Exploring calcium pillaring of O3-type NaNi0.9Ti0.1O2 cathodes to advance Na-ion battery technology

• Systematic study on 0.5–5.0 mol.% Ca 2+ substitution in NNTO. • Calcium pillaring increases the interlayer spacing, facilitating sodium diffusion and enhancing rate capability. • Ca 2+ incorporation suppresses O1 phase formation, limiting volume changes to ∼3 %, compared to ∼30 % for NNTO. • Synchrotron XAS reveals improvements in reversibility of Ni redox, ensuring better long-term cycling performance. • DEMS demonstrates mitigated lattice oxygen loss at high potentials, leading to lower CO 2 and H 2 evolution rates. NaNi 0.9 Ti 0.1 O 2 (NNTO), a promising sodium-ion cathode material, is capable of delivering high specific capacities but is compromised by structural degradation at high potentials and poor redox stability. Herein, we introduced varying amounts of Ca 2+ as pillaring ions into the intercalation sites of NNTO to examine their effect on structure and electrochemical performance. Laboratory X-ray diffraction and charge/discharge testing identified Na 0.95 Ca 0.025 Ni 0.9 Ti 0.1 O 2 (CaNNTO) as the optimal composition in terms of interlayer spacing, rate capability, and cycling stability. Specifically, the capacity retention after 200 cycles improved from 27 to 48 %. To elucidate the role of Ca 2+ , synchrotron X-ray diffraction was performed, revealing that CaNNTO undergoes only about 3 % volume change in the initial cycle, compared to about 30 % for NNTO. This reduced volume variation was confirmed by cross-sectional electron microscopy and acoustic emission measurements. Hard and soft X-ray absorption spectroscopy further indicated a more reversible nickel redox (Ni 2+ /Ni 3+ /Ni 4+ ) in CaNNTO, and differential electrochemical mass spectrometry demonstrated that Ca 2+ incorporation helps suppress oxygen loss from the lattice, thereby directly stabilizing the cathode|electrolyte interface. Overall, calcium pillaring is found to effectively mitigate structural collapse and redox instabilities, making CaNNTO a more viable candidate for practical sodium-ion battery applications.