Surface Modification Of Li-Rich Layered Cathode Material For Lithium-Ion Batteries

Li-rich layered cathodes xLi2MnO3·?1-x?LiMO₂ attract much attention due to their high capacity?over 250 mAh g-1? and high voltage?⁴.5 V?. However, the voltage plateau fade, poor cyclic and rate capability hinder their commercialization. Surface modification is an effective strategy to improve the electrochemical performances of cathodes. This dissertation focus on the improved electrochemical performances of layered Li-rich cathode Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ by surface modifications. The main research results are as follows:?1? Li-rich layered cathode Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ were modified by various amounts of AlF₃ which shows higher stability compared with traditional surface modification materials such as Al2O3 and other oxides. The cyclic and rate capability were improved by AlF₃ modification, but the capacity are reduced. 1 wt.% AlF₃ modified Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes exhibits the best cyclic. It shows a capacity of 185 mAh g-1 for the first cycle which is lower than the capacity of bare cathode and retains 80.5% after 100 cycles at 0.2 C. The lowered capacity can be attributed to the inactive surface modification layer which blocks the Li-ion and electron diffusion at interface.?2? To investigate the effects of the Li-ion diffusion at interface on the electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode, Li₃AlF₆ modification, AlF₃ modification and bare cathode were compared. Li₃AlF₆ as modification material can not only eliminate the effects of heteroatoms dopant exclude Al and F but also enhance the Li-ion conductivity of the surface modification layer. The Li₃AlF₆ surface modification amounts were optimized and the cathode with the best electrochemical performances were compared with other electrodes. The cyclic and rate performances were greatly improved by Li₃AlF₆ surface improved. The 3 wt.% Li₃AlF₆ modified Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathodes shows the best electrochemical performances. It delivers 227 mAh g-1 at first cycle and retains 90.7% after 100 cycles at 0.2 C. The comparison study indicates that a passivation layer with ultralow electron and Li-ion conductivity form at the electrode/electrolyte interface. Surface modification material with higher Li-ion conductivity can improve the electrochemical performances, but also retains even enhance the capacity after modification.?3? Li_1.3Al_0.3Ti_1.7?PO₄?₃?LATP?is a kind of solid state electrolyte which shows high Li-ion conductivity and stability. LATP were employed as surface modification material to improve the electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes and the modification amount and sintering temperature are optimized. The electrochemical performances were significant enhanced after LATP modification. The 550 °C treated 2 wt.% LATP modified Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes shows the best electrochemical performances. It delivers 209 mAh g-1 at first cycle and retains 86.3% after 100 cycles at 0.2 C. The LATP surface modification can not only inhibit the phase variation but also reduce the side reactions at cathode/electrolyte interface hence the suppressed the voltage plateau fade and improved the electrochemical performances.