Study On Surface Modification Of Co-free Lithium-rich Manganese-based Oxide Cathode Materials

With the advantages of ultra-high specific capacity(250 mAh g^-1)and low cost,Cobalt-free lithium-rich manganese-based layered oxide(LLO)has become the preferred cathode material for next-generation high-energy-density power batteries.The high capacity of such oxides materials is mainly due to the redox reaction of lattice oxygen ions at high-voltage,while this reaction can bring about severe oxygen release during the first charging process,resulting in a large amount of irreversible capacity loss and low first coulombs efficiency.Moreover,the release of oxygen also creates vacancies that drive the migration of adjacent cations,thereby promoting the transformation of the layered structure to the spinel structure.With the prolongation of cycling at high voltage,the surface interfacial reaction and electrolyte decomposition can further erode the surface structure of LLO,forming a thicker interfacial film and slowing down the normal transport of lithium ions;it also promotes the dissolution of transition metal ions and the evolution of phase structure,thereby reducing the electrochemical activity of LLO.This series of detrimental reactions causing continuous attenuation of capacity and voltage all originate from the unstable surface structure of materials.Therefore,the surface modification strategy may be an effective solution to suppress the above unfavorable reactions,this paper mainly focuses on the surface structure modification of LLO cathode materials,and draws the following conclusions:(1)A novel and facile three-in-one surface modification strategy is designed to achieve the simultaneous integration of oxygen vacancies,spinel phases,and nitrogen-doped carbon nano-coating layers on the surface of LLO materials.The thermal decomposition of urea successfully induces the release of surface-active lattice oxygen,followed by the injection of abundant oxygen vacancies;meanwhile,it also promotes the migration of adjacent cations,and in situ forms a spinel structure.In addition,the decomposition products of urea deposits on the surface of the material and forms a nitrogen-doped carbon nano-coating layer.After the three-in-one surface modification strategy,the surface structure stability of LLO materials has been significantly improved.The modified sample releases the highest specific discharge capacity of 253.5 mAh g^-1(pristine sample is only 238.8 mAh g^-1)at 1C rate,and after 500 cycles,its capacity retention rate is more than 89%,which is1.6 times that of the pristine sample.Most importantly,the voltage decay rate of the modified sample is reduced from 1.45 mV cycle^-1 to 1.09 mV cycle^-1 compared to the pristine sample.Excellent electrochemical performance is mainly due to the following three aspects:a.The surface structure of the material injects abundant oxygen vacancies,which reduces the distribution of active lattice oxygen,thereby reducing the amount of oxygen released during the first charging process;b.The integrated spinel phase on the surface structure not only improves the stability of the surface structure,but also inhibits the transformation of the phase structure;c.The nitrogen-doped carbon nano-coating layer can inhibit the organic electrolyte during high voltage and long-term cycling,prevent the formation of dense nano-corrosion holes on the surface of the material,and improve the stability of capacity and voltage.(2)We employ an improved gas-solid interface reaction for surface modification of LLO materials.Linear regulation of initial coulomb efficiency(ICE)is achieved by means of NH₄F modification and thermal treatment.NH₄F modification can control the distribution of active lattice oxygen on the surface of the material,followed by injecting of oxygen vacancies,integrating of spinel-layered coexistence structures,and doping of fluoride ions on the surface structure.By adjusting the dosage of ammonium fluoride(5 wt%,10 wt%,15wt%and 20 wt%),the ICE of LLO material can be adjusted linearly(88.41%,96.25%,103.00%and 112.35%).Thermal treatment process is used to improve the crystallinity of the spinel phase,ensure that the fluoride ions enter the lattice oxygen vacancies and form stable TM-F bonds with the cations,which greatly improves the reversibility of the oxygen anion redox reaction.Therefore,the simultaneous integration of oxygen vacancies,fluorine doping,and spinel phases together enhances the surface structural stability,improves the high reversibility of the oxygen ion reaction,and realizes the linear regulation of the ICE of LLO materials,while improving the rate performance and inhibiting the capacity and voltage attenuation.(3)Dimethyl carbonate(DMC)-assisted solvothermal method is employed to reconstruce the surface structure of LLO cathode materials.DMC modification provides a midl chemical delithium environment,inducing chemical delithiation of partial lattice lithium ions to form Li-defects;it also reduces the distribution of active lattice oxygen on the surface of the material and induces the formation of oxygen vacancies.Li-defects promote the expansion of the surface structure spacing,which helps to accelerate the diffusion of Li ions.Moreover,DMC modification reduces the amount of lithium compounds on the surface of the material and helps to reduce the transport impedence of lithium ions.Electrochemical performance tests show that the modified material releases a specific discharge capacity of221.9 mAh g^-1 at 1 C,and the capacity retention rate was as high as 99%after 250 cycles.Therefore,the cooperative construction of surface Li-defects and oxygen vacancies can improve the interface structure stability of LLO materials and significantly extend the cycle life.(4)The distinction of K_SP between Ni CO₃ and Mn CO₃is used to control the concentration of CO₃^2-ions in the reaction system to prepare assembled-microspheres with concentration-gradients,subsequently,Co-free Li-rich materials with Ni/Mn and Al dual concentration-gradients are synthesized.One concentration-gradient is the electrochemical active center(Ni/Mn),and the other is the electrochemical inert center(Al).XPS and soft/hard XAS measurement results show that dual concentration-gradients can effectively mitigates the erosion of undesirable interfacial reactions,the evolution of layered phase structure,and the generation of lower-valence Mn³⁺/Mn⁴⁺or Mn²⁺/Mn³⁺redox couples during cycling.the voltage fading rate of modified sample is as small as 0.97 mV cycle^-1 at0.5 C after 100 cycles.Furthermore,its capacity retention ratio is also as large as 84.1%after 400 cycles at 1.5 C.Therefore,the dual concentration gradients is extremely beneficial to improve the voltage and capacity stability of LLO cathode materials.After unremitting efforts in research and drawing on the essence of predecessors'research,this paper is devoted to explore the cobalt-free lithium-rich manganese oxide surface modification technology,strengthen the surface structure stability of LLO materials,improve the reversibility of oxygen anion reaction,so as to speed up the high energy density cathode materials in the pure electric vehicle and energy storage device applications.