Effect of a Two-Step Temperature-Swing Synthesis on Coarse-Grained LiNiO₂ Secondary Particles Characterized by Scanning Transmission Electron Microscopy

To enhance the range of electric vehicles, research is focused on increasing the nickel content in cathode active materials (CAM), which leads to higher practically achievable specific capacities. As a result, the material LiNiO₂ (LNO) has attracted significant interest. In this study, a two-step temperature swing synthesis is employed to produce LNO secondary particles with large primary grains as CAM for solid-state batteries (SSBs). The synthesis involves sintering the material at 800 °C for 1 h, followed by an annealing step at a lower temperature for 6 h. Different batches of annealed LNO, using temperatures of 600 and 700 °C, are compared with unannealed LNO, utilizing various transmission electron microscopy (TEM) techniques. The annealing step contributes to smoother particle surfaces, reduced residual lithium species on particle surfaces, and increased lithium occupancy in the crystal lattice, resulting in higher discharge capacity during the initial cycles. However, higher annealing temperatures also lead to the formation of a thin rock-salt layer on the surface and internal misorientation, likely caused by thermal residual stress during cooling. These effects are more pronounced in the sample annealed at 700 °C compared to 600 °C. Despite the potential drawbacks associated with these factors, LNO annealed at 700 °C achieves the highest discharge capacity, indicating that the benefits of annealing outweigh its disadvantages, at least during the initial cycles.