Structure Design And Electrochemical Performance Of Lithium-rich Manganese-based Cathode Materials

Nowadays,lithium-ion batteries?LIBs?have become the mainstream products in secondary batteries.They have been widely used in modern portable electronics such as smart phones and laptops,and promote the development of electric vehicles and large-scale energy storage industries.However,the cathode materials are gradually becoming the bottleneck of LIBs,limiting the further development of advanced LIBs.Therefore,many researchers are concentrating on exploiting the next generation of cathode materials which are environmental friendless,low cost and high specific capacities.Among the new cathode materials that have been studied,Li-rich layered oxides?LLO?are considered promising candidates for the next generation of high-energy-density LIBs because of their high reversible specific capacity and environmental friendliness.In this thesis,the structural characteristics and progress of LLO are systematically introduced.And the typical Li-rich layered cathode oxide,Li_1.2Mn_0.54Ni_0.13Co_0.13O₂,is selected as the research object in this study.Li-rich layered cathode oxides with high specific capacity,excellent rate capability and cyclability were successfully obtained via modification strategies such as surface modification and structural control.The main achievements and progress are as follows:In the first part of the work,Li₂MnO₃-coated Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ nano/micro hierarchical microrods?LC-LLO-M?were successfully synthesized.First,the Li-rich layered oxide materials of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ nano/micro hierarchical microrods are synthesized using a solvothermal method.And then Li₂MnO₃ was coated onto the surface of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ microrods by a facile sol–gel method.The test results showed that LLO modified with this strategy exhibits excellent electrochemical performance,especially the rate performance.LC-LLO-M provided very high discharge capacities of 291.8 and189.6 mAh·g^-1 at C-rates of 0.1C(20 mA·g^-1)and 5C(1000 mA·g^-1).Even at the C-rate of10 C,the discharge capacity of LC-LLO-M could also reach 159.5 mAh·g^-1.Results also showed that the Li₂MnO₃ surface-modified Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ microrods significantly decelerated voltage decay.The significant enhancement of electrochemical performance could be attributed to the combination of the nano/micro hierarchical microrods structure and the Li₂MnO₃-modified surface,which remarkably promoted the activation of LLO and maintained the structural stability during electrochemical cycling.This strategy greatly improved the rate capability,cycling stability and first coulombic efficiency of LC-LLO-M.In the second part of the work,a facile microwave hydrothermal method was used to rapidly synthesize Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ hierarchical secondary microspheres,And then those Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ microspheres was modified by an effective MOF-assisted treatment which we proposed in the first time,using a Zr-based MOF(UIO-66-F4,forming by reaction of TFBDC?2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid?and Zr²⁺),as a precursor to produce porous Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ materials with in-situ MOF-derived ZrO₂ coatings and F doping.EDS?Energy Dispersive X-Ray Spectroscopy?tests showed that F homogeneously distributed in the LLO microspheres.The rate capability,cycling stability,and first coulombic efficiency of LLO were significantly improved by the MOF-assisted treatment.The discharge capacity of the MOF-derived ZrO₂ coated LLO?MDZ@LLO?material was 279 and 110.0 mAh g^-1 at 0.1 C and 5 C,respectively.The capacity retention increased from 71.1%to 83.8%after 200 cycles at 1 C while the first coulombic efficiency increased from 62%to 72%during the first cycle,compared with that of the pristine LLO.In the third part of the work,a sol-gel method was used to synthesize nano-sized Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ particles.And then those Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ nanoparticles was modified with in situ spinel Li_1+xNi_yMn_2-yO₄ coating and organic fluorine doping by a novel TFBDC-assisted treatment.The strategy involved simultaneous in situ spinel Li_1+xNi_yMn_2-yO₄coatingusingMn-TFBDC?TFBDC:2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid?as the precursor and organic fluorine doping agent.Uniform LiNi_x Mn_2-x-x O₄ coating on the LLO surface was easily achieved via the coordination of the carboxylic acid ligands with the manganese ions and subsequent heat treatment.The results of X-ray diffraction?XRD?,field emission scanning electron microscopy?FESEM?,transmission electron microscope?TEM?,EDS?Energy Dispersive X-Ray Spectroscopy?and Raman spectrum revealed that the spinel/layered heterostructure was successfully formed on LLO and F was distributed homogeneously in the bulk of LLO nanoparticles.TFBDC-assisted treatment rendered significantly enhanced electrochemical performance to LLO.The first discharge specific capacity of LLO modified by TFBDC-assisted treatment?TA-LLO?was 302.1 mAh g^-1 at 0.1C,and the capacity retention could reach 98.9%after 100 cycles at 1C.Even at the high C-rate of 10,TA-LLO also provided very high discharge capacity of 112.6 mAh g^-1,while that of the pristine LLO?PLLO?only reached 36.2 mAh g^-1.Electrochemical impedance spectroscopy and galvanostatic intermittent titration technique confirmed that TA-LLO had a lower charge transfer resistance and higher ion diffusion coefficient than PLLO.