Operando neutron imaging-guided gradient design of Li-ion solid conductor for high-mass-loading cathodes.

High-mass-loading cathodes are crucial for achieving high energy density in all-solid-state batteries from the lab scale to industry. However, as mass-loading increases, electrochemical performance is significantly compromised due to sluggish kinetics. In this work, operando neutron imaging is deployed on a high-mass-loading NMC 811 cathode of 33 mg/cm² (5.0 mAh/cm²) and directly visualizes the lithiation prioritization of the cathode active material (CAM) from the solid electrolyte membrane side to the current collector side. In addition to the tortuosity, another key limitation on ion transfer in the cathode arises from the mismatch between the uniform distribution of the solid electrolyte (catholyte) in the conventional composite cathode and the non-uniform Li⁺ flux generated by the faradaic reaction of CAMs. Therefore, we engineer a gradient in the catholyte concentration to match the Li⁺ flux distribution as a means of eliminating the ion transfer obstacle. This approach demonstrates enhanced rate performance, even with high-mass-loading cathodes. A LiCoO₂ composite cathode with 100 mg/cm² high-mass-loading exhibits an areal capacity of 10.4 mAh/cm² at a current density of 2.25 mA/cm². This work provides insight into the ion-transport limitation in thick cathodes and demonstrates an effective gradient design to overcome the kinetic barrier and achieve high battery performance.