Abstract:

P2-type layered transition metal oxides (P2-Na_xTMO₂) have emerged as promising cathodes for sodium-ion batteries (SIBs) owing to their superior cycling stability and excellent rate capability. However, their practical application is significantly hindered by two major challenges. Firstly, irreversible phase transitions occur during high-voltage operation, which disrupt the structural integrity and deteriorate electrochemical performance. Secondly, their inherently low theoretical specific capacity fails to meet modern energy demands. To tackle these challenges, this study proposes a novel synergistic strategy that integrates high-entropy engineering with a biphasic P2/O3 structural design. An innovative cathode material, Na_0.70Ni_0.25Mn_0.35Co_0.15Fe_0.05Ti_0.20O₂ (denoted as Na_0.70NMCFT), was successfully synthesized via a high-temperature solid-state reaction. This material design critically incorporates five distinct transition metal cations into the transition metal (TM) layer, constructing a stabilized high-entropy configuration. Careful optimization of both the five TM elements and the sodium content was essential to precisely regulate the synthesis and formation of the desired integrated P2/O3 biphasic structure within this high-entropy host. Comprehensive structural characterization unequivocally confirms the successful construction of this tailored architecture. X-ray diffraction (XRD) and transmission electron microscopy (TEM) collectively confirm the successful construction of the P2/O3 biphasic architecture. The high-entropy engineering stabilizes the P2 phase through configurational entropy, effectively suppressing irreversible phase transitions and Na⁺/vacancy ordering during cycling, as evidenced by smoother charge/discharge profiles and ex-situ XRD analysis under high potentials. Meanwhile, the introduced O3 phase compensates for capacity shortages and improves cycling stability, working in tandem with the P2 phase. Critically, the interaction between the two phases enables a highly reversible transition between P2/O3-P2/P3, further enhancing the overall performance. Under the combined action of the high-entropy and biphasic strategies, Na_0.70NMCFT exhibits optimal electrochemical performance. It delivers an initial discharge capacity of 102.08 mAh g⁻¹ at 1C, retaining 88.15% after 200 cycles, demonstrating exceptional cycling stability. Moreover, even at 10C, Na_0.70NMCFT still has an initial discharge specific capacity of 85.67 mAh g⁻¹ and a capacity retention of up to 70% after 1, 000 cycles. Kinetic analyses further reveal that Na_0.70NMCFT possesses the lowest charge transfer resistance and the highest sodium-ion diffusion coefficient among the materials studied. In conclusion, this work demonstrates that the rational design of biphasic high-entropy cathodes can synergistically achieve superior rate capability, cycling stability, and maintain high theoretical capacity. It not only overcomes the key bottlenecks of P2-type oxides but also paves the way for the development of advanced SIB cathodes, establishing a new paradigm for the engineering of high-performance cathode materials in the field of sodium-ion batteries.

Key words: Sodium ion battery, Layered oxide cathode, High entropy, P2/O3 biphasic, Phase transition