Application And Energy Storage Mechanism Of Molybdenum Disulfide And Graphite In Dual-ion Batteries

The requirement of electrochemical energy storage devices is increasing.Conventional lithium-ion batteries and supercapacitors are hampered by their inability to integrate high energy density and high power density.Dual-ion battery is a new type of energy storage device in which the cathode and anode ions react with the positive and negative electrodes,respectively.Among them,the reaction potential between anion and positive electrode is relatively high?4.5V vs.Li/Li⁺?,which enables a dual-ion battery to afford a higher working voltage,which is beneficial to obtain a higher energy density.In addition,the migration path of anode and cathode ions in the electrolyte is shortened by half compared with that of lithium-ion batteries,so dual-ion batteries have higher power density.Based on the aformentioned principles,dual-ion batteries have the potential to harvest both high energy density and high power density.In the future,dual-ion batteries will position as an important catalogue among those energy storage systems.A dual-ion battery with modified molybdenum disulfide as negative electrode and graphite as positive electrode was constructed.Molybdenum disulfide?MoS₂?has a unique two-dimensional layered structure and high lithium storage capacity.However,the poor stability of MoS₂ leads to poor cycling performance.In this thesis,NaOH is mainly used to modify the morphology of MoS₂,and carbon coating is ultilized to tune MoS₂ to improve the stability of the negative electrode.Thus,it finally matches the graphite anode material to fabricate dual-ion batteries with long-term stability and high capacity.The main discussion and results are as follows:1.The morphology of MoS₂ was modified by adding NaOH with different concentrations in the preparation process.It was found that with the increase of NaOH concentration,the size of MoS₂ particles increased,the length and thickness of petals increased,the interspace between petals decreased and the specific surface area increased,which was conducive to the contact between electrolyte and active material.However,with the continuous expansion of the petal size,the interspace between the petals decrease,started to form clusters,surface area and material capacity decreased.We tested the electrochemical properties of molybdenum disulfide modified with different contents of NaOH and found that the MoS₂ sample with 1M NaOH concentration had the best electrochemical properties.At the current density of 100mA/g,the discharge capacity of the second cycle was 823.36 mAh/g,after 100 cycles,the discharge capacity was 268.67 mAh/g.After 100 cycles,the discharge capacity was reduced by 67.37%compared with that of the second cycle.2.MoS₂@C composite was prepared by carbon coating with different amount of glucose.The morphology remained as nano-flower cluster structure.With the increase of carbon content,the size of MoS₂@C particles and nanopetals decreased,and the cycling stability increased gradually.The volume retention of MoS₂ with 100 cycles without glucose was 20.99%,and that of MoS₂ with 10 g glucose increased to 52.15%.3.The graphite and carbon fiber paper were respectively assembled as the dual-ion positive electrode half cell.The test results showed that under the current density of 100mA/g,the charging capacity after 50 cycles of the graphite half battery is 48.21 mAh/g,only reduced by 5.73%.The capacity after 50 cycles of the carbon fiber paper half battery is 11.40 mAh/g,and the capacity is reduced 12.84%.The two positive electrode materials were matched with the 1M NaOH concentration MoS₂ and MoS₂@C composites respectively.We found that the working voltage of the entire MoS₂/griphite dual-ion battery was up to 5V.The current density of 10 mA/g was used for the cyclic stability test.The charging ratio capacity of the first cycle was 395.50 mAh/g,the capacity retention after 100 cycles was 72.31 mAh/g,and the capacity retention rate was 18.28%.MoS₂@C/graphite dual-ion batteries were also tested for cyclic stability at a current density of 10 mA/g.The first cycle discharge capacity was 303.02 mAh/g,the capacity after 100 cycles was 194.50 mAh/g,and the capacity retention was 64.19%.