Investigation On Design,Potassium Storage Performance And Mechanism Of Manganese-Based Layered Cathode Material

Mn-based layered transition metal oxides have been intensively investigated as one of the most promising cathode materials for potassium-ion batteries（PIBs）at the present stage due to their relatively high theoretical capacities,appropriate operating potential and the facile synthetic chemistry.However,the issues of poor cycle stability,inferior rate performance and low practical potassium storage capacity seriously hinder them further practical application in PIBs.Herein,by regulating the Mn average valence state,modulating the K content and Ti ions doping,respectively,the Jahn-Teller effect of Mn-based layered cathode materials is suppressed and the K⁺/vacancy disordered structure is obtained,which achieves long cycling life,high-rate and high-capacity PIBs.The advanced in situ and ex situ characterization techniques combined with theoretical calculations were employed,the intrinsic potassium storage mechanism of Mn-based layered cathode materials was systematically investigated,and the correlation between structural regulation and potassium storage performance optimization was revealed.This discovery opens a novel route and provides scientific foundations to layered oxide cathode materials for next-generation high-performance PIBs.The following interesting results were achieved:（1）A general novel strategy is proposed to suppress the Jahn-Teller effect of Mn-based oxide cathodes by regulating the Mn average valence state,thereby improving their cycling stability.The Mn-Co-Fe-based layered oxide K_0.5Mn_0.7Co_0.2Fe_0.1O₂was chosen,and Ti doping and Mg doping were carried out,respectively.The introduction of Ti⁴⁺ions reduce the valence of Mn,while the Mg²⁺ions increase the Mn valence.As a result,compared with K_0.5Mn_0.7Co_0.2Fe_0.1O₂and K_0.5Mn_0.6Co_0.2Fe_0.1Ti_0.1O₂with lower Mn average valence,the K_0.5Mn_0.6Co_0.2Fe_0.1Mg_0.1O₂with the highest Mn valence exhibits outstanding cycling stability with capacity retention of 91%at 0.1 A g^-1after 150 cycles.Incorporated with advanced in situ XRD and theoretical calculations,it is demonstrated that the regulation of the Mn average valence state can suppress the Jahn-Teller effect and enhance the structural stability,thereby improving the cycling stability.（2）A unique strategy was developed to form a K⁺/vacancy disordered structure by modulating the K content in the Mn-based layered oxide cathode,thus greatly improving the rate capability and practical potassium storage capacity.The Mn-Ni-based layered oxide K_xMn_0.7Ni_0.3O₂was chosen.The interlayered structure configurations evolve from K⁺/vacancy ordered structure to K⁺/vacancy disordered structure when K content increases from 0.4 to 0.7.As a result,the K⁺/vacancy disordered K_0.7Mn_0.7Ni_0.3O₂as a PIB cathode exhibits outstanding rate capability(67.8 m Ah g^-1at 2 A g^-1)and high discharge capacity(125.4 m Ah g^-1at 0.1 A g^-1).The crystal structure evolution and potassium storage mechanism of K_0.4Mn_0.7Ni_0.3O₂and K_0.7Mn_0.7Ni_0.3O₂materials were deeply analyzed through advanced in situ XRD characterization.It is demonstrated that the K⁺/vacancy disordered structure plays a unique role during the K⁺extraction/insertion process.Finally,incorporated with various experimental measurements and molecular dynamics simulation calculations,the advantages of the K⁺/vacancy disordered structure on K⁺diffusion kinetics were systematically demonstrated.The K⁺/vacancy disordered structure provides interconnected continuous channels for K⁺transport and reduces the energy difference between K⁺sites,enabling materials with fast K⁺transport kinetics and more K⁺storage sites.（3）Based on the simple Ti ions doping method,a K⁺/vacancy disordered structure was successfully designed and obtained,thereby improving the cycling stability and rate performance of Mn-based cathode materials.The Mn-Co-based layered oxide K_0.5Mn_0.9Co_0.1O₂was employed,and the K⁺/vacancy disordered K_0.5Mn_0.8Co_0.1Ti_0.1O₂was obtained by doping with Ti⁴⁺.The obtained K⁺/vacancy disordered K_0.5Mn_0.8Co_0.1Ti_0.1O₂exhibits excellent cycling stability and rate capability.Even after 800 cycles at a large current density of 1 A g^-1,the capacity retention is as high as 86.2%,and it still shows a reversible specific capacity of 61.7 m Ah g^-1at an ultra-high current density of 2 A g^-1.The improvement in cycling stability is attributed to the suppression of the complex phase transition by the K⁺/vacancy disordered structure during the K⁺extraction/insertion process,which provides higher structural stability.The optimization of the rate performance is due to the fast K⁺transport kinetics ensured by the K⁺/vacancy disordered structure.