The pursuit of efficient, scalable, and cost-effective energy storage solutions has never been more critical. While lithium-ion batteries (LIBs) have dominated the landscape for portable electronics and electric vehicles, concerns regarding the geographical concentration and long-term availability of lithium resources have spurred intense research into alternative chemistries. Among these, sodium-ion batteries (SIBs) have emerged as a particularly compelling candidate for large-scale stationary storage applications, such as grid-level energy storage, due to the natural abundance, low cost, and even global distribution of sodium resources. SIBs operate on a similar “rocking-chair” principle as LIBs, where sodium ions shuttle between the cathode and anode during charge and discharge cycles. This fundamental similarity allows researchers to leverage decades of knowledge accumulated in LIB development. However, the larger ionic radius of Na+ (1.02 Å) compared to Li+ (0.76 Å) introduces distinct challenges, primarily concerning slower ionic diffusion kinetics, more pronounced volume changes during (de)intercalation, and greater susceptibility to detrimental phase transitions and structural degradation in electrode materials.
The cathode material is a pivotal component, as it fundamentally determines the operating voltage, specific capacity, cycling stability, and overall cost of the battery. Within the spectrum of cathode candidates for SIBs—including polyanionic compounds and Prussian blue analogues—layered transition metal oxides with the general formula NaxTMO2 (where TM = Mn, Ni, Co, Fe, Cu, etc.) have garnered significant attention. This is due to their relatively high theoretical specific capacity, straightforward synthesis, and compositional versatility. These materials are typically classified into two main structural types based on the coordination environment of sodium ions: the prismatic P2 and octahedral O3 phases (the letter denotes the sodium site, and the number indicates the number of TM-O layers in the unit cell). Despite their promise, practical application of NaxTMO2 cathodes is hindered by several intrinsic issues: irreversible phase transitions (e.g., P2 to O2 or OP4 phases at high voltages), transition metal ion migration into the sodium layer (cation mixing), electrolyte decomposition at high voltages, and oxygen release, all of which lead to rapid capacity fade and voltage decay.
Cation doping, a mature and powerful materials engineering strategy, has proven to be exceptionally effective in mitigating these shortcomings. The core principle involves the partial substitution of host cations (either Na+ or TMn+) in the crystal lattice with foreign cationic species. This substitution can induce a multitude of beneficial effects, which can be described by several key mechanisms. Doping can expand the interlayer spacing, providing a wider pathway for Na+ diffusion, as described by the relationship for ionic conductivity ($\sigma$):
$$ \sigma = n z e \mu $$
where $n$ is the charge carrier concentration, $z$ is the charge number, $e$ is the elementary charge, and $\mu$ is the carrier mobility. Doping can increase $\mu$ by reducing diffusion energy barriers. It can act as a “pillar” to suppress detrimental slab sliding and phase transitions. Doping can also strengthen the TM-O bond, thereby enhancing structural integrity and suppressing oxygen loss. Furthermore, it can disrupt long-range Na+/vacancy ordering, which often creates kinetic bottlenecks, and directly block the migration pathways for TM ions, reducing cation mixing. The selection of doping elements is strategic and is broadly categorized based on their electrochemical activity within the operating voltage window of the sodium-ion battery: electrochemically active elements, inert elements, and combinations thereof (multi-ion doping). This article provides a comprehensive, first-person perspective review of recent progress in these doping strategies for layered NaxTMO2 cathodes, analyzing their mechanisms, performance outcomes, and future potential to advance sodium-ion battery technology.
Doping with Electrochemically Active Elements
Incorporating elements that can undergo reversible redox reactions during the charge/discharge cycle of a sodium-ion battery offers a dual advantage: they can contribute additional capacity while simultaneously stabilizing the host structure. The redox activity must be carefully matched to the voltage window and should ideally help suppress harmful processes in the base material.
Iron (Fe) Doping
Iron is a quintessential example of a multifunctional active dopant. Its appeal lies in its low cost, natural abundance, and the activity of the Fe3+/Fe4+ redox couple at high potentials (>4.0 V vs. Na/Na+). In many Mn-rich P2-type cathodes, the high-voltage capacity is primarily governed by anionic (oxygen) redox, which is often irreversible and leads to structural degradation. Fe doping can engage cationic redox at these high voltages, which is generally more reversible, thereby stabilizing the oxygen lattice. A seminal study on P2-Na2/3Ni1/3Mn2/3O2 demonstrated that strategic Fe substitution for Mn (e.g., Na2/3Ni1/3Mn7/12Fe1/12O2) effectively suppressed the detrimental P2-O2 phase transition observed in the pristine material above 4.2 V. In-situ X-ray diffraction revealed the complete retention of the P2 structure in the doped sample upon charging to high voltage, whereas the undoped sample underwent a complete phase transformation. This structural stabilization translated directly into superior cycling performance. The optimized material retained 85% of its capacity after 300 cycles at a high rate of 5C. Furthermore, galvanostatic intermittent titration technique (GITT) measurements confirmed a marked improvement in sodium-ion diffusion kinetics ($D_{Na^+}$), calculated from:
$$ D_{Na^+} = \frac{4}{\pi \tau} \left( \frac{n_m V_m}{A S} \right)^2 \left( \frac{\Delta E_s}{\Delta E_\tau} \right)^2 $$
where $\tau$ is the pulse duration, $n_m$, $V_m$, $A$, and $S$ are material parameters, and $\Delta E_s$ and $\Delta E_\tau$ are voltage changes. This enhanced $D_{Na^+}$ endowed the material with exceptional rate capability and remarkable low-temperature performance, a critical metric for the real-world application of sodium-ion batteries.
Copper (Cu) Doping
Copper is another attractive active dopant, typically substituting for Ni2+ due to their similar ionic radii (Cu2+: 0.73 Å, Ni2+: 0.69 Å). The Cu2+/Cu3+ redox couple provides a sloping voltage profile and contributes to capacity. More importantly, Cu doping has been shown to effectively disrupt the charge ordering and Na+/vacancy ordering that often occur in layered oxides. This disordering lowers the kinetic barriers for Na+ diffusion. Research on P2-Na0.67Ni0.33Mn0.67O2 showed that Cu substitution (e.g., Na0.67Ni0.23Cu0.1Mn0.67O2) significantly delayed the onset voltage of the irreversible phase transition from 4.2 V to 4.4 V. This was attributed to the strengthened TM-O bonds and altered electronic structure, which increased the energy barrier for layer gliding. Consequently, the Cu-doped cathode exhibited vastly improved long-cycle stability, maintaining over 78% capacity retention after 500 cycles at 2C, a performance level essential for durable sodium-ion batteries.
| Cathode Material | Doping Element | Key Mechanism | Capacity Retention (Cycles) | Rate Performance |
|---|---|---|---|---|
| P2-Na2/3Ni1/3Mn7/12Fe1/12O2 | Fe | Suppresses P2-O2 phase transition, activates cationic redox. | 85% (300 cycles at 5C) | Stable from 0.1C to 25C |
| P2-Na0.67Ni0.23Cu0.1Mn0.67O2 | Cu | Disrupts Na+/vacancy ordering, delays phase transition voltage. | 78% (500 cycles at 2C) | Improved high-rate capability |
| O3-NaNi0.5Mn0.5-xTixO2 | Ti | Stabilizes structure, mitigates Jahn-Teller distortion from Mn3+. | >90% (100 cycles) | Moderate improvement |
Doping with Electrochemically Inert Elements
Inert dopants, which do not participate in redox reactions within the operational window of the sodium-ion battery, are primarily used to enhance structural and interfacial stability. While they may cause a marginal decrease in initial specific capacity by displacing redox-active ions, the gains in cycling life and safety are often substantial.
Magnesium (Mg) and Lithium (Li) Doping
Mg2+ (0.72 Å) and Li+ (0.76 Å) are classic inert dopants often used to substitute for Ni2+ in the TM layer. Their primary role is structural stabilization. In Mn-rich systems, the presence of Mn3+ causes a Jahn-Teller distortion, leading to local structural strain and capacity fade. Introducing Li+ or Mg2+ increases the average oxidation state of Mn, reducing the Mn3+ content and mitigating this distortion. For instance, Li-doped Na2/3Ni1/5Li2/15Mn2/3O2 showed a dramatic reduction in Mn3+ content compared to its undoped counterpart, leading to significantly improved capacity retention. Furthermore, these strong M-O bonds (where M=Mg or Li) act as anchors, suppressing the slab sliding that triggers phase transitions. A study on P2-Na0.62Ni0.25-xMgxMn0.75O2 revealed that optimal Mg doping (x=0.1) effectively suppressed the high-voltage P2-O2 phase transition and regulated the oxygen redox activity, making it more reversible. This resulted in a cathode with 92% capacity retention after 100 cycles, compared to 61% for the pristine material.
Calcium (Ca) Doping
Calcium doping is unique because the large ionic radius of Ca2+ (1.00 Å) is close to that of Na+ (1.02 Å). Consequently, Ca2+ preferentially occupies sites within the sodium layer rather than the TM layer. When incorporated into the Na layer, Ca2+ ions serve as robust structural pillars, physically blocking the shear displacement of TM-O slabs and thus inhibiting phase transitions during deep desodiation. They also effectively disrupt the long-range order of Na+ and vacancies, which enhances Na+ mobility. Research on P3-type Na0.6-xCaxNi1/3Mn1/3Co1/3O2 demonstrated that Ca doping increased the phase transition voltage and improved capacity retention from 68% to 77% over 100 cycles. The strengthened Ca-O bond also contributes to stabilizing lattice oxygen, a crucial factor for high-voltage stability in sodium-ion battery cathodes.
The stabilization effect of an inert dopant ‘D’ can be conceptually framed as increasing the activation energy ($E_a$) for a detrimental process like phase transition or oxygen loss:
$$ k = A \exp\left(-\frac{E_a}{RT}\right) $$
where $k$ is the rate constant for the degradation process, $A$ is the pre-exponential factor, $R$ is the gas constant, and $T$ is temperature. Doping increases $E_a$, thereby exponentially reducing the rate $k$ of the degradation process.
| Cathode Material | Doping Element (Site) | Primary Stabilizing Role | Impact on Capacity | Cycling Stability Improvement |
|---|---|---|---|---|
| P2-Na0.62Ni0.15Mg0.1Mn0.75O2 | Mg (TM Layer) | Reduces Mn3+, suppresses P2-O2, regulates O redox. | Moderate initial decrease | 92% vs. 61% retention after 100 cycles |
| P2-Na2/3Ni1/5Li2/15Mn2/3O2 | Li (TM Layer) | Mitigates Jahn-Teller distortion, stabilizes structure. | Initial decrease | >90% retention after 100 cycles |
| P3-Na0.56Ca0.04Ni1/3Mn1/3Co1/3O2 | Ca (Na Layer) | Acts as a pillar, disrupts Na/vacancy order. | May alter voltage profile | 77% vs. 68% retention after 100 cycles |
| P2-Na0.67Zn0.05Mn0.95O2 | Zn (TM Layer) | Strong Zn-O bond, inhibits Mn dissolution. | Decrease due to inert Zn | Significant improvement in long-term cycling |
Multi-Ion Doping and High-Entropy Strategies
Recognizing that single-element doping may optimize one property at the expense of another, the field has progressively moved towards multi-ion doping strategies. The synergistic interaction between different dopants can address multiple failure mechanisms simultaneously, leading to more balanced and superior overall performance for the sodium-ion battery.
Dual-Ion Synergistic Doping
This approach combines elements with complementary functions. A prominent example is the co-doping of Ca (in the Na layer) and Mg (in the TM layer) in a P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode. Here, Ca2+ pillars the Na layer to suppress slab sliding, while Mg2+ stabilizes the TM-O framework and modulates Mn redox. This dual action more effectively inhibits the P2-O2 phase transition than either dopant alone. The optimized material Na0.64Ca0.03(Ni0.17Co0.17Mn0.66)0.9Mg0.1O2 exhibited a capacity retention of 84.9% after 300 cycles. Another effective pair is Cu and Fe. In P2-Na0.67MnO2, co-doping with Cu and Fe (Na0.67Mn0.92Cu0.04Fe0.04O2) synergistically reduced Jahn-Teller distortion, inhibited complex phase transitions, and improved the Na+ diffusion coefficient. The material delivered an excellent capacity retention of 94.35% after 100 cycles at 1C and maintained good rate performance at 5C.
High-Entropy Doping
Inspired by high-entropy alloys, the concept of high-entropy oxides (HEOs) has been introduced to cathode design for sodium-ion batteries. This involves doping the TM layer with four or more different metal cations in near-equimolar ratios. The high configurational entropy ($\Delta S_{config}$) of such a system is given by:
$$ \Delta S_{config} = -R \sum_{i=1}^{n} x_i \ln x_i $$
where $R$ is the gas constant, $n$ is the number of components, and $x_i$ is the mole fraction of component $i$. When $\Delta S_{config}$ is sufficiently high, it can stabilize a single solid-solution phase and impart exceptional structural stability due to the severe lattice distortion and “cocktail effect.” For instance, a P2-type cathode with a composition like Na0.67(Mn0.25Fe0.2Co0.2Ni0.2Cu0.15)O2 leverages multiple redox couples (Mn, Fe, Co, Ni, Cu) and enjoys immense entropy stabilization. These materials often show very smooth voltage profiles (due to distributed redox potentials), negligible phase transitions, and outstanding cycling stability. They represent a paradigm shift from tuning a single parameter to engineering the entire cationic environment for optimal sodium-ion battery performance.
| Strategy | Typical Elements | Advantages | Disadvantages/Challenges | Typical Outcome for SIB Cathode |
|---|---|---|---|---|
| Single Active Ion | Fe, Cu, Ti | Adds capacity, specific property tuning. | May not address all failure modes; can introduce new issues. | Improved capacity or rate, moderate stability gain. |
| Single Inert Ion | Mg, Li, Ca, Zn | Excellent structural stabilization, simple design. | Direct loss in specific capacity. | High cycling stability, possible rate improvement. |
| Dual-Ion Synergy | Ca/Mg, Cu/Fe, Al/Ti | Addresses multiple mechanisms, balanced performance. | More complex synthesis optimization. | High stability + good capacity/rate (balanced). |
| High-Entropy | 5+ different TMs | Exceptional phase stability, smooth voltage, multi-redox. | Complex synthesis, potential cost, characterization challenges. | Ultra-stable cycling, unique voltage profile. |
Conclusions and Future Perspectives
Cation doping has undoubtedly established itself as an indispensable and highly effective tool for engineering high-performance layered oxide cathodes for sodium-ion batteries. As reviewed, the strategic selection of dopants—whether electrochemically active like Fe and Cu, inert like Mg, Li, and Ca, or in synergistic multi-ion combinations—allows researchers to precisely tailor material properties. The core mechanisms, including expansion of Na+ diffusion pathways, suppression of phase transitions, inhibition of TM migration, and stabilization of the oxygen lattice, provide a clear roadmap for material design.
Looking forward, several promising and challenging directions emerge for the advancement of doped NaxTMO2 cathodes in sodium-ion batteries:
- Beyond Laboratory Performance: Future research must increasingly focus on parameters critical for commercialization. This includes optimizing areal loading (>3 mAh/cm²), achieving stable performance in full cells paired with practical anodes (hard carbon, etc.), and demonstrating long-term cycling under realistic conditions (e.g., extended temperature ranges, varying C-rates). The cost-benefit analysis of using elements like Co or Ni, even as minor dopants, needs careful scrutiny in favor of earth-abundant alternatives like Fe and Mn.
- Deepened Mechanistic Understanding: While the macroscopic benefits of doping are clear, atomic-scale and dynamic understanding is still evolving. Advanced in-situ/operando techniques (neutron diffraction, X-ray absorption spectroscopy, electron microscopy) and first-principles calculations are crucial to precisely map dopant locations, their evolution during cycling, and their direct role in suppressing specific degradation pathways. Understanding the dopant’s influence on the often-troublesome anionic redox process is particularly important.
- The Rise of Data-Driven Design: The vast compositional space opened by multi-ion and high-entropy strategies makes traditional trial-and-error exploration inefficient. Machine learning (ML) is poised to play a transformative role. ML models can be trained on existing datasets to predict stable phases, voltage profiles, and diffusion barriers, dramatically accelerating the discovery of optimal doping schemes for next-generation sodium-ion battery cathodes.
- Holistic Electrode Engineering: Doping is a bulk material solution. Its full potential is unlocked when integrated with other strategies. Surface coating (with oxides, phosphates, or fast-ion conductors) can protect the doped cathode from electrolyte side reactions. Morphology control (nanostructuring, single-crystal synthesis) can further enhance kinetics and mechanical resilience. The future lies in designing “core-dopant, shell-coating, ideal-morphology” hierarchical structures.
- Sustainability and Circularity: As the sodium-ion battery industry scales, the environmental footprint of cathode synthesis, including doped materials, must be considered. Developing low-temperature, aqueous, or energy-efficient synthesis routes is important. Furthermore, designing cathodes with recycling in mind—using elements that are easily separable and recoverable—will be essential for a truly sustainable sodium-ion battery ecosystem.
In conclusion, cation doping remains a vibrant and essential frontier in the quest to make sodium-ion batteries a mainstream technology for large-scale energy storage. By moving from single-element tweaks to sophisticated multi-ion and entropy-engineered designs, and by leveraging new tools like machine learning, researchers are systematically overcoming the historical limitations of layered oxide cathodes. The continued progress in this field promises to deliver the high-performance, safe, and low-cost electrodes needed to power a sustainable energy future with advanced sodium-ion batteries.