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

ABSTRACT 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 0.89 Co 0.055 Mn 0.055 O 2 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 LiCoO 2 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.