Studies On Carbon-based Anode Materials For Sodium-ion Stationary Batteries

With the rapid exhaustion of fossil fuel resources and the increase of environmental pollution,the use of renewable and cleaner energy sources,such as solar radiation,wind and waves,is becoming urgent.However,the variability of renewable resources in time,duration and location limits their developments and a wide range of efficient applications.Therefore,a large-scale energy storage system(ESS)is necessary for the utilization of renewable energies in a stable and reliable manner.Lithium-ion batteries have captured portable electronics and electric vehicles markets due to the high energy density and power density,but the low abundance in the Earth’s crust and non-uniform geographic distribution of Li limit their large-scale application in EES.Sodium-ion batteries(SIBs)have recently been considered as an ideal energy storage technology for EES application because of the huge abundance and low cost of Na.However,the anode still remains a grand challenge for the commercialization of SIBs.Based on this background,this thesis investigates three kinds of carbon-based anode materials from biomass derived hard carbons,pitch-based amorphous carbons and pyrolyzed anthracite.(1)Hard carbon anode is a promising candidate for SIBs and attracts wide interest due to the high reversible capacity,low sodium storage voltage and excellent cycling stability.In the first part,we fabricate and study two hard carbons with regular morphology from sugar and cotton,and investigate the influence of carbonized temperature on microstructure and electrochemical performance of hard carbon for the first time.Our result shows that the rate performance of hard carbon is limited by the Na insertion process.The GITT result further proves that the low Na+ apparent diffusion coefficient in plateau region of electrochemical discharge curves is mainly responsible for the poor rate capability.We also have confirmed that the sloping region corresponds to the adsorption of sodium in defected sites,edges and the surface of nanographitic domains while the plateau region is contributed to the nanovoids filling using ex situ TEM and XPS techniques.(2)Although the hard carbon shows high sodium storage performance,the high cost limits its pratical application.In the second part,we improve the disordered degree and sodium storage performance of pitch-based amorphous carbon by adding a second phase into pitch,and realize the pitch utilization in fabricating amorphous carbon anode for SIBs.Firstly,we try to add hard carbon precursors into pitch to improve the electrochemical performance,and the effect is very obvious.The amorphous carbon derived from pitch and phenolic resin shows a high reversible capacity of 284mAh/g.To further reduce the cost,the phenolic resin is replaced by abundant and low-cost lignin to obtain an amouphous carbon with lower cost,and the carbon material exhibits 259 mAh/g sodium storage capacity.Furthermore,we utilize KOH as activator to get an amorphous carbon material with a reversible capacity of 288 mAh/g,and the KOH can be recycled and reused.The application prospect of all amorphous carbons as the anode for SIBs is proved in full cells with Na0.9[Cu0.22Fe0.30Mn0.48]O2 as cathode.(3)In the third part,we report a soft carbon anode material using the abundant and low-cost anthracite as precursor through one simple carbonization process,which is further closer to the level of commercial application.The pyrolyzed anthracite anode shows promising sodium storage performance as demonstrated by prototype pouch cells with a practical energy density of 100 Wh/kg,good rate and cycling performance.Furthermore,the high safety of pouch cells with this anode is proved by a series of safety experiments including nail penetration,short-circuit and overcharge.We also prove the same sodium storage mechanism with hard cabon by using in situ XRD and ex situ XPS.