| The rapid development of portable electronic devices and electric vehicles has increased the demand for lithium-ion batteries(LIBs),but the lack of lithium resources has become a new problem.The researchers therefore focused on potassium,which has similar electrochemical behavior to lithium and is abundant in the earth’s crust.Graphite is considered a promising anode material for potassium-ion batteries(KIBs),but due to the large radius of K~+(1.38(?))and the large volume expansion(about 60%)during insertion,the cyclic stability and rate performance of the electrode are not ideal,thus hindering the practical application of graphite anode.In this paper,the working principle of graphite as potassium-ion electrode,the research status and the problems to be overcome are briefly discussed,and then some feasible solutions to these difficulties are proposed.The main research contents are as follows:(1)Using the heteroatom doping strategy,the beneficial effect of fluorine-doping on K-storage performance of graphite is explored.Microcrystalline graphite(MG)is simply purified by immersion with hydrofluoric acid.The results show that the mild purification process not only results in a high purity of 98.59 wt%for MG but also realizes the effective F-corporation in the graphite host.The content of fluoride is 1.02%,and the composition of semi-ionic bond is high,which makes the graphite interlayer distance expand from 3.356(?)to 3.461(?),alleviates the volume expansion of K-ion insertion and accelerates the diffusion kinetics.Thanks to fluorine-doping and the improvement of MG electrode structure,the reversible capacity of the modified electrode can reach 320 mAh/g for the first time at the current density of 0.1 A/g,and the capacity retention rate is74.6%after 100 cycles.Meanwhile,the rate performance is also improved.In addition,the same method is used to modify the flake graphite and the artificial graphite,and the results indicate that the interlayer distance is increased in different degrees,but the fluorine doping content is not ideal.Therefore,the comprehensive K-storage performance of the two kinds of modified graphite are inferior to those of fluorine-doped microcrystalline graphite.(2)The effect of activation temperature on the micromorphology and K-storage properties of microcrystalline graphite is investigated by chemical activation strategy.MG is activated at different temperatures using KOH as the activator.The results indicate that with the increase of temperature,the graphite interlayer distance increases,the grain size decreases,and the disorder increases,gradually.However,when the temperature is up to 650℃,MG is over-etched and impurity insoluble in acid is formed,which has a negative effect on the electrochemical performance.Therefore,550℃is the best activation temperature.The electrode can be stable at a current density of 0.1 A/g for 200 cycles and still has a reversible capacity of 171.9 mAh/g,with a retention rate of 65.5%,which is benefit for its suitable layer spacing,grain size and porous surface structure.At the same time,the modified electrode has a capacity of 104.3 mAh/g at the current density of 0.8 A/g,which shows good rate performance.(3)Using the fluorine-doped carbon coating strategy,it is proved that the reasonable design of carbon coating can improve the performance of potassium storage.The composite is prepared by coating high purity microcrystalline graphite with PVDF as precursor.The results show that the limiting effect of carbon coating on the volume expansion of graphite,increasing specific surface area,abundant mesoporous structure and fluorine-doping can maintain the structural stability of MG and enhance the charge transfer/K~+diffusion kinetics.Therefore,the long cycle stability and rate performance of the electrode are significantly improved.After 500 cycles at a current density of 0.4 A/g,the electrode still has a capacity of 131 mAh/g,and retains an excellent rate capacity retention rate of 42.2%at a current density of 1.0 A/g. |
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