Soluble Electrolyte-Coordinated Sulfide Species Revealed in Al–S Batteries by Nuclear Magnetic Resonance Spectroscopy

Abstract

Rechargeable aluminum−sulfur (Al−S) batteries have recently garnered significant interest to the low cost, earth abundance, safety, and high theoretical capacity of the electrode materials. However, Al−S batteries exhibit many challenges that plague other metal−sulfur battery systems, including significant capacity fade of the sulfur electrode due to the formation of electrolyte-soluble reaction intermediates. Here, Al−S cells using chloroaluminate-containing ionic liquid electrolytes were investigated up from the molecular level using multidimensional solid-state 27 Al MAS NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical measurements. Solid-state 27 Al single-pulse NMR measurements acquired on cycled sulfur electrodes containing electrolyte-soaked separator revealed multiple discharge products, which were distinguished into liquid-and solid-phase products based on 27 Al chemical exchange and nutation NMR experiments. During discharge, electrolyte-soluble sulfide species form that coordinate with the AlCl 4 − chloroaluminate anions, resulting in (S x AlCl 4 ) y− electrolyte complexes. These electrolyte-coordinated sulfide species persist upon charge, resulting in the loss of active mass that explains the significant capacity fade observed upon galvanostatic cycling. XPS, XRD, and solid-state 27 Al NMR measurements reveal that solid amorphous Al 2 S 3 forms reversibly upon discharge. The results highlight the technological importance of understanding how electrolyte-soluble sulfide species coordinate with the complex electroactive species used in multivalent metal−sulfur batteries, which can affect their reversibility and electrochemical properties.

Sulfur‐Tuned Advanced Carbons of Novel Properties and Scalable Productivity

“…An Al‐S cell galvanostatically cycled at 25 °C ( Figure A) exhibited a discharge plateau of 0.43 V upon first discharge and a specific capacity of 1490 mAh/g, close to sulfur's theoretical capacity of 1675 mAh/g, which is a result of the electrochemical reduction of S to Al 2 S 3 . [ 54 ] A sloping charge plateau at approximately 1.5 V was observed upon charge with a specific capacity of 1373 mAh/g, corresponding to the electrochemical oxidation of Al 2 S 3 to S. As reported in other Al‐S battery systems, [ 39,40,54,55 ] a significant voltage hysteresis was observed due to the large overpotential required to achieve the reversible electrochemical conversion of S to Al 2 S 3 at room temperature, which requires forming and breaking Al‐S bonds. Upon subsequent cycles, the capacity faded due to the formation of intermediate electrolyte‐soluble sulfide species, as revealed in our previous work by 27 Al nuclear magnetic resonance (NMR) measurements, [ 39 ] which can persist upon charge and results in loss of active mass (i.e., sulfur).…”

“…Emphasis is placed on controlling the sulfur loading, its pore-size-dependent allocation, its local structure (e.g., crystalline allotrope), and the final porosity. We character-ize in detail the physical and chemical properties of these materials, demonstrating their applications as positive electrodes in aluminum-sulfur (Al-S) batteries [39][40][41] and as adsorption materials for oil spill clean-ups. [42] Due to the novel and tunable properties of STACs, combined with their environmentally friendly and low-cost hydrothermal synthesis, we expect them to find diverse applications ranging from electrochemical energy storage to adsorption materials for environmental and medical use.…”

Beyond Composition: Surface Reactivity and Structural Arrangement of the Cathode–Electrolyte Interphase

“…In general, SSNMR spectroscopy is a nondestructive bulk analytical technique (no exposure to electron/ X-ray beams, UHV, or sputtering) that can be carried out in air-free conditions and is readily available at many research institutions. Although the methods described above are performed postmortem, samples can be prepared in such a way that they closely resemble or minimally disturb their native environment (e.g., sample rinsing/drying is not needed; the cathode sample can even be packed with liquid electrolyte and separator 95 ).…”

Three-Dimensional Nitrogen-Doped Carbonaceous Networks Anchored with Cobalt as Separator Modification Layers for Low-Polarization and Long-Lifespan Aluminum–Sulfur Batteries

“…Al–S batteries are promising energy storage devices, because of the high theoretical capacity of the low-cost electrode materials (2980 mAh g –1 and 8050 mAh cm –3 for the Al anode; 1672 mAh g –1 and 3459 mAh cm –3 for the S cathode) and the inherent safety endowed by the nonflammable ionic liquid-based electrolytes. , Nonetheless, the conversion reaction of sulfur with AlCl 4 – is rather sluggish, leading to severe cell polarization and, consequently, low energy density. Meanwhile, the notorious shuttle effect is also demonstrated in Al–S batteries, resulting in irreversible deposition of sulfide species on Al metal anodes and a severe decline in capacity. , Hence, mitigating the polarization and suppressing the shuttle effect are highly desired to improve the energy density and lifespan of Al–S batteries.…”

“…Meanwhile, the notorious shuttle effect is also demonstrated in Al−S batteries, resulting in irreversible deposition of sulfide species on Al metal anodes and a severe decline in capacity. 3,4 Hence, mitigating the polarization and suppressing the shuttle effect are highly desired to improve the energy density and lifespan of Al−S batteries.…”