The relentless pursuit of high-performance, cost-effective electrochemical energy storage systems has intensified due to the surging demand driven by electric vehicles and grid-scale applications. While lithium-ion batteries (LIBs) have dominated the market, concerns regarding the geopolitical distribution and long-term cost sustainability of lithium resources have spurred significant interest in complementary or alternative chemistries. Among the various candidates, sodium-ion battery technology stands out as a particularly promising avenue. Sodium is abundant, geographically widespread, and inexpensive, offering a compelling economic and strategic advantage. Furthermore, the working principles of sodium-ion batteries are analogous to those of LIBs, facilitating a smoother technological transition and leveraging existing manufacturing knowledge. The core challenge in realizing high-performance sodium-ion batteries lies in developing robust electrode materials that can reversibly and efficiently host the larger Na+ ion (1.02 Å) compared to Li+ (0.76 Å).
Cathode materials are pivotal in determining the energy density, cycle life, and cost of the battery. Among the diverse cathode families for sodium-ion batteries, layered transition metal oxides with the general formula NaxTMO2 (where TM = transition metal) have garnered tremendous attention. These materials are classified based on the stacking sequence of the oxygen layers and the coordination environment of the Na+ ions. The P2-type structure, where Na+ ions reside in prismatic sites and the oxygen stacking follows an ABBA sequence, is particularly attractive for its typically higher ionic conductivity and favorable Na+ diffusion pathways compared to the O3-type (octahedral Na+, ABCABC oxygen stacking). Within the P2 family, Mn-based oxides like P2-NaxMnO2 are appealing due to the high natural abundance and low toxicity of manganese, coupled with good specific capacity. However, a fundamental issue plagues these Mn-rich cathodes: the Jahn-Teller distortion associated with Mn3+ ions (high-spin d4 configuration). During electrochemical cycling, especially at high voltages or deep desodiation states, the presence of Mn3+ can lead to severe local structural distortions, irreversible phase transitions, and ultimately, structural collapse. This manifests as rapid capacity fading and poor cycle life, hindering practical application.
To mitigate the Jahn-Teller effect and stabilize the P2 structure, cation substitution or doping is a widely adopted and effective strategy. The principle involves partially replacing Mn with other metal ions that either do not exhibit Jahn-Teller activity or can act as structural pillars. Common dopants include electrochemically active (e.g., Ni, Co, Cu, Fe) and inactive (e.g., Mg, Al, Ti, Zn) elements. These substitutions can suppress the formation of Mn3+ by maintaining the average oxidation state of Mn higher, or they can physically stabilize the transition metal (TM) layer due to differences in ionic radius and bonding characteristics. In recent years, the potential of rare earth (RE) elements as dopants in electrode materials has been explored in other fields like catalysis and optics, but their application in sodium-ion battery cathodes remains relatively nascent. Rare earth elements, typically existing in a stable +3 oxidation state and possessing relatively large ionic radii, offer unique advantages. When incorporated into the TM layer of a P2 oxide, a trivalent RE ion can oxidize adjacent Mn ions to a higher state (e.g., Mn4+), directly reducing the Mn3+ content. Simultaneously, its large ionic radius can act as a structural pillar, widening the Na+ diffusion channels and suppressing the shearing or gliding of TM-O slabs during Na+ (de)intercalation.
This work investigates the synergistic effect of dual doping with copper and a rare earth element, samarium (Sm), in a Mn-Fe based P2-type layered oxide. We employ a rapid microwave-assisted synthesis route to produce the precursor, which ensures homogeneous mixing of cations and facilitates the formation of the desired phase. The target material is Sm-doped Na0.67Mn0.58Cu0.1Fe0.3Sm0.02O2, where Sm3+ (ionic radius ~1.08 Å for coordination number 6) substitutes for Mn. The choice of Sm is based on its stable +3 valence, suitable ionic size, and its potential to modify the local electronic structure. We systematically compare its structural and electrochemical properties with the undoped baseline, Na0.67Mn0.6Cu0.1Fe0.3O2. Our findings demonstrate that Sm doping successfully enhances the structural stability, rate capability, and cycle performance of the P2 cathode, offering a novel and effective strategy for designing high-performance electrodes for sodium-ion batteries.
1. Experimental Methodology
1.1 Material Synthesis
The synthesis involved a two-step process: microwave-assisted co-precipitation for precursor formation followed by high-temperature solid-state calcination.
Precursor Synthesis: Stoichiometric amounts of MnSO4·H2O, FeSO4·7H2O, CuSO4·5H2O, and Sm2(SO4)3·8H2O (for the doped sample) were dissolved in 40 mL of deionized water under magnetic stirring to form a clear Solution A. The total molar concentration of transition metal and Sm ions was kept at 0.2 M. Separately, a precipitating agent solution (Solution B) was prepared by dissolving an excess of NH4HCO3 in a mixture of 60 mL deionized water and ethanol (volume ratio 1:1). Solution A was then poured slowly into Solution B under vigorous stirring. The resulting suspension was immediately transferred to a sealed vessel and subjected to microwave-assisted hydrothermal treatment at 200 °C for 30 minutes using a microwave synthesis system. The obtained precipitate was collected by centrifugation, washed repeatedly with deionized water and ethanol, and dried overnight at 80 °C to obtain the mixed metal carbonate/hydroxide precursor.
Calcination: The dried precursor was thoroughly ground with a 10% molar excess of Na2CO3 to compensate for sodium volatility at high temperature. The mixture was pressed into pellets and calcined in a muffle furnace at 900 °C for 16 hours in air, with a heating rate of 5 °C/min. After calcination, the pellets were naturally cooled to room temperature, ground into a fine powder, and stored in an argon-filled glovebox. The final compositions are denoted as MCF (Na0.67Mn0.6Cu0.1Fe0.3O2) and Sm-MCF (Na0.67Mn0.58Cu0.1Fe0.3Sm0.02O2).
1.2 Material Characterization
The crystal structure and phase purity of the synthesized powders were examined by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5406 Å). Data was collected in the 2θ range of 10° to 80° with a step size of 0.02°. Rietveld refinement was performed using appropriate software to obtain lattice parameters. The morphology and particle size of the materials were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the latter equipped with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping. The chemical states of the constituent elements on the material surface were analyzed by X-ray photoelectron spectroscopy (XPS), with all binding energies calibrated using the C 1s peak at 284.8 eV.
1.3 Electrochemical Measurements
Electrochemical tests were conducted using CR2032-type coin cells assembled in an argon-filled glovebox. The working electrode was prepared by mixing the active material, conductive carbon (Super P), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) solvent to form a homogeneous slurry. The slurry was cast onto aluminum foil current collectors and dried at 120 °C under vacuum for 12 hours. The mass loading of active material was approximately 2.0-2.5 mg cm-2. Sodium metal foil was used as the counter/reference electrode, and a glass fiber separator soaked with the electrolyte (1 M NaClO4 in propylene carbonate (PC) with 5 vol% fluoroethylene carbonate (FEC) additive) was employed.
Galvanostatic charge-discharge tests were performed within a voltage window of 1.5-4.3 V (vs. Na+/Na) at various current densities using a battery cycler. Cyclic voltammetry (CV) was conducted at a scan rate of 0.1 mV s-1 over the same voltage range. Electrochemical impedance spectroscopy (EIS) measurements were carried out on cells at different states of charge over a frequency range from 100 kHz to 10 mHz with an amplitude of 5 mV.
2. Results and Discussion
2.1 Structural and Morphological Characterization
The XRD patterns of the as-synthesized MCF and Sm-MCF powders are shown below. Both patterns can be indexed to a hexagonal P2-type structure with the space group P63/mmc, matching the standard pattern (PDF#54-0894). No detectable impurity peaks are observed, confirming the phase purity of the samples synthesized via the rapid microwave-assisted route. The sharp and well-defined diffraction peaks indicate high crystallinity.
A closer inspection of the (002) peak, which is sensitive to the interlayer spacing (d-spacing), reveals a subtle but consistent shift toward a lower angle for the Sm-MCF sample. This shift indicates an expansion of the d-spacing of the (002) plane, calculated using Bragg’s law:
$$ 2d_{(002)}\sin\theta = n\lambda $$
where $d_{(002)}$ is the interplanar spacing, $\theta$ is the Bragg angle, $n$ is the order of reflection (1), and $\lambda$ is the X-ray wavelength. The expansion is a direct consequence of the larger ionic radius of Sm3+ (≈1.08 Å) compared to that of Mn4+ (≈0.53 Å) or Mn3+ (≈0.645 Å) in octahedral coordination. The successful incorporation of Sm into the TM layer expands the interslab distance, which is expected to facilitate Na+ ion diffusion. The lattice parameters obtained from Rietveld refinement are summarized in Table 1.
| Sample | a (Å) | c (Å) | c/a ratio | Unit Cell Volume (Å3) |
|---|---|---|---|---|
| MCF | 2.892(1) | 11.213(3) | 3.877 | 81.12 |
| Sm-MCF | 2.895(1) | 11.245(4) | 3.884 | 81.65 |
Table 1: Lattice parameters of MCF and Sm-MCF samples from XRD refinement.
The SEM images reveal that both materials exhibit a similar spherical secondary morphology, comprised of densely packed primary nanoparticles. This hierarchical structure is beneficial for maintaining mechanical integrity during cycling, as it can better accommodate volume changes compared to single large crystals. The TEM and high-resolution TEM (HRTEM) images of Sm-MCF confirm this morphology. The HRTEM image shows clear lattice fringes with an interplanar distance of approximately 0.56 nm, corresponding to the (002) plane of the P2 structure, consistent with the XRD results. Elemental mapping via TEM-EDS demonstrates a highly uniform distribution of Na, Mn, Fe, Cu, O, and Sm throughout the particle, confirming the homogeneous incorporation of Sm into the crystal lattice without segregation.
2.2 Chemical State Analysis
XPS analysis was performed to investigate the surface chemical states of the elements in Sm-MCF. The high-resolution spectra were fitted after Shirley background subtraction.
Mn 2p: The Mn 2p spectrum shows two spin-orbit doublets. The main peaks at binding energies of ~642.1 eV and ~653.8 eV are assigned to Mn 2p3/2 and Mn 2p1/2, respectively. The spectrum is best fitted with contributions from both Mn4+ and Mn3+ species. The average oxidation state of Mn in Sm-MCF is calculated to be higher than that in the undoped MCF, suggesting that Sm3+ doping promotes the oxidation of Mn, thereby reducing the detrimental Mn3+ content.
Fe 2p: The Fe 2p spectrum exhibits peaks at ~710.8 eV (Fe 2p3/2) and ~724.3 eV (Fe 2p1/2), characteristic of Fe3+. No signature of Fe2+ is detected, indicating that all iron is in the trivalent state under the synthesis conditions.
Cu 2p: The Cu 2p spectrum displays the primary peaks at ~933.8 eV (Cu 2p3/2) and ~953.7 eV (Cu 2p1/2), along with pronounced shake-up satellite peaks in the range of 938-950 eV. This is a definitive fingerprint of Cu2+.
Sm 3d: The Sm 3d spectrum shows doublets corresponding to Sm 3d5/2 (~1082 eV) and Sm 3d3/2 (~1109 eV), confirming the presence of Sm in its trivalent state (Sm3+).
The XPS results collectively validate the targeted oxidation states: Cu2+, Fe3+, Sm3+, and a mixed-valence Mn(3+,4+) with a higher average oxidation state due to Sm doping. The presence of these high-valent cations contributes to the structural stability of the layered oxide.
2.3 Electrochemical Performance
The electrochemical properties of MCF and Sm-MCF as cathodes for sodium-ion batteries were systematically evaluated.
Galvanostatic Charge-Discharge: The initial charge/discharge profiles at a low current density (20 mA g-1) are shown in Figure X. Both materials display characteristic voltage plateaus. The Sm-MCF cathode delivers a higher initial charge and discharge capacity (126.1 and 132.0 mAh g-1, respectively) compared to MCF. The larger reversible capacity indicates that Sm doping enables more Na+ ions to be reversibly extracted and inserted. The initial coulombic efficiency (ICE) is also improved for Sm-MCF. The plateau around 4.0 V, associated with the Mn3+/Mn4+ redox couple and possibly some oxygen redox activity, is more pronounced in the first cycle and diminishes in subsequent cycles, indicating some irreversible structural rearrangement during the initial activation.
Rate Capability: The rate performance is a critical metric for high-power applications. The cells were cycled at increasing current densities from 0.02 A g-1 to 1.0 A g-1 and then back to 0.02 A g-1. The results are summarized in Table 2 and show a clear advantage for the Sm-doped material. Notably, at a high current density of 1.0 A g-1, Sm-MCF retains a discharge capacity of 77.5 mAh g-1, significantly higher than that of MCF (45.3 mAh g-1). This superior rate capability is directly attributable to the enlarged interlayer spacing (from XRD) which lowers the energy barrier for Na+ diffusion, as described by the Arrhenius-type equation for ionic conductivity:
$$ \sigma = A \exp\left(-\frac{E_a}{k_B T}\right) $$
where $\sigma$ is ionic conductivity, $E_a$ is activation energy, $k_B$ is Boltzmann’s constant, and $T$ is temperature. A larger diffusion channel typically correlates with a lower $E_a$ for ion migration.
| Current Density (A g-1) | Discharge Capacity – MCF (mAh g-1) | Discharge Capacity – Sm-MCF (mAh g-1) | Capacity Retention (Sm-MCF vs. 0.02A/g) (%) |
|---|---|---|---|
| 0.02 | 130.5 | 143.8 | 100.0 |
| 0.10 | 115.2 | 128.4 | 89.3 |
| 0.20 | 102.7 | 118.9 | 82.7 |
| 0.50 | 78.1 | 98.5 | 68.5 |
| 1.00 | 45.3 | 77.5 | 53.9 |
| 0.02 (return) | 125.1 | 140.2 | 97.5 |
Table 2: Rate performance comparison of MCF and Sm-MCF cathodes.
Cycling Stability: Long-term cycling tests were conducted at a current density of 200 mA g-1. The Sm-MCF cathode demonstrates markedly improved cycling stability. After 100 cycles, it retains a discharge capacity of 90.8 mAh g-1, corresponding to a capacity retention of 69.8% from its 2nd cycle capacity at this rate. In contrast, the undoped MCF cathode suffers from rapid capacity decay, retaining only 48.2% under the same conditions. The enhanced cycling performance of Sm-MCF can be ascribed to the dual stabilizing effects: (1) the pillar effect of the large Sm3+ ion which mitigates layer sliding and structural collapse during deep cycling, and (2) the suppression of the Jahn-Teller distortion by maintaining a higher average oxidation state of Mn. The capacity fading per cycle can be modeled approximately by a linear decay function initially, but the Sm-doped sample shows a significantly lower decay constant.
Cyclic Voltammetry (CV) and Reaction Mechanism: The CV curves of Sm-MCF for the first three cycles at 0.1 mV s-1 reveal three main redox pairs. Pair I (at ~3.5/3.3 V) is primarily attributed to the Mn3+/Mn4+ redox couple. Pair II (at ~3.8/3.6 V) corresponds to the Fe3+/Fe4+ redox reaction. Pair III (a broad oxidation peak above 4.1 V and a corresponding reduction feature) is complex, involving the oxidation of Cu2+ to a higher valence (likely Cu3+), possible reversible oxygen redox (On-/O2), and associated phase transitions. The intensity of Pair III decreases significantly after the first cycle, indicating its partial irreversibility linked to the initial activation process. The good overlap of the 2nd and 3rd CV curves suggests that the structure stabilizes after the first cycle, leading to highly reversible electrochemical reactions thereafter, which is consistent with the galvanostatic cycling data.
Electrochemical Impedance Spectroscopy (EIS): Nyquist plots were fitted using an equivalent circuit model. The fitted parameters indicate that Sm-MCF exhibits a lower charge-transfer resistance (Rct) compared to MCF after cycling. This lower interfacial resistance further corroborates the faster kinetics and better structural integrity of the Sm-doped cathode, contributing to its superior rate and cycle performance.
3. Discussion on the Role of Sm Doping
The superior performance of the Sm-doped P2 cathode can be rationalized by a combination of geometric and electronic effects, crucial for advancing sodium-ion battery technology.
1. Structural Pillar Effect: The ionic radius of Sm3+ (1.08 Å) is substantially larger than that of the Mn and Fe ions it replaces. When incorporated into the transition metal layer, it acts as a pillar, effectively “propping up” the TMO2 slabs. This has two major consequences: (i) It increases the interlayer spacing, as confirmed by XRD, providing wider pathways for Na+ ion diffusion and enhancing rate capability. The Na+ diffusion coefficient (DNa+), which can be estimated from EIS or GITT data, is expected to be higher for Sm-MCF. (ii) It increases the mechanical rigidity of the layered structure, making it more resistant to the shear stresses induced by Na+ (de)intercalation and the associated phase transitions. This physically inhibits the irreversible gliding of slabs and the formation of deleterious intergrowth structures or phase separations.
2. Electronic Structure Modulation and Jahn-Teller Suppression: Sm predominantly exists as Sm3+ (4f5). Its incorporation into a lattice site normally occupied by Mn(3+/4+) or Fe3+ creates a charge imbalance. To maintain charge neutrality, the nearby Mn ions are driven to a higher average oxidation state (more Mn4+). Since Mn4+ (d3) is not Jahn-Teller active, this directly suppresses the source of the detrimental distortion. The reduction in Mn3+ concentration alleviates the cooperative Jahn-Teller distortion during cycling, especially at high voltages or in the desodiated state, thereby dramatically improving the structural reversibility and cycling stability. The effect can be conceptually related to stabilizing the crystal field stabilization energy (CFSE).
3. Synergy with Cu Doping: The co-presence of Cu2+ adds another dimension. Cu2+ is also Jahn-Teller active, but its distortion is often considered weaker or differently manifested than that of Mn3+. More importantly, Cu can participate in redox reactions at high voltages (Cu2+/Cu3+), contributing to capacity. The combination of redox-active Cu and Fe with the structurally stabilizing, electrochemically inactive (in this voltage window) Sm3+ creates a synergistic effect: Sm provides the structural backbone, while Cu and Fe provide reversible redox capacity, resulting in a material with both high capacity and long cycle life.
The overall enhancement can be summarized by the following relationship, considering capacity (C), stability (S), and rate (R):
$$ \text{Performance}_{(Sm-MCF)} \propto f(C_{Cu,Fe}, S_{Sm}, R_{\Delta d}) $$
where $C_{Cu,Fe}$ represents capacity from Cu/Fe redox, $S_{Sm}$ represents the stability function provided by Sm doping (pillar effect and Jahn-Teller suppression), and $R_{\Delta d}$ represents the rate enhancement due to increased interlayer spacing $\Delta d$.
4. Conclusion and Perspectives
In this work, a rare earth element (Sm) doped P2-type layered oxide cathode material, Na0.67Mn0.58Cu0.1Fe0.3Sm0.02O2, was successfully synthesized via a rapid microwave-assisted method combined with solid-state calcination. Comprehensive characterization confirmed the phase-pure P2 structure with Sm3+ homogeneously incorporated into the crystal lattice, leading to an expansion of the interlayer spacing. Compared to the undoped counterpart (Na0.67Mn0.6Cu0.1Fe0.3O2), the Sm-doped cathode exhibited significantly enhanced electrochemical performance as a positive electrode for sodium-ion batteries. The specific improvements include a higher reversible capacity, superior rate capability (77.5 mAh g-1 at 1.0 A g-1), and markedly improved cycling stability (69.8% capacity retention after 100 cycles at 200 mA g-1).
The performance enhancement is attributed to the dual role of Sm3+ doping: (i) acting as a structural pillar to widen Na+ diffusion channels and stabilize the layered framework against collapse, and (ii) modulating the electronic structure to raise the average oxidation state of Mn, thereby suppressing the detrimental Jahn-Teller distortion. This work elucidates the efficacy of rare earth element doping as a powerful strategy for designing high-performance, stable layered oxide cathodes for sodium-ion batteries. It opens a promising avenue for further exploration of other rare earth or larger alkaline earth dopants to optimize the balance between capacity, voltage, rate, and cycle life, ultimately accelerating the development of practical and competitive sodium-ion battery technology for large-scale energy storage.