Research On Anion Redox Transition Metal Sulfur-rich Aluminum-ion Cathodes

Rechargeable aluminum metal batteries（RAMBs）have become one of the most promising battery systems for large-scale energy storage in the future,owing to the high theoretical energy density,natural abundances,and low cost.However,the development of RAMBs is still in its infancy,in which the actual energy density and cycling stability have not reached the application requirements.The main reason is that most traditional transition metal compound cathodes only rely on the cation redox to provide one charge transfer,which is challenging to maintain the charge balance of the local structure in time during the Al³⁺inserted,resulting in low specific capacity and large polarization of the cathode.Besides,the inserted Al³⁺diffuses slowly in the cathode with poor reversibility due to the strong interaction with the anion of cathodes and the limitation by the fixed lattice structure in the crystal.Therefore,developing novel cathode materials that can achieve multi-electron transfer to improve energy density and cycle stability is a major challenge and a critical issue for developing RABs.This dissertation focuses on improving the energy density and kinetics of anion redox transition metal sulfur-rich aluminum-ion cathodes via regulating the electronic band structure,local crystal structure and ion storage mechanism,respectively.Meanwhile,the relevant mechanisms are also revealed.The main results are listed as follows:（1）Aiming at the low capacity of cathode caused by the difficulty of single charge transfer to meet the Al³⁺storage requirements,we proposed regulating the electronic band structure to introduce the anion redox,thus realizing the multi-electron transfer.S₂^2–is introduced into crystalline Ti S₂(Ti⁴⁺S^2–S^2–)to synthesize crystalline Ti S₃(Ti⁴⁺S^2–S₂^2–),forming stable ligand-hole chemistry in the energy band structure,thus inducing the sulfur anions redox.Compared with the crystalline Ti S₂that based on localized single electron transfer of titanium cations,crystalline Ti S₃could achieve the S₂^2–/S^2–anion redox in aluminum batteries with multi-electron transferred in local structure,exhibiting a higher specific aluminum storage capacity（98 m Ah/g）.Experiments and theoretical calculations confirmed the intercalation of Al³⁺in crystalline Ti S₃ for the first time,and the migration along the b direction with a diffusion energy barrier of 418 e V.（2）Aiming at the large polarization and low capacity of cathode caused by the diffusion limitation of Al³⁺in the fixed lattice structure of crystalline materials,we proposed amorphization and anion-enrichment on crystalline materials.Specifically,crystalline Ti S₂(Ti⁴⁺S^2–S^2–)is amorphized by high-energy ball milling with many defects being introduced,promoting the ionic diffusion dynamics.Meanwhile,with the weakening of the Ti-S local bonding structure after amorphization,the proportion of S₂^2-can be further increased,significantly improving the specific capacity.Thus,the synthesized amorphous Ti S₄(Ti⁴⁺S₂^2–S₂^2–)exhibits excellent cycling stability（206m Ah/g,1000 cycles）and good rate capability（150 m Ah/g,under a current density of2 A/g）.The mechanism was analyzed and revealed that the amorphous structure of amorphous Ti S₄ is always maintained during cycling.As Al³⁺intercalated,the local charge compensation is mainly provided by the reversible redox reaction between S₂^2–/S^2–while the coordination number of titanium cation on the local structure decreased.（3）Aiming at the poor cycle stability and low capacity caused by the sluggish kinetics of the conversion reaction and the insulation of sulfur cathode,we proposed a composite intercalation-conversion storage mechanism for aluminum ions.The Mo₆S₈cathode with intercalation reaction and the S cathode with conversion reaction are combined,in which Mo₆S₈ could adsorb the Al_2/3S₆ in the electrolyte to inhibit shuttle effect,improving the cycling stability.Meanwhile,Mo₆S₈ could also catalyze the decomposition of the final product of Al₂S₃,improving the reversibility of the redox and specific capacity of the sulfur cathode.In addition,benefiting from the excellent electrical conductivity and low porosity of Mo₆S₈,the composite cathode can significantly reduce the inactive components（conductive carbon and electrolyte）,improving the energy density of electrode.Therefore,both the S⁰/S^2–and S₂^2–/S^2–redox reactions are achieved in this work via Mo₆S₈/S composite cathode with excellent kinetics and cycle stability.A high energy density of 371 Wh/kg at electrode level was obtained,significantly promoting the development of high-energy-density rechargeable aluminum ion battery.