Study On Highly Stable Layered Manganese Oxides Cathodes For Aqueous Mg²⁺/Ca²⁺ Batteries

Layered manganese oxides,as a typical layered material,have many advantages such as large ion diffusion channels,high theoretical capacities and flexible morphological plasticity,showing great potential as cathode materials for Mg²⁺and Ca²⁺storage.However,their practical application and development are greatly limited by the poor structural stability.Besides,cation（de）intercalation,Jahn-Teller effect and layered-to-spinel phase transition are the main factors that affect the structural stability of layered manganese oxides.Therefore,in order to improve the structural stability of layered manganese oxide cathode materials for aqueous Mg²⁺and Ca²⁺batteries,this paper investigates the causes and mechanisms of above problems and puts forward three modification methods from the perspective of morphological modification,heterogeneous element doping and defect engineering.The main contents are as follows:Hierarchical-morphology Mg/Ca-birnessite（electro-conversion Mg-birnessite and electro-conversion Ca-birnessite,denoted as ECMB and ECCB,respectively）nanowalls arrays,self-assembled from tiny nanosheets,were prepared via electrchemical conversion of nanoscale Mn₃O₄ parcitles in aqueous Mg²⁺and Ca²⁺electrolyte.Compared with the large-sized HT-BIR（hydrothermal birnessite）prepared by traditional hydrothermal methods,the special morphology of tiny nanosheets shortens Mg²⁺/Ca²⁺diffusion length,which is conducive to the uniform distribution of Mg²⁺and Ca²⁺in the nanosheets.Thus,the structural damage caused by uneven Mg²⁺and Ca²⁺（de）intercalation could be reduced and the structural stability of birnessite is enhanced.In addition,this hierarchical morphology with enriched active（010）planes provides more open channels for Mg²⁺/Ca²⁺（de）intercalation.Meanwhile,kinetic analysis shows that this hierarchical morphology can effectively improve the contribution of surface pseudocapacitance.Taking advantage of increased diffusion channels and enhanced pseudocapacitance contribution,the energy storage performance of materials is greatly improved.Therefore,both ECMB and ECCB electrodes exhibit outstanding reversible capacity and superb cycle stability.For aqueous Mg²⁺battery,ECMB cathode exhibits a large specific reversible capacity of about 255.1 m Ah g^-1 at a current density of 0.2 A g^-1,and outstanding cycling stability with 73.6%capacity retention after 3000 cycles at 2 A g^-1.For aqueous Ca²⁺battery,ECCB electrode shows an specific capacity of 175 m Ah g^-1 at 0.1 A g^-1and 89 m Ah g^-1 at 2 A g^-1.In addition,ECCB electrode also delivers an ultralong cycling performance of 83.2%retention after 2000 cycles at 1 A g^-1.Importantly,ECMB cathode and ECCB cathode were successfully matched with polyimide（Polyimide,denoted as PI）anode to construct ECMB//PI aqueous Mg²⁺battery and ECCB//PI aqueous Ca²⁺battery,both of which showed excellent cycling performance.By virtue of morphology modification,this series of work successfully alleviates the structural destruction caused by ion（de）intercalation and effectively improves the structural stability of layered manganese oxides,which provides a reference for the preparation of ultrastable layered materials.（2）2%Co-doped K-birnessite K_0.11Co_0.02Mn_0.98O₂·1.4H₂O(2%Co doped birnessite,denoted as CB-2,was synthesized by a one-step low-temperature hydrothermal method.The CB-2 exhibits enhanced Ca²⁺storage capability.Moreover,both experiments and calculations show that the doping of 2%Co could significantly shorten the average length of Mn-O bond and greatly increase the binding energy of Mn-O bond,which effectively inhibits the Jahn-Teller effect and thus results in improved structural stability.Thus,in aqueous Ca²⁺system without considering layered-to-spinel phase transition,CB-2 cathode delivers an excellent rate performance and structural stability of 181.2 m Ah g^-1 at 0.2 A g^-1 and 1000 cycles at 2 A g^–1with the capacity retention of 90.4%.For demonstrating application prospect,an ultrastable aqueous Ca²⁺batteries of CB-2//PI was constructed by using CB-2 cathode and polyimide anode.The coulomb efficiency and capacity retention of CB-2//PI after 1000 cycles are both close to 100%at a current density of 1 A g^-1.This work analyzes the reasons why the cycling performance of birnessite is restricted in aqueous Ca²⁺system and provides a superior 2%Co-doped K-birnessite cathode for Ca²⁺storage in aqueous batteries.As far as we know,CB-2 is the most outstanding layered manganese-based cathode material for aqueous Ca²⁺batteries with best reversible capacity and cycling stability,which provides a feasible idea for the development of high-performance layered manganese-oxide in Ca²⁺batteries.（3）Na₂Mn₃O₇-conversion Mg birnessite,denoted as NCMB,were prepared by electro-conversion of Na₂Mn₃O₇.Compared with Na-birnessite（denoted as Na-bir）prepared by conventional low-temperature method,NCMB delivers better Mg²⁺storage ability and structural stability.The experimental results show that NCMB electrode exhibits 180.1 m Ah g^-1 at a current density of 0.2 A g^-1,and almost no capacity decay after 1000 cycles at a current density of 1 A g^-1.According to the analyses,NCMB electrode with nano-flower morphology not only enhances surface pseudocapacitance capability,but also facilitates the Mg²⁺diffusion.This synergistic effect provides NCMB electrode with an excellent Mg²⁺storage ability.Moreover,NCMB with nano-flower morphology inherits the intrinsic regular vacancy structure of Na₂Mn₃O₇.Compared with Na-bir,this intrinsic vacancy structure can effectively alleviate the influence of ion（de）intercalation,Jahn-Teller effect and layered-to-spinel phase transition on layered manganese oxide structure,and ensure its structural stability as cathode for aqueous Mg²⁺batteries.Finally,this paper successfully constructed a stable aqueous Mg²⁺battery of NCMB//PI by matching NCMB cathode and PI anode.This battery demonstrates excellent rate performance and good cycling performance of nearly 89%capacity retention for 500 cycles at a current density of 0.5 A g^-1.In this work,this paper analyze the reasons for the limitation of structural stability of normal birnessite in aqueous Mg²⁺batteries,and adopt a novel cathode of NCMB with high capacity and good stability.Taking advantages of its nano-flower morphology and regular intrinsic vacancy structure,NCMB alleviates the damage of Mg²⁺（de）intercalation,Jahn-Teller effect and layered-to-spinel phase on the layered structure.Therefore,this work provides a forward direction for the realization of high-performance layered manganese oxide cathode with both capacity and circulation for aqueous Mg²⁺batteries.