Decoupling Irreversible and Reversible Inefficiencies at Dynamic Interfaces in Sulfide Solid State Battery Positive Electrodes

Battery systems present a uniquely difficult environment for disentangling sources of inefficiency. As an example, positive electrode-electrolyte interfaces incur simultaneous charging-induced side reactions, surface area change, and lithiation-state dependent intercalation rates. For sulfur-based composite electrodes in solid state batteries (SSBs) these parallel processes present a formidable challenge to addressing first cycle Coulombic inefficiency.¹ Without fully decoupling inefficiencies, it is unclear which degradation mechanism is most critical to address and, further, how applied solutions impact each mechanism in turn. For example, to mitigate unwanted side reactions between sulfide solid electrolytes (SEs) and positive electrode materials like LiNi_XMn_YCo_1-X-YO₂ (NMC), the field has implemented interfacial coatings to increase Coulombic efficiency.^2-4 While coating design focuses on thermodynamic stability at the interface, the coatings could be improving battery performance by addressing other inefficiencies in parallel, such as increasing the rate of the intercalation reaction or reducing ionic constriction at the NMC/SE interface.<br/> <br/>This work systematically decouples the impacts of ionic constriction, electrolyte redox and NMC/SE interphase formation in sulfide-SE SSB first cycle electrochemistry to better inform design of solutions for the NMC/SE interface. Using uncoated LiNi_0.5Mn_0.3Co_0.2O₂ we demonstrate a method to delineate contributions to Coulombic inefficiency between slow kinetics and irreversible reactions. To determine the reversibility of interphase products and the impact of cycling history we track lithiation state and generation of interphase products during cycling. We determine the ability of generated interphase products to contribute to self-discharge of NMC and the resultant impact on Coulombic inefficiency. Using two different electrolytes, Li₃PS₄ and Li₅PS₆Cl, which have well-defined but unique thermodynamically-predicted redox reactions and reaction potentials, we separate contributions to Coulombic inefficiency between electrolyte redox reactions and reactions involving transition metals and oxygen from NMC. Finally, by comparing borate-coated and uncoated NMCs, we investigate the coating’s impact on interphase formation, ionic constriction and intercalation reaction rate.<br/> <br/>[1] Janek, J., Zeier, W.G. Challenges in speeding up solid-state battery development. <i>Nat Energy</i> <b>8</b>, 230–240 (2023). https://doi.org/10.1038/s41560-023-01208-9<br/>[2] Morino, Y., Kanada, S. Degradation Analysis by X-ray Absorption Spectroscopy for LiNbO3 Coating of Sulfide-Based All-Solid-State Battery Cathode. <i>ACS Applied Materials & Interfaces</i> 15 (2), 2979-2984 (2023). https://doi.org/10.1021/acsami.2c19414<br/>[3] Walther, F. <i>et al.</i> The Working Principle of a Li2CO3/LiNbO3 Coating on NCM for Thiophosphate-Based All-Solid-State Batteries. <i>Chemistry of Materials</i> 33 (6), 2110-2125 (2021) https://doi.org/10.1021/acs.chemmater.0c04660<br/>[4] Zhang, Y.Q. <i>et al.</i> Direct Visualization of the Interfacial Degradation of Cathode Coatings in Solid State Batteries: A Combined Experimental and Computational Study. <i>Adv. Energy Mat.</i> 10 (27), 1903778 (2020). https://doi.org/10.1002/aenm.201903778