Pillaring Effect of Li+ in a Mn‑Rich P2-Type Layered Oxide Cathode with Ultra-Low Volume Strain for Sodium Storage

Abstract

Mn-rich P2-type oxides offer high capacity via Mn redox, yet their commercial viability is stalled by Jahn−Teller distortions and structural instability. To overcome these hurdles, a pillaring strategy is employed to anchor the layered framework, effectively suppressing lattice strain and ensuring long-term cycling stability. Here, we regulate the coordination chemistry of a multielement co-doped P2 framework by coupling Ca/Mg/Ni/Ti substitution with controlled Li+ occupation in the Na layer and systematically tune the composition as Na0.8Li0.2Ca0.025Mg0.05Ni0.1Mn0.75Ti0.05O2 (NMNMT-20Li). The optimized NMNMT-20Li activates the reversible anionic redox above 4.0 V and delivers the highest reversible capacity of 120 mAh g−1. In situ XRD evidences a solid-solution-like evolution without a phase transition and an ultra-low unit-cell volume change of ∼0.9% within 2.0−4.3 V. In situ distribution of relaxation times (DRT) reveals highly reversible charge-transfer and cathode−electrolyte interphase resistances. Density functional theory (DFT) calculation identifies interlayer Li as a “pillar” that smooths electrostatic potential between layers and lowers Na+ migration barriers, while excessive Li induces transport blocking. Furthermore, the NMNMT-20Li || HC full cell exhibits a high initial energy density of 241.3 Wh kg−1 and retains 97.71% capacity after 200 cycles, demonstrating its strong practical promise.