Pseudocapacitive Nanomaterials For Sodium-ion Storage:Design,Preparation And Electrochemical Mechanism

The development of high-energy density,high-power density and low-cost sodium-ion storage devices,is of great significance for alleviating the shortage of lithium resources and large-scale energy storage.Motivated by solving the problem of slow sodium-ion diffusion kinetics in electrode materials,a strategy for developing pseudocapacitive nanomaterials with high electronic conductivity is proposed.Taking materials design and preparation,morphology and phase characterization,electrochemical performance and mechanism analysis as research ideas,transition metal phosphides and nitrides anodes with high electronic conductivity and layered birnessite with carbon cathode are synthesized,and correlations between the reaction mechanism and material structure are revealled.The details are summarized briefly as follows:（1）Mo P nanocrystallines embedded in conducting carbon nanowire scaffolds（Mo P/C-NWs）with high electron conductivity are synthesized as the anode,achieving high-rate sodium-ion storage performance.There is an amorphous molybdenum oxides layer on the surface of the Mo P nanocrystallines,and the content of molybdenum oxides could be adjusted by controlling the calcination time.The surface pseudocapacitive sodium-ion storage mechanism is demonstrated through in-situ/ex-situ characterizations,kenitics analysis and theoretical calculation.The phase and morphology structure of electrode materials remain unchanged,while the amorphous molybdenum oxides layers on the Mo P nanocrystallines undergo surface faradaic reaction.The sodium-ion storage performance of the electrode materials is related to the content of molybdenum oxides.The electrode materials with the highest content of molybdenum oxide exhibit superior rate capability(even at 20 A g^?1,it can still display the reversible specific capacity of 71 m Ah g^?1)and cycling performance（after 3500 cycles,the capacity attenuation per cycle is only 0.013%）.An assembled sodium-ion capacitor displays high energy density(157 Wh kg^?1)and high power density(9316 W kg^?1).（2）The mesoporous Mo₃N₂nanowires（Meso-Mo₃N₂-NWs）with high electron conductivity are synthesized as the anode,achieving high-rate sodium-ion storage performance.The Meso-Mo₃N₂-NWs consist of interconnected nanocrystallines and inner slit-like mesoporous.Amorphous molybdenum oxides layers exist on the surface of the Mo₃N₂nanocrystallines and the content of molybdenum oxides could be adjusted by synthesizing electrode materials with different specific surface area.The detailed sodium-ion storage mechanism is demonstrated as surface pseudocapacitance through in-situ/ex-situ characterizations and kenitics analysis.It is found that the phase and morphology structure of the electrode materials remain unchanged,while the surface molybdenum oxides layers undergo a faradaic redox reaction.The crystal structure does not affect the sodium-ion storage mechanism of molybdenum nitrides.The sodium-ion storage performance of the electrode materials is related to the content of molybdenum oxides.The electrode materials with the highest content of molybdenum oxide exhibit superior rate capability(even at 16 A g^?1,it can still display the reversible specific capacity of 87 m Ah g^?1)and cycling performance（after 800 cycles,the capacity attenuation per cycle is only 0.027%）.However,Meso-Mo₃N₂-NWs undergo a conversion reaction for lithium-ion storage.The mechanism of the Meso-Mo₃N₂-NWs for sodium-ion storage is different from that for lithium-ion storage.The possible reason is that lithium ions with a smaller radius are prone to be intercalated in the crystal structure,leading to conversion reaction.（3）Intercalation pseudocapacitive materials could make full use of bulk phase for charge storage.Based on these,layered binrssite nanosheets supported by the CNTs&KBs conductive networks（b-NMO/C）are synthesized as the cathode,achieving high-rate sodium-ion storage performance.Through ex-situ tests,the total lattice expansion between the birnessite layers is just about 2.4%,and the lattice breathing is highly reversible during de/intercalation of sodium ions.At the same time,the nanosheet morphology and layered structure are well maintained.Further kinetic analysis proves that electrode materials exhibit intercalation pseudocapacitive sodium-ion storage mechanism.Benefiting from structured water and carbon materials with high electron conductivity,b-NMO/C exhibits excellent rate capability(even at 100 C,it also retains a high capacity of 43 m Ah g^?1)and cycling performance（after 1000 cycles,the capacity attenuation per cycle is only 0.043%）,which are better than reported manganese-based cathodes.An assembled sodium-ion capacitor displays a high energy density(84 Wh kg^?1)and high power density(5816 W kg^?1).The above research provides new insights for the development of high-energy density,high-power density and low-cost sodium-ion storage devices.