Synergistic high-entropy engineering in biphasic layered oxides enables high-rate sodium-ion cathodes

Abstract

Layered oxide cathodes for sodium-ion batteries (SIBs) are plagued by irreversible oxygen redox and structural degradation at high voltages, leading to severe capacity fade. To address this challenge, we synthesized a high-entropy biphasic oxide cathode, P2/O3-Na_0.6Mn_0.44Ni_0.2Fe_0.09Cu_0.09Ti_0.1Li_0.04Mg_0.04O₂ (MNFCTLM) via a solid-phase method. The high-entropy composition effectively stabilizes the transition metal (TM)–O bonds, suppresses oxygen activity, and mitigates O₂ precipitation. Concurrently, the biphasic structure offers additional Na⁺ migration channels, significantly enhancing ion diffusion kinetics. As a result, the MNFCTLM cathode delivers a specific capacity of 134.5 mAh g⁻¹ at 0.2C and exhibits exceptional cycling stability, retaining 81.3% of its capacity after 200 cycles at 2C. It also achieves excellent rate performance (83.2 mAh g⁻¹ at 20C) and exceptional air stability, retaining 86.4% of its initial capacity after seven days of air exposure. Ex situ XPS analysis revealed that, compared to the low-entropy P2-Na_0.6Mn_0.7Ni_0.21Fe_0.09O₂ (MNF) cathode, MNFCTLM suppresses the formation of unstable Oⁿ⁻ species at high voltage. Ex situ XRD combined with DFT calculations further demonstrated the superior structural stability of MNFCTLM. In a full cell with a hard carbon anode, it demonstrates a high capacity of 160 mAh g⁻¹ at 0.2C and retains 81.2% of its capacity after 200 cycles at 2C. This work not only presents a high-performance cathode for SIBs but also provides fundamental insight into stabilizing anionic redox through entropy engineering, offering a generalizable design strategy for advanced layered oxide electrodes.

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1. Introduction

As new energy technologies advance rapidly, lithium-ion batteries (LIBs) have established themselves as the prevailing technology for sustainable electrochemical energy storage, attracting extensive research interest. Nevertheless, the scarcity of lithium and its escalating cost have driven the development of sodium-ion batteries (SIBs) as a focal point for new-generation energy storage research. SIBs, with their abundant sodium reserves and operational mechanisms analogous to those of LIBs, hold significant promise for both academic and industrial applications.^1,2 Currently, Prussian blue analogs, layered oxides, and polyanionic compounds are extensively employed as positive electrode materials in SIBs.³ Among these, layered oxides Na_xTMO₂ (0 < x ≤ 1, where TM indicates transition metal elements such as Ni, Fe, Mn, etc.) have become some of the most studied cathode materials owing to their exceptional energy density and facile synthesis.⁴ The diverse crystal structures and chemical compositions of layered oxides offer extensive opportunities for designing novel cathode materials with enhanced rate performance, specific capacity, and cycling stability.^5,6 Na_xTMO₂ can be categorized into various phase structures based on the coordination environment of Na⁺ ions (P for prismatic sites and O for octahedral sites) and the arrangement of oxygen layers. The P2 and O3 phases, as representative layered oxides, present distinct advantages and challenges. In the P2 phase, Na⁺ occupies triangular prismatic sites, thereby enabling direct diffusion to adjacent sites through a low energy barrier, which facilitates rapid Na⁺ migration.^7,8 However, the lower Na content (<0.8) in the P2 phase limits its capacity, and phase transitions in the high-pressure region induce significant volume changes, reducing cyclic stability.^9,10 Conversely, the O3 phase, with its higher sodium content (≈1), offers increased capacity, but the octahedral sites hinder Na⁺ diffusion, thereby limiting electrochemical kinetics. In addition, the O3 phase undergoes a complex multi-step phase transition (O3 → O3′ → P3 → P3′ → P3″) during (de)sodiation, resulting in poor structural reversibility.¹¹

The entropic stability and interactions between different doping elements confer unique structural stability to high-entropy materials. The selection of appropriate transition metals is essential for creating structurally stable high-entropy layered cathodes. To improve cycling stability without reducing capacity, inactive metals are commonly used to stabilize the phase structure, while active metals contribute to reversible capacity. Hu et al. successfully prepared O3-NaNi_0.25Mg_0.05Cu_0.1Fe_0.2Mn_0.2Ti_0.1Sn_0.1O₂, which delivered 130.8 mAh g⁻¹ at 0.1C between 2.0 and 4.0 V while maintaining excellent cycle stability.¹² Tian et al. developed a novel O3-NaFe_0.2Co_0.2Ni_0.2Ti_0.2Sn_0.1Li_0.1O₂, achieving reversible capacities of 112.7 mAh g⁻¹ at 0.1C and 80.8 mAh g⁻¹ at 2C.¹³ To improve structural stability and prolong cycling durability, constructing P2/O3 biphasic oxides effectively merges their benefits, thereby enhancing the overall electrochemical performance.^14,15 These multiphase structures maximize the benefits of each phase. For instance, P-type cathodes containing P2 and P3 phases demonstrate outstanding structural stability and performance.^16,17 The P2/P3 composite with a spinel structure integrates the wide Na⁺ channels of the P-type structure with the excellent electrical conductivity of the spinel phase, markedly enhancing electrode performance.¹⁸ The P/O biphasic structure synergistically facilitates Na⁺ diffusion kinetics while simultaneously boosting capacity.^19,20 Three primary methods exist for synthesizing P/O biphasic materials: adjusting sodium content,²¹ substituting with Li⁺/TM ions,²² and varying sintering temperature.²³ For example, P2/O3-Na_0.85Ni_0.34Mn_0.33Ti_0.33O₂, prepared by Chang et al., delivered 97.8 mAh g⁻¹ at 0.1C, while maintaining 85.5% capacity retention after 500 cycles at 1C.²⁴ Li et al. prepared P2/O3-Na_0.8Ni_0.23Fe_0.34Mn_0.43O₂, which achieved an ultrahigh specific capacity of 146.4 mAh g⁻¹ at 0.1C with 93.1% capacity retention after 200 cycles at 1C.²⁵ However, the unpredictability of existing multiphase structures during early preparation persists, largely because of the absence of effective guiding models or empirical rules. The composition of transition metals is a key factor in determining the phase structure. Defining the relationship between elements and phase structure could enhance the efficiency of multiphase design. Moreover, the potential benefits of integrating high-entropy strategies with multiphase structures, which could enhance stability via rational design, remain underexplored. Therefore, investigating the synergistic effects of multiphase structures and high-entropy on electrochemical performance is crucial for designing advanced cathodes.

In this work, the high-entropy strategy was innovatively integrated with a dual-phase structure design (Fig. 1a), leading to the preparation of a high-entropy biphasic cathode material, P2/O3-Na_0.6Mn_0.44Ni_0.2Fe_0.09Cu_0.09Ti_0.1Li_0.04Mg_0.04O₂ (MNFCTLM), via a straightforward high-temperature solid-state method. The experimental process is depicted in Fig. 1b. This design synergistically combines the high capacity of the O3 phase and the rapid ion transport of the P2 phase, establishing dual Na⁺ migration channels. The resulting material significantly outperforms the single-phase P2-Na_0.6Mn_0.7Ni_0.21Fe_0.09O₂. The composition of the high-entropy material emphasizes low-cost Mn and Fe alongside high-capacity Ni as primary redox centers, Mg and Cu for structural stability, Li for enhanced air stability due to its high redox potential, and Ti for elevating the average voltage. Electrochemical testing revealed that this cathode exhibits an initial coulombic efficiency (ICE) of 103.8%, delivering capacities of 134.5 mAh g⁻¹ at 0.2C and 113.8 mAh g⁻¹ at 2C, while retaining 81.3% of its capacity after 200 cycles. It also exhibits outstanding rate performance, maintaining 83.2 mAh g⁻¹ at 20C. In particular, MNFCTLM demonstrates excellent air stability, showing an initial capacity of 133.2 mAh g⁻¹ at 0.2C even after seven days of air exposure and 86.4% capacity retention after 150 cycles at 2C. Structural evolution was examined via ex situ XRD, while electrode kinetics were analyzed using galvanostatic intermittent titration technique (GITT), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). These studies confirm a high Na⁺ diffusion coefficient in the high-entropy biphasic cathode. Furthermore, full cells assembled with the P2/O3-MNFCTLM cathode and hard carbon (HC) anode displayed exceptional electrochemical performance. Specifically, the full cell achieved an initial discharge capacity of 160 mAh g⁻¹ at 0.2C, corresponding to an energy density of 457.8 Wh kg⁻¹ (relative to cathode mass) and an ICE of 121%. After 200 cycles at 2C, the full cell retained 81.2% of its initial capacity. This research offers a promising approach for designing high-stability, high-rate cathode materials for SIBs.

Fig. 1 (a) The combined contribution of the multiphase structure and high-entropy configuration. (b) Flow chart for the preparation of MNFCTLM.

2. Experimental section

2.1. Material preparation

P2/O3-Na_0.6Mn_0.44Ni_0.2Fe_0.09Cu_0.09Ti_0.1Li_0.04Mg_0.04O₂ (MNFCTLM) and P2-Na_0.6Mn_0.7Ni_0.21Fe_0.09O₂ (MNF) were synthesized via a conventional high-temperature solid-state method. Stoichiometric quantities of Na₂CO₃ (Sinopharm Chemical Reagent Co., AR, 99%), Li₂CO₃ (Aladdin, AR, 98%), Mn₂O₃ (Aladdin, AR, 98.0%), NiO (Aladdin, AR, 99.0%), CuO (Aladdin, AR, 99.0%), Fe₂O₃ (Aladdin, AR, 99.0%), MgO (Aladdin, AR, 99.0%), and TiO₂ (Aladdin, 99.0%) were weighed, with a 5 wt% excess of Na₂CO₃ to compensate for volatilization losses during calcination. The precursors were homogenized via ball-milling in anhydrous ethanol (500 rpm for 8 h) followed by vacuum drying at 60 °C for 8 h. The resulting powder was thoroughly ground and then calcined in a muffle furnace with a controlled heating program (5 °C min⁻¹ ramp to 900 °C; 15 h dwell time). After natural cooling to ambient temperature, the product was finely pulverized, sieved through a 200-mesh sieve (<75 µm), and stored in an argon-atmosphere glovebox (H₂O/O₂ levels <0.1 ppm) to prevent atmospheric degradation.

2.2. Materials characterization

Structural characterization was carried out using X-ray diffraction (Bruker D8 Discover, Cu Kα radiation, λ = 1.5406 Å), with Rietveld refinement analysis conducted via GSAS II software. Morphological evaluation employed high-resolution scanning electron microscopy (JEOL-7500F, 5 kV acceleration voltage). Atomic-scale imaging and elemental distribution mapping were performed using aberration-corrected transmission electron microscopy (FEI Talos F200S) equipped with energy-dispersive X-ray spectroscopy (EDS). Surface chemistry analysis was performed by X-ray photoelectron spectroscopy (Thermo Scientific ESCALAB 250Xi, monochromatic Al Kα source). Bulk elemental composition was quantified by inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 8000). For ex situ analyses, electrode samples at various charge–discharge states were disassembled in an Ar-filled glovebox, rinsed with propylene carbonate, and vacuum dried for at least 10 h.

2.3. Electrochemical tests

The positive electrode material (80 wt%), polyvinylidene fluoride (PVDF, 10 wt%) as a binder, and acetylene black (AB, 10 wt%) as a conductive agent were thoroughly mixed and then homogeneously dispersed in N-methylpyrrolidone (NMP) solvent. The resulting slurry was then coated onto aluminum foil. Subsequently, the coated electrodes were dried at 110 °C for 10 h, rolled, sliced into 10 mm diameter discs, with an active loading of approximately 1.2 mg cm⁻². CR2032 coin cells were assembled with metallic Na as the negative electrode, a porous glass fibre separator (Whatman GF/D), and an electrolyte composed of 1 M NaClO₄ dissolved in a 1 : 1 volume mixture of diethyl ethyl carbonate (DEC) and ethyl vinyl carbonate (EC). All assembly procedures were performed in a glovebox filled with argon. Charge/discharge tests and GITT measurements were conducted at indoor temperature using a LAND battery test system (model CT2001A). In the GITT experiments, the charge/discharge cycle was performed at a rate of 0.1C for 15 minutes, followed by a 1 hour rest period to achieve equilibrium. The Na⁺ diffusion coefficients were calculated according to the following equations:

In the diffusion coefficient equation, the variables are defined as follows: τ – current pulse duration (s), m_B – active material mass (g), V_M – molar volume per unit cell (cm³ mol⁻¹), M_B – compound molar mass (g mol⁻¹), S – electrode geometric surface area (cm²), ΔE_s – steady-state voltage variation during current pulse, and ΔE_τ – total cell voltage change. CV tests were conducted employing a CHI760E electrochemical workstation (Shanghai Chenhua Instruments) at a 0.1 mV s⁻¹ scan rate. EIS data were acquired on the same setup, with a 5 mV amplitude and a frequency range spanning from 1 MHz to 0.01 Hz. Half-cell testing employed a voltage range of 2.0–4.3 V vs. Na⁺/Na. The full-cell configurations followed the same procedures for cathode preparation and used the same separator and electrolyte composition as those used in the half-cells. The negative electrode was prepared similarly to the positive electrode, comprising 80 wt% HC, 10 wt% AB, and 10 wt% PVDF. These materials were homogeneously blended and coated onto Cu foil. Prior to constructing the full cell, the HC negative electrode was pretreated to form a stable solid electrolyte interphase (SEI), minimizing the low coulombic efficiency of HC. Full cell tests were conducted within a voltage range of 1.5–4.0 V.

3. Results and discussion

Fig. 2a illustrates the crystal structure models for the P2 and O3 phases. The XRD patterns (Fig. S1) indicate that MNF corresponds to a pure P2 phase (P6₃/mmc, PDF# 54-0887), while MNFCTLM exhibits a biphasic P2/O3 structure (R3̄m, PDF# 70-3726). No impurity peaks were detected, indicating phase purity. Rietveld refinement profiles (Fig. 2b and c) demonstrate excellent fit quality (R_wp < 10%), with crystallographic parameters detailed in Table S1. The c-axis lattice parameter of MNFCTLM (11.20252 Å) exceeds that of MNF (11.10713 Å), attributable to the larger ionic radii of Cu²⁺, Li⁺, Mg²⁺, and Ti⁴⁺ relative to Mn⁴⁺ (Table S2). The expanded interlayer spacing facilitates Na⁺ ion diffusion. ICP-OES analysis (Table S3) verifies that the chemical compositions align with theoretical stoichiometry.

Fig. 2 Structural characterization of MNF and MNFCTLM. (a) Schematic crystalline structures of P2 and O3 phases. XRD patterns and Rietveld refinement profiles of (b) NMF and (c) MNFCTLM. (d) SEM image, (e) HRTEM image, (f) SAED pattern, and (g) EDS mappings of MNFCTLM.

The morphology and microstructure of both MNF and MNFCTLM were examined using SEM and TEM. Fig. S2a and d reveal that both MNF and MNFCTLM consist of lamellar particles with diameters of 2–5 µm and a thickness of approximately 1 µm. MNFCTLM particles exhibit a cleaner and smoother surface compared to MNF, indicating improved crystallinity. The high-resolution TEM (HRTEM) of MNFCTLM (Fig. 2e) confirms layered structures with interplanar spacings of 2.82 Å (O3 (004)) and 2.68 Å (P2 (006)). The selected area electron diffraction (SAED) pattern shown in Fig. 2f verifies the biphasic nature, with diffraction spots corresponding to O3 (004) and P2 (006) planes. For MNF, HRTEM (Fig. S2b) displays a 2.482 Å spacing matching the P2 (100) plane, corroborated by SAED (Fig. S2c). TEM-EDS mapping in Fig. S2d and 1g confirms uniform elemental distribution in both materials, although Li remained undetectable due to its low energy level.

The chemical states of elements in MNF and MNFCTLM were examined using XPS. The results are presented in Fig. S3, 3, and S4. Survey spectra (Fig. S3a and S4) confirm the presence of all targeted elements, thereby verifying the successful integration of high-entropy elements into the MNFCTLM lattice. The Mn 2p spectra reveal Mn³⁺ (at 641.88 eV) and Mn⁴⁺ (at 643.58 eV), indicating the coexistence of both oxidation states in MNF and MNFCTLM.²⁶ The Ni 2p_3/2 peak at 854.48 eV corresponds to Ni²⁺. The Fe 2p binding energy is consistent with Fe³⁺, with Fe 2p_3/2 positioned at 711.08 eV.²⁷ In addition, in MNFCTLM, the peaks of Cu 2p_3/2, Cu 2p_1/2, Ti 2p_3/2, Ti 2p_1/2, Li 1s, and Mg 2p are located at 933.2 eV, 952.8 eV, 457.8 eV, 463.6 eV, 55.2 eV, and 49.4 eV, respectively, corresponding to valence states of +2, +4, +1, and +2.²⁸ Finally, the O 1s spectrum is deconvoluted into three distinct components: a lattice oxygen peak at 528.97 eV, and two peaks at 531.2 eV and 535.48 eV corresponding to surface-adsorbed oxygen species.²⁹

Fig. 3 XPS spectra of (a) O 1s, (b) Na 1s, (c) Mn 2p, (d) Ni 2p, (e) Fe 2p, (f) Cu 2p, (g) Ti 2p, (h) Li 1s, and (i) Mg 2p for MNFCTLM.

To examine the impact of high-entropy and multiphase structures on the electrochemical properties of SIBs, galvanostatic charge–discharge testing was performed on MNF and MNFCTLM. Fig. 4a and b present the charge–discharge curves for the first three cycles at 0.2C (1C = 100 mA g⁻¹). The initial specific discharge capacity of MNF was 152.6 mAh g⁻¹, while MNFCTLM showed a slightly lower value of 134.5 mAh g⁻¹. Notably, MNF shows a significant high-voltage plateau above 4 V, attributed to oxygen redox reactions and structural phase changes induced by Na⁺/vacancy ordering,³⁰ which adversely affects cycling stability. In contrast, the charge–discharge curve of MNFCTLM lacks this high-voltage plateau, displaying a smoother profile. The absence of this plateau in MNFCTLM can be ascribed to the high-entropy doping, which introduces multiple components that effectively suppress Na⁺/vacancy ordering. This modification promotes solid-solution-like discharge behavior and enhances structural stability. The voltage plateaus at approximately 2.8 V and 2.1 V are attributed to the redox reactions of Ni and Mn, respectively, contributing to the relatively high specific capacity of MNFCTLM.

Fig. 4 Electrochemical performance of MNF and MNFCTLM. Charge/discharge curves of (a) MNF and (b) MNFCTLM at 0.2C. (c) Cycling performance of MNF and MNFCTLM cathodes at 2C. dQ/dV curves collected within 2.0–4.3 V for (d) MNF and (e) MNFCTLM. (f) Rate capabilities and coulombic efficiency of MNF and MNFCTLM at different current densities. (g) Cycling performance of MNF and MNFCTLM cathodes at 5C. (h) Radar summary chart displaying the comprehensive performance comparison.

Fig. 4c illustrates the cycling capability of MNF and MNFCTLM at 2C. MNFCTLM exhibits an initial capacity of 113.8 mAh g⁻¹ with 81.3% capacity retention after 200 cycles. In contrast, MNF, which initially displays a higher capacity of 123.2 mAh g⁻¹, retains only 36.7% after 200 cycles. Fig. S5a and b present the charge–discharge curves for MNF and MNFCTLM at 0.2C across various cycle numbers. Repeated cycling progressively damages the structural integrity of MNF, exacerbating voltage hysteresis and leading to rapid capacity decay. Conversely, the high-entropy dual-phase structure of MNFCTLM confers excellent structural integrity, significantly mitigating voltage hysteresis and extending cycle life. Furthermore, we performed a semiquantitative assessment of the contributions of cationic and anionic redox reactions to the discharge capacity at 0.2C across the voltage ranges of 2.0–4.0 V and 4.0–4.3 V (Fig. S5c and d). The initial specific capacities of MNF and MNFCTLM are 149.4 mAh g⁻¹ and 132.6 mAh g⁻¹, respectively, with cationic redox contributions of about 131.4 mAh g⁻¹ and 121.9 mAh g⁻¹, and anionic redox contributions of about 18.0 mAh g⁻¹ and 10.7 mAh g⁻¹. During the electrochemical activation process, the cationic redox capacity of both materials gradually increased within 2.0–4.0 V.³¹ However, cycling induces a decline in MNF's properties, attributed to intensified side reactions, crystal microcracking, and surface remodeling. Concurrently, the capacity of MNF decreases rapidly in the 4.0–4.3 V high-voltage range, likely due to the irreversible anionic redox reaction that results in oxygen deficiency. In contrast, the high-entropy biphasic structure of MNFCTLM retains a relatively high capacity even after 50 cycles, attributable to its stable structural framework and inherent anionic redox chemistry.

The dQ/dV curves for MNF and MNFCTLM are displayed in Fig. 4d and e, respectively. The peaks in these curves correspond to the plateau regions observed in the charge/discharge profiles of both materials. Notably, above 4.0 V, MNF exhibits a pronounced oxidation peak compared to MNFCTLM, while its corresponding reduction peak is markedly weak, indicating diminished reversibility of redox reactions at high voltages. Below 4.0 V, the dQ/dV profile of MNFCTLM reveals two distinct redox pairs. The redox couple observed at 2.95 V/2.9 V is associated with Ni²⁺/Ni³⁺ redox centers, while the Mn³⁺/Mn⁴⁺ redox pair operates at a lower voltage of approximately 2.6 V.³² These observations suggest that although MNF initially delivers a higher capacity, its poor reversibility at elevated voltages and the ordering of Na⁺/vacancies contribute to structural degradation. Conversely, MNFCTLM leverages the high-entropy effect to suppress Na⁺/vacancy ordering, thereby improving structural stability and preserving capacity through charge compensation involving Mn and Ni redox couples.

The rate capability of MNF and MNFCTLM was further investigated across a range of current densities, from 0.2C to 20C, as depicted in Fig. 4f. While MNF exhibits superior rate performance at low current densities (0.2C to 1C) owing to its higher initial capacity, MNFCTLM outperforms MNF at higher current rates. Specifically, MNFCTLM achieves specific capacities of 111.6, 102.3, 93.3, and 83.2 mAh g⁻¹ at 2C, 5C, 10C, and 20C, respectively, significantly surpassing its single-phase low-entropy counterpart. This enhanced performance is because of the dual-phase structure of MNFCTLM, which offers additional Na⁺ diffusion pathways. The incorporation of multiple atoms further expands these channels, while the high-entropy effect contributes to improved structural stability. Remarkably, the reversible capacity of MNFCTLM exceeds its initial capacity (134.5 mAh g⁻¹) upon returning to 0.2C, likely due to a cell activation process during high-rate cycling.

To assess the electrochemical cycling stability of the electrode materials, long-term cycle tests were conducted on MNF and MNFCTLM, as shown in Fig. 4g. At 5C, MNFCTLM delivered a capacity of 74.8 mAh g⁻¹ following 300 cycles, corresponding to a capacity retention of 79.3%. This performance is considerably higher than that of MNF, which delivered a capacity of merely 51 mAh g⁻¹ and a retention rate of 45.4% under the same cycling conditions. Furthermore, MNFCTLM demonstrated exceptional air stability, as illustrated in Fig. S6a and b. After being exposed to air for 7 days, the initial discharge capacity of MNF decreased by 18 mAh g⁻¹, whereas MNFCTLM experienced a mere 10 mAh g⁻¹ capacity loss. Fig. S6c depicts the cycling stability of both air-exposed materials at 2C. MNFCTLM retains 86.4% of its initial capacity after 150 cycles, whereas MNF exhibits a much lower capacity retention of merely 53.7%, consistent with previous results. Upon exposure to air, Na⁺ ions are spontaneously released from the lattice, leading to Na deficiency within the electrode structure. MNFCTLM shows minimal capacity decay, suggesting reduced spontaneous Na⁺ extraction and effective preservation of the positive electrode's original architecture. Conversely, MNF shows significant Na deficiency. Fig. S7 reveals that MNF reacts with CO₂ and H₂O in air. Although the main P2 phase is retained in MNF, the P2 (002) peak exhibits a slight shift toward a smaller angle, accompanied by the appearance of noticeable Na₂CO₃ impurity peaks, indicating poor air stability. In contrast, the XRD pattern of MNFCTLM after 7 days of air exposure shows neither structural changes nor new impurity peaks. This exceptional air stability arises from the strong electronegativity of Cu/Ti, which transfers some of the negative charge from the lattice oxygen to Cu/Ti. As a result, the bonding interactions between the lattice oxygen and H₂O/Na⁺ are weakened, thereby lowering the likelihood of H⁺/Na⁺ exchange.³³ In addition, multi-element incorporation enhances the metal–oxygen (TM–O) bonding energy, further improving structural stability. Interestingly, recent investigations have demonstrated that recalcination can restore the structural and electrochemical properties of degraded materials. However, this process consumes a lot of energy and undoubtedly increases production costs. Hence, electrode materials with excellent air stability are preferable for commercial-scale production. The analysis and performance comparison of MNF and MNFCTLM summarized in Fig. 4h suggest that merging a high-entropy structure with a composite phase represents a viable strategy to enhance the electrochemical stability of layered cathodes.

Given the superior rate performance of the MNFCTLM cathode, in situ EIS measurements were performed to examine impedance and dynamic changes during cycling. Based on the equivalent circuit model (Fig. S8), the Nyquist profiles are composed of three sections: the high-frequency intercept with the real axis corresponds to the ohmic resistance (R_s); the two mid-frequency semicircles are assigned to the interfacial contact impedance (R_sf) and the charge transfer resistance (R_ct); and the low-frequency tail is associated with Na⁺ diffusion. R_s is negligible compared to R_ct and R_sf, as it is merely 4–7 Ω and exhibits minimal variation. The impedance values of R_ct and R_sf are detailed in Table S4, with trends illustrated in Fig. S9. A comparison of the Nyquist plots (Fig. 5a–e) shows that both samples display similar impedance evolution patterns. However, MNFCTLM exhibits smaller semicircle radii than MNF at the same state of charge, indicating lower impedance and superior kinetics. Notably, both R_sf and R_ct decrease significantly as the cell is charged from the initial voltage to 3.2 V, implying that the O3-to-P3 phase transition facilitates wider Na⁺ diffusion channels and increases the interlayer spacing. Although R_ct varies considerably with Na⁺ de/intercalation, R_sf remains relatively stable even at 4.3 V, suggesting that the cathode electrolyte interphase (CEI) formed on MNFCTLM is highly stable and robust.³⁴ This stable CEI continuously suppresses TM cation dissolution and ensures fast Na⁺ diffusion kinetics. During discharge, R_ct and R_sf increase, mainly due to the reduced lattice spacing upon Na⁺ insertion and the restructuring of the stabilized CEI. The analysis indicates that the high-entropy composition and biphasic structure substantially promote electron transfer during the anion redox reaction. However, distinguishing the resistance values of R_sf and R_ct in the Nyquist diagrams is challenging due to potential overlap between Na⁺ interphase transport and charge transfer within the mid- and high-frequency region. Therefore, the relaxation time distribution (DRT) technique was utilized to directly identify the time scales of fundamentally specific electrochemical processes, especially to decouple overlapping electrochemical processes (Fig. 5c and f). Four original peaks evolve within the discharge voltage range of 4.3–2.0 V, primarily falling into three categories, the CEI region, charge transfer region, and Na⁺ diffusion region, with corresponding relaxation times spanning from 10⁻⁴ to 10³ s. MNFCTLM shows lower resistance across these three regions, further validating its smaller impedance. In particular, the MNFCTLM electrode exhibits lower intensity in the CEI region and maintains minimal changes during discharge, indicating its more stable CEI. The impedance area of the Na⁺ diffusion region gradually increases as discharge proceeds, which can be attributed to the narrowing of the Na intermediate layer.

Fig. 5 In situ EIS Nyquist plots during the charge process of (a) MNF and (d) MNFCTLM, and during the discharge process of (b) MNF and (e) MNFCTLM. The corresponding DRT transformation for (c) MNF and (f) MNFCTLM. (g) CV curves at various scan speeds and (h) the linear fitting results of peak current versus the square root of the scan rate for MNFCTLM. (i) The calculated Na⁺ diffusion coefficients.

The CV curves of MNF and MNFCTLM for the first three cycles at a sweep rate of 0.1 mV s⁻¹ are shown in Fig. S10. The redox peak of MNF in the low-voltage range (2.0–2.5 V) is primarily attributed to the Mn³⁺/Mn⁴⁺ redox couple, while MNFCTLM significantly inhibits this reaction. Instead, a new redox pair emerges in the 3–3.5 V range, which can be attributed to the Ni²⁺/Ni³⁺ redox couple and serves as the primary capacity contributor. This suggests that the synergistic high-entropy and multiphase structure of MNFCTLM inhibits the Jahn–Teller distortion induced by Mn³⁺, thereby mitigating capacity loss. A notable oxidation peak emerged for MNF above 4.0 V, suggesting a more pronounced oxygen redox activity. Conversely, the redox peak intensity for MNFCTLM was markedly reduced within the same voltage range, reflecting suppressed oxygen redox processes consistent with the dQ/dV analysis. In addition, the strong overlap of the first three cycles for MNFCTLM indicates excellent cycling reversibility. The CV tests for MNF and MNFCTLM at different scan rates between 0.1 and 1.0 mV s⁻¹ (Fig. 5g and S11) were conducted to quantitatively evaluate their Na⁺ transport kinetics.³⁵Fig. 5h displays the linear relationship obtained by fitting the square root of peak current (v^1/2) against the scan rate for MNF and MNFCTLM. The fitting results (Table S5) reveal that the slopes of the MNFCTLM curves, with its high-entropy biphasic structure, are consistently steeper than those of MNF. This suggests that the entropy-driven layered oxide cathode effectively enhances ion transport kinetics.

To gain deeper insights into the diffusion behavior of Na⁺ during cycling, GITT measurements were performed³⁶ (see Fig. S12 for a detailed view of the GITT test). The Na⁺ diffusion coefficients under various states were calculated. As shown in Fig. 5i and S13, the GITT curves and the corresponding calculated Na⁺ diffusion coefficients (D_Na⁺) for both MNF and MNFCTLM are presented. The curves exhibit a marked decline as the charge increases, especially within the high-voltage region of 4.0–4.3 V. Both MNF and MNFCTLM exhibit a sharp decrease in D_Na⁺. Notably, MNFCTLM consistently shows a higher D_Na⁺ compared to MNF. This variation is presumably attributed to the larger phase transition induced by sliding of the TMO₂ layer during Na⁺ extraction in MNF, whereas MNFCTLM undergoes a smaller phase transition due to its structural attributes. Throughout the process, MNFCTLM exhibits a significantly higher average D_Na⁺ (6.35 × 10⁻¹¹ cm² s⁻¹) compared to MNF (3.79 × 10⁻¹¹ cm² s⁻¹), suggesting that the high-entropy and dual-phase structure of cathode materials significantly enhances Na⁺ diffusion kinetics.

To examine the structural evolution of MNF and MNFCTLM during the initial charge/discharge cycle at 0.2C, ex situ XRD characterization was carried out (Fig. 6a and b). Notably, the P2 (002) and O3 (003) diffraction peaks of MNFCTLM, along with the P2 (002) peak of MNF, exhibit a shift toward smaller angles at the onset of Na⁺ extraction. This shift suggests lattice expansion along the c-axis, which is due to an increasing repulsive force between the TMO₂ layers. Furthermore, MNFCTLM demonstrates a smaller initial peak shift angle compared to MNF, suggesting a more robust crystal structure. Upon further discharge to 2.0 V, the P2 (002) peak in MNF fails to revert to its original position, indicating inadequate structural recovery during the charge–discharge process. Such significant lattice changes may induce anisotropic stresses and cracks, potentially compromising long-term cycling stability. Conversely, while the P3 phase emerged in MNFCTLM upon charging to 4.3 V, it vanished upon discharge to 3.8 V. After further discharge to 2.0 V, all the peaks returned to their original positions, with no new peaks detected. This observation unequivocally demonstrates that the high-entropy biphasic phase significantly enhances the structural stability of MNFCTLM.

Fig. 6 Ex situ XRD patterns of (a) MNF and (b) MNFCTLM during the initial charge/discharge procedure. Ex situ XPS spectra of O 1s for (c) MNF and (d) MNFCTLM. (e) Diagram showing the mechanism of lattice oxygen inhibition.

To delve deeper into the charge compensation mechanism, ex situ XPS measurements were conducted. Fig. 6c and d present the O 1s spectra for MNF and MNFCTLM in their pristine state, during charging to 4.3 V, and upon discharging to 2.0 V, respectively. The spectra show peaks corresponding to lattice oxygen at 528.97 eV and two peaks associated with surface oxygen at 531.2 eV and 535.48 eV. Notable variations were observed in the oxygen characteristic peaks of MNF and MNFCTLM during initial charging and discharging at varying voltages. In the case of MNF (Fig. 6c), the O²⁻ peak diminishes significantly at 4.3 V, while a new peak emerges at 530.5 eV, indicative of anionic peroxide Oⁿ⁻, signifying a redox reaction of anionic oxygen at high voltage. Upon discharging to 2.0 V, the lattice oxygen content significantly decreases from its initial level, with Oⁿ⁻ still constituting a substantial portion, suggesting an irreversible transformation of O²⁻ to Oⁿ⁻.³⁷ Further oxidation of unstable Oⁿ⁻ leads to an irreversible loss of O²⁻ and the production of O₂.³⁸ Conversely, in MNFCTLM (Fig. 6d), the reduced Oⁿ⁻ peak at 4.3 V is attributed to the increased TM–O bond energy and altered electron cloud density resulting from the incorporation of multiple elements, which modifies the local oxygen environment and inhibit the formation of unstable Oⁿ⁻. Interestingly, the intensity of the lattice oxygen (O²⁻) peak in MNFCTLM increases upon completion of the discharge process, indicating that the high-entropy effect promotes the reversibility of the oxygen redox reaction, which in turn enhances the rate capability and cycling stability of the electrode material. Fig. 6e illustrates the mechanism underlying the suppression of lattice oxygen evolution in MNFCTLM.

To explore the structural stability of MNFCTLM in more depth, density functional theory (DFT) calculations were conducted on both MNF and MNFCTLM. The results are shown in Fig. 7 and S12. Fig. 7a illustrates the formation energy (ΔE_formation) and configurational entropy (ΔS_config) for both materials. The data reveal that multi-element doping significantly elevates the ΔS_config of MNFCTLM while increasing the absolute value of ΔE_formation. This suggests that the thermodynamic benefit arising from the high-entropy effect reinforces the stability of the crystal structure, contributing to the material's exceptional properties. Fig. S14 shows the total density of states (TDOS) of MNF and MNFCTLM, with MNF exhibiting a well-defined band gap of 0.817 eV near the Fermi energy level, consistent with its semiconductor properties. In contrast, the band gap of MNFCTLM, formed through elemental doping into a high entropy structure, is significantly reduced, suggesting improved electronic conductivity.

Fig. 7 (a) The calculated formation energies and configurational entropies of MNF and MNFCTLM. PDOS of (b) MNF and (d) MNFCTLM. Schematic band structure of (c) MNF and (e) MNFCTLM. (f) Post-cycling SEM images of MNF and MNFCTLM after 50 cycles.

Fig. 7b and d present the partial density of states (PDOS) for MNF and MNFCTLM, respectively. Compared with MNF, the 3d orbitals of Mn, Fe, and Ni in MNFCTLM are close to the Fermi energy level, and the PDOS at the top of the valence band is mainly occupied by the 3d electronic states of Mn, Cu, Fe, and Ni. This indicates that the 3d electrons from these transition metals likely contribute to charge compensation during electrochemical reactions. Ti 3d, Li 2s, and Mg 3s contribute minimally to the density of states near the Fermi energy level, indicating that these elements exhibit low electrochemical activity during charge/discharge cycles without engaging in redox reactions; hence, they are omitted from the figure. Fig. 7c and e show the band structure diagrams of MNF and MNFCTLM, respectively. The findings reveal a clear 3d–2p orbital hybridization between redox-active elements and oxygen. The O 2p orbitals in MNFCTLM are farther from the Fermi energy level (0.95 eV), making it more difficult to extract electrons from oxygen. This suggests that MNFCTLM exhibits stronger stability under high pressure compared to MNF. This finding aligns with the ex situ XPS analysis, implying that MNFCTLM demonstrates an enhanced capability to suppress oxygen release. In addition, orbital hybridization near the Fermi energy level forms bonded TM 3d e_g-O 2p (located below the Fermi energy level) and high-energy antibonded states (located above the Fermi energy level). In MNFCTLM, there is a larger overlap between O 2p and TM 3d electronic states on the TM 3d e_g-O 2p orbitals, while the overlap on the orbitals is decreased, indicating stronger hybridization between the O 2p orbital and the 3d orbital of the redox-active elements. This phenomenon indicates an increase in the TM–O bond energy in MNFCTLM.

To assess the morphological stability of MNF and MNFCTLM after prolonged cycling, SEM images were acquired after 50 cycles at a 0.2C rate (Fig. 7f). The morphology of MNF particles underwent significant alterations after long-term cycling, with substantial swelling and cracking observed following repeated Na⁺ insertion and extraction. This structural deterioration led to a marked decline in electrochemical performance. In contrast, the morphology of MNFCTLM particles remained intact, preserving a distinct blocky structure, which is indicative of internal stability. This finding aligns with electrochemical data, thus reinforcing the exceptional structural stability of MNFCTLM. To further assess the impact of the high-entropy biphasic structure on electrode dissolution, we dissected MNF and MNFCTLM button cells after 200 cycles at 2C to inspect the glass fiber diaphragms. Fig. S15 reveals that the MNF diaphragm exhibited pronounced yellow discoloration, potentially resulting from the dissolution of transition metals from the cathode due to severe side reactions. Conversely, the yellowing on the diaphragm associated with the MNFCTLM electrode was minimal, providing additional evidence that the high-entropy biphasic structure significantly improves the material's stability.³⁹

To assess the real-world applicability of MNFCTLM, we constructed a full cell configuration employing MNFCTLM as the cathode and HC as the anode, followed by performance evaluation. The performance of HC is shown in Fig. S16. Prior to assembly, the HC anode was preactivated to establish a robust SEI layer. The schematic structure of the full cell is shown in Fig. 8a. Fig. 8b presents the initial charge/discharge profiles of the MNFCTLM cathode and HC anode used to determine the voltage window of the full cell. Based on the charge/discharge curves of the electrodes, the optimal voltage range for the full cell was found to be 1.5–4.0 V, effectively leveraging the electrochemical performance of both electrodes. Fig. 8c displays the initial three charge–discharge profiles of the full cell. Operating at 0.2C, the full cell achieved a first-cycle discharge capacity of 160 mAh g⁻¹, corresponding to an energy density of 457.8 Wh kg⁻¹ (cathode mass-based) with an ICE of 121%. Fig. 8d presents the discharge capacities at various current densities (0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C), which were 159.6, 130.2, 116.6, 103.3, 85.8, 68, and 41 mAh g⁻¹, respectively. After 200 cycles at 2C, the full cell retained 81.2% of its initial capacity, as shown in Fig. 8e, indicating excellent long-term stability. This performance surpasses that of most previously reported layered oxide sodium-ion full cells, as depicted in Fig. 8f.^40–48 This further validates the potential of MNFCTLM for practical applications.

Fig. 8 Electrochemical measurements of the MNFCTLM‖HC full cell. (a) Schematic of the full cell configuration. (b) Charge–discharge curves of the MNFCTLM cathode and HC anode for establishing the voltage range for the full cell. (c) Charge/discharge plot of the full cell at 0.2C. (d) Rate performance of the full cell at different rates. (e) Long-term cycling capability of the full cell at 0.2C. (f) Comparisons of the electrochemical performance with previously reported studies.

4. Conclusions

In summary, we have successfully prepared an MNFCTLM cathode with reduced Ni content via a conventional solid-phase technique. This was achieved through a high-entropy approach that incorporates a P2/O3 biphasic structure design. Multi-element synergy increases TM–O bonding energy, modulating the oxygen electronic environment to prevent the formation of unstable Oⁿ⁻ ions and mitigate issues related to O₂ release and irreversible TM-layer migration during cycling. This fundamentally improves structural integrity, while the biphasic structure provides more Na⁺ diffusion pathways for excellent rate performance. Electrochemically, MNFCTLM exhibits a capacity retention of 81.3% after 300 cycles at 5C and achieves a specific capacity of 83.2 mAh g⁻¹ at a high rate of 20C, outperforming its low-entropy MNF counterpart. Remarkably, it retains 86.4% of its capacity after 150 cycles at 2C following seven days of air exposure, demonstrating exceptional air stability. The MNFCTLM‖HC full battery offers an initial discharge capacity of 160 mAh g⁻¹ at 0.2C, corresponding to an energy density of 457.8 Wh kg⁻¹, and retains 81.2% capacity after 200 cycles at 2C. This work establishes a high-entropy multiphase design paradigm for developing high-stability and high-performance layered oxide cathodes for SIBs.

Author contributions

Jing Sun: experiment design, data collection, writing-original draft. Cailing Liu and Hongbo Huang: experimental design, funding, writing-review & editing. Xiaohong Liu and Huan Liu: data collection. Meilan Xie, Lingling Liu, Juntong Huang and Dui Ma: discussion, writing-review & editing. Peixun Xiong and Xiao Liang: supervision, writing-review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The article and Supplementary Information (SI) contain all the experimental and computational data. Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta07188g.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52004129), and the Natural Science Foundation of Jiangxi Province (No. 20224BAB214034, 20242BAB25249, and 20252BAC240380). Thanks to Scientific Compass (http://www.shiyanjia.com) for the XPS analysis.

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