Preparation And Electrochemical Performances Of Sucrose-based Hard Carbon Anode Materials For Sodium Ion Batteries

Sucrose is widely used in the synthesis of hard carbon materials due to its advantages of low cost,green regeneration and high carbon content.However,when sucrose is dehydrated at high temperature,it is often accompanied by serious foaming phenomenon,resulting in a large number of holes and defects,which reduces the coulombic efficiency of hard carbon anode in the first charge and discharge process and hinders its practical application in sodium ion batteries.In order to synthesize high-performance sucrose based hard carbon anode materials for sodium ion batteries,sucrose based hard carbon with high Na+ storage capacity,high first coulombic efficiency,excellent rate and cycle properties were successfully prepared by carbon coating,introducing sodium salt additives to control the size of carbon microcrystals and constructing composite carbon materials.(1)3-Hydroxytyramine hydrochloride(PDA)was self polymerized under alkaline conditions,which made it adhere to the surface of sucrose based carbon microsphere.The coating structure of PDA pyrolytic carbon@sucrose based carbon microsphere was obtained by high temperature pyrolysis.The coating layer of PDA pyrolytic carbon can effectively reduce the holes and defects on the surface of carbon microspheres,which can significantly improve the first coulomb efficiency of electrode materials.In addition,PDA pyrolytic carbon with disordered carbon layer provides more active sites for the storage of Na+,and expands the carbon layer spacing of hard carbon materials,which makes a great contribution on improving the sodium ion storage capacity of carbon anode.PDA pyrolytic carbon can also be coated on the surface of carbon microspheres as an elastic buffer layer,which can effectively retard the destruction and pulverization of crystal structure of materials caused by the volume expansion during the Na+ insertion/extraction process,and improve the cycle stability of sucrose based hard carbon.When the coating ratio of PDA pyrolytic carbon is 5%,the Na+storage capacity of 5%PDA-S-1200 sample can reach 309.2 mAh g-1 at 0.1 C,and the coulombic efficiency of the process of initial charge can reach 85.66%,which shows great cycle and rate properties.(2)In the process of dehydration and carbonization of sucrose with concentrated sulfuric acid,sodium salt additives(NaCl and Na2SO3)were introduced to regulate the microcrystalline structure and carbon micro size of sucrose based hard carbon,and the relationship between the structure of sucrose based hard carbon and electrochemical properties was studied.The introduction of sodium salt additive can effectively change the arrangement and lateral widths(La)of carbon layer in the pseudographitic domains of hard carbon,improve the disorder degree of carbon materials and shorten the lateral widths(La)of carbon layer,thus exposing more sodium storage active sites.In addition,the decrease of the lateral widths(La)of the carbon layer can avoid a large number of holes and defects caused by disorderly stacking,which is helpful to improve the initial coulombic efficiency of the electrode material.Compared with NaCl,on the one hand,the addition of Na2SO3 can decompose to produce SO2 gas in the process of sucrose carbonization and improve the disorder degree of carbon layer;on the other hand,the addition of trace S element can expand the interlayer distance of carbon layer and improve the conductivity of electrode material.Therefore,Na2SO3 additive can more significantly improve the structure and sodium storage performance of hard carbon.By optimizing the feed ratio,0.04 M-Na2SO3-S-1200 has a high Na+storage capacity of 329.1 mAh g-1 at 0.1 C,the initial coulombic efficiency can reach 80.79%,and shows excellent rate performance.The sodium storage capacity can still reach 150 mAh g-1 at 5 C.Thanks to the optimized microcrystalline structure,the carbon anode material also shows good cycle stability.After 1500 cycles of charge discharge test at 2 C,the sodium storage capacity can still reach 280 mAh g-1,and the capacity remains about 100%.(3)The three-dimensional conductive network of carbon fiber@carbon microsphere was constructed by impregnating absorbent cotton with sucrose solution and carbonizing at high temperature.The sucrose@cotton composite carbon materials with nano microstructure were prepared.At the same time,the tight connection between the tubular carbon fibers and the sucrose based carbon microspheres provides a directional channel for ion/electron transport,which improves the electrochemical reaction dynamics of carbon materials.In addition,the nanotube structure of carbon fiber and the internal space of carbon fiber@carbon microsphere can effectively buffer the strain caused by sodium ion insertion/extraction and improve the structural stability of electrode materials.In addition,the graphitization degree of cotton pyrolytic carbon is higher,which can significantly reduce the polarization phenomenon during the charging and discharging process,and improve the sodium storage capacity of low potential platform area.The electrochemical properties of S-C-6:4-1200 showed a reversible capacity of 303.9 mAh g-1 at 0.1 C,and the capacity conversion efficiency in the first process of charge/discharge reached 79.53%.At the same time,S-C-7:3-1200 had a higher rate capacity when the proportion of absorbent cotton pyrolytic carbon was 30%,and its specific capacity maintained at 160 mAh g-1 at 2 C.