Synthesis And Mechanism Studies Of Layered Oxide Cathode Materials For Lithium/Sodium-Ion Batteries

Lithium-ion batteries have been widely used in human social life and production due to their high energy density,excellent energy conversion efficiency,and better safety performance.At the same time,greater demands are being placed on the lithium-ion battery cathode materials by the fields of power batteries such as electric vehicles.Lithium-rich manganese-based layered oxide cathode materials have become important candidates for the development of high energy density lithium batteries due to their excellent capacity retention.Unfortunately,it usually produces a higher irreversible capacity resulting from the activation in the first cycle.Indeed,the irreversible phase transition from the layered structure to the spinel phase structure also leads to serious defects such as voltage attenuation,which limits its practical application in industrialization.The scientific researchers have turned their attention to the sodium-ion batteries because sodium and lithium are in the same group on the periodic table of the elements with the similar chemical properties.As one of the earliest research systems,layered oxide sodium-ion cathode materials have good commercial prospects.Regrettably,the layered structure will suffer the distortion to a certain extent during charging and discharging,and it is more sensitive to water molecules in air and electrolyte,which seriously affects the cycle performance.We not only choosed the layered lithium-rich manganese-based lithium-ion battery cathode material Li_1.2Ni_0.2Mn_0.6O₂ and the layered manganese-based sodium-ion battery cathode material Na_0.7MnO₂ as reference materials but also designed their local structures and compositions.Then,we explored the electrochemical performance of lithium/sodium-ion battery materials in order to have a better understanding of the mechanism during the charge and discharge process.The main work includes:?1?Electrochemical performance and mechanism of Ni-Mn-based lithium-ion battery material Li_1.2-xK_xNi_0.2Mn_0.6O₂First,the cathode material Li_1.2Ni_0.2Mn_0.6O₂ was synthesized by a simple solid-phase method,and the test was performed for 100 cycles at a voltage range of 2.0-4.8V and a current density of 20 mAh g^-1.However,the capacity retention rate of the material after cycling was only 62.2%.Generally,the lattice structure of the materials can be adjusted by the element replacement strategy,thereby improving the dynamic performance.In this experiment,the Li site was doped with a larger radius of K⁺to support the stability of the layer spacing and inhibit the migration of transition metals from octahedral sites to tetrahedral sites.As a result,the dopted batteries obtained a better capacity of 265 mAh g^-1 under the same conditions.The results of XRD refinement proved that K⁺doping expands the interlayer distance of LiO₂ at the Li site.The in-situ/ex-situ electron paramagnetic resonance?EPR?spectroscopy was carried out to comprehend the valence state of Mn in the materials.In a word,the charge-discharge mechanisms of K⁺-doped Li_1.2Ni_0.2Mn_0.6O₂ were explained by the high resolution transmission electron microscopy?HRTEM?and electron paramagnetic resonance?EPR?spectroscopy.?2?Electrochemical performance and mechanism of Cu-Mn-based sodium-ion battery material Na_0.7Mn_0.9Cu_0.1O₂First,Na_0.7Mn_0.9Cu_0.1O₂ and Na_0.7MnO₂ were prepared by a single sol-gel method and their capacity were 87 mAh g^-1 and 39.7 mAh g^-1 after 100 cycles at a voltage range of 2.0-3.8 V with a current density of 20 mAh g^-1,respectively.We had explored the variation of valence state for the transition metals?Mn/Cu?,with the result that the capacity was mainly provided by the redox reaction of Mn³⁺/Mn⁴⁺according to the synchrotron radiation X-ray absorption near edge structure spectroscopy?XANES?and soft X-ray absorption spectroscopy?sXAS?.It was demonstrated that Mn²⁺was generated from the surface during charging and discharging process,but the doping of Cu²⁺can effectively inhibit the generation rate of Mn²⁺on the surface.On account of ex-situ XRD study,it was found that the Na_0.7MnO₂ charged electrode materials showed some hydrate phase pesks related to Na-vacancy ordering,but no peaks were found during the testing of the Na_0.7Mn_0.9Cu_0.1O₂ charged state samples.It was presumed that the doping of Cu²⁺could affect the rearrangement related to Na-vacancy ordering and inhibit water molecules from embedding into the sodium layer structure.The XRD patterns of the two materials were collected after exposing to the same air environment.It was found that the XRD patterns of Na_0.7MnO₂ exposed to air for 5 days showed mixed peaks around 14°and 25°,while Na_0.7Mn_0.9Cu_0.1O₂ showed no mixed peaks in the same environment.