Solvothermal Route to Model Single-Crystalline Co-Free Li- and Mn-Rich Layered Oxides from Micrometer-Sized Carbonate Precursors

To facilitate the small-scale synthesis of model single-crystalline LMR-MN (Li_1.17Mn_0.58Ni_0.25O₂) cathode active materials (CAMs), we employ a solvothermal method to prepare dense, monodisperse, Mn- and Ni-carbonate precursors with a secondary particle size of ≈ 1 μm. The reaction kinetics, determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), indicate that the particles comprise a Ni/Mn-carbonate core and a Ni-rich shell, arising from the preferential complexation of Ni²⁺ over Mn²⁺ by NH_3, thereby delaying NiCO₃ precipitation. Subsequent calcination with Li₂CO₃ between 950 and 1100 °C yields phase-pure CAMs at temperatures above 1000 °C, with rock-salt present only in the material calcined at 950 °C, confirming temperature-dependent crystal domain growth, which directly affects the accessible surface area. After calcination at 1100 °C, the crystal domain size is comparable to the precursor secondary particle size, indicating the formation of single crystals with ≈ 87% of the primary crystallite surface exposed. Electrochemical testing of all CAMs in coin cells reveals reduced capacities with increasing particle size, governed primarily by the accessible surface area that determines the effective current density across the surface of the CAMs. As expected, the presence of the rock-salt outer shell in the CAM calcined at 950 °C negatively affects its rate capability.