A Cellulose-Derived Polymer Additive for Stabilizing Thick Cathodes in All-Solid-State Batteries

All-solid-state batteries (ASSBs) offer enhanced safety and energy density over conventional lithium-ion batteries. However, achieving high active material loading remains challenging due to poor interfacial contact from cold-pressing and the incompatibility of solvent-based processing with advanced solid-state electrolytes. Herein, we report a cellulose-derived polymer additive (CA-MDI) that establishes intimate solid-solid interfacial contact while ensuring continuous electron/ion transport in the composite cathodes. The efficacy of CA-MDI is ascribed to the urethane-linked cellulose framework, which is synthesized via the polymerization of cellulose acetate (CA) and methylene diphenyl diisocyanate (MDI). The as-constructed ASSBs incorporating a CA-MDI-modified LiNi_0.89Co_0.055Mn_0.055O₂ cathode achieve a high areal capacity of 6.4 mAh cm^-2, delivering an initial discharge capacity of 136.6 mAh g^-1 at 0.3C and retaining 91.1% of the capacity after 100 cycles, whereas additive-free cells show rapid degradation. At a lower areal capacity of 1.8 mAh cm^-2, the CA-MDI-modified cell maintains 80% of its initial capacity for over 620 cycles at 1 C. The applicability of the CA-MDI additive is further demonstrated using LiCoO₂ and Li-rich layered oxide cathodes. These results show that a mechanically adaptive polymer additive can improve the cycling stability of thick composite cathodes and provide a useful approach for developing high-energy-density ASSBs.