The development of single-crystal nickel-rich layered LiNixCoyMn1-x-yO2 materials (S-NCMs) represents the most significant progress for the electrification applications of nickel-rich ternary materials. There has been prior research on the important role of transition metal elements in agglomerated materials, supplemented by surface and internal lattice optimization to drive the performance improvements. However, studies on S-NCMs, especially on the role of transition metals (TM, i.e., Co and Mn), have not been reported. In this study, we synthesized four kinds of S-NCMs with different Co/Mn contents and studied their structural, electrochemical, kinetic, and thermodynamic properties with different Co/Mn contents. The results were as follows: (1) Electrochemically, Co was more effective than Mn at 25 °C at enhancing the intercalation/deintercalation kinetics, which resulted in an increased discharge capacity, an improved rate capability, and a reduced energy loss. (2) Thermodynamically, Mn was more effective at maintaining a higher thermal stability than Co, especially at a low cutoff voltage, but at a high cutoff voltage, the difference between the action of Co and Mn decreased. The main finding of this work was the enhanced structural stability provided by Co, which could be attributed to the following: (i) the absence of the H2/H3 phase transformation when Co exceeded 15%, which inhibited the irreversible phase transformation and reduced the volume strain, and (ii) the lower degrees of decrease in the cell parameters a and c with higher contents of Co, which contributed to a low cracking degree along the (003) crystal plane. The current work provides an important reference for the single-crystallization strategy of nickel-rich materials.
Keywords: Co/Mn content; H2/H3 transition; electrochemical kinetics; lattice parameter; single-crystal NCM.