| With the continuous development of new energy systems,diversified demands have been put forward for energy storage technologies in different scenarios.Secondary batteries,as an energy storage device with high-efficiency,low-cost and environmental-friendliness,have been widely concerned.Lithium secondary batteries have high energy density and high technical maturity,but the cost is difficult to control due to limited lithium reserves and uneven distribution.Sodium has similar physicochemical properties with lithium,and the raw materials are cheap and easy to obtain so that it is of great attention in the field of research and development of secondary battery.However,the theoretical specific capacity of sodium is not as good as that of lithium.Lithium batteries and sodium batteries have their own strengths and can complement each other in different energy storage fields.Currently,the performance of commercialized secondary batteries is approaching the upper limit of energy density,and there is an urgent need to develop secondary batteries with higher energy density,longer service life,and lower overall cost.The current secondary battery anode materials with high energy density,such as conversion-alloy materials and lithium/sodium metal itself,will face large volume changes and dendrite growth during cycling,resulting in electrode failure due to pulverization or short circuit.In this dissertation,mesoporous structure is introduced in the process of constructing nano-electrode materials,and nanosizing and porosizing can mitigate the volume deformation of the electrode during the charging and discharging process,and the increased specific surface area is conducive to reducing the effective current density and alleviating the rate of nucleation and growth of dendritic.In addition,the mesoporous structure also shortens the diffusion free path of the lithium/sodium ions in the electrode material,which helps to enhance the degree of activation of the material and the rate of the electrochemical reaction of the battery performance,and optimize the overall performance of the battery.The main research contents and results are as follows:（1）Nitrogen-doped mesoporous carbon microsphere（meso-N-C）with a high specific surface area of 413.9 m2 g-1,a large pore size of～5.5 nm,and abundant nitrogen content of 7 wt%is constructed by microemulsion method with dopamine hydrochloride and histidine as the nitrogen and carbon sources.As a lithium metal anode host material,the coulombic efficiency of meso-N-C is maintained above 99%in more than 3600 cycle tests at 0.5 m A cm-2,0.5 m Ah cm-2.The mesoporous nitrogen-doped carbon is used as the coating shell to construct r GO@meso-N-C and CNT@meso-N-C,and the cycle life of the coated electrodes is 3～5 times of that of the electrodes before coating.The test and characterization results demonstrate that the abundant lithophilic nitrogen atoms in the mesoporous nitrogen-doped carbon materials can effectively promote the uniform deposition of lithium metal and stabilize the solid electrolyte interphase.And the high specific surface area shown by the interconnected mesoporous structure can reduce the effective current density,alleviate the growth of lithium dendrites and the volume change,and effectively improve the cycling stability of the lithium-metal batteries.（2）Bi2O3/mNC composite is fabricated by using mesoporous nitrogen-doped carbon（mNC）as the carrier and impregnated with nano-Bi2O3 via the microemulsion method.The mesoporous pore structure not only effectively controls the size of Bi2O3nanoparticles,but also provides a great buffer space for subsequent alloying.At the same time,the nitrogen-doped carbon materials can effectively improve the overall electronic conductivity of the material.As a result,the first discharge capacity of Bi2O3/mNC could reach 762.2 m Ah g-1 at 100 m A g-1 with the first irreversible capacity loss of 33.7%.And the capacity could still respectively reach 358.8 and 296.5 m Ah g-1 when Bi2O3/mNC is recycled to 100 and 1000 m A g-1 after cycling at 100～5000 m A g-1.After 100 cycles at1000 m A g-1,the capacity remains 273.1 m Ah g-1,which are significantly better than the performance of the commercial Bi2O3 and the comparative samples of Bi2O3@NC.indicating that constructing a mesoporous confined structure of the conversion-alloy anode electrode can effectively optimize the sodium storage performance of the battery.（3）Bi2O3-mNb2O5/C composite with specific surface area of 104.8 m2 g-1 as well as pore size of～5.6 nm is constructed via solvent volatilization induced self-assembly strategy.Bi2O3-mNb2O5/C could reach a first discharge capacity of 649.9 m Ah g-1 at 100m A g-1,with a first irreversible capacity loss of less than 40%.After a rate cycle from 100to 5000 m A g-1,the capacity retention is 86.2%and 88.2%when it is restored to 100 and1000 m A g-1,respectively.After 200 cycles at 1000 m A g-1,the capacity retention rate is maintained at 91.0%,which is much better than the other two sets of comparison samples with different feed ratio of Bi.The test and characterization results exhibit that the carbon framework and mesoporous structure effectively reduce the material charge transfer impedance,enhance the sodium ion diffusion coefficient,and increase the proportion of surface sodium storage process.Mesoporous Nb2O5 substrate is effective in buffering and suppressing the volume change generated by Bi-based nanomaterial during sodium storage process,thus achieving excellent long cycle performance of sodium storage.（4）Mesoporous SnO2 nanoparticle（meso-SnO2）with a pore size of～3.7 nm and a specific surface area of 162.1 cm2 g-1 with a tunable concentration of oxygen vacancies is constructed by a surfactant micelle-mediated assembly method.The concentration of oxygen vacancies can be modulated by varying the hydrolysis rate of the Sn precursor and the calcination conditions.meso-Sn O2 with the highest oxygen vacancy concentration obtained in air atmosphere at 300°C,exhibits more than 500 times of reversible sodium deposition/stripping at 2.0 m A cm-2 and 1.0 m Ah.In the symmetric cell tests,the meso-Sn O2/Na symmetric cell shows only～20 m V of sodium deposition overpotential at 5 m A cm-2,5 m Ah cm-2,and could operate stably for more than 1000hours.The test and characterization results illustrate that the oxygen vacancies in meso-Sn O2 can significantly accelerate the interfacial charge transfer.Meanwhile,the unique mesoporous structure provides a high active surface space,which not only helps to reduce the effective current density,but also alleviates the volume effect during charging and discharging,thus realizing the cyclic stability of sodium metal batteries. |
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