Understanding and optimizing Li substitution in P2-type Sodium Layered Oxides for Sodium-Ion Batteries

In the quest for novel cathode materials for Na-ion batteries, we studied 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). We explored the consequences of Li substitution, as well as the challenge of elevating the Na content in the pristine materials through solid-state synthesis. By in situ temperature-resolved x-ray diffraction (XRD) we observed the synthesis process, clearly showing the formation of an intermediate Li2MnO3 structure with increasing Li content. Structurally, honeycomb ordering is observed in all samples, while we show that Li induces the loss of Na+/vacancy ordering hence it leads to more disordered Na positions. Electrochemically, this family of materials exhibits an increasing trend of polarized hysteresis in the first cycle, suggesting the contribution of oxygen redox. We coupled semi-simultaneous operando x-ray absorption near edge structure (XANES) and XRD to appreciate the structural evolution and redox behavior during this process. We verified that Li in the transition metal site eliminates phase transitions at high voltage and modifies the activation of O-redox. As confirmed by our XANES analysis and by Density Functional Theory calculations, the Li-free sample already surprisingly show anionic redox despite the residual availability of Ni-redox, due to the peculiar density of states dominated by Ni-O hybridized states at high state of charge. On the other hand, Li-containing samples have O non-bonding states that lead to increasing O-redox contribution as expected due to the limited Ni content. One composition (Na0.745(6)Li0.164(4)Ni0.238(1)Mn0.599(3)O2) proves to have the lowest proportion of O-redox among all samples, coupled with reduced phase transitions, disordered occupancy of Na sites, and small volume change during cycling (4.8%). This material hence delivers the best balance of cycling stability (∼92% after 100 cycles in half cells), capacity (> 100 mAh/g) and rate capability. Determining this optimal compositional range is a promising starting point for further development of P2 layered oxides as cathode materials for Na-ion batteries and can be generalized to other families of Na-based layered oxides with redox-inactive dopants.