Systematic Study on Alkaline Earth Metal Doping in P2-Type Na 2/3 Fe 0.5 Mn 0.5 O 2 Cathodes for Na-Ion Batteries

Na-ion batteries (NIBs) are considered as a promising energy source alongside Li-ion batteries owing to their low-cost, abundant, and widely available resources. For the positive electrode, P2-type layered oxides ( i . e . P2-Na x TMO 2 , TM = transition metal) have garnered significant interest with their excellent rate capability and relatively high operating voltage. 1 Among various modification strategies aimed at improving their properties as cathode materials for NIBs, alkaline earth metal doping has emerged as an effective approach to regulate their electrochemical/physicochemical properties. Our previous work demonstrated that 1 mol% Ca doping in P2 type Na 2/3 Fe 0.5 Mn 0.5 O 2 (P2-FeMn) markedly improved both its rate performance and air stability. We believe that improved rate performance is related to the pillar effect of Ca doped within the Na layer, thereby suppressing the excessive slab gliding during rapid Na + extraction/insertion, while the air stability was linked to the migration of Ca ions towards the particle surface when exposed to humid air. 2 Nevertheless, a general understanding of how dopants govern site preference, lattice parameters, phase stability, and sodium ion transport kinetics remains unclear. In this work, we systematically investigate the role of various alkaline-earth metal ion doping, i.e. , Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ , on P2-FeMn structure, air stability and cycling performance. When doping with Mg 2+ , we find that its small ionic radius (~0.72 Å) leads to preferential substitution at the transition-metal site. Consequently, although no secondary phase or P2/O3 biphasic formation is observed in the SXRD patterns of the 0.5 mol% Mg 2+ -doped sample (Fig. 1a), the lattice parameter of the a-axis increases while the c-axis decreases upon Mg doping (Fig. 1b). Meanwhile, doping with the larger ionic radius of Ba 2+ (~1.35 Å) tended to show significant formation of impurity phases (Fig. 1a) which are likely linked to severe lattice mismatch upon incorporation. In contrast, Ca 2+ (~1.00 Å) and Sr 2+ (~1.18 Å) have similar ionic radii and are expected to preferentially occupy the sodium (prismatic) site. We find the Ca 2+ -doped sample exhibits a moderate expansion of the c-axis (Fig. 1b), whereas Sr 2+ induces the most pronounced c-axis expansion. The expansion correlates well with improved Na + kinetics and superior rate capability(Fig. 2) suggesting a stronger pillar effect for Sr 2+ compared to other alkali metal ions. Overall, our results clearly indicate that c-axis expansion is key for attaining superior rate performance, but at the same time, the size of the dopant can prevent stable incorporation into the sodium layer as well as destabilize the material also. Building on these insights, our ongoing work extends alkaline-earth metal doping across a series of P2 layered oxides to develop general design principles that connect ionic radius and dopants distribution to air stability and sodium ion transport. References: Yabuuchi, N. et al. P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries. Nature Mater 11 , 512–517 (2012). Mahapatra, M. et al. Enhanced air stability by calcium doping in Na 2/3 [Fe 1/2 Mn 1/2 ]O 2 cathode material for Na-ion batteries. J. Mater. Chem. A 10.1039.D5TA04742K (2025) doi:10.1039/D5TA04742K. Figure 1