Manganese-based Materials Designed As High-performance Cathodes For Lithium/Sodium Ion Batteries

Manganese-rich oxide is a promising cathode material for lithium/sodium ion batteries because of its low cost,incorporation of highly abundant Mn,and large theoretical capacity associated with the Mn³⁺/Mn⁴⁺redox couple(above 200 m Ah g^-1).However,Mn-rich based oxides cathodes are limited to structural transformation and large unit cell volume changes during charge/discharge process.This is mainly due to the cooperative Jahn-Teller distortion of Mn³⁺.In this paper,heterostructures are constructed to suppress Jahn-Teller-active Mn³⁺and improve the structural stability of manganese-rich oxides for electrochemical lithium/sodium storage.First,Li Mn O₂,either in a layered or spinel structure,offers a substantial cost advantage over its analog Li Co O₂ as a cathode material for lithium-ion batteries.The application of Li Mn O₂ cathodes,however,is greatly impeded by the cooperative Jahn-Teller distortion of Mn³⁺,which induces a large structural variation,resulting in very limited cycle life.In this work,we report the preparation of spinel-layered hybrid Li Mn O₂ via in situ electrochemical conversion from spinel Mn₃O₄.Through this conversion,interfacial orbital ordering is induced between the layered and spinel domains with Mn-O octahedra intertwined at 83.8°.This nearly orthogonal interfacial orbital ordering can effectively suppress Jahn-Teller distortion in Li Mn O₂,resulting in greatly enhanced structural stability and outstanding cycle performance.This work provides a new strategy for interface engineering and heterostructure design,possibly stimulating new research on Li Mn O₂ as a practical cathode material for lithium-ion batteries.Second,sodium-ion batteries are promising candidate to replace lithium-ion batteries for large-scale energy storage.Sodium transition metal oxides with layered structures(O3 or P2)are very attractive cathode materials due to their large theoretical specific capacities.However,these layered oxides suffer from poor cycle performance and rate performance because of structural instability and sluggish electrode kinetics.In the present work,we report a monoclinic Na Mn_2-y-?(OH)_2y comprising new layered polymorph with H'3 stacking via Na/Mn(OH)₈hexahedra and O'3 stacking via Na/Mn O₆ octahedra,demonstrating large specific capacity up to 211.9 m Ah g^-1,excellent cycle performance(94.6%capacity retention after 1000 cycles),and outstanding rate capability(156.0 m Ah g^-1 at 50 C).With introduction of H'3 stacking,the monoclinic Na Mn O_2-y-?(OH)_2y exhibits enlarged interlayer distance with greatly increased c value of about 7?,favoring fast Na⁺migration in the Na slabs.The expanded layered structure is ultrastable during Na⁺intercalation/deintercalation with only?2%volume change and no phase transition.This study demonstrates effective structure regulation for monolicinc Na Mn O₂to achieve superior cathode performance for sodium-ion batteries,making it a general strategy for developing high-performance layered oxide cathodes.Finally,typical birnessite is very attractive cathode material for aqueous sodium-ion batteries due to their large theoretical specific capacities.However,the typical birnessite suffer from poor electrochemical performance and narrow potential window because of low Na content,Mn or O defects,and highly catalytic activity.In the present work,we report a high Na content monoclinic birnessite,demonstrating large specific capacity up to 199.9 m Ah g^-1,excellent cycle performance(82.9%capacity retention after 20000 cycles),and outstanding rate capability(125.2 m Ah g^-1 at 10 A g^-1).The high Na content monoclinic birnessite exhibits the fast Na⁺diffusion in the Na slabs and ultrastable structure during Na⁺intercalation/deintercalation with only?1.8%volume change and no phase transition in aqueous Na₂SO₄ solution.This study demonstrates that it is difficult to occur Mn ions migration from Mn O₆ octahedron with high sodium and little crystal water content in birnessite,because of the short average Mn-O bond length and large bond energy of Mn-O,which leads to good cycling stability.