Understanding and Optimizing Li Substitution in P2‐Type Sodium Layered Oxides for Sodium‐Ion Batteries

In the quest to improve cathode materials for Na-ion batteries, a family of Li-substituted P2 layered oxides with nominal stoichiometry Na5/6LiyNi5/12-3y/2Mn7/12+y/2O2 (y = 2/18, 3/18, 4/18, 5/18) is studied. The consequences of Li substitution and the challenge of elevating the Na content are explored. Structurally, honeycomb ordering is observed in all samples, while Li induces the loss of Na+/vacancy ordering. Electrochemically, the materials exhibit an increasing trend of polarized hysteresis in the 1st cycle. Semi-simultaneous operando x-ray absorption and diffraction are coupled to appreciate the structural evolution and redox behavior during this process. Li in the transition metal site eliminates phase transitions at high voltage and modifies the activation of O-redox. All samples show anionic redox: as confirmed computationally, in the Li-free sample this is rooted in Ni─O hybridized states, while in the Li-containing samples in O non-bonding states. Composition Na0.745(6)Li0.164(4)Ni0.238(1)Mn0.599(3)O2 proves to have the least O-redox among all, coupled with reduced phase transitions, disordered occupancy of Na sites, and small volume change during cycling, leading to the best balance of cycling stability (≈92% after 100 cycles), capacity (> 100 mAh g−1) and rate capability. This can pave the way for further development of P2 layered oxides with redox-inactive dopants.