Study On Structural Design、Preparation And Performance Of Lithium-rich Layered Cathode Materials

Lithium-rich layered cathode materials have become the mainstream of lithium ion batteries due to their high discharge capacity, unique charge/discharge mechanism, low cost, environmental benignity and so on. Nevertheless, the practical uses of these cathodes have so far been hindered by their cyclic durability problems, serious voltage fading and poor rate capability during high voltage cycling.In an attempt to improve the properties of lithium-rich layered cathode materials, in this dissertation, lithium-rich layered cathode materials with core-shell, core-gradient shell and full concentration-gradient structure have been designed, synthesized and investigated in detail. And the main works are as follows:1. Spherical LiNi0.5Co0.2Mn0.302@Li2Mn03core-shell composite, that is, high capacity LiNi0.5Co0.2Mn0.3O2as the core and high stable Li2MnO3as the shell, was synthesized by a simple sol-gel deposition method. The research results showed that the core-shell composite exhibited excellent cycling performance with a capacity retention of98.8%after200cycles at a rate of0.5C in the voltage range of2.0-4.4V, while the single LiNio.5Co0.2Mn0.3O2core had only75.3%capacity retention during the same cycles. In addition, when cycled at high temperature of55℃, the core-shell composite exhibited only2.3%capacity loss after100cycles, which was much lower than that of the core material (36.2%). The improved electrochemical properties of the core-shell composite are mainly attributed to the stable Li2MnO3shell, which is electrochemical inert between2.0and4.4V and can protect the LiNi0.5Co0.2Mn0.3O2core material from eroded by the electrolyte.2. Spherical lithium-rich layered Li1.15[(Ni1/3Co1/3Mn1/3)0.5(Ni1/4Mn3/4)0.5]0.8502with core-shell structure was synthesized by a co-precipitation route. The influence of preparatory technological conditions on structures and electrochemical properties were systematically studied. The results showed that best electrochemical properties of the core-shell structured material were obtained when calcined at850℃for12h. The core-shell structured material showed a high initial discharge capacity of242.3mAh/g at a rate of0.1C between2.0～4.8V. In addition, it showed an improved cyclic stability with capacity retention of95.6%than that of the single Li1.15[Ni1/3Co1/3Mn1/3]0.85O2core (74.3%) after100cycles at0.5C between2.0and4.6V. Furthermore, the core-shell structured material delivered242.3,224.4,202.2, 172.6,146.0,121.4and96.3mAh/g when cycled at the rates of0.1C,0.2C,0.5C,1C,2C and5C, respectively. However, the single Li1.15[Ni1/4Mn3/4]0.85O2shell only delivered a specific capacity of63.7mAh/g at10C rate. A synergetic effect of the positive attributes of the core and shell materials is achieved by the formation of the core-shell architecture, which can achieve outstanding comprehensive properties comparing with any single material.3.In order to prevent large void from appearing at the core-shell interface owing to the difference in expansion coefficient between the core and the shell, a lithium-rich layered Li1.13[Mn0.534Ni0.233Co0.233]0.87O2with concentration-gradient shell was reasonably designed and successfully synthesized. It was found that the particle was consisted of an inner core and a stable concentration-gradient shell in which Mn content was gradually increased. The core-gradient shell Li1.13[Mn0.534Ni0.233Co0.233]0.87O2showed better electrochemical properties than conventional lithium-rich layered material, which had the same chemical composition. In the voltage range of2.0-4.6V, the core-gradient shell material exhibited an initial discharge capacity of220.2mAh/g at a rate of0.1C, and it showed excellent cycling performance with capacity retention of97.2%after100cycles at0.5C, while the conventional material showed only83.4%during the same cycles. Additionally, the core-gradient shell material showed improved rate capability and thermal stability.4. In order to further improve the electrochemical properties, a spherical lithium-rich layered Li1+Jx[Mn0.60Nio.25Co0.15]1-xO2with full concentration-gradient structure was designed and synthesized. The optimized preparation conditions of the full concentration-gradient material were as follows:the x value (lithium content) was0.14, the temperature of reaction was900℃, and the reaction time was16h. The results showed that, the chemical composition was not uniformity distribution in the whole Li1.14[Mno.60Ni0.25Co0.15]0.86O2particles. Namely, the Mn content gradually increased from the spherical particle center to the outer layer, while the Co concentration showed the reverse. Meanwhile, the Ni content remained almost constant throughout the particle. Electrochemical testing showed that the full concentration-gradient material exhibited stable discharge capacity and voltage comparing with the conventional material.In the voltage range of2.0～4.6V, the concentration-gradient material showed outstanding cyclic stability with capacity retention of93.8%after200cycles at0.5C and95.2%after100cycles at5C, which were much higher than that of the conventional material. The mechanism of the improved electrochemical properties of the concentration-gradient material was interpreted for the cycled material by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscope, and transmission electron microscopy and so on.