| With the proposal and steady progress of carbon peak and carbon neutral national strategy,it is very important to design and develop large-scale and efficient energy storage system.Compared with traditional secondary batteries(such as Ni/Cd batteries and Ni/MH batteries),lithium ion batteries have the advantages of high energy density,wide operating temperature range and long cycle life,and have been widely used in the field of energy storage.At the same time,due to the similar physical and chemical properties of sodium,potassium and lithium,sodium/potassium ion batteries have become a hot spot of researchers'attention.However,due to the large size of sodium/potassium ion,electrode materials will undergo significant volume changes in the cycle process,resulting in the application of most lithium ion battery anode materials is limited.Carbon material has the characteristics of abundant resources,low cost and stable structure,and has great potential as anode material of alkali metal ion battery.However,pure carbon materials have low reactivity and poor charge-discharge performance under high current density.Introducing heteroatoms into carbon materials can increase effective active sites and improve electrochemical performance.Boron,as a neighboring element of carbon,has many advantages such as light weight,large storage,low cost and non-toxicity,and is an excellent heteroatom.At the same time,reducing the size of boron to the quantum scale can not only improve the electrochemical activity of boron,but also provide more active sites for ion adsorption.In view of this,the following research work is carried out in this paper focusing on the boron-carbon structure electrochemical energy storage materials:(1)In this paper,boron quantum dots and starch carbon nanocomposites(B dots@BC)were prepared by simple hydrothermal reaction and high temperature carbonization.The composite has a large layer spacing and microporous hierarchical structure,which can promote the rapid transport of ions and electrons.As an anode material for lithium-ion batteries,it exhibits an initial discharge capacity of 928 m Ah g-1,which remains at 238 m Ah g-1 after 1000 cycles.At the same time,in order to further analyze the energy storage mechanism of B dots@BC,its kinetic study was carried out.The results show that the capacitance contribution of surface capacitance is dominant,and the capacitance contribution reaches 96.6%when the scanning rate increases to 5.0 m V s-1.Moreover,the nanoparticle and abundant pore structure can effectively shorten the diffusion length of lithium ions,and the initial diffusion coefficient is 8.61×10-10 cm~2 s-1.(2)Boron carbon nanocages(BCNCs)were synthesized by hydrothermal reaction and high temperature carbonization in order to explore suitable anode materials for the slow kinetics and volume change of electrodes caused by the large radius of sodium/potassium ions.The interconnecting cage-like structure of the material effectively alleviates volume changes during circulation,and the mesoporous and fully open channels increase the contact area between electrode and electrolyte,promoting the insertion and release of sodium/potassium ions.BCNCs has a sodium storage capacity of 769 m Ah g-1,which remains at 177 m Ah g-1 after 1000 cycles,and a potassium storage capacity of 2493 m Ah g-1,which remains at 212 m Ah g-1 after 500 cycles.Meanwhile,in order to reveal the storage mechanism of BCNCs electrode,the kinetic storage process of sodium/potassium ions was further studied.It is found that charge storage mainly comes from the contribution of surface capacitance behavior.In addition,mesoporous channels facilitate ion transport,and the edge defects caused by boron doping increase the active sites in the electrochemical reaction process.The synergistic effect of the two promotes the rapid transport of Na+/K+.The theoretical calculation shows that the boron doping enhances the absorption of sodium/potassium on the carbon matrix and improves the conductivity of the carbon matrix. |
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