Preparation And Morphology Modulation Of Layered Na_0.67MnO₂ Cathode Materials For Enhanced Performances Of Sodium-ion Batteries

The booming market of electric vehicles and the increasing demand for large-scale energy storage render the development of low-cost and high-performance rechargeable batteries much more important today than any time in history.Facing the challenge,tremendous efforts have been directed toward the development of advanced Li-ion battery（LIB）technology over the last several decades,whereas the rarity of lithium is posing a threat to its sustainable development.In this context,Na-ion battery（NIB）has emerged as a promising alternative to the LIB technology due to the abundant resources of Na and its low price relative to Li.Nevertheless,akin to LIB,the NIB technology is significantly hindered by the development of advanced cathode materials.Compared with the small radius of Li⁺（0.76（?））,the larger Na⁺（1.02（?））would trigger sluggish ion diffusion,serious volume change and complicated phase transitions,leading to low rate performance and severe capacity fade and hence making the exploration of advanced NIBs cathode materials even more challenging.Among the NIB cathode materials,layered Na_xMnO₂ represents a promising candidate owing to its low cost,high theoretical capacity and especially its open framework structure that facilitates the diffusion of Na⁺ion.In this paper,P2-Na_0.67MnO₂layered cathode material was selected as the basic research object.Due to its advantages of mild and controllable synthesis conditions,low preparation cost and high capacity performance,morphology regulation and element incorporation were adopted to improve the cyclic stability of this kind of material,inhibit phase transformation and improve the magnification performance.The main tasks are as follows:1.The co-precipitation method was employed to prepare precursor via controlling rotating speed and temperature,and Na₂CO₃ was used as precipitator to prepare spherical MCO₃·n H₂O（M=Mn,Zn,Mg）precursors with radial growth structure after adsorption and growth of metal oxalate in citric acid and sodium citrate solution.Then the precursors were calcined in the air through sodium mixture.The precursor decomposed CO₂ gas during the calcination process.At the same time,under the action of hot atmosphere,the loose microporous and radial growth like Na_0.67MnO₂ cathode material was finally prepared,and the Zn/Mg double-doped modification study was carried out.The test results show that the spherical structure with loose micro-aperture direction of Na_0.67Mn Zn_0.15Mg_0.15O₂ cathode material can inhibit the volume change of the material during charging and discharging.Meanwhile,the introduction of Zn/Mg double cation can improve the stability of the crystal structure of the cathode material and inhibit the phase transformation to a large extent.The lattice volumetric strain is only 0.55%during charging and discharging.The contribution rate of Na⁺in Na_0.67Mn_0.7Zn_0.15Mg_0.15O₂ cathode material is 66%after doping with Zn/Mg double cation,which improves the rate performance of the material.Therefore,the rate performance and cycle performance of Na_0.67Mn_0.7Zn_xMg_0.3-xO₂ cathode material are improved obviously.Among them,Na_0.67Mn_0.7Zn_xMg_0.3-xO₂（x=0.15）cathode material has the best cycling and rate performance.At 1 C rate,the discharge capacity of 100cycles is 134 mA h g^-1,and the capacity retention rate is 93.8%,The discharge capacity is still 112.9 mA h g^-1after 300 cycles and also 67.2 mA h g^-1 at a high rate of 10 C.2.The micron rod-like oxalate precursor MC₂O₄·2H₂O（M=Li,Mn）was successfully prepared by co-precipitation method at room temperature on the basis of the regulation of water-ethanol solvent system.The morphology of the precursor can be gently controlled by the difference of reaction kinetics between different metal acetate and oxalic acid.The prepared precursor was mixed with a sodium source and calcined in a muffle furnace,and the Na_0.67Mn_0.95Li_0.05O₂ cathode material with a micron rod-like structure was successfully obtained.At the same time,the P2-Na_0.67Mn_1-xLi_xO₂（x=0,0.05）cathode material was prepared,namely,NMO and NMLO material was obtained.The electrochemical properties and structural evolution of Li doped P2-Na_0.67Mn_0.95Li_0.05O₂ micron rod-like cathode materials were investigated by using Na_0.67Mn_0.95Li_0.05O₂ micron rod-like cathode materials as blank control.The surface of micron rods is loose and porous,which is conducive to ion conduction kinetics.Li doped P2-type Na_0.67MnO₂,that is,Li doped P2-Na_0.67Mn_0.95Li_0.05O₂ micron rod cathode material,compared with the blank sample Na_0.67MnO₂ micron rod cathode material,It was found that Li doped Na_0.67Mn_0.95Li_0.05O₂ significantly expanded the plane spacing.Because Na_0.67Mn_0.95Li_0.05O₂ has a wide ion diffusion channel,it is more conducive to ion exiting/embedding.More importantly,non-in-situ X-ray diffraction（XRD）results show that Li addition plays an important role in inhibiting the phase transition of P2-O2 during charge and discharge.Thanks to the above good properties,Li doped gs Na_0.67Mn_0.95Li_0.05O₂ micron rod-like cathode material has superior cycling stability and rate performance.