Modification And Properties Of Manganese-Based Layered Oxide Cathode Materials In Sodium-ion Batteries

Na-ion batteries have attracted a lot of attention and research by researchers due to their wide source of raw materials,easy availability,low price,and the"rocking-chair"charge-discharge principle similar to that of lithium-ion batteries.Manganese-based oxides benefit from the environmental friendliness of manganese element,rich valence,and high theoretical specific capacity are an important object of exploration in cathode materials for Na-ion batteries.However,a larger radius of sodium ions will lead to the collapse of the manganese base oxide structure during the continuous de-embedding of sodium ions,which will affect the performance of materials.The selection of appropriate experimental methods,the use of non-electrochemically active element doping and the construction of alkali metal ion pre-intercalation structures can effectively stabilize the changes in the material structure caused by the deintercalation of sodium ions to achieve the purpose of improving the material properties.Starting from the preparation of P2-Na_0.67Fe_0.33Mn_0.67O₂（NFMO）manganese base oxide with high reversible capacity,this thesis determines the experimental route of sol-gel synthesis of the target material,and on the basis of this experimental method,lithium and aluminum are co-doped to optimize the performance of P2-NFMO materials and the manganese base oxide materials that construct potassium ion pre-interpolation structure.The main findings are as follows:（1）In the process of preparing P2-Na_0.67Fe_0.33Mn_0.67O₂（NFMO）material by sol-gel method using citric acid as chelating agent,when the total metal ions in the P2-NFMO material and citric acid molar ratio is 1:1.5,the two sintering temperatures and their holding time are 350°C,5h and900°C,10h,respectively,the discharge specific capacity of the first turn of the prepared P2-NFMO material reaches 203.8m Ah·g^-1 at a magnification of 0.1C.The material is a typical P2phase structure,the spatial group is P6₃/mmc and the microparticle distribution is uniform,and the EDS test proves that the four elements of Na,Mn,Fe and O are uniformly distributed in the material.（2）Because the non-electrochemically active lithium and aluminum elements do not participate in the electrochemical reaction that occurs when the battery works,it can be stably present in the transition metal layer,which plays a role in increasing the lattice spacing inside the material,thereby inhibiting the material structure changes caused by the de-embedding of sodium ions during charge and discharge,and achieving the purpose of improving the material performance.When the lithium element doping amount is 0.15 and the amount of aluminum element doping is 0.1,the P2-Na_0.67Li_0.15Fe_0.18Al_0.1Mn_0.57O₂（NLFAMO）material prepared by the sol-gel method has excellent electrochemical properties,and the discharge specific capacity of the first turn at 0.1C is 210.8m Ah·g^-1,and it still maintains 67.4m Ah·g^-1 at a high magnification of10C,while the lithium mono-doped P2-Na_0.67Li_0.15Fe_0.18Mn_0.67O₂（NLFMO）material has a capacity of only 14.6m Ah·g^-1.The capacity retention rate of the P2-NLFAMO material after 100cycles at 5C magnification is 80.0%,which is much higher than the capacity retention rate of the lithium single-doped P2-NLFMO material of 51.1%.It is obtained by transmission electron microscopy that the lattice spacing of P2-NLFAMO materials（0.4962nm）is nearly 2.5 times higher than that of P2-NLFMO material（0.2039nm）.（3）The construction of potassium ion pre-intercalation structure benefits from the advantage that the radius of potassium ions is larger than that of sodium ions.In the process of material preparation,potassium ions are introduced as the structural support of manganese-based oxides,which promotes the free deintercalation of sodium ions.The material structure can be effectively avoided during the working process of the battery,so as to achieve the purpose of improving the performance of the material.Pre-intercalated manganese-based oxides were prepared by sol-gel method.When the molar amount of potassium ions was 0.60,K_0.6Al_0.05Fe_0.05Mn_0.9O₂（KAFMO）material had the best electrochemical performance.The reversible capacity of 141.7m Ah·g^-1 can be stably exhibited at the rate of 0.1C,and the capacity after 200 cycles at the rate of 1C and 5C is maintained at 77.2%and 76.4%of the original,respectively.The sodium ion diffusion coefficient of this material is as high as 4.54·10^-11cm²/s.The above results demonstrate that the potassium ion pre-intercalation structure can improve the electrochemical performance and kinetic performance of the material.