CuTCNQ And Red Phosphorus As Advanced Electrode Materials For Potassium-ion Batteries

Lithium-ion batteries(LIBs)are widely used in portable electronic devices and electric vehicles because of its outstanding performances.Due to the scarcity and uneven distribution of lithium(Li)resources,the price of LIBs may rise sharply in the future.It is urgent to find alternative energy-storage systems.As one of the alkaline metal elements,potassium(K)is naturally abundant compared with Li and they have similar physicochemical properties.In addition,a conventional graphite anode for commercial LIBs can also reversibly store and release K~+ions.All of these have prompted K-ion batteries(KIBs)to gain increasing attention.But the large radius of K~+cation makes it difficult to seek suitable electrode materials for KIBs.For cathode materials,inorganic materials exhibit high redox potential but low capacity,organic materials are the opposite.Metal-organic frameworks(Prussian blue analogs)are ideal candidates because they possess the advantages of both high potential and moderate capacity.For the anodes,carbon materials and organic materials exhibit lower capacity,and most alloy-type materials have drawn extensive attention owing to their high capacity and suitable operating voltages.Among these alloy-type materials,phosphorus-based materials are promising anodes since they deliver the highest theoretical specific capacity of 865mAh/g.Therefore,this thesis studied the electrochemical properties and K~+-storage mechanism of copper-tetracyanoquinodimethane(CuTCNQ,a metal-organic material)and red phosphorus.Firstly,high-conductivity phase-I of CuTCNQ was prepared successfully by localized-corrosion-deposition method,and its K~+-storage performance and mechanism were evaluated.Ex-situ XRD,XPS experiments indicated that both cuprous cations(Cu~+)and organic anions(TCNQ~-)were electrochemically active in terms of a three-electron redox mechanism,thereby enabling CuTCNQ to yield a high capacity of 244 mAh/g.By optimizing the carbon film interlayer,CuTCNQ achieved good cycle stability and maintained a practical capacity of 170 mAh/g after 50 cycles in the voltage range of 2-4.1 V.XRD experiment showed that the crystal structure of CuTCNQ became amorphous around 2 V.Thus,after narrowing the voltage range to 2.4-4.1 V,CuTCNQ delivered better cycling stability with negligible decay during 150 cycles.When the current density was elevated to 1 A/g,CuTCNQ still exhibited excellent rate performance with the discharge capacity of 125 mAh/g.The rate performance of CuTCNQ was comparable or even superior to that of most cathode materials.Secondly,low-conductivity phase-II of CuTCNQ was synthesized successfully,and its electrochemical performance was investigated.Though the electrical property and crystal structure were quite different,reversible capacity,rate and cycle performance of phase-I and phase-II were similar.In addition,CV test showed that there were some differences in electrochemical parameters such as redox potential between phase-I and phase-II,indicating that deeper studies of isomers were important.Moreover,K-ion full cells were assembled with phase-II CuTCNQ as cathode and graphite as anode.The full cell delivered a high capacity of 170 mAh/g(based on cathode)and good rate performance,suggesting the potential application of CuTCNQ phase-II as an advanced cathode material.Finally,red phosphorus/Super P and red phosphorus/graphene composites were prepared by simple mechanical ball-milling.They can both store K~+reversibly and show obvious charge/discharge slopes.Red phosphorus/Super P delivered reversible capacity of 262 mAh/g(deducting the contribution of Super P),and the capacity retention ratio was only 27%after 30 cycles.Compared with red phosphorus/Super P,after being composited with 2D graphene nanosheets,red phosphorus/graphene exhibited a high reversible capacity of 378 mAh/g(deducting the contribution of graphene),capacity retention ratio was increased to 62%after 30 cycles.CV curves at different scan rates showed that red phosphorus/graphene exhibited higher apparent K~+diffusion coefficient and better kinetics.This study demonstrated the importance of choosing suitable carbon matrix to improve the electrochemical performance of alloy materials.