Structure Control And Performance Optimization Of Sodium-doped Lithium-rich Manganese-based Layered Oxides

The rapid development of society has made energy and environmental protection problems more and more serious,so efficient and green use of energy has become an urgent core issue.Among many energy storage/conversion devices,lithium-ion batteries are widely used as energy storage devices with high energy/power density,high efficiency and environmental protection,especially in the field of new energy electric vehicles.However,in order to meet the increasing demand of society,people urgently need a lithium-ion battery with higher performance to improve the current low endurance of electric vehicles,so researchers are trying to find a low-cost,high-capacity,high-voltage cathode material for lithium-ion batteries.Among all kinds of cathode materials,lithium-rich materials are considered to be the next-generation cathode materials with development potential because of their high theoretical specific capacity of 300 m Ah g^-1and wide voltage range of 2.0-4.8 V.However,because it also has the disadvantages of poor cycling performance,serious voltage decay,large initial irreversible capacity loss and difficult to match the rate performance of traditional cathode materials,it seriously restricts the development path of lithium-rich materials into commercialization for further large-scale applications.In the thesis,Li_1.2Ni_0.13Co_0.13Mn_0.54O₂is used as the object of investigation,and different synthetic strategies are employed to achieve the objectives of Na⁺introduction,oxygen vacancy induction generation,surface protection layer,and gradient construction,which in turn effectively improve the rate performance,cycling performance and significantly mitigate the voltage decay of the material.The main works are as follows:2.1D hollow square rod-shaped Mn O₂was synthesized by hydrothermal method,and then Na⁺lattice doped-oxygen vacancy lithium-rich layered oxide（LLO-Na-OV）was obtained by a simple molten salt template method.Different from the traditional synthesis method,the hollow square rod-shaped Mn O₂provides many anchor points for Li,Co,and Ni ions in the Na Cl molten salt environment,so that LLO can be directly prepared on the original morphology.Meanwhile,Na⁺is also introduced for lattice doping and induces the formation of oxygen vacancy（OV）.The resulting LLO-Na-OV not only inherits the 1D stick structure,but also achieves Na⁺lattice doping and oxygen vacancy endowment,which facilitates Li⁺diffusion and improves the structural stability of the material.To this end,characterizations such as TEM,HAADF-STEM,XPS and Raman spectrum were used for analysis.In addition,density functional theory calculations（DFT）are used to further analyze the influence of OV generation on local Co and Mn,and theoretically explain the electrochemical performance mechanism of the samples.Therefore,the modulated LLO-Na-OV has a high discharge capacity of 282 m Ah g^-1and a high capacity retention of 90%after 150cycles.At the same time,the voltage attenuation per cycle is only 0.0028 V,which is much smaller than that of Pristine-LLO prepared without this strategy（0.0038 V attenuation per cycle）.This synthesis strategy can realize the controllable morphology of LLO,Na⁺lattice doping and induced oxygen vacancy generation,which provides a feasible idea for related exploration.2.It is a challenging task to endow lithium-rich manganese-based cathode materials with high capacity and low voltage decay characteristics,and reasonable design and modulation are the key to endow lithium-rich materials with excellent electrochemical performance.Na-doped lithium-rich layered oxide（Na-LLO）was obtained by co-precipitation-calcination method,and then it was used as a high-manganese core and NCM811(Li Ni_0.8Co_0.1Mn_0.1O₂)as a high-nickel shell.Na-doped-manganese/nickel reverse gradient LLO（Na-NCM-LLO）can be modulated by simple processing.Na-NCM-LLO-6%not only has a high discharge capacity of275 m Ah g^-1,but also has a high capacity retention rate of 95%after 150 cycles.At the same time,the voltage attenuation per cycle is only 0.0026 V,much smaller than the 0.0039 V of Pure-LLO.The above properties show that reasonable cation regulation can realize the expansion of Li plate layer distance and the inhibition of Li₂Mn O₃activity,which provides a boost for promoting the charge transfer reaction kinetics of Na-NCM-LLO and hindering the transformation of layered structure to spinel phase.Therefore,it is ensured that the material has the characteristics of low voltage attenuation while having high capacity,and its rate and cycle performance are improved.In addition,the important role of Mn/Ni reverse gradient and Na doping in improving the electrochemical performance is also strongly confirmed by characterization and DFT calculations.The excellent electrochemical performance proves the feasibility of the cation regulation strategy to prepare lithium-rich materials with high capacity-low voltage decay characteristics,and also provides a clear direction for the subsequent exploration of high capacity-low voltage decay systems.3.Dual-gradient surface coatings and lattice-doped layered Li-rich oxides were synthesized stepwise through a facile method.Rod-like LLO with rich defect structure and Na pinning effect was first induced by molten salt method,and then Li⁺/Na⁺exchange was performed between the Na F coating and the bulk phase interface of LLO by high-temperature treatment,so as to realize the Na_1-xLi_xF gradient on the outer surface of LLO coating and LLO subsurface Na⁺gradient doping.In order to clarify this mechanism,XPS,TEM,HAADF-STEM and Raman spectrum were used for analysis.In addition,DFT calculations are used to guide the experiments and explain the mechanism of the electrochemical performance of the samples.Due to the gradient Na_1-xLi_xF coating as a protective layer for LLO,it prevents the electrolyte erosion,stabilizes the electrode/electrolyte interface,and reduces the side reactions at the interface.At the same time,the gradient lattice Na⁺doping can act as a pillar of the layered structure during the charge and discharge process,maintaining the stability of the material structure.Therefore,after the modification of this strategy,the optimized GLLO-0.5%sample has the best electrochemical performance,which not only significantly alleviates the voltage drop during long-time cycling,but also achieves a gratifying improvement in cycle stability（the voltage decay per cycle is only 0.0024 V and the capacity retention rate is as high as 99%）.This strategy provides a feasible idea for the preparation of lithium-rich manganese-based cathode materials with high electrochemical performance,revealing its great application potential in high-performance energy storage materials.