Understanding the Formation of Antiphase Boundaries in Layered Oxide Cathode Materials and Their Evolution Upon Electrochemical Cycling

Abstract

Layered Li(Ni_1-x-yCo_xMn_y)O₂ (NCM, with Ni ≥ 0.8) cathode materials are essential to achieve high energy densities in the next generation of lithium-ion batteries. This increased performance comes at the expense of stability. To extend the materials’ lifetime, it is necessary to understand the role that crystal defects play in the degradation during electrochemical cycling. In this study, NCM851005 (85% Ni) is investigated in the pristine state and after 100 and 200 cycles using scanning transmission electron microscopy (STEM), with the focus being on the defects in the material. The formation of antiphase boundaries (APB) from a dislocation in a pristine sample is proven. After 100 cycles, the APBs’ length and width are enlarged compared to the pristine state. After 200 cycles, APBs further evolve into an intragranular rock salt-like phase, distorting the nearby layered structure. It is suggested that the behavior of APBs plays a critical role in determining the performance of this cathode material with prolonged electrochemical cycling. These findings will help to understand better the role of dislocations and antiphase boundaries with electrochemical cycling, and the role of dopants may then be explored to avoid them.

Keywords: Batteries; cathode active material; dislocations, antiphase boundaries, twin boundaries, nanopores, layered phase, rock salt-like phase