@article{CAO2025, 
author = {Wendong CAO and Dan ZHOU},
title = {Research Progress on Doping Modification Strategy of Layered Oxide Cathode Materials for Sodium-Ion Batteries},
year = {2025},
journal = {Journal of the Chinese Ceramic Society},
volume = {53},
number = {12},
pages = {3798-3832},
keywords = {sodium-ion batteries, cathode materials, layered oxides, doping modification},
url = {https://www.sciopen.com/article/10.14062/j.issn.0454-5648.20250079},
doi = {10.14062/j.issn.0454-5648.20250079},
abstract = {Sodium-ion batteries (SIBs) have emerged as a research hotspot in electrochemical energy storage due to their abundant raw material resources and low cost. As a critical battery component, cathode materials directly govern the energy density and dictate the cycling stability and safety of batteries. Consequently, in-depth research and development of high-performance cathode materials are pivotal to advancing SIBs and enhancing their electrochemical properties. The existing research efforts predominantly focus on several categories of SIBs cathode materials, i.e., layered oxides, polyanionic compounds, and Prussian blue analogues (PBAs). Among these, layered oxide cathodes exhibit structural similarities to their lithium-ion battery counterparts, demonstrating significant advantages in energy density. Their high theoretical specific capacity enables the realization of high-energy-density batteries. Moreover, these materials (i.e., NaxTMO2, where TM denotes transition metal) combine low-cost synthesis with facile manufacturability, aligning with future development trends. Thus, layered oxides represent the existing core cathode system for SIBs and a primary focus on future research endeavors.Layered oxides in sodium-i on batteries undergo complex phase transitions during charge/discharge cycles due to Na+ intercalation/deintercalation, resulting in electrochemical performance degradation. Furthermore, electrostatic interactions between Na+—Na+ and Na+—TMn+ ions trigger the formation of Na+/vacancy ordering, which alters in-plane Na+ diffusion mechanisms and imposes elevated diffusion energy barriers. These phenomena ultimately diminish the energy density of cathode materials. It should be noted that such phase transitions predominantly occur at high voltage ranges. This approach sacrifices specific capacity and consequently compromises overall battery performance, while restricting the cutoff voltage can suppress phase transitions Elemental doping thus emerges as an effective strategy for mitigating high-voltage phase transitions. Doping significantly enhances key electrochemical metrics including cycling stability and rate capability via regulating complex phase evolution, eliminating Na+/vacancy ordering, and optimizing Na+ transport kinetics. Despite substantial recent advancements in doping modification research for layered oxide cathodes, a systematic review of this domain remains notably absent.This review categorizes ion doping into metallic and non-metallic elements based on dopant types. Introducing trace dopant ions (i.e., Cu2+, Ti4+, Zn2+, Mg2+, F–, etc.) into the layered oxide cathode matrix enables partial or complete substitution at transition metal sites, sodium sites, or oxygen sites within NaxTMO2. This modification alters chemical bonding configurations and bond strength, thereby modulating interlayer distances in transition metal slabs.Consequently, it optimizes the crystalline and electronic structures of the material, suppresses Na+/vacancy ordering and structural phase transitions during charge/discharge cycles, and ultimately enhances cycling stability.Nevertheless, studies reveal that most single-element doping strategies exhibit inherent limitations, typically enhancing either specific capacity or cycling stability individually, with few elements capable of holistically optimizing overall performance. Consequently, multi-element doping emerges as an essential approach for superior cathode enhancement, predominantly employing co-doping of electrochemically active and inactive elements to achieve synergistic performance gains. Although this strategy outperforms conventional single-element doping in comprehensive property enhancement, the modification efficacy of simplistic elemental combinations remains constrained and highly sensitive to doping ratios. Thus, high-entropy layered oxides are synthesized either by doping low/medium-entropy oxide frameworks or via direct synthesis to harness combined high-entropy effect and doping modification effect for the breakthrough performance.Studies confirm that the incorporation of multiple elements in high-entropy layered oxides substantially increases a lattice disorder and reduces the Gibbs free energy, thereby enhancing structural stability and suppressing phase transitions. Furthermore, the high-entropy effect facilitates ion diffusion through lowered migration energy barriers, elevates ionic conductivity, and ultimately improves rate capability and long-term cycling stability. This design strategy achieves a synergistic enhancement across multiple performance metrics.Summary and ProspectsSystematic statistical analysis reveals that distinct doping elements exert element-specific regulatory effects on the key performance metrics of layered oxide cathodes for sodium-ion batteries (i.e., capacity, rate capability, long-cycle stability, and structural integrity). Electrochemically inactive dopants enhance structural stability at the expense of reducing the number of redox-active sites due to lattice occupation, leading to capacity decay. Conversely, electrochemically active dopants improve capacity through redox reactions yet modify electron cloud distribution (i.e., exacerbating electronic disorder and altering localization/delocalization behavior), concurrently inducing lattice distortion and additional stress that compromise structural stability. Consequently, synergistic modification strategies integrating high-entropy design with doping effects emerge as a pivotal approach to transcend comprehensive performance limitations. However, this strategy confronts triple challenges, i.e., 1) synthesis requiring prolonged high-temperature annealing with precise multicomponent stoichiometric control, significantly amplifying manufacturing complexity and cost; 2) cost escalation from precious/rare-earth dopants restricting industrial scalability; and 3) insufficient mechanistic understanding of the synergistic interactions between dopant elements. A future research must prioritize developing low-cost facile synthesis techniques, advancing computational materials science-guided elemental combination optimization, and elucidating multiscale synergistic mechanisms, thereby establishing definitive composition–structure–property relationships to direct targeted fabrication of high-performance materials.}
}