Study Of Cathode Materials For Aluminium-based Ion Batteries Based On AlCl₃-based Ionic Liquid Electrolyte

Rechargeable aluminum-based ion batteries have been receiving great attention as a promising candidate technology to address future electrical energy storage needs of large scale mobile and stationary devices,due to the low cost and low flammability of aluminum metal and trivalent character of aluminum ion.However,the scarcity of sui Tablecathode materials and the controversial energy-storage mechanism of the reported cathode materials hinder their application.In this thesis,a new strategy was proposed to prepare small graphite nanoflakes（SGN）from natural graphite using an electrochemically assisted pulverization.In this strategy,natural graphite was lithiated firstly,followed by violent reaction with aqueous solution to pulverize the natural graphite to SGN.The as-prepared SGN shows much higher discharge capacity and much better rate performance than pristine natural graphite.The excellent electrochemical performance of SGN can be attributed to the following reasons:（i）the small size and thickness of SGN makes[Al Cl₄]^-intercalation/deintercalation easier,resulting in more efficient utilization of the material and larger capacity.（ii）SGN with large end surface provides a number of active sites for[ACl₄]^-intercalation/deintercalation,in favor of rate performance;（iii）the small particle size of SGN makes the ion diffusion distance much shorter and improve the rate performance,because the diffusion time t（t=ɑ²/D）in a solid material is proportional to the square of the diffusion distance,ɑ,and reversely proportion to the diffusion coefficient,D.（iv）high crystallinity and low defect concentration contribute to its good cycle stability.The SGN material and its synthetic strategy is significant to the further development of graphite based materials for AIBs cathodes.The energy storage mechanism of a promising Co Se₂-based cathode involved incorporation of Al³⁺into Co Se₂ to generate Al_mCo_nSe₂(i.e.,partial substitution of Co²⁺by Al³⁺)and elemental Co,while the dissolution of active cobalt species into the electrolyte and the pulverization of the Co Se₂ phase severely deteriorate their capacity retention.Once understood the capacity-deterioration mechanism of the cathode,a Co Se₂/carbon nanodice@r GO composite material was designed and fabricated to suppress dissolution of Co,enhance the structural stability,and increase composite conductivity,resulting an superior cycle performance（after 500 cycles）of 143 m Ah g^-1 at 1000 m A g^-1.These findings provide useful insight into and guidelines for designing novel electrode materials for the further development of high-capacity RAIBs.Red phosphorus（RP）was selected as a positive electrode to create a new Al-RP battery in this thesis.As expected,RP has been proved to be quite s Tablein the lewis acidic ionic liquid-based electrolyte.Remarkably,detailed characterizations revealed that the energy storage involved the RP-based electrochemical oxidation reaction with a five-electron transfer reaction.Based on the five‐electron redox reaction,electrochemically active PCl₄⁺can be produced with an unprecedented TSC of 4322m Ah g^-1.Subsequently,the capacity fading reasons,related to the poor conductivity,“dead phosphorus”,and especially“shuttle effect”of PCl₄⁺,were well understood.Then,based on this capacity-deterioration mechanism and with the help of density functional theory（DFT）calculations,graphene aerogel coating/red phosphorus（GA-C/RP）composites,and graphene aerogel separator interlayer（GA-SI）were predesigned and synthesized to assemble an Al//GA-SI//GA-C/NRP battery for resolving these issues.As an consequence,this battery afforded an ultra-high initial capacity beyond1512 m Ah g^-1 with an energy density of 1156 Wh Kg^-1.This work makes it intriguing to use RP as promising positive electrodes to create a new Al-RP energy storage system,making a huge step forward for the practical application of AIBs with ultra-high capacity.