Structural and interfacial stabilization of sodium-deficient O3-type layered oxide cathodes via optimized lithium doping

• Designed cobalt-free, low-nickel Fe-Mn-Ni-Cu layered oxide cathodes with controlled sodium deficiency. • Optimized lithium substitution strengthens TM-O bonds and suppresses Jahn-Teller distortions. • Operando XRD of the optimized material reveals reversible O3-P3-OP2 phase transitions with minimal lattice volume change. • XPS, ToF-SIMS, and FIB-SEM confirm reduced electrolyte decomposition and enhanced interfacial stability in the optimized material. • Optimized Na-deficient Li-doped cathode delivers 88% capacity retention after 200 cycles at 0.2C and excellent rate capability. Abundant and inexpensive, sodium-ion batteries are promising for large-scale energy storage. However, the cathodes often suffer from poor cycling stability caused by structural transitions and interfacial degradation. Among cathode candidates, cobalt-free and low-nickel Fe-Mn-based layered oxides offer an attractive, cost-effective, and environmentally friendly choice, yet their structural instability and high surface reactivity remain as major challenges. This work introduces a design strategy that combines sodium deficiency with optimized lithium substitution in O3-type Fe-Mn-Ni-Cu layered oxides. Lithium doping strengthens TM-O bonds, suppresses Mn 3+ -induced distortions, and the optimal amount mitigates parasitic electrolyte surface decomposition. In-operando and ex-situ XRD measurements reveal reversible O3-P3-OP2 transitions with minimal lattice-volume change in the optimally doped composition, whereas under- and over-doped samples show significant strain and poor phase reversibility. Post-cycle studies using various analytical techniques confirm improved interfacial and structural stability in the optimized sample, resulting in 88% retention after 200 cycles. Overall, balancing sodium deficiency with optimized lithium substitution offers a practical and scalable pathway to stabilize cobalt-free layered oxide cathodes for durable, cost-effective sodium-ion batteries. Na-deficiency combined with optimized Li-substitution strengthens TM-O bonds and limits Mn 3+ distortions, stabilizes O3-P3-OP2 transitions with minimal volume change, and reduces surface degradation for durable Co-free SIB cathodes.