Structure Designing Of Porous Carbon Anode For Excellent Sodium-ion Storage Performance

With the rapid development of renewable energy,sodium-ion batteries?SIBs?are believed to be one of the most promising systems for large-scale energy storage owing to the natural abundance and the low cost of sodium reserves.However,the lack of suitable anode materials has greatly slowed their commercialization.Due to its low cost,highly adjustable structure and performance,hard carbons are still the most promising anode materials for the industrialization of SIBs.But the initial coulomb efficiency and rate performance of them are low.In this dissertation,biomass carbon materials with high initial coulomb efficiency and 3D porous carbon materials with excellent rate performance were fabricated by adjusting the graphite crystallite structures and pore structures.Meanwhile,an ion-conducting SnS-SnS₂ hybrid was designed to coat the outer surface of the porous carbon to prevent the contact of the electrolyte with the inner pore surface,achiving the high initial coulomb efficiency and excellent rate performance.The main content and conclusion are as follows:A biomass-derived hard carbon material with ultra-low specific surface area(3.70m² g^-1)was fabricated by the one-step carbonization method.This hard carbon exhibited high initial coulomb efficiency?69.8%?and excellent reversible capacity(286.7 mAh g^-1).We systematically investigated the influences of heat treatment temperature on the microstructure and electrochemical performance of hard carbon.The results show that with the increase of heat treatment temperature,the specific surface area and interlayer distance become lower,the edges and defects of graphite crystallite become fewer,while the number of nanovoids and layers increases,leading to the rapid slope capacity decay and plateau capacity increase.The biomess hard carbon exhibits higher initial coulomb efficiency and reversible capacity as the heat treatment temperature increases.Secondly,a 3D porous hard-soft composite carbon?3DHSC?with controlled porous and disordered structures was synthesized via a facile NaCl template-assisted method.This 3DHSC is rich in mesopores and macropores,and the porosity?macropores and mesopores?is beneficial to large contact area between the electrolyte and active materials,shortening sodium-ion diffusion distances.In addition,the NaCl template can impede the development of ordered structure during the carbonization process.The formation of graphitized carbon has been influenced and the active sites on the carbon surface for sodium ion storage have increased.This 3DHSC exhibited high reversible capacity(215mAh g^-1)and good rate performance(90 mAh g^-1 at 5 A g^-1).Lastly,we chose commercial activated carbon?AC?as raw material,an effective approach using an ion-conducting SnS-SnS₂ hybrid coating was proposed to decrease the accessibility of the electrolyte to the large inner surface of the porous carbon,irreversible reactions between the electrolyte and the carbon surface was greatly reduced.The formation of the SEI only occurred on the SnS-SnS₂ surface which improved the ICE and helped retain the active sites on the carbon surface for sodium ion storage,which improved the reversible capacity.Moreover,as a good ion conductor after alloying with sodium,the SnS-SnS₂ hybrid ensured the fast desolvation of sodium ions and sodium ion diffusion through the coating layer.As a result,with a SnS-SnS₂ hybrid coating,a commercial activated carbon exhibited high initial coulomb efficiency?68.6%?and excellent rate performance(110 mAh g^-1 at 5 A g^-1),exhibiting great potential to practical application.