Visualization of Lithium Transfer Resistance in Secondary Particle Cathodes of Bulk-Type Solid-State Batteries

The rate capability of bulk-type solid-state Li-ion batteries is limited by the solid-state diffusion of Li ions in the cathodes that typically form micrometer-sized polycrystalline particles composed of aggregated nanocrystals. This necessitates nanoscale and dynamic observations of Li ions to establish a detailed understanding of Li conduction in the complicated particle structure. This study demonstrates Li-ion dynamics in particle cathodes under low- and high-rate charge-discharge reaction conditions using operando scanning transmission electron microscopy coupled with electron energy-loss spectroscopy. Direct observation results reveal the slow diffusion through nanocrystal grain boundaries as the primary factor limiting Li conduction in particle cathodes. Therefore, rate-capability enhancement of Li-ion batteries requires either optimization of the interfacial structure along with nanocrystal orientation or the use of large single crystals without boundaries.