Formulating cathode materials based on high-entropy strategies for sodium-ion batteries

Abstract

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries (LIBs) owing to abundant resources and cost-effectiveness. However, cathode materials face persistent challenges in structural stability, ion kinetics, and cycle life. This review highlights the transformative potential of high-entropy (HE) strategies that leveraging multi-principal element synergies to address these limitations via entropy-driven mechanisms. By establishing thermodynamic criteria for high-entropy materials (HEMs), we elucidate the universal principles whereby configurational entropy mitigates lattice distortion, suppresses phase transitions, and enhances Na⁺ diffusion kinetics via multi-element interactions. HE design demonstrates unique advantages for layered oxides, Prussian blue analogues (PBAs) and polyanionic cathode systems: it alleviates Jahn-Teller distortion through dopant synergy to stabilize layered structures; optimises ion migration channels by tuning exposed crystal facets; suppresses irreversible phase changes and mechanical strain to enable reversible structural evolution; and enhances redox reversibility via multi-site charge compensation among transition metals. Furthermore, reasonable design principles for the HE strategy in cathode materials for SIBs were proposed, along with the future expansion of theoretical calculations and the application of the HE strategy in the future. At the same time, potential challenges that may occur during this process and the current viewpoints and methods for solving these problems were emphasized. Overall, this review provides valuable guidance for the further exploration of the HE strategy in the field of SIBs.

Fig. 1. (a) Distribution of global lithium resources. Reprinted with permission from ref. . Copyright 2024 John Wiley and Sons. (b) Research hotspots in the HE cathode of SIBs.

Fig. 2. Timeline of representative works on HEMs for sodium-ion batteries. All the illustrations are from the literature. Reprinted with permission from ref. . Copyright 2020 Wiley VCH. Reprinted with permission from ref. . Copyright 2021 Springer Nature. Reprinted with permission from ref. . Copyright 2022 John Wiley and Sons. Reprinted with permission from ref. . Copyright 2023 Elsevier. Reprinted with permission from ref. . Copyright 2024 Wiley VCH. Reprinted with permission from ref. . Copyright 2025 RSC.

Fig. 3. Thermodynamic analyses of the HE mixing considering both (a) entropy and (b) enthalpy, which are mainly determined by the composition of the HEMs. Reprinted with permission from ref. . Copyright 2024 Elsevier.

Fig. 4. The following diagram provides an overview of the four core effects of HE that are employed in SIB cathodes.

Fig. 5. (a) The conventional O3-type Na-based layered oxides contain three different TM elements. (b) The proposed HEO cathodes have multiple TM elements (TM₂ representes the redox elements and is marked in blue). Reprinted with permission from ref. . Copyright 2020 Wiley VCH.

Fig. 6. HE affects structural transformation. (a) XRD of Na_0.7Mn_0.4Ni_0.3Cu_0.1Fe_0.1Ti_0.1O_1.95F_0.1 at different temperatures during calcination. (b) The ratios of P2/O3 phases in NaMnNiCuFeTiOF sintered at various temperatures. (c) Schematic illustration of the structural change mechanisms of Na_0.7Mn_0.4Ni_0.3Cu_0.1Fe_0.1Ti_0.1O_1.95F_0.1. Reprinted with permission from ref. . Copyright 2023, Elsevier. (d) Schematic diagram of the charging and discharging behavior of Na_2/3Li_1/6Fe_1/6Co_1/6Ni_1/6Mn_1/3O₂. (e) The bond length changes of TM–O (where TM represented Ni, Fe, Co, and Mn) varies during the charging process from 2.0 to 4.5 V. Reprinted with permission from ref. . Copyright 2022 John Wiley and Sons. Schematic diagram of the DOS for (f) LMM and (g) LMCNM. Reprinted with permission from ref. . Copyright 2025 American Chemical Society.

Fig. 7. Some effects of HE on the crystal surface. (a) Schematic diagram of the morphological structure of the HEM compared with the normal material. (b) HAADF-STEM images of HEO424. (c) The HEM possesses a variety of cations that provide highly stable structures and diffusion channels, making more (010) active crystalline surfaces for ion transport. Reprinted with permission from ref. . Copyright 2022 American Chemical Society. (d) The six (010) active crystalline planes of HEM. (e) Variation of (010) active crystalline facet content with the change of ΔS_config. Reprinted with permission from ref. . Copyright 2022 Springer Nature. (f) In situ XRD patterns and corresponding lattice parameter changes of TMO5. Reprinted with permission from ref. . Copyright 2025 Elsevier.

Fig. 8. Suppression of phase transition through HE design. (a) In situ XRD patterns of NaNi_0.12Cu_0.12Mg_0.12Fe_0.15Co_0.15Mn_0.1Ti_0.1Sn_0.1Sb_0.04O₂ within 2.0–3.9 V at 0.1C. (b) Crystal structure evolution of the HE layered oxide cathode. Reprinted with permission from ref. . Copyright 2020 Wiley VCH. (c) In situ XRD patterns of NaMn_0.2Fe_0.2Co_0.2Ni_0.2Ti_0.2O₂ during charge/discharge cycling. (d) Variations in the content of O3 and P2 phases during the first charge/discharge cycle. (e) Graphical visualization of O3- and P3-type crystal structures. Reprinted with permission from ref. . Copyright 2022 Elsevier.

Fig. 9. Inhibition of grain surface strain by HE. (a) Schematic diagram of designing HE O3-type layer cathode material. (b) Crystal structure optimization diagram of HE cathode materials, showing the (001) crystal facet andELF. (c) The curves of Na⁺ ion formation energy and ΔS_config during extraction and intercalation in HEMs. (d) Schematic diagram of reversible phase transition of O3-type HE cathode materials. Reprinted with permission from ref. . Copyright 2024 Wiley VCH. (e) SEM images of HEO and NMO before and after 200 cycles. (f) Calculation of hardness and Young's modulus for both materials. Reprinted with permission from ref. . Copyright 2023 Wiley VCH. (g) Structural model of O3 phase and schematic representation of the correlation between strain distribution and ionic displacement for NCFMT and NCFMS viewed in perpendicular and parallel to the TMO₂ layers. Reprinted with permission from ref. . Copyright 2024 Springer Nature.

Fig. 10. Ex situ XAS spectra of TMs in NaMn_0.2Fe_0.2Co_0.2Ni_0.2Ti_0.2O₂ in both TEY (a) and PFY (b) modes during charging and discharging. Reprinted with permission from ref. . Copyright 2022 Wiley VCH. (c) Normalized in situ XANES spectra and the corresponding voltage curve of the NCNFMT at Ti, Mn, Fe, Ni, and Cu K-edges in various states. (d) K-edge XANES spectra of Ti, Mn, Fe, Ni, and Cu and the layered structure scheme where M–O and M–M bond lengths were optimized in NCNFMT. Reprinted with permission from ref. . Copyright 2022 Wiley VCH.

Fig. 11. (a) Schematic diagram of the crystal structure of multi-element doping. (b) In situ DEMS test of the HEM during the second charge/discharge. (c) In situ XRD patterns during charging and discharging. Reprinted with permission from ref. . Copyright 2021 Wiley VCH. (d) The structures of PBAs with three different entropies were simulated by using DFT calculations. Reprinted with permission from ref. . Copyright 2022 Wiley VCH.

Fig. 12. (a) Comparison of XAS data for HEM and Mn-HCF. Comparison of XAS data for HEM, Mn-HCF, and Fe-HCF. Reprinted with permission from ref. . Copyright 2022 Wiley VCH. (b) Schematic diagram of the structural simulation of SC-HEPBA during the cycling process. (c) Schematic representation of the unit volume change of SC-HEPBA during charging and discharging. (d) The TM contents in the electrolyte after more than 100 cycles. Reprinted with permission from ref. . Copyright 2023 Elsevier. (e) Schematic diagram for the chemical etching process of HEPBA-Etched-0. Reprinted with permission from ref. . Copyright 2024 American Chemical Society.

Fig. 13. (a) The XRD pattern of HE-NVPF. (b) The structures of p-NVPF and HE-NVPF. (c) GCD curves within the potential window of 2.0–4.3 V. (d) Cs values and capacity contributions during discharge. (e) Na⁺ migration pathways in p-NVPF. Reprinted with permission from ref. . Copyright 2022 Wiley VCH. COMSOL simulation and postmortem analysis. (f) Stress field analysis at different depths of discharge (DOD) states based on the COMSOL platform. Reprinted with permission from ref. . Copyright 2025 RSC.

Fig. 14. The research content and an overview of HEMs are presented.

Fig. 15. The application of HE strategies in the field of SIBs is summarised on the left, while the future research directions that require to be explored are showed on the right.