Study On Electrochemical Performance Modification Of Li-Rich And Mn-Based Cathode Materials

With the rapid development of portable mobile electronic devices,electric vehicles and large-scale energy storage grids,the demand for high-performance lithium-ion batteries is also growing.However,at present,the cathode materials have been blocked in the capacity increase,which limits the further improvement of the energy density of lithium-ion batteries.It is urgent to develop new cathode materials with higher capacity.The Li-rich and Mn-based(LRM)cathode materials has a unique transition metal(TM)cation redox simultaneous with reversible oxygen-anion redox,which provides a high specific capacity of more than 300 mA h g-1.Therefore,it is an ideal cathode substitute material for the next generation lithium-ion batteries.However,LRM cathode materials also suffer from low initial Coulombic efficiency,poor rate performance,and serious capacity and voltage fading during cycling,which hinders their practical applications.These problems are closely related to TM ion migration and structural transformation in the bulk phase,as well as TM ion dissolution and interface side reactions at the surface/interface.Therefore,in this doctoral thesis,the problems in bulk phase and surface/interface of LRM cathode materials are improved.Firstly,the structural stability and Li+diffusion ability are enhanced by the bulk Na+ doping,and the defective structure of the material can also provide compensation for the capacity loss.Moreover,the surface is coated with spinel phase to inhibit oxygen release and stabilize the interface between electrode material and electrolyte.Finally,on the basis of the above work,a multi-strategies integrated method has been proposed,combining the cation/polyanion doping and defect regulation with stress engineering,taking into account the problems existing in the bulk material and the surface/interface,to achieve the comprehensive improvement in the performance of LRM cathode materials,and revealing the mechanism of performance improvement.The main research contents and results are as follows:(1)The surfactant is introduced as dispersant to ensure the uniform distribution of Na+ in the bulk phase when preparing the precursor.Moreover,doped Na+won’t participate in the redox reaction,and plays the role of "pinning effect" and "pillar effect"in the high charging state,preventing crystal plane slip and layer structure collapse,inhibiting the dissolution of TM ions in the electrolyte and phase transition in the cycling,significantly enhancing the capacity and voltage stability.Besides,abundant stacking faults are further introduced to compensate the capacity loss of the LRM cathode material.The specific capacity of modified sample is about 240 mA h g-1 after 200 cycles at 0.5 C(1C=200 mA g-1)rate.Therefore,the results of this study can provide a reference for uniform doping and defect construction in LRM cathode materials.(2)The lattice oxygen that activated during the cycling process of LRM cathode materials contributes a huge amount of capacity,but also leads to irreversible oxygen release,structural transition and surface reconstruction,and deteriorating the kinetics properties.Due to the anion-redox-free and the excellent structural consistency of LiNi0.5Mn1.5O4 to the LRM cathode materials,a dense and stable LiNi0.5Mn1.5O4 coating layer has been in-situ constructed on the surface of LRM cathode materials to stabilize the surface lattice oxygen,and inhibit the O2 release during the cycling.At the same time,the coating layer has unique 3D ion diffusion channels,which can promote the rapid Li+transport and the rate performance.Moreover,the interface stability between the electrode material and the electrolyte is significantly enhanced,and the electrochemical performance is greatly improved.After 1000 cycles at 2C rate,the specific capacity of 135.5 mA h g-1 is still maintained,and the voltage fading is only 0.67 mV per cycle.This strategy that in-situ constructing the oxygen-passivation-layer provides a new idea for stabilizing the surface lattice oxygen of LRM cathode materials(3)Based on the above research,a simple and effective "three-in-one" multi-strategy integrated modification method has been proposed by simultaneously considering the internal and surface/interface problems of LRM cathode materials in practical applications,and utilizing the self-assembly behavior of surfactant in water phase and the subsequent high-temperature calcination process.The introduced cationic/polyanion codoping can enhance the structural stability,and inhibit the capacity and voltage fading.The pre-constructed unique alternately distributed defect bands and crystal bands structure can offer lots of longitudinal ion diffusion channels in addition to the conventional 2D ion diffusion channels,which can effectively improve the Li+diffusion kinetics.This special structural design also relieves stress accumulation during cycling.By integrating cationic/polyanion co-doping,defect regulation and stress engineering,the comprehensive modification of bulk phase and surface/interface of LRM cathode materials is realized.This multi-strategy integration method breaks through the functional limitations of single modification strategy and provides an important experimental and theoretical basis for the development of the next generation of high capacity cathode materials.