High-capacity Lithium-excess Cathode Materials: Fabrication And Lithium-ion Storage Properties

In order to meet the needs of high-power batteries, the lithium-rich manganeselayered compounds as cathode materials have attracted much attention due to their highspecific capacity of more than250mAh g-1. However, the poor rate capability and cyclingstability limit their large-scale application in lithium-ion batteries. It is generally knownthat particle size reduction and surface modification are effective ways to improve theelectrochemical performance （lithium storage performance） of the material. In this paper,we focused on fabrication of nanomaterials and surface modification, and the results are asfollows:First, we use the gel-assisted combustion method and template method to prepare thedisordered and ordered porous lithium-rich manganese layered compounds. Forgel-assisted combustion method （polyvinylpyrrolidone （PVP, Mw=130W） as theauxiliary combustion agent）, the disordered porous0.4Li₂MnO₃·0.6LiNi_2/3Mn_1/3O₂sampleprepared at600℃shows the best electrochemical performance because of theappropriate degree of crystallinity and the particle size. For instance, the electrode delivers291mAh g-1at the current density of15mA g-1within2.0–4.8V and the capacity retains92.3%after100cycles. In contrast, lithium-rich manganese layered compound withordered porous structure, using the ordered porous SiO₂（KIT-6） as template, shows betterrate and cycle performance, i.e., it shows a higher capacity of293.6mAh g-1and highercapacity retention of93.9%after100cycles. When cells were charged at15mA g-1anddischarged at200,500and1500mA g-1, ordered （disordered） porous electrodes show229.8（208）,112.5（82） and84.7（40.6） mAh g-1, respectively. Obviously, the orderedporous sample shows better electrochemical performance than the disordered.As we all known, the templates in traditional template method usually need to beremoved, but the templates in self-template method can also serve directly as the source ofcertain elements in final product. The latter was more simple and controllable. In thispaper, porous MnO₂amicrospheres （obtained by calcining MnCO₃microspheres） wereused as template to prepare hollow0.3Li₂MnO₃·0.7LiNi0.5Mn0.5O₂microspheres, whichcan be explained by Kekendaer effect （different metal ions with different diffusion velocity）. The hollow0.3Li₂MnO₃·0.7LiNi0.5Mn0.5O₂microspheres electrode shows highreversible specific capacity （initial capacity of295mAh g-1）, outstanding cycleperformance （278mAh g-1after200cycles） and superior rate performance （213.9mAh g-1when discharged at200mA g-1）. In addition, the micro-spheroidal morphology can stillmaintain even after200charge/discharge cycles.Moreover,1-D nanorod structure and3-D microsphere0.3Li₂MnO₃·0.7LiNi0.5Mn0.5O₂were also prepared by using1-D nanorod-MnO₂and3-D thorn ball-MnO₂as templates, and the two lithium-rich manganese layeredcompounds show excellent lithium storage performance. Compared with the1-D nanorodssample, the3-D microspheres sample consisting of1-D nanorods delivers a much highervolumetric capacity. For example, the compact density of the3-D microspheres sampleand the1-D nanorods sample are1.68g cm-3and1.16g cm-3, respectively. So the3-Dmicrospheres sample is almost1.5times the volume specific capacity of the1-D nanorodssample at the same current density.At last, surface modification is also used to improve lithium storage performance ofthe lithium-rich manganese layered compounds. The0.4Li₂MnO₃·0.6LiNi_2/3Mn_1/3O₂particles were coated with a thin layer of Al₂O₃by atomic layer deposition technology（ALD）, and a typical core-shell structure (i.e.,0.4Li₂MnO₃·0.6LiNi_2/3Mn_1/3O₂as core, andAl₂O₃as shell) was obtained. In comparison with the pure0.4Li₂MnO₃·0.6LiNi_2/3Mn_1/3O₂sample, the core-shell structure0.4Li₂MnO₃·0.6LiNi_2/3Mn_1/3O₂@Al₂O₃shows bettercycling performance. After150charge/discharge cycles, the capacity retains94.5%forthe Al₂O₃modified electrode, while only retains87.5%for the pristine electrode.