Lithium-ion batteries (LIBs) are one of the most promising energy storage systems for power grid load leveling and transportation power supply. To realize these foreseen applications, the energy density, the lifetime and the cost-effectiveness of the batteries need to be further improved. This thesis focuses on Li-rich layered oxide, one of high energy density cathode materials, and aims to address some of challenging issues of this material such as capacity fade, voltage decay and rate performance declination by modifications including metal-doping and surface-coating.

The cycling stability and the rate performance of the material are found substantially enhanced in Li_1.17Mg_0.03Mn_0.54Ni_0.13Co_0.13O₂ obtained by Li substitution with Mg. Some 86% of capacity is retained after 50...[ Read more ]

Lithium-ion batteries (LIBs) are one of the most promising energy storage systems for power grid load leveling and transportation power supply. To realize these foreseen applications, the energy density, the lifetime and the cost-effectiveness of the batteries need to be further improved. This thesis focuses on Li-rich layered oxide, one of high energy density cathode materials, and aims to address some of challenging issues of this material such as capacity fade, voltage decay and rate performance declination by modifications including metal-doping and surface-coating.

The cycling stability and the rate performance of the material are found substantially enhanced in Li_1.17Mg_0.03Mn_0.54Ni_0.13Co_0.13O₂ obtained by Li substitution with Mg. Some 86% of capacity is retained after 50 cycles at 0.1 C after doping, significantly higher than 76% obtained for Mg-free sample. Moreover, the doped sample is able to achieve 72% of increase in the capacity at 5 C compared with the undoped counterpart. This is owing to the improved conductivity and facilitated Li ion diffusion which is resulted from the presence of Mg ions and the lattice expansion from doping.

Cu-doping and Zn-doping are also found effective to stabilize the electrodes. The capacity retention after 50 cycles at 0.1 C for Cu-doped and Zn-doped samples are 93.7 and 96.9%, respectively, in contrast to 58.6% of the undoped sample. The voltage decay is mitigated as well after doping, suggesting the presence of Cu or Zn suppresses the structural transformation of Li-rich layered oxide during cycling.

Meanwhile, the surface of the material is coated with conductive polyaniline by a simple oxidative polymerization approach using oxide itself serving as an oxidant. The reaction time of 30 minutes is identified as the optimal duration for the polymerization, which yields about 2% of polyaniline with 5-10 nm thickness on the surface. Some 67.7% of capacity is retained with polyaniline coating after 500 cycles at 1 C, higher than 44.2% retained for the pristine sample. The enhanced cycling ability and mitigated voltage decay are resulted from the prevention of corrosion by electrolyte and the inhibition of surface passivation. The activation process at initial stage of cycling for coated material is found related to the sluggish decomposition of Li₂MnO₃ component.

The combination of Mg-doping and polyaniline-coating is then realized. Further improvement has been made as the sample modified by this combination is able to achieve 76.2% of original capacity after 500 cycles at 1 C, higher than that of either modification alone.