Atomistic Insights into Solid-State Phase Transition Mechanisms of P2-Type Layered Mn Oxides for High-Energy Na-Ion Battery Cathodes

Mn-based layered oxides hold great promise as high-energy, cost-effective cathodes for sodium-ion batteries (NIBs), but repetitive Na+ cycling induces harmful phase transitions. Understanding these mechanisms is essential for designing better performing NIB cathodes. Applying density functional theory (DFT) and variable cell-nudged elastic band (VC-NEB) calculations, we provide atomistic insights into phase transformation pathways and energy barriers in P2-Na x MnO2 material and its Ni-doped variant. We reveal the key P2-to-OP4/O2 and P2-to-P2' transitions that occur across various sodiation levels, involving substantial rearrangements around the transition metal sites, with tetrahedral transition states accountable for energy barriers. Our analysis of bond length and angle distortions highlights that shear deformations are pivotal in triggering P-to-O gliding at low sodium levels. Based on these insights, our structural distortion metrics offer a straightforward and computationally efficient descriptor to evaluate structural integrity for these layered oxides, enabling the design of NIBs with improved stability and extended lifespan.