Synthesis of Co-Free Ni-Rich Single Crystal Positive Electrode Materials for Lithium Ion Batteries: Part I. Two-Step Lithiation Method for Al- or Mg-Doped LiNiO₂

Increasing the Ni content of a Ni-rich layered positive electrode material is one common way to improve energy density of Li-ion cells but normally leads to shorter cell lifetimes. Single crystalline materials have been shown to improve the cell lifetime by reducing the degree of material degradation. This first study in a two part series investigates the synthesis of Co-free single crystalline LiNi_0.95Al_0.05O₂ and LiNi_0.975Mg_0.025O₂ via a two-step lithiation method. This method consists of a first step heating of the precursors at high temperatures but with deficient Li to grow crystalline particles and then a second step at lower temperature to fully lithiate the material. The synthesized materials were characterized by scanning electron microscopy and X-ray diffraction to understand the impact of synthesis conditions. Single crystal materials were successfully synthesized, and Mg-containing single crystal materials achieved micron-sized particles with as low as 2% Ni in the Li layer. Al-containing single crystal materials could not avoid the formation of Li₅AlO₄ impurity for all conditions tested. The presence of Li or Mg and high temperatures were identified as factors that promote crystallite growth. Selected samples were characterized electrochemically and compared to their polycrystalline counterparts. Mg-containing single crystal materials are not yet competitive with their polycrystalline counterparts yet, and further understanding and development is needed.