Is Cobalt Needed in Ni-Rich Positive Electrode Materials for Lithium Ion Batteries?

As a derivative of LiNiO2, NCA (LiNi1-x-yCoxAlyO2) is widely used in the electric vehicle industry because of its high energy density. It is thought that Co and Al both play important roles in enhancing NCA material properties. However, there is no solid evidence in the literature that clearly shows that Co is required in NCA with high nickel (e.g. when 1-x-y > 0.9) content. Therefore, a systematic study on the roles of different cation substituents in a series of LiNi1-nMnO2 (M = Al, Mn, Mg, or Co) materials was made. In-situ X-ray diffraction (XRD) and differential capacity versus voltage (dQ/dV vs. V) studies showed that the multiple phase transitions in LixNiO2 during charge and discharge, thought to cause poor charge-discharge capacity retention, were suppressed in LixNi0.95M0.05O2 (M = Al, Mn, or Mg), while 5% Co failed to suppress the phase transitions. First principles calculations were made to understand the function of each substituent. Accelerating rate calorimetry shows that unlike Al, Mn, or Mg, Co has no contribution to safety improvement. Therefore, we believe that Co brings little or no value at all to NCA-type materials with high Ni content (> 90% Ni in the transition metal layer) and we hope this paper will spur more interest in Co-free materials.