Preparation Of Graphene-based Anode Material For Potassium Ion Battery And Research On Its Potassium Storage Mechanism

With the promotion of new energy electric vehicles and wind-solar power stations,the market demand for large energy storage devices is increasing day by day.The scarce lithium resources on the earth cannot afford such a huge market demand.People urgently need to find a kind of alkali metal with abundant reserves to replace the role of lithium.Therefore,sodium and potassium,which samely belong to the first group of metals and have abundant reserves,have attracted people’s attention.Although the reserves of sodium are more abundant,potassium-ion batteries,which are closer to lithium-ion batteries in energy density,can be more line with the standards for large-scale energy storage devices.There are many types of materials that can be used for the anode of potassium ion batteries,such as carbon-based materials,potassium alloys,transition metal oxides and sulfides,and some conductive organic materials.Similar to the intercalation reaction with lithium,graphite can intercalate with potassium at 0.24（V vs.K/K⁺）to form the intercalation compound KC8,corresponding to a theoretical capacity of 279 m Ah·g^-1..This is also the main potassium storage mechanism for other carbon-based materials.Even non-graphite carbon materials,such as soft carbon hard carbon,etc.,rely on its internal graphite crystallites as the main potassium storage site.Therefore,the carbon-based anode of potassium ion battery has the advantages of high energy density and high conductivity,and is one of the most promising anode materials.However,the application of carbon-based anodes for potassium ion batteries is faced with the challenges of low rate performance and rapid capacity decay when cycling.The reason is that the reaction process of potassium intercalation into graphite is accompanied by huge volume expansion.The stress generated by the expansion will cause the material to rupture and fail and hinder the diffusion of potassium ions into the material.In response to this phenomenon,this thesis selected graphene oxide as a precursor,starting from two aspects of material compounding and structural regulation,to construct a new type of carbon-based negative electrode:（1）Take a few layers of exfoliated graphite（EG）as the other part and mix it well with graphene oxide（GO）.After 180°C hydrothermal,GO is reduced and translated into reduced graphene oxide（r GO）,then composite EG/r GO is obtained.Electrochemical test results indecats that EG/r GO can reach a specific capacity of up to 433 m Ah·g^-1,which is 1.6 times and 3.2 times the capacity of EG and r GO,respectively.It can still provide 196 m Ah·g^-1at a high current density of 800m A·g^-1.specific capacity.After being characterized by SEM,XRD and Raman,it was found that r GO wrapped the surface of EG and cross-linked each other,which not only avoided the loss of activity after stacking of EG,but also provided double electron and ion transport channels for the intercalation reaction of EG,as well as offering volume buffer.Thanks to the triple optimization of EG by r GO,the excellent potassium storage performance of EG has been fully utilized,thereby enhance the energy density of composite materal.（2）Using coaxial electrospinning technology,the continuous assembly of GO in the tubular nano-cavity constructed by the outer polymer layer is realized.Then,the outer shell of the polymer was removed by the moethod of solidifying-washing and high-temperature pyrolysis respectively to obtain reduced graphene oxide fibers（r GONFs）.After the electrochemical performance test,we found that the fiber obtained by the solidifying-washing method has not high potassium storage performance.The analysis of XRD and Raman characterization results indecats that this is related to their low degree of graphitization.The graphene nanofibers obtained by the high-temperature pyrolysis method have good performance in three aspects:capacity,rate performance and cycle stability.By comparison,we found that although the potassium storage performance of these fibers fluctuates due to high temperature graphitization,the range is not large.This is because the gaps of graphene nanofibers can be used as ion transport channels to shorten the diffusion path of K+and their own flexibility buffers the double effect of volume expansion,which makes up for the lack of graphite potassium insertion in the reaction kinetics.