Toughened Bimodal Cathodes for All-Solid-State Batteries via Controlled Interfacial Heterogeneity

All-solid-state batteries (ASSBs) employing composite electrodes require a significant fraction of heavy solid electrolytes (SEs), which poses a challenge to improving their energy density. In this context, a bimodal cathode design, incorporating mixed cathode active materials of different particle sizes, is proposed to enhance the packing density of the cathode layer. This study reveals the mechanism of fracture-induced failure in bimodal cathodes, triggered by interfacial heterogeneity, and discusses the fundamental requirements for achieving high-performance, long-cycling ASSBs. The cycling performance of polycrystalline LiNi_0.88Co_0.09Al_0.03O₂ (NCA)|Li₆PS₅Cl (LPSCl)|Li-In full cells is evaluated using two cathode configurations: a unimodal cathode composed solely of 3 μm polycrystalline NCA (U-NCA) and a bimodal cathode comprising a mixture of 3 and 10 μm NCA particles (B-NCA). Although B-NCA offers improved packing density and electronic conductivity, it suffers from rapid capacity decline under external pressures. Comprehensive impedance analysis and microstructural characterization combined with mechanical simulations reveal that the accelerated degradation of B-NCA arises from cracking in the larger NCA particles: the reaction heterogeneity at the NCA/LPSCl interface incurs the grain boundary damage of NCA, which is more severe in larger particles compared to smaller ones. Introducing a Li₂ZrO₃ nanolayer to mitigate the interfacial heterogeneity effectively prevents the mechanical failure of the NCA particles, leading to stable cycling performance with B-NCA. This study provides an in-depth understanding of the degradation characteristics of bimodal cathodes for sulfide-based ASSBs with high energy densities and elucidates the critical factors for their design.