SnO₂ Anchored On Nitrogen-doped Porous Carbon As Anode Materials For Sodium Storage

As an emerging hybrid electric energy storage device,sodium ion capacitors have been paid much attention in recent years.In the sodium ion capacitors,the battery-type anodes provide high energy density by reversible sodium-ions storage,while the capacitive cathodes provides high power density and long cycling performance through rapid capacitive response on electrode surface.Although sodium ion capacitors inherit the advantages of batteries and capacitors,the performance is still limited by the design and fabrication of high performance electrode materials.As one of the anode materials for sodium-ion storage,SnO2 has the advantages of high specific capacity,good safety and easy preparation.However,SnO2 anodes commonly exhibit poor rate capability and cycle stability due to the low electric conductivity and large volume change during charge-discharge process.In this paper,we realized the sythesis of the amorphous and SnO2(A-SnO2)sub-nano clusters using nitrogen-doped porous carbon nanosheets as support material.The carbon-supported SnO2(A-SnO2@PCNS)was futher used as anode material in half cells for perfomrance evaluation.The relationship between structural composition and electrochemical performance for A-SnO2@PCNS was detaily analyzed by electrochemical characterization and theoretical calculations.Then,a high-performance sodium-ion capacitor was assembled using A-Sn02@PCNS anode.Finally,we expored the formation mechanism of A-SnO2 subnano clusters and a universal synthetic method for carbon-supported A-SnO2 subnano clusters was proposed.The following results was achieved:(1)A simple and efficient method was presented to prepare A-SnO2 subnano clusters.We used biomass-derived nitrogen-doped porous carbon nanosheet as support material.By simply mixing the support material with tin salts followed by heat treatment,we successfully obtained A-Sn02 subnano clusters due to the strong adsorption and confinement effect from the micropores of carbon support.Benefitial from the synergistic effect of micropore confinement and chemical bonding(Sn-N-C and Sn-O-C bonds),the A-Sn02 subnano clusters(with average particle size of 0.93 nm)were uniformly anchored on the carbon support and exhibited good structure stability.(2)In half cells(using metallic sodium as counter and reference electrode),the A-SnO2@PCNS anode exhibited much better rate performance than graphene-supported SnO2 nanocrystal anode.The reversible capacity of A-SnO2@PCNS anode was 380 mAh g-1 at a current density of 0.05 A g-1,and maintained 184 mAh g-1 even at a high current density of 5 A g-1.In addition,the A-SnO2@PCNS anode also showed an extremely long cycling performance,that capacity can maintain 93 and 92%compared with the first cycle after 1000 cycles at 1 A g-1 and 1400 cycles at 5 A g-1.(3)Electrochemical characterization and first-principles calculations showed that the unique SnO2 structure and superior N-doped porous carbon support enhanced the storage and diffusion of sodium ions,endowing the A-SnO2@PCNS anode with high reversible capacity and excellent rate performance.In addition,the amorphous and subnano structure accommodated the volume change and stabilized material structure of Sn02 anode during charge-discharge process,leading to the superior cycling perfomrance of A-SnO2@PCNS anode.Due to the superb performance of SnO2@PCNS anode,the assembled sodium-ion capacitor using A-SnO2@PCNS anode showed high energy density(196.4 Wh kg-1),high power density(28.1 kW kg-1)and long lifetime(capacity retention rate is about 80%).(4)Through specific design and preparation methods,we also prepared carbon materials with different pore structures,morphological dimensions and hetero atom content,and used them to support SnO2 using the same sythetic conditions.We found that A-SnO2 subnano clusters could obtained when the carbon supports simultaneously had the following properties:abundant microporous pores,numerous surface heteroatoms and two-dimensional nanosheet structures.Based on this conclusion,we successfully realized the universal synthesis of carbon-supported A-SnO2 subnano clusters.