Researches On Synthesis And Sodium Storage Of Biomass-based Hard Carbon Anode Materials

As the next generation of new energy storage devices,the sodium-ion batteries are expected to be complementary to lithium-ion batteries in the fields of low-speed vehicles and large energy storage power stations due to their advantages of low operating voltage,low cost,rich sodium resources,and working principle of rocking chair battery.Hard carbon has a high specific capacity of sodium storage,low potential,and low coefficient of expansion,which is the most promising material for sodium-ion batteries.Biomass is a natural porous and heteroatomic selfenriched carbon precursor.It is an inexpensive means to improve the sodium storage capacity of hard carbon by introducing heteroatomic doping and closed pore in biomass-derived carbon materials.The electrolyte is also an effective way to improve the properties of hard carbon materials.In this work,based on this,a variety of biomass as the precursor of carbon,the carbonyl group,and the structure of closed pore are introduced into the biomass-derived hard carbon at the same time.A variety of high-performance biomass-derived hard carbon anode materials for sodium-ion batteries are prepared.Also,the sodium storage capacity of hard carbon in ester and ether-based electrolyte systems is explored.The microstructure of the material and solid electrolyte interface(SEI)is measured by SEM,TEM,XRD,Raman,XPS,SAXS,and other characterization methods,and the electrochemical properties of the material are tested by the charge-discharge equipment and an electrochemical workstation.The main research results are as follows:(1)Two-step carbonization is an effective means to increase the obturator structure of hard carbon by studying pine-derived carbon.The carbonyl group is introduced through 6 mol L-1 H2SO4 oxidation,the closed pore is increased by two-step carbonization,and high-performance pine-derived carbon anode materials were prepared.SAXS and XPS results show that both carbonyl and the closed pore structure are introduced into the carbon material,and the modified hard carbon anode has a capacity of 350 mAh g-1 at 30 mA g-1 and an ICE of 88.1%.(2)The storage capacity of hard carbon in the ether-based and the ester-based electrolyte is compared.The GITT and CV results show that hard carbon in the ether-based electrolyte delivers decreased polarization and improved charge transfer.The SEI film in the ether is thinner than in the ester and has more inorganic composition evidenced by SEM and XPS results.The electrochemical results show that hard carbon has excellent long cycle stability(retaining 243 mAh g-1 after 5000 cycles at 1 A g-1)and superior rate properties in ether-based electrolytes.The energy density of～166.2 Wh kg-1 is achieved in the ether-based electrolyte full battery.(3)The applicability of the scheme in a variety of biomass precursors(Bamboo,Coconut Shell,Camellia Seed Shell,Litchi,Poplar,and Chinese fir)is compared.The electrochemical results show that the capacity of all samples has an increase of 20 to 50 mAh g-1 and the reversible specific capacity over 320 mAh g-1.Notably,the bamboo-derived carbon shows superior sodium storage capacity(～355 mAh g-1).Physicochemical characterization confirms the carbonyl and obturator structures.By comparing the microstructure differences between biochar-derived materials,it is found that the biochar-derived anode materials with a better storage capacity than others have larger layer spacing and carbon defects.