| Graphite oxide was first produced by strong oxide agents of concentrated sulfuric acid and nitric acid in a low temperature; and pyrolytic GOs were produced at different temperatures, which used as anode for lithium-ion batteries. Commercial hard carbon was used as anode for lithium(sodium)-ion batteries. The morphology and structure of pyrolytic GOs and hard carbon were characterized by X-ray diffraction(XRD), scanning electron microscope(SEM), Brunauer-Emmett-Teller(BET) measurements and Raman spectroscopy;the electrochemical performance was tested by constant charge-discharge,cyclic voltammetry and electrochemical impedance spectroscopy.The results showed that there was a broad peak at 23.05° which meant a smaller layer distance(0.748 nm) than Hummers’ method, but much lower peak than graphite which indicated a low microcrystalline based stacking thickness( Lc) and fewer layers. With the temperature increasing, water molecules between the layers and oxygen containing functional groups were removed, which was caused layer distance decreased and stacked gradually, but Lc turned much larger,(002) peak turned sharper and shifted right. There were some structural defects of obtained pyrolytic GOs on the edge of the carbon surface caused by distortion and chemical treatment, which were difficulty to recover to complete crystal structure of graphite’s. In the electrochemical performance, LTGO-250 anode delivered the highest first lithium insertion capacity of 1816.9 m Ah/g, and the largest reversible capacity of715.4 m Ah/g, which resulted from larger layer distance and smaller crystalline size providing more activated positions. LTGO-800 anode had the same layer distance with graphite, but it delivered a reversible capacity of 477.3 m Ah/g, which was higher than that of graphite anode’s theoretical capacity(373.2 m Ah/g). At large current density(1A/g), all these samples showed a huge capacity fading, but the capacity retention of LTGO-400 and LTGO-800 were30 % and 32 %. At current density of 100 m A/g, all these samples kept the reversible capacity around 200 m Ah/g after 50 cycles; LTGO-400 anode delivered the highest reversible capacity of 600 m Ah/g with the capacity retention ratio of 99% after 50 cycles..Hard carbon with turbostratic disordered porosity structure had a surface area of 2.2m2/g, an average pore diameter of 11.3 nm and much larger layer distance(0.38 nm) than that of graphite( 0.335 nm). This kind of hard carbon based lithium(sodium)- ion batteries had a better performance on capacity of lithium(sodium)- ion insertion and extraction, rate and cycle performance. In the first sodium-ion insertion and extraction progress, the capacity were361.7 and 259.8 m Ah/g, respectively, while delivered an efficiency of 72 % at the current density of 20 m A/g. In the first lithium-ion insertion and extraction progress, the capacity were 358.1 and 213.2 m Ah/g, which delivered an efficiency of 59 % also at the current density of 20 m A/g. On the rate performance, between the current density from 20 m A/g to1A/g, sodium-ion battery had the capacity decreased from 262.7 m Ah/g to 32.9 m Ah/g and the capacity retention ratio droped to 12.5%; lithium battery had the capacity decreased from 273 m Ah/g to 138.8 m Ah/g and the capacity retention ratio droped to 50.8%. Because sodium had a larger radius than lithium, sodium-ion battery showed obviously capacity fade. When the current density up to 20 m A/g, the capacity of sodium-ion and lithium-ion batteries were up to230 and 359.1 m Ah/g. At the current density of 40 m A/g, sodium and lithium-ion batteries had the capacity of 250 m Ah/g and 260 m Ah/g, both of which showed the capacity retention up to 99 % after 100 cycles. |
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