Air-stability And Electrochemical Properties Of Sodium Nickel Manganese For Sodium-ion Batteries

Sodium-ion batteries are considered as a promising alternative to lithium-ion batteries due to their similar charge storage mechanism and the low cost of Na resources.Cathode materials are the key component to improve the energy density for sodium-ion batteries.Layered transition metal oxides represent one of the most attractive cathode materials because of their large theoretical capacity,diverse crystal phase,and facile synthesis.However,the poor air stability and rate performance limit their practical applications in commercialized batteries.Due to the limitation of interlayer distance,there is trade-off between air stability and rate performance.Among the layered structures,P3 and P6 phases of Na_xNi_1-yMn_yO₂（NNMO）,with large interlayer distances,have attracted great attention due to their superior rate performance.In this thesis,the air stability and rate performance of P3-NNMO and P6-NNMO as cathodes for sodium-ion batteries are systematically investigated through theoretical calculations and control experiments.In this thesis,the vacancy formation energy of Ni in transition metal layer,the vacancy formation energy of Na in Na layer,and Na⁺diffusion energy barrier in P3-NNMO and P6-NNMO were analyzed by density function theory（DFT）calculations to investigate their air stability and rate performance.The DFT calculation results show that P6-NNMO possesses better air stability than P3-NNMO,as revealed by the relatively higher Ni vacancy formation energy（+0.215 e V）and Na vacancy formation energy（+0.152 e V）,respectively.Interestingly,P6-NNMO also exhibits better rate performance with lower Na⁺diffusion barrier（-0.94 e V）.Therefore,P6-NNMO is theoretically considered as superior cathode to P3-NNMO due to the improved air stability and rate performance.To verify the DFT calculation results,P3-NNMO and P6-NNMO powders were synthesized by high temperature solid state sintering,and their air stability and electrochemical performance in sodium-ion batteries have been systematically investigated.It has been found that P6-NNMO exhibits significantly improved air stability as compared to P3-NNMO.During the water soaking experiments,the composition of P3-NNMO changes from Na_0.68Ni_0.52Mn_0.48O₂ to Na_0.15Ni_0.44-y?_yMn_0.56O₂while the composition of P6-NNMO changes from Na_0.64Ni_0.55Mn_0.45O₂ to Na_0.51Ni_0.54-y?_yMn_0.46O₂,demonstrating superior air stability of P6-NNMO.A large amount of Ni²⁺dissolve from P3-NNMO after the water soaking experiments while P6-NNMO presents negligible Ni dissolution.For electrochemical measurements,the as-prepared P6-NNMO shows enhanced rate performance（85.3%capacity retention at 10 C）and cycle performance（77.6%capacity retention after 350 cycles at 0.2 C）compared to P3-NNMO.This work provides effective structural regulation strategy to design advanced layered materials with both high air stability and electrochemical performance for sodium-ion batteries.