Synthesis of High-Quality and Low-Cost Li₆PS₅Cl via Naphthalene Assisted Li₂S-Free One-For-All Method for Commercial Synthesis

The commercialization of sulfide-based all-solid-state batteries (ASSBs) has been hindered by the lack of sustainable synthesis techniques for sulfide-based solid electrolytes (SSEs) [1]. While mechano-chemical and solid-state sintering methods are energy-intensive and commercially non-viable, solvent/wet chemistry-based routes involving organic solvents have become the focus of extensive research [2]. Thus, it is crucial to develop a viable process that eliminates the need for Li₂S as a starting reagent.<br/><br/>In this study, we present a Li₂S-free one-for-all (OFA) method capable of producing high-quality Li₆PS₅Cl, a key SSE material, at the world's lowest cost of $47/kg. The OFA method offers a universal approach for the synthesis of Li_xMS₄ (x = 3 or 4, M = P, Sb, or Si) and Li₆PS₅X (X = Cl, Br, or I) in a single-step technique, eliminating the need for pre-synthesized Li₂S. This process uses lithium metal (Li) and, sulfur powder (S) as a precursor for Li₂S along with LiCl and P₂S₅ in tetrahydrofuran (THF), Li₆PS₅Cl can be synthesized within 9 hrs. The synthesis of Li₆PS₅Cl was carried out using the OFA method, and the resulting material exhibited exceptional ionic conductivity of 2.5 mScm^-1 and electronic conductivity below 10^-7 Scm^-1. The low-cost production of Li₆PS₅Cl was achieved by utilizing readily available precursors, and the process cost was reduced to $47/kg, making it highly attractive for large-scale manufacturing.<br/><br/>The synthesis mechanism of Li₆PS₅Cl was investigated using electrospray ion mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy, revealing the formation pathway of the desired Li₆PS₅Cl structure. The wet synthesis started from spontaneous Li oxidation and naphthalene reduction, and the resulting naphthalene radical anion initiated the reduction reaction of elemental S to form polysulfide anions (S_n^2-). Subsequently, P₂S₅ reacted with polysulfide anions and co-precipitated in the form of Li₃PS₄, Li₂S, and LiCl at the end. Electrospray ion mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy are employed to elucidate the underlying chemistry between Li-NAP solution with S and P₂S₅ with LiCl during Li₆PS₅Cl synthesis. We found that S₇^2- and S₉^2- are predominantly formed in the polysulfide solution, and the additional LiCl, P₂S₅, and Li made PS^7- and PS^9- at the initial stage. With increasing Li-ion concentration in the solution (as the lithium dissolved further in the reaction mixture), PS^3- and PS^4- coordinated with THF remained in the supernatants while Li₃PS₄, Li₂S, and LiCl were co-precipitated at the end in a Li₃PS₄○x(Li₂S-LiCl) complex. To further explain the argyrodite growth X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) are used to investigate the phase growth of complex as-prepared product Li₃PS₄○x(Li₂S-LiCl) to final argyrodite Li₆PS₅Cl from the OFA method.<br/><br/>To evaluate the electrochemical performance of the OFA Li₆PS₅Cl, it was paired with a high nickel cathode Li(Ni_0.8Mn_0.2Co_0.2)O₂ (NMC-811) cathode. Galvanostatic cycling tests showed excellent cycling stability, with a capacity retention of over 90% after 1000 cycles at 2C (100 mAh g^-1) with a high 1^st discharge capacity of 180 mAhg^-1 at 30 ^oC. The OFA Li₆PS₅Cl-NMC811 system demonstrated superior performance, highlighting the effectiveness of the Li₂S-free synthesis method in achieving high-performance solid-state batteries.<br/><br/>We believe that the results of this study have the potential to revolutionize SSE and ASSLB production, significantly contributing to the scientific community's understanding of cost-effective synthesis methods and paving the way for future advancements in one-pot co-precipitation techniques.<br/><br/>References:<br/>1. Miura, A. <i>et al.</i>, <b><i>Nat Rev Chem,</i></b> <b>3</b>, 189–198 (2019). <b>https://doi.org/10.1038/s41570-019-0078-2</b><br/>2. Ali, M. <i>et al.</i>, <i><b>J. Mater. Chem. A., </b></i><b>10</b>, 25471-25480 (2022). <b>https://doi.org/10.1039/D2TA06632G</b>