2D Graphene/MnO Heterostructure with Strongly Stable Interface Enabling High‐Performance Flexible Solid‐State Lithium‐Ion Capacitors

The delicate structural engineering is widely acknowledged as a powerful tool for boosting the electrochemical performance of conversion‐type anode materials for lithium storage. Here, a general electrostatic self‐assembly strategy is proposed for the in situ synthesis of MnO nano‐cabbages on negatively charged reduced graphene oxide (rGO/MnO). The strong interfacial heterostructure and robust lithium storage mechanism related to fast Li+ diffusion kinetics and high Li‐adsorption ability of rGO/MnO heterostructure are confirmed through operando experimental characterizations and theoretical calculation. Owing to the rapid charge transfer, enriched reaction sites, and stable heterostructure, the as‐synthesized rGO/MnO anode delivers a high capacity (860 mAh g−1 at 0.1 A g−1), superior rate capability (211 mAh g−1 at 10 A g−1), and cycle stability. Notably, the as‐assembled flexible pouch cell of activated carbon//rGO/MnO solid‐state lithium‐ion capacitors (LICs) possesses an exceptional energy density of 194 Wh kg−1 and power density of 40.7 kW kg−1, both of which are among the highest flexible solid‐state LICs reported so far. Further, the LICs possess an ultralong life span with ≈77.8% retention after 10 000 cycles and extraordinary safety, demonstrative of great potential for practical applications. A general electrostatic self‐assembly strategy is proposed for synthesizing nano‐cabbages like MnO anchored on reduced graphene oxide (rGO/MnO). Benefited from the strong interfacial interactions, fast Li+ diffusion kinetics, and high Li‐adsorption ability, the rGO/MnO heterostructure possesses impressive capacity and rate performances. Further coupled with activated carbon to assemble high‐performance flexible solid‐state lithium‐ion capacitors, demonstrating its feasibility for practical applications.