Direct observation of Li6PS5Cl–NMC electrochemical reactivity in all-solid-state cells

Abstract

Sulfide solid electrolytes have demonstrated some of the highest known lithium ion conductivities and are expected to participate in the incoming generation of all-solid-state lithium-ion batteries (ASSLIB). Amongst the best promising electrolytes, several lithium thiophosphates have been extensively studied for their outstanding performances. However, they also suffer from their narrow electrochemical stability window, leading to a range of various redox reactions during cycling when used in electrochemical cells. As a result, losses of capacity and increases of cell impedance are systematically observed. As already pointed out, Li 6 PS 5 Cl argyrodite-type electrolyte undergoes a series of decomposition reactions during cycling, previously investigated by several spectroscopic, electrochemical and DFT studies. However, this work provides a direct high resolution observation of the decomposition products of Li 6 PS 5 Cl and their spatial distribution in a composite cathode based on Ni-rich NMC compound (LiNi x Mn y Co 1– x – y O 2 , x > 0.8) through scanning/ transmission electron microscopy (S/TEM). This study focuses on the understanding of the oxidative mechanisms and reports the detailed observations of LiCl, Li 3 PS 4 , P 2 S 5 , P 2 O 5 , NiS x and Li 2 O compounds. Transmission electron microscopy is used to identify different phases produced during the charge of cells based on LPSCl–NMC cathodes. Li 6 PS 5 Cl reacts to Li 3 PS 4 , then to P 2 S 5 while MO x and Li 2 O are produced at the surface of NMC during charge. Sulfide products and MO x react to form P 2 O 5 and MS x /SO x compounds.