Synthesis And Performance Of Conversion-type Inorganic Electrode Materials With High Capacity For Sodium Storage

With the development of the world economy and the progress of science and technology,the demand for energy is increasing day by day.However,traditional fossil energy is increasingly depleted,which also brings serious environmental pollution.In recent years,lithium-ion battery and other electrochemical energy storage devices have received widespread attention,becoming the next generation of green energy that is expected to replace traditional energy sources.However,the total lithium resources on Earth are low and severely unevenly distributed,leading to the increasing cost of lithium-ion batteries and limiting their widespread application in large-scale energy storage systems.Compared to lithium,the sodium element is more abundant in the earth’s crust,less costly and more suitable for large-scale energy storage applications.The development of electrode materials for sodium-ion batteries with high specific capacity and high stability is one of the key points in the development of high energy sodium-ion batteries.In this paper,the charge transfer performance and structural stability of various inorganic materials are improved through the design of microstructure and composition.A variety of new structure and high specific capacity conversion-type sodium storage electrode materials were obtained and applied to sodium-ion battery and room temperature sodium-sulfur battery.The research contents are summarized as follows:2D MoO₂-C@MXene nanohybrids were synthesized via an in situ complexation and polymerization reactions of ammonium molybdate and dopamine in the presence of the 2D MXene templates,and followed by annealing.Highly conductive MXene nanosheets can provide a stable 2D conductive path.The small size of MoO₂ nanoparticle not only afford decreased length for Na⁺diffusion but also can effectively accommodate the volume expansion through the size effect.The introduction of carbon accompanied with MoO₂ can prevent MoO₂ nanoparticles from agglomeration.Furthermore,MoO₂ nanoparticles also can impede the MXene flakes restacking.The open two-dimensional structure of composites can also provide more active sites for electrochemical reactions.When employed as the anode materials for sodium-ion batteries,it delivers high discharge capacity of 245 mAh g^-11 after 500cycles at 1 A g^-1.Even at an ultrahigh current density of 20 A g^-1,specific capacity of 197.2mAh g^-1 is achieved.Also,it delivers ultra-long life of 5000 cycles with an extremely slow capacity loss of 0.002%per cycle at high current rate.Using calculations,we reveal that the pseudocapacitance significantly contributed to the sodium-ion storage,especially at high current rates,leading to a high rate capability.The Na₂S embedded N-doped porous carbon matrixes（Na₂S-Co@NC）were designed and constructed by encapsulating Na₂SO₄ in a molecular cage of ZIF-67 followed by a high-temperature annealing approach.The spatial and chemical limiting effects of the ZIF-67molecular cage enable Na₂S to form low-scale,highly active nanostructures.The carbon structure formed by the decomposition of ZIF-67 provides a good conductive network in which the highly dispersed Co nanoparticles can also anchor soluble polysulfides efficiently,significantly improving the electrode cycling stability by inhibiting the polysulfide ion shuttle effect.When employed as the cathode materials of room temperature sodium-sulfur cells,these electrode materials have a specific capacity of 337.9 mAh g^-1 at a current density of 0.1C and exhibit excellent cycling stability at a high current density of 2 C with a stable cycle of400 cycles.