Studies On Preparations And Na-storage Properties Of Layered Manganese Based Solid Solution Cathode Materials For Sodium Ion Batteries

In this paper,P2-type Manganese-based solid-solution cathode materials with excellent high-rate cycling stability have been synthesized by solid-state method.The effects of Nb⁵⁺and Si⁴⁺doping,Co₃O₄ surface modification,and La₂O₃ and TiO₂co-modification on the microstructures and sodium-storage properties of pristine P2-type Na_2/3Co_0.25Mn_0.75O₂?NCM?were studied,respectively,by XRD,FE-SEM,TEM,EDS,XPS,EIS,CV and galvanostatic charge-discharge tests.The experimental results showed that the high-valence Nb⁵⁺and Si⁴⁺doping enlarged the Na-layer spacing and lattice volume,reduced the bond energy of Na–O,and improved the dynamic properties of NCM,meanwhile suppressed the Jahn–Teller distortion of layered Mn-based solid-solution materials.And Nb⁵⁺and Si⁴⁺substituted the Mn site of[Co_0.25Mn_0.75]O₂?T_M–O₂?in P2-NCM lattice,which could not only introduce high binding energy of Nb–O and Si–O bonds,and enhance the bond energy of T_M–O and O–O,but also promote the structure stability and cycle life of NCM.In the voltage range of 1.8–4.0V,P2-type Na_2/3Co_0.25Mn_0.705Si_0.045O₂ delivered 144.0 mAh g^–1 at 0.1 C,and its capacity retention rate at the 100th cycle is up to 80.1%,which is 74.1%higher than that of the pristine.Surface modified P2-NCM benefited from the synergism of the modification and doping.On the one hand,the modified layer could reduce the contact area of P2-NCM and electrolyte,and ameliorate the interface state of cathode materials,thereby reducing the resistance of Na⁺migration and increasing the interface reaction activity.And the pseudo-capacitance effect and Co³⁺/Co²⁺ions of the Co₃O₄ modified layer have enhance the reaction activity of P2-NCM.On the other hand,the high concentration of metal cations in modified layers have doped into the solid solution of P2-NCM and improved its internal structure.For example,a few La³⁺and Ti⁴⁺in La₂O₃ and TiO₂ modified layer have substituted the Na and Mn sites,respectively,which can not only lead to the shrinkage of T_M–O₂ layer and the increase of Na⁺insertion site,but also promote the structural stability and dynamic characteristics of P2-NCM sample.The maximum discharge capacity of P2-NCM co-modified with 1 wt%La₂O₃ and TiO₂?1:1,wt%?is 164.3 mAh g^–1with the capacity retention of 81.7%after 100 cycles,and its capacity retention rate can reach up to90.1%after 240 cycles at 10 C,which is much better than that of the parent material.The ex-situ XRD results showed that the Na⁺content of P2-type Manganese-based solid solutions could decrease/increase with the reaction of Na⁺?de-?intercalation,which resulted in highly reversible P2–P3–P2 phase transition.In a single P2 or P3 phase domain,the phase-transition process still has obvious solid solution-characteristics.In addition,element doping and surface modification have not only ameliorated the behavior of Na⁺?de-?intercalation,but also increased the reversibility of P2–P3 transition,thus improving the charge-discharge reversibility of samples.