Study On The Preparation And Lithium/sodium Storage Performance Of Polyacrylonitrile Based Hard Carbon

As a kind of organic polymer pyrolytic carbons, the preparation temperature of polyacrylonitrile based hard carbon materials(PAN-HC) is low. PAN-HC with a large layer spacing and disorder structure has been regarded as a promising anode material of lithium/sodium-ion battery for its high carbon residue rate, good conductivity, effective charging/discharging and high cycle life. In this paper, we prepared a series of PAN-HC with different morphologies and microstructures and PAN/lignin composite hard carbon materials. The relationships between the morphology,microstructure and sodium/lithium storage performance were studied in detail and the insertion-extration mechanisms for lithium and sodium ion in hard carbon materials were explored.In this paper, we prepared novel hard carbon microspheres(HCS) with a uniform walnut kernel structure from PAN by a simple stabilization–carbonization method and used them as anode materials for lithium/sodium-ion batteries(LIBs/SIBs). As carbonization temperature increases,the reversible lithium storage capacity decreases while the reversible sodium storage capacity firstly increases,then decreases. The initial columbic efficiency and rate performance are improved. Among all the samples, the sample carbonized at 1250 ? exhibits the best electrochemical performance: At a current density of 0.02 A/g, the first reversible lithium storage capacity is 348 mAh/g with a initial columbic efficiency of 85%, the first reversible sodium storage capacity is 182 mAh/g with a initial columbic efficiency of 74%; The reversible lithium storage capacity is 89 mAh/g at 0.8 A/g, but the reversible sodium storage capacity is reduced to 20 mAh/g at 0.2 A/g.PAN based carbon nanofibers(PAN-CNFs) were fabricated by the combination of electrospinning method and heat treatment process in our study. The unique nanofiber morphologies and 3-D network structures of PAN-CNFs make its electrochemical properties in LIBs and SIBs better than PAN-HCS. By comparing lithium with sodium insertion behaviors in PAN-CNFs treated at different carbonization temperatures, we demonstrate that the PAN-CNFs anodes in LIBs and SIBs show similar mechanisms : intercalation between graphene layers at high potential sloping region and insertion into nanopores at low potential plateau region. However,due to the differences of ionic radius and ionic electronegativity between Li and Na, they exhibit differemt rate and cycling performance.By adding lignin in the polyacrylonitrile solution, the electrospun polyacrylonitrile/lignin-carbon nanofibrous webs(PL-CNFs) contain more defect and pore structure, which could increase the sodium storage capacity and improve the cycling stability and rate capability. The PL-CNFs anode obtained at 1300? with 50 wt% lignin exhibits a high reversible capacity of 292.6 mAh/g with an initial efficiency of 70.5%,high rate capability(80 mAh/g at 1 A/g, respectively) and excellent cycle stability(247 mAh/g reversible capacity at 0.1 A/g over 200 cycles). It is a very promising electrode material for SIBs.