cathodes deliver among the most promising rate performance characteristics in the literature, achieving 987 mAh g⁻¹at 1 A g⁻¹, 818 mAh g⁻¹at 3 A g⁻¹, and 621 mAh g⁻¹at 5 A g⁻¹. The cells deliver superior cycling stability, retaining 650 mAh g⁻¹after 1000 cycles at 1.5 A g⁻¹, corresponding to 0.028% capacity decay per cycle. High mass loading cathodes (64 wt% S, 12.7 mg cm⁻²) also show cycling stability. Density functional theory demonstrates that formation energy of Na₂S_x(1 ≤x ≤ 4) on surface of MoC/Mo₂C is significantly lowered compared to analogous redox in liquid. Strong binding of Na₂S_x(1 ≤x ≤ 4) on MoC/Mo₂C surfaces results from charge transfer between the sulfur and Mo sites on carbides’ surface. 

Abstract This is the first report of a multifunctional separator for potassium‐metal batteries (KMBs). Double‐coated tape‐cast microscale AlF₃on polypropylene (AlF₃@PP) yields state‐of‐the‐art electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm⁻²and 0.5 mAh cm⁻², with 0.042 V overpotential. Stability is maintained at 5.0 mA cm⁻²for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite‐free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and “dead metal” and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF₃@PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF₃partially reacts with K to form an artificial SEI containing KF, AlF₃, and Al₂O₃phases. The AlF₃@PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K⁺transference number is ascribed to the strong interaction between AlF₃and FSI⁻anions, as revealed through¹⁹F NMR. The enhancement in wetting and performance is general, being demonstrated with ester‐ and ether‐based solvents, with K‐, Na‐, or Li‐ salts, and with different commercial separators. In full batteries, AlF₃prevents Fe crossover and cycling‐induced cathode pulverization.