Physico-chemistry of ceria-coated nickel rich manganese cobalt oxides as cathode materials for lithium-ion batteries

Abstract

The layered cathode materials, LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) have been lauded as robust and fitting candidates for the development of the next generation lithium-ion batteries for electric vehicles and other high energy demanding technological applications because of their high-energy density. Ni is responsible for the high-capacity/energy density, Mn contributes to structural and thermal stability, while Co contributes to the rate capability by mitigating the Ni/Li ion mixing. It is therefore not surprising that the widespread commercialisation of NMC622 and NMC811 cathode materials have been frustrated by structural and thermal instability, capacity decay, and poor rate capability. To overcome the poor electrochemical performance of the NMC622 and NMC811, this thesis investigated the use of ceria (CeO2, a rare earth metal oxide) to coat or/and dope these two cathode materials. First, commercially available NMC622 was chemically modified with a very low amount of CeO2 (~ 2%) to form an ultrathin CeO2-coated NMC622 surface (NMC622-c). The NMC622-c exhibited enhanced electrochemical performance compared to the pristine counterpart (NMC622) in terms of i) the first cycle Coulombic efficiency was reduced by ~58% compared to the NMC622, and ii) capacity retention of 98 % after 50 cycles at 1C rate compared to the 82 % of the NMC622-c. This enhancement has been made possible by the NMC622-c overcoming the destructive H2/H3 phase transition, reduced Ni/Li mixing, and improved ordered hexagonal structure. The results show that the electrochemical performance of the conventional commercial NMC622 cathode material can be significantly improved by optimising the CeO2 surface coating. Next, NMC811 (which is more energetic than the NMC622) was synthesized using the Taylor-Laminar flow method combined with microwave-assisted surface-coating and bulk-doping with about 2% CeO2. Six samples were investigated, which are the pristine and microwaved samples (NMC811-p and NMC811-p_MW), CeO2-coated and microwaved samples (NMC811-c and NMC811-c_MW), and CeO2-doped and microwaved samples (NMC811-d and NMC811-d_MW). The NMC811-d and NMC811-d-mw outperformed other samples investigated in terms of the first cycle Coulombic efficiency and capacity retention. For example, NMC811-d_MW delivered a capacity of 220 mAhg-1 with a capacity retention of 90 % over 50 cycles followed by the non-microwaved NMC811-d. The CeO2 coating/doping eliminated the H2/H3 phase transition, which is known to lead to microcracks that propagate the degradation process. In summary, CeO2 treatment offers alternative treatments that can be used to overcome degradation problems that are inherent in Ni-rich cathodes for Li-ion batteries, which can pave way to applications in electric vehicles and related high-energy demanding applications.