Study On The Performance Of Manganese-Based Oxide Cathode Materials For Sodium-Ion Batteries

Thanks to high abundance,low cost and high safety,sodium-ion batteries have attracted extensive attention.Manganese-base oxides are considered as one of the most promising cathode materials for sodium-ion batteries due to its high specific capacity and low cost of manganese.However,the large sodium radius and the slow transport kinetics of manganese-based oxides during charge and discharge severely restricted the development of sodium-ion batteries.More importantly,there are two serious problems during electrochemical sodium storage process of P2-type manganese-based transition metal oxides:（1）Due to the sodiation/desodiation at high potentials,the irreversible phase transition of P2-OP4 can lead to the sliding of the Mn O₂-metal slabs;（2）the Jahn-teller effect of Mn³⁺can cause the structural distortion to form the P’2 phase,which results in broken layered structure and low capacity retention at low potentials during charging and discharging process.To solve the above problems,this thesis is mainly focuses on exploring suitable elemental doping of manganese sites to suppress the two phase transition problems existing in manganese-based oxides.The main contents of this thesis are summarized as follows:（1）The layered P2-Na_0.67Mn_1-xMo_xO₂cathode materials were synthesized by high-temperature solid-phase method.The P2-Na_0.67Mn O₂ electrode undergo two phase transformation processes（P2-OP4 and P2-P’2）during charging and discharging based on the In-situ XRD results,resulting in the unstable layered structure and poor cycling stability and rate performance.The Mo-doping P2-Na_0.67Mn_0.97Mo_0.03O₂ cathode has a large Na-layer spacing,which significantly increases the Na⁺transport rate and suppresses the phase transition of P2-OP4.This research reveals that no phase transition occurs at high voltage by doping Mo into the Mn O₆ layer.The initial discharge capacity of P2-Na_0.67Mn_0.97Mo_0.03O₂ cathode was 169.7 m Ah·g^-1 at current density of 20 m A·g^-1,and the average operating voltage was 2.61 V.The capacity retention rate was 86.4%after100 charge/discharge cycles at the same current density.It can be concluded that the P2-Na_0.67Mn_0.97Mo_0.03O₂ cathode has excellent cycle stability and superior rate performance.（2）The layered P2-Na_0.67Ga_xMn_1-xO₂ cathode materials were synthesized via high-temperature solid-phase method.The Ga-doping within P2-Na_0.67Ga_xMn_1-xO₂enhances the specific discharge capacity and cycling performance of P2-type layered transition metal oxides.An optimal doping amount with a stoichiometric ratio of 0.08 was selected by XRD refinement and preliminary electrochemical testing results.The Ga doping enlarged the Na layer spacing and widened the Na⁺transport channel,which are beneficial to the Na⁺transport during sodium storage process.XPS shows that the Ga is an inactive element,which does not participate in the redox reaction during sodium storage.In situ XRD shows that Ga-doping P2-Na_0.67Ga_0.08Mn_0.92O₂ can effectively suppress the Na⁺/vacancy disordering during sodium storage and the phase transition of P2-OP4 at high potential,thus improve the structural stability of the cathode material and increase the Mn³⁺/Mn⁴⁺redox potential.The initial charge and discharge specific capacities of P2-Na_0.67Ga_0.08Mn_0.92O₂ were 70.7 m Ah·g^-1 and 165.1 m Ah·g^-1 at a current density of100 m A·g^-1,respectively.The capacity retention rate was as high as 88%after 200cycles.The discharge capacity of P2-Na_0.67Ga_0.08Mn_0.92O₂ is 123 m Ah·g^-1 at a current density of 500 m A·g^-1,demonstrating an excellent rate performance and long cycling performance.This work gives new insights for development higher performance cathode materials toward next-generation advanced sodium-ion batteries.