Evolution of a Radially Aligned Microstructure in Boron-Doped Li[Ni<sub>0.95</sub>Co<sub>0.04</sub>Al<sub>0.01</sub>]O<sub>2</sub> Cathode Particles
Ye Sung Kim, Ju Hwan Kim, Yang‐Kook Sun, Chong Seung Yoon
Abstract
Boron (B) (1.5 mol %) is introduced into Li[Ni0.95Co0.04Al0.01]O2 (NCA95) to create a radially oriented microstructure with a strong crystallographic texture. The cathode microstructure allows dissipation of the abrupt lattice strain near the charge end and improves the cycling stability of the NCA95 cathode (88% capacity retention after 100 cycles at 0.5 C). Transmission electron microscopy (TEM) analysis of the B-doped NCA95 cathode during lithiation reveals that the highly oriented microstructure is provided by a hydroxide precursor. Boron prevents random agglomeration of the primary particles and keeps them elongated through (003) faceting. The selected-area electron diffraction analysis shows that the structure of the lithiated oxide undergoes subtle structural changes even after the crystal structure is fully converted from P3̅m1 to R3̅m at 600 °C. Li+/Ni2+ intermixing is prevalent due to the slow oxidation of Ni2+ to Ni3+. Li+ and Ni2+ do not randomly occupy the Ni and Li layers; instead, these ions occupy their sites in an ordered pattern, forming a superlattice. The superlattice gradually disappears as the lithiation temperature is increased. One peculiar structural feature observed during lithiation is the prevalence of twin defects that preexist in the hydroxide precursor as growth twins. The twin defects, which could serve as nucleation sites for intraparticle cracks, also gradually anneal out during lithiation. TEM analysis substantiates the importance of the hydroxide precursor microstructure in a coprecipitation process and provides a basis for choosing the appropriate lithiation temperature and soaking time to obtain the desired cathode structure and primary particle morphology.