Role of Ni to Mn ratio in Ca-pillared O3-type cathodes: decoupling structural dynamics and electrochemical performance via operando characterization

O3 phase NiFeMn- based layered transition metal oxides have attracted interest for positive electrode materials for Na-ion batteries. However, they generally suffer from challenges like phase transitions and Fe migration. Recently, the substitution of Ca into the Na layer, serving as a ‘pillar’, has proven to be an effective approach to overcome these challenges. Here, we systematically studied the composition-dependent Ca pillaring effect on the electrochemical performance and structure evolution of two O3 phase NiFeMn-based layered transition metal oxides. It is found that, although moderate Ca doping in high-Ni system - Na_1-2xCa_xNi_0.25Mn_0.25Fe_0.5O₂ (x = 0.00, 0.03) enhances cycling stability and reduces polarization, excessive doping compromises rate capability and does not effectively prevent Fe migration. Conversely, high-Mn system - Na_1-2xCa_xNi_0.17Mn_0.33Fe_0.5O₂ (x = 0.00, 0.04) exhibits a more robust and beneficial response to Ca incorporation, showing enhanced structural integrity, improved redox reversibility, and effective suppression of Fe migration. This study provides insights into the tunable chemical environments of transition metal oxides, thereby advancing the design of high-performance positive electrode materials and contributing to the development of next-generation sodium-ion batteries.