Layered transition metal oxides with the general formula NaxMO2, where M represents transition metals such as Fe, Mn, Ni, or Co, have emerged as promising cathode materials for sodium-ion batteries. These materials exhibit structural and electrochemical characteristics that make them suitable for energy storage applications. The performance of NaxMO2 cathodes is closely tied to their crystallographic phases, sodium diffusion mechanisms, and stability during cycling. Understanding these factors is critical for advancing sodium-ion battery technology.

Structural classifications of NaxMO2 cathodes primarily include P2 and O3 phases, distinguished by their stacking sequences and sodium coordination environments. The P2 phase has a prismatic coordination of sodium ions between transition metal layers, with an ABBA oxygen stacking sequence. In contrast, the O3 phase adopts an octahedral sodium coordination and an ABCABC oxygen stacking. The P2 phase typically offers higher ionic conductivity due to more favorable sodium diffusion pathways, while the O3 phase often provides higher capacity but with slower kinetics. The choice between these phases depends on the intended balance between energy density and rate capability.

Electrochemical performance of NaxMO2 cathodes is influenced by the transition metal composition and structural stability. For example, NaxMnO2 demonstrates a high theoretical capacity but suffers from Jahn-Teller distortion due to Mn³⁺, leading to structural degradation. In contrast, NaxFeO2 is cost-effective and environmentally benign but exhibits lower energy density. Mixed transition metal systems, such as NaxNi1/3Mn1/3Co1/3O2, have been explored to combine the advantages of individual metals, improving capacity retention and cyclability. The average operating voltage of these materials typically ranges between 2.5 and 3.5 V versus Na/Na⁺, depending on the transition metal redox couples involved.

Sodium diffusion in layered oxides occurs through interstitial sites within the transition metal layers. In P2-type structures, sodium ions migrate through adjacent prismatic sites with lower energy barriers compared to O3-type structures, where diffusion involves more complex pathways through octahedral sites. This difference in diffusion kinetics explains why P2 phases generally exhibit better rate performance. However, both phases are susceptible to phase transitions during sodium extraction and insertion, which can lead to volume changes and structural degradation. For instance, P2-to-O2 transitions at high voltages can cause irreversible capacity loss.

A major challenge with NaxMO2 cathodes is their moisture sensitivity. These materials tend to react with water and carbon dioxide in ambient air, forming sodium carbonate and hydroxide species on the surface. This degradation not only reduces the active material content but also increases interfacial resistance, impairing electrochemical performance. Strategies to mitigate this issue include protective coatings, such as aluminum oxide or carbon layers, and storage under inert atmospheres. Additionally, careful control of synthesis conditions can minimize residual alkali content, which exacerbates moisture reactivity.

Phase transitions during cycling are another critical issue. As sodium ions are extracted, the remaining Na⁺ ions redistribute, often leading to ordered phases or lattice distortions. These transitions can cause abrupt voltage changes, hysteresis, and mechanical strain, ultimately reducing cycle life. For example, P2-Na2/3MnO2 undergoes multiple phase transitions upon cycling, resulting in step-like voltage profiles. Doping with electrochemically inactive elements like Mg or Ti has been shown to stabilize the host structure and suppress detrimental phase transformations.

Comparing NaxMO2 cathodes to their lithium-ion counterparts reveals key differences. Layered lithium oxides (LiMO2) generally exhibit higher operating voltages (3.5–4.5 V vs. Li/Li⁺) and energy densities due to the lighter mass and smaller ionic radius of lithium. However, sodium-based systems benefit from the abundance and lower cost of sodium resources. Additionally, some NaxMO2 compositions display better thermal stability than high-nickel lithium oxides, which are prone to oxygen release at elevated temperatures. The trade-offs between performance, cost, and sustainability make sodium-ion batteries attractive for large-scale energy storage despite their lower energy density.

Recent material innovations have focused on improving the structural and electrochemical stability of NaxMO2 cathodes. One approach involves the design of high-entropy oxides, where multiple transition metals are incorporated to create configurational disorder, suppressing phase transitions and enhancing cyclability. Another advancement is the development of P2/O3 intergrowth structures, which combine the fast kinetics of P2 phases with the high capacity of O3 phases. Surface modifications, such as cation doping and nanoscale coatings, have also proven effective in reducing interfacial side reactions and improving rate capability.

Industrial adoption of NaxMO2 cathodes is progressing, with several companies piloting sodium-ion battery production. These efforts are driven by the need for cost-effective and sustainable alternatives to lithium-ion batteries, particularly for grid storage and low-speed electric vehicles. While current energy densities remain below those of commercial lithium-ion systems, ongoing research aims to close this gap through compositional optimization and engineering solutions. The scalability of layered oxide synthesis, leveraging existing manufacturing infrastructure from lithium-ion technology, further supports their commercialization prospects.

In summary, layered transition metal oxide cathodes for sodium-ion batteries present a viable pathway for next-generation energy storage. Their structural diversity, combined with ongoing advancements in material design, offers opportunities to address existing limitations related to phase stability and moisture sensitivity. As research continues to refine these materials, their integration into commercial battery systems is expected to grow, contributing to a more diversified and sustainable energy storage landscape.