Basic Research On The Application Of Layered Transition Metal Oxide Na₂Mn₃O₇ In New Battery Systems

Energy storage battery is one of the most promising energy storage technologies due to its higher energy density,long cycle life,better flexibility and free geological limits.Based on the type of electrolyte,types of batteries can be roughly divided into non-aqueous and aqueous electrolyte rechargeable batteries.In consideration of resource sustainability,cost,and safety,new battery systems such as sodium ion batteries,aqueous sodium ion batteries and aqueous zinc ion batteries have gradually become research hotspots.In this case,we hope to design versatile electrode material to host different alkaline ions.And the electrode material exhibits stable electrochemical activities in both aqueous and non-aqueous electrolytes.The layered transition metal oxide has the advantage of high capacities and low over potentials.In addition,the manganese element has the characteristics of abundance,low cost,non-toxicity,and multivalent state.Therefore,the layered sodium manganese oxide is favored among many electrode materials.Na₂Mn₃O₇(Na_4/7[□_1/7Mn_6/7⁴⁺]O₂,in conventional Nax MO2 notation)consists of Na+and□[₁₇Mn₆₇^4（10）]layers that stack alternately.[Mn₃O₇]_?^-2layers built up with edge sharing Mn O₆octahedra and have one-seventh of the Mn vacancies.Na⁺ions between the transition metal layers have two unusual local coordination environments with neighboring oxygens,in which the effective coordination numbers are 5 and 6.Because of triclinic structure involving weak bonding of Na between two adjacent lamellar sheets,which is beneficial to the diffusion and storage of cations between layers,Na₂Mn₃O₇can act as a versatile insertion host for multiple cations.In this paper,Na₂Mn₃O₇is used as the active material in（non-aqueous and aqueous）sodium ion batteries,aqueous zinc ion batteries,respectively.At the same time,we reduced the Jahn-Teller distortion by substituting Mn ions with other metal ions and studied the electrochemical performance and the crystal structure.The specific research contents are as follows:1.Using solid-phase method to synthesize Na₂Mn₃O₇.Na₂Mn₃O₇is used as a cathode material for sodium ion batteries,the specific charge capacity of Na₂Mn₃O₇can reach 149.9m Ah g^-1in the voltage range of 1.5～3 V（vs.Na⁺/Na）.At a high rate of 10 C,the capacity reaches 80 m Ah g^-1and can be cycled steadily for more than 2800 cycles,Na₂Mn₃O₇exhibits good cycling stability and rate capability.During the cycle,Na₂Mn₃O₇experienced single-phase reaction and two-phase reaction,keeping the original overall structure basically unchanged,which is beneficial to achieve a long cycle life of the battery.2.In order to reduce the adverse influence of the Jahn-Teller effect of Mn³⁺,Mn sites of Na₂Mn₃O₇was substituted by Ti or Cu.For Ti-substituted Na₂Ti Mn₂O₇（abbreviated as NTMO）,Ti substitution changes the crystal structure of the material.NTMO is a two-phase mixture of lamellar phase and tunnel phase.The disordered arrangement of Mn/Ti suppresses Na-vacancy ordering in Na layers.Ti substitution suppresses the Jahn-Teller distortion of Mn³⁺,inhibits the dissolution of Mn ions,further improves the cycle stability.Therefore,NTMO has wide operating voltage range of solid-solution reaction and has better cycle stability than Na₂Mn₃O₇at low current density.For Cu-substituted Na₂Cu Mn₂O₇（abbreviated as NCMO）,Cu substitution does not change the crystal structure of the material but expands the layer spacing,which is beneficial to the de/intercalation of Na⁺.Cu as an active element undergoes a redox reaction at a higher potential.By replacing Mn with Cu in the lattice,more of the Mn ions are in the+4 oxidation state,which not only reduces the Jahn-Teller distortion,but also reduces the Mn redox reaction which is rate-limiting,thereby improving the Na⁺transport performance.Therefore,NCMO has more excellent high-rate performance and has a higher average potential than Na₂Mn₃O₇.3.The electrochemical behavior of Na₂Mn₃O₇in aqueous batteries was preliminarily studied.Studies have shown that in the voltage range of-1.0～0 V vs.SCE（1.95～2.95 V vs.Na⁺/Na）,Na₂Mn₃O₇undergoes an irreversible phase change and loses its electrochemical activity in neutral 1 M Na₂SO₄solution.In the 1 M Na OH+1 M Na₂SO₄solution or 2 M Na OH solution or 5 M Na OH solution,Na⁺is released due to the over-intercalation of H⁺during the negative scan.The initial scanning was found to trigger an irreversible phase transition from the Na₂Mn₃O₇to Mn₃O₄,which has electrochemical activity in this voltage range.On this basis,the cycle performance of Na₂Mn₃O₇as a cathode material for aqueous zinc ion batteries in different electrolytes was investigated.In 3 M Zn（CF₃SO₃）₂aqueous solution,Na₂Mn₃O₇has better cycle performance.In 2 M Zn SO₄aqueous solution,regardless of whether the additive contains 0.1 M Mn SO₄,Na₂Mn₃O₇has a higher capacity but poor cycle performance.In addition,compare the influence of materials structures on the cycle performance.Under the same conditions,due to the synergistic effect of the layered and tunnel structure,the cycle stability of layered-tunnel hybrid NTMO is better than that of layered Na₂Mn₃O₇and tunnel Na_o.44Mn O₂.