Study On Cation And Anion Co-doping Modification Of Lithium And Manganese-enriched Layered Solid-solution Li_1.2Ni_0.2Mn_0.6O₂ Cathode Materials

Lithium-rich manganese-based layered structure cathode materials are arousing significant attention in the next-generation commercial lithium ion batteries owing to their high capacity and energy density.However,the poor rate performance and inferior cycle stability stand in the way of its large-scale applications.In this work,an innovative strategy with cation and anion doping was proposed to enhance the electrochemical performance of the lithium-rich manganese-based layered oxide Li_1.2Ni_0.2Mn_0.6O₂.The cathode material Li_1.12Na_0.08Ni_0.2Mn_0.6O_1.95F_0.05 with best electrochemical performance has been obtained.In addition,the mechanism of co-doping process and the effect of co-doping on the microstructure,morphology and electrochemical performance of Li_1.2Ni_0.2Mn_0.6O₂ are thoroughly studied here.The primary contents are as follows:CH₃COONa and Na₂CO₃ were selected as the different Na source to introduced Na ions.As the results revealed,Na-doping process could raise the reversible discharge capacity,enhance the structural stability and improve the electrochemical performance.Na-doped sample with CH₃COONa as Na source displayed a better performance than Na₂CO₃.Li_1.2?xNa_xNi_0.2Mn_0.6O₂（x=0,0.03,0.05,0.08,0.10）were synthesized by introducing Na ions into the Li layer through a facile ball-milling method.XRD and TEM results revealed that the cathode materials Li_1.2-xNa_xNi_0.2Mn_0.6O₂ display a typical layered structure.The enlarged Li layer spacing was confirmed by the characterization of morphology and structure.The Li_1.12Na_0.08Ni_0.2Mn_0.6O₂ electrode shows excellent electrochemical performance including enhanced rate capability(112 mAh·g^-1 at 5 C)and superior cycling stability（100%capacity retention after 50 cycles,96%capacity retention after 100 cycles）.EIS results confirmed that Na doping effectively decreases the charge transfer resistance and facilitates the Li diffusion of the as-prepared cathode material.XRD and HR-TEM results revealed that Na-doping stabilizes the host layered structure by suppressing the phase transformation from a layered to spinel structure during cycling.LiF,NH₄HF₂ and PVDF were selected as the different F source to introduced F ions to synthesize the cathode materials Li_1.2Ni_0.2Mn_0.6O_1.95F_0.05,respectively.XRD,SEM and TEM results revealed that the F was successfully introduced into the bulk phase,all samples with different F source display a typical layered structure.Electrochemical test show that the different F source has a big influence on the electrochemical performance.PVDF has been proved as the best F source.The sample Li_1.2Ni_0.2Mn_0.6O_1.95F_0.05 using PVDF as F source displayed the excellent electrochemical performance including high reversible discharge capacity(240mAh·g^-1)and good rate behavior(120 mAh·g^-1 at 5 C).Na&F co-doping was proposed to improve the electrochemical performance of the Li_1.2Ni_0.2Mn_0.6O₂.XRD,SEM and TEM results revealed that the co-doped cathode materials display a typical layered structure,all samples show a similar morphology as nano-particles with distinct edges and corners with the unified size distribution of200-800 nm.Rietveld refinement results confirmed that the co-doped material possesses the most superior inherent crystal structure with lower disorder for electrochemical lithium intercalation and deintercalation.XPS results revealed that Na and F were successfully introduced into the cathode material as Na⁺and F^-.F-doping process might induce partial reduction of nickel ions to keep electronic neutrality.The co-doped Li_1.12Na_0.08Ni_0.2Mn_0.6O_1.95F_0.05.05 exhibits an excellent cycling stability（100%after 100 cycles at 0.2 C）and a superior rate performance(167 mAh·g^-1 at 5 C).EIS measurement proved that the F-doping process could improve electronic and ionic conductivity more easily.Furthermore,the HR-TEM analysis and Raman test of cycled electrodes showed that the Na-doping process could prohibit the phase transformation from layered to spinel-like structure which improve the structural stability effectively.