Research On Sodium Ion Anode Material Based On Biomass Hard Carbon

Although hard carbon is considered as a commercially available sodium ion battery anode materials,there are still two major problems that limit the application of hard carbon.First,the initial coulomb efficiency of hard carbon is low;second,the storage mechanism of sodium on hard carbon is still controversial.In this paper,the hornet’s nest and canola pole are selected as precursor materials for preparation of hard carbon.The effects of different inert atmospheres on the properties of prepared hard carbon and the exploration of Na storage mechanisms are investigated.The specific research results are as follow:Firstly,the hard carbon is prepared using the hornet’s nest under a nitrogen and argon atmosphere which properties are investigated.XRD（X-ray diffraction）,cyclic voltammetry and charge/discharge curve analysis at 0.1 C confirm that the materials prepared under both nitrogen and argon atmospheres are hard carbon.At 1000 ℃,carbon materials synthesize under Ar have higher specific surface area,however the carbon materials under nitrogen are even larger when the carbonization temperature increases.According to experiments results,materials with large specific surface areas have lower coulombic efficiency at the same carbonization temperature.At a suitable carbonization temperature（1200 ℃ and 1400 ℃）,the hard carbon under argon possesses lower specific surface area,while,higher initial coulomb efficiency（4-6%）and higher capacity retention（3-6%）.Thus,it is inferred that high-performance hard carbon can be obtained under argon atmosphere.Secondly,the prepared hard carbon material,used the hornet’s nest as a precursor,has poor storage properties.The hard carbon material is prepared under the argon atmosphere using the canola pole which has excellent electrochemical performance.The hard carbon prepared at 1400℃has the best electrochemical performance.Its initial charge capacity is 330.7 mAh g^-1,the initial coulombic efficiency is 75.68%,and the cycle retention rate is 83.33%after 100 cycles.At higher current densities,the capacity is also significant,with a reversible capacity of 165.3 mAh g^-1 at a current density of 1 C.Thirdly,the material prepared at 1400℃ is selected as the material for exploring the sodium storage mechanism.The microstructure of the material has changed by ball milling.According to the characterization of the material,the specific surface area and pore volume of the material increase as the milling time increases.TEM（transmission electron microscopy）analysis indicated that the graphite-like structure is significantly reduced,and the degree of disorder is significantly increased.At the current density of0.1 C,the increase of ball milling time causes the reversible charging capacity and platform capacity of the material constantly decrease while the slope capacity gradually increases.Therefore,the capacity of the slope region（high potential）is mainly derived from the adsorption of Na-ion in defected sites,edges.The platform region（low potential）is due to Na-ion insertion into the graphite sheets.This is consistent with the mechanism of"adsorption-intercalation".