Molten-salt Electrolytic Graphitization Of Amorphous Carbon And Its Electrochemical Aluminum Storage Behaviors

In view of the difficulty of graphitization transformation of amorphous carbon materials and low added value of products after graphitization,an electrochemical graphitization method for amorphous carbon materials in calcium chloride based molten salt system has been developed in this dissertation,and the process of molten-salt electrolytic graphitization has been systematically studied.The relationship between the graphitization degree and structure characteristics of graphitic carbon materials and the electrochemical performance and reaction mechanism of the battery was analyzed in detail,in order to improve the cycle stability and cycling life of aluminum-ion batteries.In this dissertation,the following aspects are systematically studied:graphitization process of amorphous carbon materials and preparation of graphitized carbon materials,correlation mechanism between graphitization degree and electrochemical behavior of carbon materials,evolution of electrochemical behavior of flakes-stacked graphite with appearance,and influence of microporous structure of ultra-thin graphite on electrochemical behavior of molten-salt aluminum battery.The specific conclusions are as follows:(1)Through systematically investigating the relationship between the electrolytic parameters,such as electrolysis temperature,voltage and time,and the crystal structure and morphology of graphite products,it was revealed that higher electrolysis temperature,higher electrolysis voltage and longer electrolysis time would endow the products with much thinner nanoflakes and high graphitization.The graphitization conversion of amorphous carbon could be realized even at a lower temperature of 700?.The electrochemical graphitization mechanism confirmed that the O-containing functional groups(C-O)from the amorphous carbon materials could be removed by the electrochemical reduction process,and the disordered C atoms were then rearranged in a long range,making them accumulate into highly ordered graphitic structure.In addition,the porous graphite nanosheets prepared at 850?,2.8 V and 2 h showed the most obvious nanoflakes-like structure,the thinnest nano flake thickness(12.7 nm),and the highest graphitization degree(46.07%).Most importantly,the electrochemical graphitization method can be extended to other amorphous carbon precursor materials for the graphitization conversion.(2)The structure-activity relationship between graphitization degree and electrochemical properties was studied by using the graphite nanosheets obtained by the molten-salt electrolysis as the positive electrode.The structure-activity relationship between graphitization degree and electrochemical properties was confirmed.The results confirmed that the higher the graphitization degree of graphitized carbon materials,the more remarkable the intercalation/desorption reaction,the higher the discharge plateau and the larger charge/discharge capacity.The obtained high-graphitization porous graphite nanosheets exhibited an initial discharge specific capacity of 72.7 mAh g-1 at 500 mA g-1.After 1000 cycles at 200 mA g-1,the specific discharge capacity was decreased from 63.6 to 55.5 mAh g-1,the capacity retention rate was reached at 87.4%,and the Coulombic efficiency was stable at 95.4%,showing good rate capability,high capacity and long cycle stability.Moreover,the charge/discharge mechanism of amorphous carbon materials and graphitized carbon materials was clarified.The adsorption and desorption process of AlCl-anion was the main reaction process of amorphous carbon materials.The graphite nanosheets not only showed the reversible adsorption and desorption process of AlCl4-anion in the micropores,but also achieved the reversible intercalation/deintercalation reaction between the graphene layers.(3)The appearance evolution of graphite and its influence on the electrochemical properties of the graphite were detailedly studied by taking the flakes-stacked graphite as the positive electrode.When the acidic leaching time was 6 hours,the obtained graphite possessed the most abundant edge-rich sheet-like morphology,the largest thickness expansion rate(1.4 times)and the lowest content of C-O group(4.2%).In situ XRD revealed that the intercalant gallery height was large enough to ensure the reversible intercalation/deintercalation of AlCl4-anion between graphite layers.After charging and discharging,the thickness of graphite electrode would increase significantly,which was more than 8 times and 3 times of the original thickness,respectively.Because of the edge-rich stacking characteristics,AlCl4-anion could migrate directly from the edge and transfer into the internal graphite layer.This would greatly shorten the diffusion path and promote the rate capability.The specific capacity was kept at?87 mAh g-1 at 50 mA g-1,and?40 mA g-1 at 200 mAh g-1,which was significantly higher than that of the untreated electrode.In addition,due to the ultra-high surface density(14.2 mg cm-2)of graphite paper electrode,the cell displayed a superior areal specific capacity(1.2 mAh cm-2).(4)Based on the advantage of high ionic conductivity of molten salt electrolyte,the relationship between the microporous structures of graphite and the electrochemical behaviors of molten-salt aluminum-ion battery was further studied.Taking flake graphite and expandable graphite as examples,the difference of cell performance caused by the change of graphite micropore structure was explained.It was clarified that the reaction mechanism of flake graphite consisted of not only the intercalation of anions between graphene layers but also the adsorption of anions within mesopores,while expandable graphite was mainly involved in the intercalation and deintercalation of AlCl4-anions.In the AlCl3-NaCl molten electrolyte system,the high discharge capacity of?219 mAh g-1 could be maintained after 1200 cycles even at a high current density of 5 A g-1,with the Coulombic efficiency of 94.1%.Since the flake graphite stacked of smaller and thinner nanosheets exhibited higher mesoporous structures,more ionic transport channels,shorter ionic diffusion paths and faster diffusion kinetics,which was conducive to the adsorption and intercalation of AlCl4-anions,thus obtaining high capacity and good rate capability.