The Synthesis And Sodium Storage Properties Of Layered Mn-based Oxides Electrode Materials

Among various Li-ion battery candidates（sodium,magnesium,aluminum-ion batteries,supercapacitors,etc.）,sodium-ion batteries have a strong development momentum,not just because sodium shares the same working principle as lithium,but benefits from its natural advantages of rich and prolific.However,each coin has two sides,it’s an inevitable trend to develop materials with high capacity,large energy density,and high stability owing to material owing to lithium storage materials are not necessarily suitable for storage sodium with larger radius.Particularly,layered transition metal oxides,due to high capacity and high voltage,as well as safe and easy to synthesize,have been a class of materials with unique application advantage among cathode materials.Both P2,O3 and tunnel materials have their strengths and weaknesses.In this paper,we focus on Mn-based layered oxide materials,explores the sodium storage characteristics and conduct modification research in view of its poor cyclic stability and rate capability.The following state the main contents and conclusions:To start with,the bulk material,Na Mn_2/3Ni_1/3O₂,was synthesized by co-precipitation method,which showed P2 phase structure and lamellar morphology.The CV test showed that the material exhibits complex electrochemical behavior,indicates that the material undergoes multi-stage phase transition.In a voltage range of 2.0-4.2 V,the bulk material achieves an initial discharge capacity of 124 mA h g^-1and a first-week Coulomb efficiency of 68.3%.When the current is increased to 1C(100 mA g^-1)and 10C,the material exhibits reversible discharge capacity of 62.4 mA h g^-1and 37.0 mA h g^-1,respectively.It can be seen that the capacity decay occur between 0.1 and 1C.When carried on cycles test at 0.2C,the reversible capacity is 43.8%of the initial value after 100 cycles.Ex situ XRD and FTIR affirmed that material showed better air stability,although the capacity reduced was observed by electrochemical test,the aged-material remained initial structure and no impurity phase was formed.In summary,the material is less sensitive to air.Secondly,on the basis of bulk materials,lithium doping is used in this paper to suppress the multi-stage phase transition of the material during sodium storage,thereby improving the cycling stability of the material.Li-doped Na_1-xLi_xMn_2/3Ni_1/3O₂（x=0.05,0.1,0.2）materials were synthesized by oxalate co-precipitation.The doping of Li introduces a small amount of O3 phase,and the mixed phase structure can increase the difficulty of phase transition,thereby improves the structural stability.With the increase of Li doping amount,the phase transition of the material is gradually suppressed,and Na_0.8Li_0.2Mn_2/3Ni_1/3O₂exhibits the most excellent electrochemical comprehensive performance.At 0.1C,0.2C,0.5C,and 1C,the material delivered reversible discharge capacity of 107.3,114.3,105.7,100.0 mA h g^-1,respectively.Compared with the bulk material,the Li-doped material possessed a good capacity retention.Finally,this paper also explored the effects of structure and electrochemical behavior of the bulk material by Cu-and Mg-doping.The synthesized Na Mn_2/3Ni_1/6Mg_1/6O₂,Na Mn_2/3Ni_1/6Cu_1/6O₂have a P2 pure phase,while Na Mn_2/3Ni_1/6Cu_1/12Mg_1/12O₂has a P2+O3mixed structure.The charge-discharge curves show that Cu-and Mg-doping also inhibit the irreversible phase transition of the material.Among them,Na Mn_2/3Ni_1/6Cu_1/6O₂shows the best rate performance(48.2 mA h g^-1,10C)and cycle stability（77.2%,0.2C,100 cycles）,while Na Mn_2/3Ni_1/6Cu_1/12Mg_1/12O₂has the highest initial discharge capacity(90.7 mA h g^-1,0.1C),which proves that copper doping is a good method to improve the structural stability of the material and remainthe capacity.