Preparation And Electrochemical Performances Of P2-type Layered Manganese-based Sodium-ion Battery Cathode Materials

Sodium-ion batteries（SIBs）have become the competitive electrochemical energy storage technology in the post-lithium-ion battery era because of their outstanding advantages of low cost,high safety,and wide operating temperature range.As an important component of SIBs,the capacity,rate performance,and cycling stability of cathode materials directly determine the energy density,power density,and cycling life of the full battery.Among various types of cathode materials for SIBs,P2-type layered manganese-based oxide is considered to be a promising cathode material for SIBs owing to its low cost,simple preparation,and high theoretical capacity.However,the electrochemical performance is seriously affected by the structural phase transition,the Mn³⁺-inducing Jahn-Teller effect and the Mn³⁺disproportionation-indcucing dissolution of Mn²⁺ions in the electrolyte during the insertion/extraction of sodium ions.To address these issues,this thesis is to optimize the P2-type layered manganese-based oxides by means of coating and entropy modulation and then to focus on investigating the relationship between microstructures,compositions,and electrochemical properties.The following research achievements were made:（1）To address the Jahn-Teller effect by Mn³⁺and Mn³⁺disproportionation in layered manganese-based oxides,the non-electrochemically active sodium ion conductor Na₃Zr₂Si₂PO₁₂ was coated on P2-Na_0.612K_0.056Mn O₂ by the wet chemical method.The electrochemical properties of the material were optimized by regulating the content of Na₃Zr₂Si₂PO₁₂.The introduction of Na₃Zr₂Si₂PO₁₂ can reduce the Mn³⁺content and increase the content of non-Jahn-Teller effect Mn⁴⁺.Meanwhile,due to its high ionic conductivity,the introduction of Na₃Zr₂Si₂PO₁₂ can significantly improve the diffusion rate of ions in the P2 phase cathode.The optimized P2-Na_0.612K_0.056Mn O₂@Na₃Zr₂Si₂PO₁₂ exhibited a high initial discharge specific capacity of 145.7 m Ah g^-1 at a current density of 500 m A g^-1 and possessed a capacity retention of 69%（100 m Ah g-1）after 100 cycles.（2）To address the issue of structural phase transformation caused by Na⁺/vacancy rearrangement in layered manganese-based oxides,the concept of medium entropy was applied to P2-type layered manganese-based oxides by doping different ratios of Co,Ni,and Ti elements into Na_0.7Mn O₂ and preparing a series of single-phase medium entropy solid-solution compounds by high-temperature solid-phase method.Among those dopants,Co elements can improve the electronic conductivity of the material and strengthen the rate performance;Ni can improve the Na⁺storage activity of the material;Ti can stabilize the lattice oxygen and prevent the material from being oxidized under high voltage.The optimized P2 phase Na_0.7Mn_0.4Co_0.4Ni_0.1Ti_0.1O₂ cathode material has a quasi-slope charge/discharge curve,which indicates that the complex structural phase transition is effectively suppressed during the charge/discharge process;the material has a specific capacity of 82.8 m Ah g^-1 in the first cycle at a current density of 100 m A g^-1 and a capacity retention rate of 96.7%after 100 cycles;in the rate performance test,the specific capacity remained a value of 48.8 m Ah g^-1 even at a high current density of 1000 m A g^-1.（3）To address the issue of structural phase transition due to Na⁺/vacancy rearrangement in layered manganese-based oxides,the high entropy concept was applied to P2-type manganese-based oxides by doping different ratios of Co,Ni,Ti,and Li elements into Na_0.7Mn O₂.Based on the mentioned–above results,Li was further introduced to the Ni lattice sites,enabling an effective increase of the specific capacity.In-situ XRD tests verified that the optimized P2-Na_0.7Mn_0.4Co_0.4Ni_0.1Ti_0.05Li_0.05O₂material always maintained the P2 phase structure throughout the Na⁺insertion/extraction process without complex phase transitions;P2-Na_0.7Mn_0.4Co_0.4Ni_0.1Ti_0.05Li_0.05O₂ achieved a discharge capacity of 99.2 m Ah g^-1 in the first cycle and a capacity retention rate of 93.7%after 100 cycles under the test conditions of 100 m A g^-1.