| Transition metal carbon/nitrides(MXenes)have the characteristics of abundant and adjustable surface functional groups,large specific surface area,adjustable interlayer spacing,and high conductivity,and have been widely studied as anode materials for sodium ion batteries.However,MXenes itself has many problems,such as:uneven interlayer spacing and unavailability of internal active sites;complex surface functional groups,and different functional groups have different effects on capacity;van der Waals forces between layers cause self-stacking,which hinders ion/electron transport and makes it The capacity is not ideal when used directly as an electrode material.Therefore,this paper takes Ti3C2 as the research object,and adjusts the intrinsic structure of Ti3C2 to alleviate the above-mentioned problems:increasing the nanosheet spacing to reduce the self-stacking effect;building a three-dimensional porous structure to reduce the ion/electron transport barrier;and then combining with high-capacity nano-silicon(Si)compound to increase the electrochemical sodium storage capacity.The specific content is as follows:(1)In this study,the multi-layer Ti3C2 was stripped into a single/small-layer structure,which not only effectively relieves self-stacking,but also changes the surface potential of nano-Si.Under electrostatic self-assembly,Si is uniformly loaded on Ti3C2 nano-sheets,and then introduced"Carbon cage"hard carbon to achieve the purpose of fixing and alleviating the volume expansion of Si.The synthesized Si/(d-Ti3C2)-HC composite material is used as a sodium ion anode material,which significantly improves the electrochemical performance and also obtains the optimal loading of Si.At a current density of 50 m A g-1,the sodium storage capacity can reach 209 m Ah g-1,and the capacity continues to increase with the increase in the number of cycles;after a high current density charge and discharge cycle,the capacity can still be maintained at 207 m Ah g-1(50 m A g-1),showing good cycle stability.This excellent performance can be attributed to:the excellent conductivity of Ti3C2 itself is conducive to the rapid transfer of electrons;the alloyed sodium storage of Si contributes part of the capacity;the introduction of hard carbon not only helps to alleviate the volume expansion of Si,Improve stability,but also contributes to the pseudocapacitance behavioral sodium storage capacity.(2)In order to further study the sodium storage mechanism of Si/(d-Ti3C2)-HC,the electrode kinetics analysis was carried out in this study.When the scanning speed is 0.3 m V s-1,the surface-induced diffusion control behavior contributes 44%of the sodium storage capacity;as the scanning speed continues to increase,the proportion of the capacity contributed by the diffusion control behavior gradually decreases,and the capacity contributed by the capacitive behavior begins dominates,reaching 88%when the sweep speed is 3.0 m V s-1.Then the diffusion coefficient of sodium ion is analyzed:during the discharge process,with the decrease of voltage,the diffusion coefficient decreases rapidly,and finally enters the slow changing region,and the diffusion coefficient is 0.7×10-10 cm2 s-1;the initial diffusion coefficient is 2.4×10-10 cm2 s-1?(3)Using the hydrothermal synthesis method,under the condition that the Ti3C2 structure is not changed,the large layers of Ti3C2 are broken into small sizes and then restacked to form a three-dimensional nano bouquet-like Ti3C2 material with high conductivity and high specific surface area.The constructed three-dimensional Ti3C2 exposes most of the active sites of the material,improves the utilization of surface active sites,shortens the ion/electrolyte transmission distance,and ultimately improves the sodium storage performance.Based on this,a three-dimensional Si/FL-Ti3C2 composite structure is prepared after compounding with Si.At a current density of 0.1 A g-1,the capacity of 200 cycles remains at 353 m Ah g-1,and the number of cycles increases.The capacity continues to rise. |
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