Studies Of The Structural Stability Of Layered Oxide For Sodium-ion Batteries

Layered oxide is one of the most ideal cathode materials due to its ideal theoretical specific capacity,efficient and diverse synthesis methods.However,the air-sensitive characteristics commonly possessed by layered oxides make storage in practical applications a major problem,and the air-sensitive characteristics are closely related to the type of layered structure(mainly P2 or O3),but no corresponding attention has been paid by researchers.On the other hand,P2-Na_0.67Ni_0.33MnO₂ is an ideal layered cathode material for sodium ion batteries owing to its high operating voltage.However,the superlattice structure associated with Na⁺/vacancy ordering usually appears during the charging-discharging process,resulting in lower Na diffusion kinetics and cycling stability.Aim at above problems,in this paper,P2-and O3-type Na_xMnO₂are selected as research objects,through parallel experimental comparison,it is clear that their electrochemical failures after air exposure are caused by different aging mechanisms.In addition,by lattice regulation to expand the spacing between Na layers,the Na⁺/vacancy ordering is effectively suppressed,and the electrochemical properties of the materials are significantly improved.The main research contents are as follows:First,two kinds of Na_xMnO₂(P2-type Na_0.50MnO₂ and O3-type Na_0.85MnO₂)were synthesized by solid state method to explore their distinction in air aging mechanisms.The experimental results showed that under the same air exposure conditions,for P2-type Na_0.50MnO₂,after H₂O were inserted into the Na layers,the specific capacity of first charge-discharge between the fresh sample and the air-exposed sample is almost equal.The electrochemical failure after exposure to air is relatively slow and requires a certain electrochemical cycle to be observed.However,after high-temperature heat treatment,the collapse of the structure due to the extraction of H₂O molecules easily leads to poor electrochemical properties.Different from P2-Na_0.50MnO₂,the air aging mechanism of O3-type Na_0.85MnO₂ is more complicated.After being exposed to air,it is vulnerable to corrosion by H₂O and CO₂,leading to spontaneous release of Na⁺from Na layers and the produce of impurities,as well as O3-P3 phase transformation.And,the charge-discharge specific capacity degradation during the first charge-discharge process can be clearly observed.Besides,SEM test results show that the air exposure sample of Na_0.85MnO₂were transformed from the original particles into agglomerated and folded chunks,while the surface of the air exposure sample of Na_0.50MnO₂ was still smooth.In addition,X-ray photoelectron spectroscopy analysis shows that,For Na_0.85MnO₂,due to the crystal lattice sodium release after air exposure,in order to maintain charge balance,part of Mn³⁺occurs spontaneous oxidation behavior to Mn⁴⁺,resulting in lattice distortion.The above results indicate that the air aging mechanism of Na_xTMO₂ is closely related to the structure type.And a reasonable explanation is given for the different air aging mechanism of P2 and O3 type layered oxides by combining with electrochemical and high-resolution transmission electron microscopy(HR-TEM).Second,we prepared a series of P2-Na_xNi_0.33Mn_0.67O₂ cathodes via adjusting the initial Na contents in the structure,realized the maximum interlayer spacing within the experimental range,and optimized the Na⁺/vacancy ordering sequence.Part of the additional electrochemical platforms caused by the existence of Na/vacancy disappeared,and the diffusion kinetics of Na ions were significantly improved with higher discharge specific capacity.As a consequence,the optimized material Na_0.50Ni_0.33Mn_0.67O₂ exhibits the most excellent cycle performance and rate performance(92.03%capacity retention after 150 cycles at a current density of 20 m A/g).We believe that the adjustment of the layer spacing plays a key role in eliminating the ordering of Na⁺/vacancy in the layered oxide cathode materials,and it must be explored in more detail.After modification,the Na⁺/vacancy disordering degree in the material is enhanced,and the additional voltage platform disappears.Hnece,the diffusion kinetics of Na ions were significantly improved.As a consequence,the optimized material Na_0.50Ni_0.33Mn_0.67O₂ exhibits the most outstanding electrochemical properties(92.03%capacity retention after 150 cycles at a current density of 100 m A/g).It is proved that the Na⁺/vacancy ordering can be eliminated by increasing the spacing of Na layers.