Preparation And Characterizations Of Li₃VO₄Coated Li-excess Layered Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries have been widely applied in laptop computers, cell phones,MP3and other electronic devices owing to the high energy/power densities, longcycle life and low toxicity. Cathode material, as one of an important part of Li-ionbattery, determines the electrochemical performance and cycle life of Li-ion battery.So it is crucial to study and exploit advanced cathode materials with high performanceand low cost. Compared with traditional cathode materials, Li-excess layered cathodematerials exhibit several advantages of high reversible capacity (more than250mA hg-1), environment friendly and low cost. However, some serious issues, related to thelarge irreversible capacity loss and low coulombic efficiency in the initial cycle, poorrate capability, severe cyclic capacity degradation, and poor thermal stability, alsolimit their practical applications. In this dissertation, we systematically improve theelectrochemical performance of Li-excess layered cathode materials by adjusting thepreparation method and surface coating. The major research results are listed asfollowing:Firstly, well crystallized Li1.18Co0.15Ni0.15Mn0.52O2cathode material was preparedby the sol–gel method. Then, the electrochemical performance ofLi1.18Co0.15Ni0.15Mn0.52O2material was studied after coated with Li3VO4. XRD andTEM demonstrated that the Li-excess compound showed a higher ordered hexagonallayered structure, and less Li+/Ni2+cation mixing after surface coating. And thesurface of the coated material was composed of Li3VO4poly-crystals, which wereimpregnated into the bulk of the active material. The electrochemical performance ofLi1.18Co0.15Ni0.15Mn0.52O2was also significantly enhanced after Li3VO4coating. The discharge capacities of the Li3VO4coated sample are214.3and126.2mA h g-1at thecurrent density of200mA g-1and1A g-1, respectively, well over the manifestedcapacities of195and74.3mA h g-1for the pristine sample. When the current densityincreases to2A g-1, a discharge capacity of5mA h g-1is recorded for the pristinesample, but the coated sample still shows a high discharge capacity of47.8mA h g-1.Moreover, the cycle stability of Li1.18Co0.15Ni0.15Mn0.52O2was also obviouslyenhanced after Li3VO4coating. A discharge capacity of99.2mA h g-1was obtainedafter100cycles at the current density of1A g-1, which is much higher than that of thepristine material,38.6mA h g-1. In regard of safety, the main exothermic peak of thepristine sample is205.2oC, with another small peak at274.5oC, while the Li3VO4coated sample only shows one exothermic peak at232.8oC. In addition, the heatamount is significantly reduced from807.5J g-1to551.0J g-1. The effects of Li3VO4surface coating can be concluded as following:(i) protecting the material from contactwith CO2in the air thus prohibiting the formation of an Li2CO3layer;(ii) improvingthe activation of Mn4+ions leading to a higher discharge capacity;(iii) enhancing theelectrochemical kinetic properties of the material which improves the rate capabilityand cycle stability of the material, and (v) retarding the side reactions of the activematerial with electrolyte thus improving the thermal stability of the material.Furthermore, we also investigate the effects of the coating technique on rate andcycle performance of Li-excess layered cathode material. TEM measurementindicated that Li3VO4layer of1nm and coupling transition area of2nm wereproduced on the surface of cathode material using the precursor after pretreat viadirect reaction with NH4VO3, and Li3VO4layer was impregnated into the bulk ofcathode material. Correspondingly, the performances of the cathode materialLi1.18Co0.15Ni0.15Mn0.52O2, such as the capacity, rate and cycling performance at highcurrent density, were also found to be improved after Li3VO4coating. The samplewith “impregnated coating” delivers a discharge capacity of145.4and106.6mA h g-1at the1A g-1and2A g-1, respectively, compared to118.2and42mA h g-1for the pristine sample. A discharge capacity of86.6mA h g-1and retention of63.4%areobtained after200cycles at the current density of2A g-1for the coated sample, whichis almost twice as much as the pristine material(46.1mA h g-1and38.4%).In this dissertation, we not only enhanced the electrochemical performance ofLi1.18Co0.15Ni0.15Mn0.52O2layered cathode materials by Li3VO4coating, but alsoproposed “impregnated coating” for the first time, and obtained the advanced cathodematerials with superior cycling stability and rate capacity. Moreover, this worksenrich the fundamental research of Li-excess layered cathode materials, and alsoprovide a theoretical guidance and reference for their practical applications inlithium-ion batteries.