Dual-Functional Lanthanum Doping: Stabilizing Cathodes by Simultaneously Mitigating Na ⁺ /Vacancy Ordering and Oxygen Release

P2-type Na _<i>x</i> Li _<i>y</i> Mn_1-<i>y</i> O₂ layered oxides represent a promising class of cathode materials for sodium-ion batteries, capable of delivering a high specific capacity by leveraging anionic redox reactions. However, their practical application is often hampered by issues such as Na⁺/vacancy ordering and irreversible oxygen release, which significantly impair the rate capability and cycling stability. To address the aforementioned problem, this work demonstrates that introducing trace La into P2-Na_0.6Li_0.2Mn_0.8O₂ fundamentally alters the Na⁺ distribution between Na_e and Na_f sites in the sodium layer and suppresses Na⁺/vacancy ordering. More importantly, the high bond energy of La-O significantly enhances the stability of lattice oxygen, suppressing irreversible oxygen release during cycling and thereby simultaneously improving the reversibility of redox reactions and structural integrity. Electrochemical tests demonstrate that the optimized LM-La0.04 cathode delivers a high discharge capacity of 193.5 mAh g^-1 at 0.2 C and retains a reversible capacity of 86.8 mAh g^-1 even at 10 C, highlighting its superior rate performance. Furthermore, the material exhibits remarkable capacity retention in long-term cycling. A combination of <i>in situ</i> and <i>ex situ</i> characterization techniques and theoretical calculations was employed to elucidate the charge compensation mechanism and structural evolution of the La-doped system during electrochemical cycling. This work provides new insights into the regulatory role of lanthanum in layered oxides and offers a feasible doping strategy for developing anion-redox-based cathode materials with a high energy density and long cycling stability.