Design And Theoretical Study Of High-Performance Layered Manganese-Based Cathodes Of Sodium-Ion Batteries

To address the issue of global energy shortage and environmental pollution caused by excessive use of fossil energy,global governments have been vigorously developing clean energy as a substitute of fossil fuels.However,clean energy such as wind energy,solar energy and tidal energy are discontinuous in time and space,so large-scale energy storage devices are needed to store energy first,and then transmit it in the form of electric energy at the appropriate time.Sodium-ion batteries are capable as clean energy storage devices owing to their low cost,high energy density and ease of research,and their cathode materials are a significant factor to batteries' energy density,service life and cost,which makes it a key step to develop suitable cathode materials for commercialization of sodium ion batteries.Being environmental-friendly and uncostly,manganese-based layered oxides among various materials draw much attention from worldwide researchers.Nevertheless,manganese-based layered oxides still possess several shortcomings.First,due to Jahn-Teller distortion,they usually suffer from the severe manganese dissolution and leads to severe capacity fading and structural deterioration.Second,their specific capacity is relatively low.Per these two issues,we conducted a series of research and summarized the result in this essay.Firstly,we synthesized spherical P2-Nao.86Coo.5Mno.5O2 and P3-Nao.86Co0.5Mno.5O2 by hydrothermal method and solid-state reactions method.From the results of electrochemical tests,it can be seen that when the voltage range is 1.5-4.0V,the capacity of P2-Nao.86Coo.5Mno.5O2 at 1C is 105 mAh/g which is lower than that of P3-Nao.86Co0.5Mn0.5O2.But capacity retention of P2 phase after 100 cycles at 1C is 68%,and the discharge capacity at 2C is about 70 mAh/g,which are far better than electrochemical performance of P3 phase.However,Cyclic performance is still not up to standard.Then,we synthesized Na1.2Mno.4Ru0.4O2 by the method of secondary solid-state reactions.When the voltage range is 1.5 V-4.0V,the discharge capacity of the first cycle can reach 184.6 mAh/g at 1C rate,and the capacity is greatly improved compared with the conventional manganese-based layered oxide.However,at 1C rate,the capacity retention after 289 cycles is only 23.5%,and the cycling performance still needs to be improved.So,the method of doping inert elements is used to try to improve the cycle performance of manganese-based layered oxides.P2-Na0.86 Co0.5Mno.5O2?P2-Na0.86 Co0.475Mn0.475Ti0.05O2 are synthesized by solid-state reactions.Via a combined experimental and theoretical study it can be found that small amounts of Ti-doping could prevent the Jahn-Teller effect of Trivalent manganese ions by restrain the elongation and shrinkage of Mn-O octahedron,minimize the dissolution of Mn ions.And eventually P2-Na0.86Co0.475Mn0.475Ti0.05O2 exhibits outstanding electrochemical performance:when voltage range is 1.5-4.0V,the discharge capacity at 0.5C is 111.8 mAh/g,and the capacity retention after 200 cycles at 5C is 81.1%.