Effects of catholyte aging on high-nickel NMC cathodes in sulfide all-solid-state batteries

Sulfide solid-state electrolytes (SSEs) in all-solid-state batteries (SSBs) are recognized for their high ionic conductivity and inherent safety. The LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode offers a high thermodynamic potential of approximately 3.8 V <i>vs.</i> Li/Li⁺ and a theoretical specific capacity of 200 mA h g^-1. However, the practical utilization of NMC811 in sulfide SSBs faces significant interfacial challenges. The oxidation instability of sulfide solid electrolytes against NMC811 and the formation of the cathode electrolyte interphase (CEI) during cycling lead to degradation and reduced cell performance. Volumetric changes in NMC during lithiation and de-lithiation can also cause detachment from sulfide electrolytes or internal particle cracking. Despite extensive galvanostatic cycling studies to address the issues, the calendar life of sulfide SSBs remains poorly understood. Here, we systematically studied the effects of four different catholytes on the calendar aging of LiNbO₃ (LNO)-coated NMC811, including Li₆PS₅Cl (LPSCl), Li₃InCl₆-Li₆PS₅Cl (LIC-LPSCl), Li₃YCl₆-Li₆PS₅Cl (LYC-LPSCl), and Li₁₀GeP₂S₁₂ (LGPS). Our results indicate that LPSCl provides optimal capacity retention when stored at high state-of-charge (SOC) at room temperature, but the LIC-LPSCl cathode shows significant capacity degradation and chemical incompatibility. We also established an effective electrochemical calendar aging testing protocol to simulate daily usage, enabling quick inference of the calendar life of SSBs. This new testing approach accelerates materials selection strategies for high-nickel NMC composite cathodes in sulfide SSBs.