Quantifying Resistive and Diffusive Kinetic Limitations of Thiophosphate Composite Cathodes in All-Solid-State Batteries

Increasing the specific capacity and rate performance of thiophosphate composite cathodes is a major challenge in solid-state battery development. Understanding the charge transport kinetics and rate-limiting mechanisms within the cathode is key towards efficient optimization. Herein, we use quantitative analysis of chronoamperometric rate performance data to differentiate between resistive and diffusive rate limitations of NCM811-Li6PS5Cl cathodes with different morphology and composition. We substantiate our findings with separate measurements of the effective ionic conductivity and Li+ diffusion coefficients using impedance spectroscopy and GITT techniques, respectively. Increasing the active material to solid electrolyte ratio is found to increase diffusive limitations, which originate from the small contact area between the active material and solid electrolyte. The diffusive limitation is especially pronounced for single crystal NCM811 cathodes at over 84% AM. Employing fine particle catholyte significantly increases the contact area, alleviates the diffusive limitation, and increases rate performance. These results provide guidelines towards bringing the solid-state battery performance levels closer to practical targets. Chronoamperometry rate tests enable characterization of resistive and diffusive rate performance limitations and bottlenecks. Solid-state diffusion in the active material limits the rate performance of sulfide cathodes. Diffusive limitations are caused by low contact surface area between the sulfide electrolyte and active material. Fine particle sulfide increases the contact surface area between the sulfide electrolyte and active material.