Modification Study On Rate Capability And Cycleability Of Lithium-rich Cathode

In recent years, layered lithium-rich cathodes Li2MnO3·(1-x)LiMO2(M=Ni0.5Mn0.5orNi1/3Co1/3Mn1/3), have attracted significant attention for use as promising cathode materialsfor next-generation high-performance lithium-ion batteries, owing to their much highercapacity (>250mAh g-1), reduced cost and environmental benign. Although the layeredcathodes exhibit several merits, they suffer from some intrinsic problems, such as severecapacity loss during the initial cycle, poor safety, poor rate capability with modest cyclestability. With the aim to enhance the rate capability and cycle stability of theLi1.2Ni0.2Mn0.6O2binary lithium-rich cathode and the Li1.2Ni0.13Mn0.54Co0.13O2ternarylithium-rich cathode, several studies were conducted in this research as follows. First,rod-like hierarchical nano/micro Li1.2Ni0.2Mn0.6O2cathode materials were synthesized by ahydrothermal reaction. Second, Li1.2Ni0.2Mn0.6O2cathode materials with enhaced ratecapability were synthesized by a hydrothermal method assisted with carbon spheres. Third,hierarchically porous micro-rod lithium-rich cathode material, Li1.2Ni0.13Mn0.54Co0.13O2cathode materials were synthesized by a hydrothermal reaction followed by a calcinationprocess. Finally, sphere shaped hierarchical Li1.2Ni0.13Mn0.54Co0.13O2cathode materials withenhanced growth of nano-crystal planes were synthesized by a precipitation methodfollowed by an ionic interfusion method.Rod-like hierarchical nano/micro Li1.2Ni0.2Mn0.6O2cathode materials are preparedthrough a hydrothermal reaction followed by a calcination process. First, rod-likemicro-sized oxalates precursors are formed during the hydrothermal treatment. Then theoxalates precursors are decomposed during the calcination process, and the rod-likehierarchical nano/micro Li1.2Ni0.2Mn0.6O2cathode materials are fabricated. The resultsindicate that this lithium-rich material as the cathode exhibits good cycleability and ratecapability. It delivers an initial discharge capacity of297.1mAh g-1at0.1C rate, a highcapacity of266mAh g-1is maintained at the completion of50th cycles. It retains a superiordischarge capacity of212.5mAh g-1(97%of the first discharge capacity) after30cycles at1C rate, and it yields a high discharge capacity of214.3mAh g-1at2C rate.Li1.2Ni0.2Mn0.6O2cathode materials with enhaced rate capability are prepared througha hydrothermal method assisted with carbon spheres. First, the carbon spheres are preparedby a hydrothermal method with glucose as carbon source. Then different amounts of carbon spheres are added into the mixed solution of acetates and oxalate acid. Finally, theLi1.2Ni0.2Mn0.6O2cathode materials are obtained through a hydrothermal reaction followedby a calcination process. The effects of different amounts of carbon spheres on strcutral andelectrochemical performances are evaluated. The results show that the Li1.2Ni0.2Mn0.6O2material with10wt%carbon spheres possesses the best electrochemical performance. Thismaterial delivers high maximal discharge capacities of238.7,219.3and204.8mAh g-1at1C,2C and5C rates, while only205.8,187.4and137.5mAh g-1for the Li1.2Ni0.2Mn0.6O2material without carbon spheres.Hierarchically porous micro-rod Li1.2Ni0.13Mn0.54Co0.13O2cathode materials aresynthesized through a hydrothermal reaction followed by a calcination process. Micro-rodoxalates precursors with rough surface are formed during the hydrothermal reaction, andthen the product with hierarchically porous structures constructed of nanoparticles issynthesized during the sintering process at high temperatures. The electrochemicalperformance results show that this material delivers a high discharge capacity of280.7mAh g-1at0.1C rate, and maintains a capacity of245.7mAh g-1after50cycles. Inaddition, the micro-rod structures are preserved. It delivers high discharge capacities of225.6,201.7and172.7mAh g-1at1C,2C and5C rates, respectively. Both thecycleability and the rate capability are superior to the Li1.2Ni0.13Mn0.54Co0.13O2materialsynthesized by a sol-gel method.Sphere shaped hierarchical Li1.2Ni0.13Mn0.54Co0.13O2cathode materials with enhancedgrowth of nano-crystal planes were synthesized through a precipitation method followed byan ionic interfusion method. First, urchin-like-MnO2precursors are prepared by aprecipitation method, and effects of reaction time on morphology of the precursors areinvestigated. Then, the precursors can be converted to the objective hierarchical structuredspheres after interfused with Co2+, Ni2+and Li+salts and followed sintering process, via gasrelease and fracturing effect. The hierarchical sphered cathode is assembled with primarynano-plates with enhanced growth of nano-crystal planes in favor of Li+intercalation/deintercalation. The results show that the-MnO2precursor with reaction timeof24h has the optimum morphology, and the corresponding Li1.2Ni0.13Mn0.54Co0.13O2material with unique structural features exhibits outstanding rate capability, cycleability.When charged at1C rate and discharged at1C,2C and5C rates, it retains dischargecapacities of212.7,190.9and164.3mAh g-1in completion of100cycles, respectively, andthe capacity retentions are all above90%compared with the maximal capacities. Amazingly, it can achieve around70%（177.3mAh g-1）of the capacity at0.1C rate withinabout2.1minutes of ultrafast charging.