Electrochemical Characterization and Microstructure Evolution of Ni-Rich Layered Cathode Materials by Niobium Coating/Substitution

The high nickel layered mixed metal oxides, such as LiNizCoyMn1-z-y-qAlqO2, are the most utilized cathode materials in Li-ion batteries for electric vehicles due to their high energy density. However, as the nickel content increases, they suffer from poor capacity retention and from voltage fading due to interfacial/structural instability. In this paper, a series of Nb-coated/substituted LiNi0.9Co0.05Mn0.05O2 (NMC 9055) were synthesized by reacting the Nb precursors, Ni0.9Co0.05Mn0.05(OH)(2), and LiOH. Nb is found in the NMC structure and also on the grain boundaries between the primary particles. These Nb-modified materials showed improved capacity retention and charge/discharge voltage profiles over the untreated material; the capacity retention was 86.4% (0.7 Nb-NMC 9055) vs 93.5% (1.4 Nb-NMC 9055) vs 99.7% (2.1 Nb-NMC 9055) vs 75.3% (NMC 9055) after 200 cycles and nearly no voltage fading (1.4 Nb-NMC 9055 and 2.1 Nb-NMC 9055) during cycling. High-angle annular dark-field (HAADF) scanning transition electron microscopy (STEM) images showed that the added niobium (Li-Nb-O phase) is located in the boundaries between the primary particles. This transferred obvious interparticles/intraparticles cracking into tiny intraparticles cracking, which benefits the release of strain/stress, maintains the mechanical integrity of secondary particles, and inhibits the structural transformation from the layer structure to rock-salt phase supported by large reduced charge transfer resistance, thus enhancing the electrochemical performance of NMC 9055.