Femtosecond-Laser Microstructuring Improves Solid-State Electrolyte/Cathode Ionic Transport

Abstract

All-solid-state batteries (ASSBs) provide the opportunity for substantially enhanced gravimetric and volumetric energy and power density and improved safety. However, these gains will not be realized without first overcoming substantial challenges, notably, the high interfacial resistance between electrodes and the solid-state electrolyte (SSE) which requires high stack pressures, on the order of 10,000 psi, to achieve high performance metrics.[1,2]. Already there is a large and growing body of research demonstrating the advantages of laser structuring of Li-ion battery materials for enhanced kinetics and shorter diffusion pathways [3,4]. This work extends these advantages to ASSBs through lowering the catholyte-SSE interfacial impedance by increasing its surface area and providing less torturous paths for ion diffusion.The work presented here uses a near-IR (1030 nm) femtosecond laser (pulse duration approx. 250 fs) to ablate microstructures into the surface of a slurry-cast LiNi0.8Mn0.1Co0.1O2 (NMC811):Li6PS5Cl catholyte. Micrometer-sized channel structures were ablated at various depths and pitch spacings to assess the effect of patterning on cell performance. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) measurements were acquired on both pristine and patterned samples to ensure that the femtosecond laser pulses do not cause any residual compositional or phase changes, respectively. Critically, low interfacial resistance will require that an SSE layer be applied to the patterned catholyte such that the microstructure is preserved while maintaining intimate contact between the catholyte and SSE layer. SSE layers were both slurry cast and pellet-pressed onto both pristine and patterned electrodes. Cross-sectional SEM was used to investigate the degree of interfacial contact, and relate interface quality to parameters such as SSE application technique (e.g., pellet pressing vs. slurry casting) and slurry properties (i.e., viscosity). The improvement to interfacial ionic conductivity is assessed using electrochemical impedance spectroscopy (EIS). The effect of stack pressure is analyzed in order to assess the degree to which laser patterning can alleviate stack pressure requirements in ASSBs.