Synthetic Approaches And Structural Modifications Of Lithium-rich Manganese-based Oxide Cathode Materials

Next generation of portable electronics and electric vehicles (EV) require higher energy density lithium-ion batteries than ever before. Currently, lithium-ion batteries suffer from the low specific capacities of cathode materials. Therefore, the development of high-capacity cathode material is a critical key for upgrading the lithium-ion batteries. Among the Li-storage cathode materials, Li-rich layered oxides xLi2MnO3-(1-x)LiMO2(M=Co?Ni?Mn) have received particular attention because of their high capacity of250-300mAh·g-1, better safety and low cost. However, the xLi2MnO3-(1-x)LiMO2materials suffer from several disadvantages such as low initial Coulombic efficiency, poor rate capability, capacity fading and voltage degradation during cycling. To solve these problems, this Ph.D work was focused on synthetic chemistries and structural modifications of these materials, aiming at enhancing the electrochemical performance of the xLi2MnO3-(1-x)LiMO2materials. The main contents and results of this paper are as follows:1. Comparative studies of the synthetic chemistries of the materials. Layered Li-rich oxides0.5Li2Mn03-0.5LiNio.33Mno.33Co0.3302,(Li[Li0.2Mn0.54Ni0.13Co0.13]02) have been prepared by three different routes:Polymer-Pyrolysis (PP), Machanochemical synthesis(MC) and Co-Precipitation(CP) approach. The structural and morphological characterizations reveals that PP method could maintained a uniform distribution of the metal ions during the preparation process, resulting in a small particle sizes (100-150nm) with good crystallinity. The electrochemical characterization shows that the material prepared by PP method can exhibit a high discharge capacity (291mAh·g-1), good rate performance (210mAh·g-1at1C), but a poor cyclability (20%capacity loss during100cycles). Though the material prepared by MC method shows a relatively low capacity of260mAh·g-1, it can exhibit an excellent cyclability (only13%capacity loss during500cycles). Since MC method is simple, green and easy to control, it is suitable for large-scale industrial production. Though the material prepared by CP method shows spherical morphology with a higher tap density, its preparation process was very complex and inconvenient for the material modification. So we adopt the PP method to prepare the material for performance enhancement of the material in this work.2. Surface modification of the layered material:Irreversible interfacial reactions in the high potential region seriously limited the application of the Li-rich layered oxides. In this work, we studied the feasibility and impact of surface modification with non-conductive oxides (AI2O3, TiO2, etc.) and conductive oxide (FTO) to inhibit the decomposition of electrolyte on the surfaces of layered compounds. The experimental results showed the AI2O3-coated material could realize a high initial coulombic efficiency of96%with high reversible capacity (302mAh·g-1), rate performance (235mAh·g-1,1C) and cyclability (capacity retention of86%over100cycles). The improved electrochemical performance was mainly due to the inert coating layer, which effectively reduced irreversible decomposition of the electrolyte in high potential region and resulted in a stable interface SEI film. Based on the above studies, we also used conductive glass FTO (fluorine-doped SnO2) to modify the surface of Li-rich oxides. FTO-coated materials exhibited an excellent electrochemical performance:a high initial coulombic efficiency (88%) with high reversible capacity (296mAh·g-1), rate performance (251mAh·g-1,1C) and cyclability (capacity retention ratio of82%over300cycles). Such an amazing electrochemical performance originated from not only the protection effect of the coated FTO surface layer for alleviating the structural change of the material surface, but also from the good conductivity of the coated surface for effectively improving the kinetics of materials.3. Structural stabilization of bulk Doping. A phase transformation of lithium-rich layered oxide from layer structure to spinel-like one occurs during cycling, which results in the performance degradation of material. To mitigate this issue, we discussed the effect of doping Mg2+and Na+on structural stability of material. The experimental results showed that Mg2+could partially substitute for Li+in the transition metal layer and Na+could partially substitute for Li+in the Li layer. The doped materials effectively enhanced the stability of the layered structure of material and realized an greatly improved cyclability. For example,4%Mg2+doping materials could deliver a reversible capacity of220mAh·g-1at the current density of100mA.g"1and showed a stable capacity during300cycles. Beside,3%Na+doping materials exhibited high capacities of266mAh·g-1at current density of100mA·g-1and a capacity retention of86%over150cycles.4. Strategy for suppression of voltage decay. Voltage decay of lithium-rich layered oxide materials (xLi2MnO3-(1-x)LiMO2) during cycling limited the practical application. To mitigate this issue, we explored the mechanisms of voltage decay during cycling. The experimental results showed that controlling the charge-discharge potential range, adjusting the Co content in LiMO2component and the ratio between Li2MnO3and LiMO2could be effectively suppressed the capacity fading and voltage drop during cycling. For example, the Li[Li0.2Mn0.54Nio.13Co0.13]02materials can deliver a capacity of172MAh·g-1, a capacity retention ratio of92%and particularly no voltage decay during430cycle when cycled between3.0-4.4V. Based on the above works, we prepared a high-performance lithium-rich layered oxide material with low-cobalt (Li1.17Mn0.54Nio.21Co0.08O2). The results showed the material could deliver a relatively high capacity of202mAh·g-1, a high capacity retention ratio of about98%and a small voltage decay of only42mV during100cycles when cycled between2.8-4.4V.