Exposed high-energy facets in ultradispersed sub-10 nm SnO 2 nanocrystals anchored on graphene for pseudocapacitive sodium storage and high-performance quasi-solid-state sodium-ion capacitors

The development of sodium (Na) ion capacitors marks the beginning of a new era in the field of electrochemical capacitors with high-energy densities and low costs. However, most reported negative electrode materials for Na+ storage are based on slow diffusion-controlled intercalation/conversion/alloying processes, which are not favorable for application in electrochemical capacitors. Currently, it remains a significant challenge to develop suitable negative electrode materials that exhibit pseudocapacitive Na+ storage for Na ion capacitors. Herein, surface-controlled redox reaction-based pseudocapacitance is demonstrated in ultradispersed sub-10 nm SnO2 nanocrystals anchored on graphene, and this material is further utilized as a fascinating negative electrode material in a quasi-solid-state Na ion capacitor. The SnO2 nanocrystals possess a small size of <10 nm with exposed highly reactive {221} facets and exhibit pseudocapacitive Na+ storage behavior. This work will enrich the methods for developing electrode materials with surface-dominated redox reactions (or pseudocapacitive Na+ storage). The electrode based on ultradispersed sub-10 nm SnO2 nanocrystals anchored on graphene nanosheets can reversibly store Na ions through both a surface-controlled pseudocapacitive reaction and a diffusion-limited alloying reaction. The fabricated Na ion hybrid capacitor with ultradispersed sub-10 nm SnO2 nanocrystals anchored on graphene nanosheets as negative electrode exhibits superior electrochemical performance.