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

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 27Al MAS NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical measurements. Solid-state 27Al 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 27Al chemical exchange and nutation NMR experiments. During discharge, electrolyte-soluble sulfide species form that coordinate with the AlCl4– chloroaluminate anions, resulting in (SxAlCl4)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 27Al NMR measurements reveal that solid amorphous Al2S3 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.