Synthesis, Characterization And Modification Of Manganese-based Layered Lithium-rich Oxide Cathodes

To meet the demands of electric vehicles running in long mileage, the energy density oflithium ion batteries should be further improved. The energy density of currently commerciallithium ion battery is mainly limited by the cathode materials of low capacity density. In thisregard, manganese-based layered lithium-rich oxides would be a promising cathode candidateas they offer a high capacity of more than200mAh g-1in the range of2.0-4.8V. However,they still suffer from several problems such as huge irreversible capacity, poor cyclicperformance and low rate capability. With an aim to solve these problems, several newmethods were proposed in this thesis to synthesize manganese-based layered lithium-richoxides, and the composition, structure and electrochemical performances of the resultingproducts were investigated by SEM, XRD, Raman, EIS and charge/discharge test.The contents of this thesis include:(1) Synergic dispersion of PVP and EG is proposed tosynthesize Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.3) with size-uniform particles. It is found that thesynthesized oxide can deliver a capacity of205mAh g-1at0.1C, and retain87%of thecapacity after60cycles. The decayed capacity is attributed to inactivation of partialtransition-metal ions.(2) Influence of Li concentration on chemical composition, crystalstructure and electrochemical performance of manganese-based layered lithium-rich oxides isinvestigated. It is demonstrated that lower Li concentration increases the reversible capacitybut decreases the cyclic performance. Additionally, too low Li concentration would lead tophase separation and reduce the capacity of the material.(3) A fast-evaporation approach isproposed to synthesize manganese-based layered lithium-rich oxide Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.5) with low structural defects. The synthesized oxide exhibits good cyclic performance.No capacity degradation is found after50cycles at0.05C, and the capacity retention after300cycles at1C is higher than80%.(4) A self-directed chemical method is proposed tofabricate Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.5) with a micro-nano structure. The synthesizedoxide delivers a capacity of222mAh g-1at0.05C, with a capacity retention of93%after50cycles. Besides, its capacity at2C is up to110mAh g-1.(5) The influence of Co3+substitution on electrochemical properties of Li[Li0.2Ni0.2-x/2Mn0.6-x/2Cox]O2cathodes isconsidered. With increasing the substitution, the oxygen loss and reversible capacity arelargely enhanced, but the capacity stability and voltage stability are severely weakened.(6)The influence of Fe3+substitution on electrochemical properties of high capacityLi[Li0.2Ni0.13Mn0.54Co0.13]O2cathode is investigated. It is found that the substitution would lead to the decrease of the reversible capacity, but the substitution for Co3+is able to suppressthe voltage decay with extended cycling.(7) The influence of chemical composition andsynthesis temperature on electrochemical properties of xLi2MnO3-(1-x)LiCoO2cathodes isconsidered. The results show that the maximum charge capacity appears at x=0.6, but themaximum discharge capacity locates at x=0.4～0.5. The materials synthesized at800℃have a two-step reaction process at the first charge, but those synthesized at900℃～1000℃possess a three-step reaction process.(8) Redox behavior of different cations M (M=Co3+,Ni3+, Cr3+, Fe3+,[Ni0.5Mn0.5]3+) in Li[Li0.2Mn0.4M0.4]O2, and their influences oncharge/discharge performance of Li[Li0.2Mn0.4M0.4]O2cathodes are investigated.Li[Li0.2Mn0.4Co0.4]O2displays a long charge plateau, Li[Li0.2Mn0.4Ni0.4]O2shows a longcharge slope, and Li[Li0.2Mn0.4Cr0.4]O2exhibits a short charge plateau. The slope and plateaulengths of Li[Li0.2Mn0.4(Ni0.5Mn0.5)0.4]O2are located between those of Li[Li0.2Mn0.4Co0.4]O2and Li[Li0.2Mn0.4Ni0.4]O2. The Li[Li0.2Mn0.4Fe0.4]O2synthesized with a sol-gel method at900℃does not show a traditional “manganese-based layered lithium-rich oxide” structure.Additionally, substitution of M (M=Co3+, Ni3+, Cr3+,[Ni0.5Mn0.5]3+) for Co3+is able toimprove the reversible capacity of Li[Li0.2Mn0.4Co0.4-xMx]O2These investigations provide new pathways and theoretical fundamentals for optimizingmanganese-based layered lithium-rich oxides or improving their electrochemical performance,and enrich the synthesis approaches and structure-performance relation of materials forlithium ion batteries.