Structural and Electrochemical Investigation of Ni- and Mn-Based Disordered Rock Salt Cathode Materials without d⁰...

Disordered rock salt (DRX) compounds are increasingly investigated as promising alternatives to conventional layered oxide cathodes due to their intrinsic cation-disordered structure that enables extensive chemical tunability, mitigating reliance on critical elements traditionally used in lithium-ion batteries. In this work, we present a new class of DRX materials obtained via mechanochemistry without any d⁰ stabilizing element, with the target composition Li_2yMn_yNi_2-3yO₂ (0.50 ≤ y ≤ 0.67). We design the materials with Ni as the main redox-active species, while Mn acts as a charge compensator and structural stabilizer with moderate redox activity. Structural characterization was performed using X-ray diffraction (XRD), neutron powder diffraction (NPD), and scanning and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (EDX) and X-ray fluorescence spectroscopy (XRF) to assess the morphology and confirm the transition metal composition. Electrochemical testing revealed promising specific capacities approaching 191 mAh/g for Li_1.2Ni_0.2Mn_0.60O₂, alongside, however, a significant voltage hysteresis and polarization. The dQ/dV curves suggest multiple redox processes, whose evolution upon cycling suggests irreversible phase transformations. The charge compensation mechanism is further clarified by semi-simultaneous operando XRD and X-ray absorption near edge structure (XANES) spectroscopy, enabling the investigation of lattice evolution and electronic changes at the Ni and Mn K-edges during the charge, in fact proving that Ni and Mn are both redox active, as is oxygen, and that the structure evolves toward a spinel phase over prolonged cycling.