Operando Measurement of Lithiation Gradients in NMC111-Argyrodite All-Solid-State Composite Cathodes

Abstract

Achieving the high energy density targets in all-solid-state batteries (ASSBs) will require thick cathodes optimized for full utilization of active material. In composite cathodes with sulfide SSEs, the exclusion of carbon additives makes cathode design to balance ionic and electronic conductivities all the more important. Li-Ni1/3Mn1/3Co1/3O2 (NMC111) is a widely studied cathode material for its high energy density and high working voltage. The lattice parameters of this well-studied structure directly correlate to the amount of Li in the material, allowing for very accurate measurements of lithium-ion concentration gradients (lithiation gradients) throughout the cathode depth.Spatially resolved lithiation gradients in thick NMC111/ Li6PS5Cl composite cathodes were measured using operando energy dispersive X-ray diffraction (EDXRD) on sealed and pressurized ASSBs with varying relative amounts of cathode active mass (CAM) and SSE. Electrochemical impedance spectroscopy (EIS) and transmission line modeling were employed to test the ionic and electronic conductivities of composite cathodes with similar varied CAM and SSE ratios. The balance of ionic and electronic conductivities was found to influence not only the lithiation gradients and utilization of active material in the cell, but also the NMC structural change throughout the depth of the cell resulting in peak bifurcation in the NMC (003) diffraction pattern.Sulfide ASSBs suffer from a large capacity loss on the first cycle due to formation of the cathode electrolyte interface (CEI). To study this interface formation and capacity loss, in situ EIS was performed during the first charge and discharge of the full cell (Li/In| Li6PS5Cl| NMC111/Li6PS5Cl). This piecewise EIS allows us to analyze both time dependent and state-of-charge dependent interfacial processes while the battery cycles with no modifications to cell design. Acknowledgments We acknowledge financial support from the National Science Foundation under Award Number CBET-ES- 1924534. This research used resources of the Advanced Photon Source beamline 6-BM, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.