Supramolecular‐Derived Heterostructured Carbon Supported with Mo Single Atoms for High‐Capacity and Long‐Cycle Sodium Dual‐Ion Batteries

ABSTRACT

Owing to their inherent advantages of high operating voltage, low cost, and environmental friendliness, all‐carbon sodium dual‐ion batteries (SDIBs) are highly competitive in the post‐lithium era and hold great promise for large‐scale energy storage. The long‐range ordered graphitized carbon and amorphous non‐graphitized carbon exhibit distinct Na⁺ storage behavior due to their fundamentally different structural configurations. While the diverse structures and physicochemical properties of supramolecular precursors offer a novel approach for the construction of heterostructured carbon with superior performance, which can integrate the respective advantages of graphitized and non‐graphitized carbon to achieve multifunctionality. Moreover, single atoms supported on the carbon matrix can further enhance the kinetic properties, promote ion/electron transport, and accelerate the redox conversion. Herein, supramolecular‐derived nitrogen‐doped heterostructured carbon (Mo‐NHC) supported with Mo single atoms is synthesized and demonstrates a breakthrough achievement for carbonaceous materials in Na⁺ storage. Proof‐of‐concept SDIBs exhibit a high discharge capacity of 222.3 mA h g⁻¹ with a retention of 80.4% after 10,000 cycles. Even under high mass loading and high‐temperature conditions, it still achieves extraordinary reversible capacity and cyclic stability. This work represents one of the best performances among all‐carbon SDIBs, providing an advanced material paradigm for heterostructured carbon in electrochemical energy storage.

During the charging process, TFSI⁻ and Na⁺ from the electrolyte migrate toward the GP cathode and Mo‐NHC anode, respectively, and participate in electrochemical reactions. Conversely, during the discharging process, TFSI⁻ and Na⁺ are reversibly desertion from cathode and anode, respectively, and return to the electrolyte. Specifically, during the sodiation process, the Mo single‐atom catalysts and N‐dopants in Mo‐NHC can effectively anchor Na⁺ and catalyze their redox conversion, while the heterojunction interface accelerates the transport and reaction kinetics of Na⁺.

1. Introduction

With the increasing global energy demand and the growing severity of environmental issues, it is imperative to develop efficient and sustainable electrochemical energy storage devices. Lithium‐ion batteries (LIBs) are widely applied in portable electronic devices and electric vehicles due to their high energy density, long cyclic life, and non‐memory effect, occupying the major share of the commercial market [1, 2, 3, 4]. However, the high cost, low working voltage, and poor safety of LIBs make it difficult to achieve grid‐scale applications. With similar physicochemical properties and reaction mechanisms to Li⁺, coupled with low reduction potential and abundant resources, sodium‐ion batteries (SIBs) have garnered extensive attention in recent years [5, 6, 7]. Moreover, thanks to the unique dual‐intercalation mechanism, dual‐ion batteries (DIBs) exhibit inherent advantages such as low cost, high operating voltage, and environmental friendliness, which have attracted significant research interest and driven rapid development [8, 9, 10]. In this context, by combining the advantages of both, sodium dual‐ion batteries (SDIBs) have emerged with promising application prospects are poised to become a powerful alternative to lithium energy storage systems [11, 12, 13]. However, the relatively large ionic size of Na⁺ (1.02 Å) tends to induce notable volume expansion and sluggish reaction kinetics, resulting in poor cycling performance and low theoretical capacity for electrode materials. Therefore, developing anode materials with low cost, high reversible capacity, and stable structure is crucial for the advancement of SDIBs.

Carbonaceous materials are considered as the potential candidate anodes for SDIBs due to their high cost‐effectiveness, abundant sources, tunable interlayer spacing, and environmental friendliness [14, 15, 16]. Specifically, graphitized carbon exhibits superior electrical conductivity, and the long‐range ordered nanographene domains in their structure are conducive to promoting the uniform and rapid transport of Na⁺, thereby improving the rate capability [17, 18]. Nevertheless, the limited active sites, small interlayer spacing, and susceptibility to severe structural degradation during Na⁺ insertion/desertion remain great challenges. Non‐graphitized carbon typically displays high mechanical strength and active specific surface area, as the short‐range amorphous regions within its structure can buffer the pronounced volume strain, thereby demonstrating high theoretical capacity and structural stability [19, 20]. Unfortunately, the kinetics of non‐graphitizable carbon is relatively slow, with a low initial Coulombic efficiency and poor rate performance. Therefore, exploring innovative high‐performance carbon electrodes that simultaneously leverage the strengths of the above two distinct carbon materials is crucial for accelerating the practical application of dual‐carbon‐based sodium dual‐ion batteries. The diverse structural and chemical properties of carbon material precursors make it possible to design robust heterostructured carbons that integrate the aforementioned advantages and exhibit enhanced electrochemical performance. Notably, hydrogen‐bonded organic frameworks (HOFs) and metal‐organic frameworks (MOFs) represent a special type of crystalline porous supramolecular materials characterized by well‐defined structures and flexible functionality, and are considered as potential platforms for developing functional carbonaceous materials [21, 22, 23]. The mild synthesis conditions, tunable structures, and thermodynamic instability at elevated temperatures of these supramolecular materials render them ideal precursors for synthesizing heterostructured carbons that integrate the merits of graphitized and non‐graphitized carbon. Furthermore, by selecting appropriate organic ligands and metal ions according to the target requirements, followed by specific synthesis and pyrolysis conditions, corresponding heteroelement doping can be introduced and functional metal single atoms can be successfully loaded onto the carbon matrix, which is highly intriguing and creative [24, 25, 26]. However, no such research has been reported in the field of SDIBs.

Herein, through a sequence of solvothermal synthesis, freeze‐drying, and high‐temperature pyrolysis, a kind of supramolecular‐derived nitrogen‐doped heterostructured carbon (Mo‐NHC) anode supported with Mo single atom was successfully synthesized at the nanoscale with outstanding electrochemical performance under both high mass loading and temperature. The Mo‐NHC with nano‐flower‐like morphology exhibits a hierarchical porous structure integrating graphitized carbon with long‐range ordered graphitic domains and non‐graphitized carbon with short‐range amorphous regions. This configuration achieves high reversible capacity and superior rate capability while imparting excellent structural and thermal stability. The functional nitrogen doping improves the intrinsic conductivity, adsorption capability, structural stability, and provides additional active sites, while the introduction of Mo single atoms further enhances the reaction kinetics of heterostructured carbon, promotes the ion/electron transport, catalyzes the redox conversion, and accelerates the electrochemical reactions. Moreover, by combining in situ characterization with theoretical calculations during the electrochemical reaction process, the sodium storage mechanism of Mo‐NHC at the heterostructured interface and the action mechanism of Mo single atom were explored from an atomic level perspective, and the structural evolution patterns of Mo‐NHC and its internal structure‐activity relationship were analyzed. This work first demonstrates the core competitiveness of nitrogen‐doped heterostructured carbon supported with Mo single atoms in SDIBs and presents an advanced electrochemical energy storage paradigm, highlighting its exceptional practicality under high mass loading and elevated temperatures.

2. Results and Discussion

2.1. Materials Synthesis and Theoretical Prediction

The nitrogen‐doped heterostructure carbon supported with single Mo atoms (Mo‐NHC) was synthesized following the technical route depicted in Figure 1a. Detailed synthesis procedures and conditions are available in the Supporting Information. First, a Mo‐doped metal‐organic framework (Mo‐ZIF‐8) was synthesized based on the coordination reaction between metal ions and organic ligands. Subsequently, a hydrogen‐bonded organic framework (HOF) was self‐assembled onto the surface of Mo‐ZIF‐8 via the solvothermal method and intermolecular hydrogen bonding, followed by freeze‐drying to prepare a type of unique MOF@HOF supramolecular heterojunction precursor. Finally, the supramolecular precursor was carbonized at 800°C for 2 h to obtain the target product as Mo‐NHC. To facilitate comparative analysis, Mo doping was not introduced during the aforementioned synthesis process, and the target product obtained after pyrolysis was designated as NHC. Similarly, the target product collected without introducing HOF was designated as Mo‐NC. That is, the nitrogen‐doped heterostructured carbon framework of NHC lacks Mo single atoms, whereas the structure of Mo–NC, although supported with Mo single atoms, does not possess the heterostructured carbon framework. Conversely, Mo‐NHC integrates long‐range ordered graphitized carbon with short‐range disordered non‐graphitized carbon and various types of nitrogen doping, with Mo atoms uniformly distributed across the heterostructure carbon matrix. The ordered graphitized nanodomains exhibit excellent conductivity, which facilitates the rapid transport of Na⁺ [27, 28], while the disordered non‐graphitized carbon demonstrates high mechanical hardness, with its amorphous regions capable of buffering the volume expansion triggered by the insertion of Na⁺ [29]. The introduction of nitrogen doping can enhance the intrinsic conductivity, Na⁺ binding energy, structural stability and yield abundant defects and edge active sites [30]. Meanwhile, the incorporation of Mo single atoms further strengthens the kinetic and thermodynamic stability of the heterostructured carbon, creating a more favorable environment for catalyzing Na⁺ redox conversion and accelerating electrochemical reactions [31, 32]. To validate the above analysis, the physicochemical properties and Na⁺ storage behavior of Mo‐NHC, Mo‐NC, and NHC were investigated through density functional theory (DFT) calculations. First, the density of states for these three materials was calculated with an in‐depth investigation into the effects introduced by the Mo single atom. NHC shows a relatively small peak intensity near the Fermi level and lacks a distinct band gap (Figure 1b), indicating that the heterostructure carbon exhibits excellent conductivity, which is beneficial for the fast transport of Na⁺ and electrons [33]. Compared to NHC, Mo‐NC exhibits a relatively higher peak intensity, indicating that the introduction of Mo single atoms can enhance conductivity. Notably, the Fermi level of Mo‐NHC shifts sharply upward with the highest intensity, suggesting that the synergistic interaction between heterostructured carbon and Mo single atoms can further improve conductivity and reaction kinetics.

FIGURE 1.

The synthesis and theoretical guidance of Mo‐NHC. (a) Schematic diagram of Mo‐NHC synthesis. (b) Comparison of density of states for Mo‐NHC, Mo‐NC, and NHC. (c) The charge density difference between graphitized carbon and non‐graphitized carbon. (d) Schematic diagram of the band structure before and after contact between graphitized carbon and non‐graphitized carbon. (e) The adsorption energy of Na⁺ in different systems. (f) Diffusion energy barriers of Na⁺ in Mo‐NHC, Mo‐NC, and NHC structures.

The difference between intrinsic Fermi levels serves as the driving force for interfacial electron transfer. Therefore, the charge density difference between ordered graphitized regions and disordered non‐graphitized regions within the heterostructured carbon was analyzed from an atom‐level perspective. As evidenced by Figures 1c and S1, when the long‐range ordered graphitized carbon comes into contact with the defective amorphous carbon, the charge density integral in the down layer (the non‐graphitized carbon region) exhibits a positive value, representing electron aggregation [34]. Conversely, the charge density integral in the up layer (the graphitized carbon region) reveals a negative value, reflecting electron depletion. As a result, the charge will be transferred from the up layer to the down layer. According to the Bader charge analysis, 0.54 electrons are transferred from the graphitized region to the non‐graphitized region, resulting in the formation of numerous holes within the graphitized carbon structure and the accumulation of substantial electrons within the non‐graphitized carbon structure. This causes the potential of the graphitized carbon region to be higher than that of the non‐graphitized carbon region. Based on the above conclusions, a simplified schematic diagram of the energy band structure between graphitized carbon and non‐graphitized carbon before and after contact is depicted (Figure 1c). When the heterostructured carbon is created, due to the absence of electrons, an electron depletion zone is formed on the side of graphitized carbon, while on the non‐graphitized carbon side, an electron hole depletion zone is formed due to the accumulation of electrons. Consequently, it can be concluded that an intrinsic electric field oriented from graphitized carbon to non‐graphitized carbon will spontaneously establish at the heterostructured interface, thereby promoting the aggregation and directional diffusion of ions and electrons [35, 36]. Based on the outstanding advantages of the heterostructured carbon supported Mo single atoms, the adsorption energy of Na⁺ in the Mo‐NHC structure was further calculated. Figure S2 displays the optimized models for different systems before and after Na⁺ adsorption, with the corresponding calculated adsorption energies (absolute value) are shown in Figure 1e. The Na⁺ adsorption energy in the pure graphene model without any modification is −1.18 eV. When only nitrogen doping exists, the adsorption energy under this model increases to −1.4 eV, indicating that the nitrogen doping effect is beneficial to anchoring Na⁺ and promoting its intercalation and diffusion. When only Mo single atoms exist, the adsorption energy in this model increases to −1.53 eV, demonstrating that the introduction of Mo single atoms is more effective than nitrogen doping in improving Na⁺ diffusion and reaction kinetics. To validate the above conclusions, the diffusion energy barriers of Na⁺ in Mo‐NHC, Mo‐NC, and NHC structures were investigated. Figure 1f illustrates the migration energy between equilibrium sites for Na⁺ in the three materials, calculated based on the selected optimal diffusion pathways. Compared to the maximum diffusion energy barriers of 0.494 eV and 0.413 eV under the Mo‐NC and NHC systems, respectively, the maximum diffusion energy barrier under the Mo‐NHC system decreased to 0.306 eV. This corroborates the aforementioned analysis, demonstrating that Mo‐NHC indeed exhibits optimal Na⁺ transport kinetics.

2.2. Physicochemical Characterization of Mo‐NHC

Comprehensive characterization analysis of the synthesized Mo‐NHC was conducted using multiple advanced techniques. First, the morphology and structure of the material were thoroughly characterized by the combined utilization of field emission scanning electron microscopy (FSEM) and transmission electron microscopy (TEM). Figures S3 and 2a display the overall morphology at different scales, revealing that Mo‐NHC exhibits a hydrangea‐like structure composed of irregularly arranged nanosheets. Atomic force microscopy (AFM) characterization confirmed that the thickness of an individual nanosheet is ∼4 nm (Figure S4). SEM–energy‐dispersive X‐ray spectroscopy (EDS) color mapping reveals a homogeneous distribution of C, N, O, and Mo elements (Figure 2b). Inductively coupled plasma optical emission spectrometry (ICP‐OES) indicates that the Mo content is ∼1.02 wt%. TEM images and corresponding EDX‐mapping further confirm the microscale hydrangea‐like morphology of Mo‐NHC, which facilitates the uniform loading of Mo single atoms and nitrogen dopants while accelerating the diffusion of active ions and electrolytes (Figures 2c and S5). Remarkably, high‐resolution transmission electron microscopy (HRTEM) reveals the unique heterostructure of Mo‐NHC, consisting of long‐range ordered graphitized nanodomains and short‐range disordered non‐graphitized regions (Figure 2d). The interlayer spacing of graphitized carbon is 0.34 nm, while the non‐graphitized carbon exhibits a larger spacing of 0.41 nm. This is highly desirable, as the long‐range ordered graphitized nanodomains facilitate the directional, uniform and rapid transport of active ions, whereas the short‐range disordered non‐graphitized regions provide sufficient insertion and adsorption sites for active ion storage, and can effectively suppress the volume expansion caused during the insertion process [31, 33, 37]. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) clearly reveals that individual Mo atoms are finely dispersed on the heterostructured carbon substrate (Figure 2e), indicating the successful formation of Mo‐single‐atom catalysts (Mo‐SACs) within both nitrogen‐doped graphitized carbon and non‐graphitized carbon networks.

FIGURE 2.

Physicochemical characterization of Mo‐NHC. (a) SEM image of Mo‐NHC and (b) corresponding EDX mapping. (c) TEM and (d) HRTEM images of Mo‐NHC. (e) HAADF‐STEM image of Mo‐NHC. (f) XRD and (g) Raman spectrum of Mo‐NHC. High‐resolution (h) N 1s and (i) Mo 2d spectra of Mo‐NHC. (j) XANES spectra, (k) FT‐EXAFS curves, (l) r space EXAFS fitting curve, and (m) k²‐weighted WT‐EXAFS fitting curve of Mo‐NHC and reference samples at Mo K‐edge. (n) The N₂ adsorption/desorption isotherm and pore size distribution diagram of Mo‐NHC. (o) The EPR spectra of Mo‐NHC, Mo‐NC, and NHC.

The crystalline nature and phase analysis of Mo‐NHC, Mo‐NC, and NHC were determined via X‐ray diffraction patterns. The characteristic peak at 25.2° corresponds to the (002) crystal plane of carbon (Figure 2f), which displays a broad full width at half maximum (FWHM), indicating a high degree of disorder attributed to defects introduced by nitrogen doping. Furthermore, characteristic peaks attributed to β‐Mo₂C and α‐MoO₃ are detected in Mo‐NHC and Mo‐NC, respectively, indicating that Mo single atoms exist in distinct forms and are stabilized via covalent coordination or ionic interactions with adjacent C and O atoms. Raman spectra show characteristic peaks at 1345 cm⁻¹ and 1582 cm⁻¹ (Figure 2g). The D peak at 1342 cm⁻¹ is related to the A₁g breathing mode of carbon atoms associated with structural defects or disorder [38]. While the G peak at 1582 cm⁻¹ corresponds to the E₂g vibrational mode of sp²‐bonded carbon atoms in a two‐dimensional hexagonal lattice, representing the in‐plane C–C stretching vibration of carbon. The I _D/I _G ratios of Mo‐NHC, Mo‐NC, and NHC are 1.004, 1.000, and 0.999, respectively, indicating a high degree of disorder consistent with the XRD results. This is mainly attributed to the formation of numerous defect structures in the non‐graphitized carbon regions within the heterostructured carbon after pyrolysis, as well as the abundant edge defects introduced by Mo single atoms and N‐dopants, which can act as additional adsorption active sites for Na⁺ storage. The chemical composition and elemental valence states of Mo‐NHC were characterized by X‐ray photoelectron spectroscopy (XPS). The XPS survey spectrum in Figure S6 shows characteristic peaks of C 1s, N 1s, O 1s, and Mo 3d, confirming the presence of C, N, O, and Mo elements with a high N content of 12.58 wt%. The high‐resolution C 1s, N 1s, O 1s, and Mo 3d spectra were sequentially deconvoluted and fitted. The high‐resolution C 1s spectrum was deconvoluted into three individual peaks (Figure S7), corresponding to C═C/C–C (∼284.8 eV), C–O (∼286.6 eV), and C═O (∼288.7 eV), respectively [39]. Similarly, the deconvolution of the high‐resolution O 1s XPS spectrum reveals three oxygen species (Figure S8), assigned to Mo–O (∼530.4 eV), C═O (∼532.1 eV), and C–O (∼533.5 eV), respectively, which originate from the pyrolysis products of oxygen‐containing ligands in the supramolecular precursor at high temperatures [40]. The presence of the Mo–O peak confirms the occurrence of coordination between Mo and O, indicating that most individual Mo atom sites are directly bonded to O sites as evidenced by XRD [41]. The deconvoluted N 1s spectrum displays five categories of dopant species (Figure 2h), namely graphitized N (N–Q, 401.9 eV), oxidized N (N–O, 400.1 eV), pyridinic N (N‐6, 399.2 eV), pyrrolic N (N‐5, 397.5 eV), and Mo–N (396.4 eV), with corresponding proportions of 21.7%, 7.9%, 28.1%, 32.3%, and 10%, respectively [41, 42]. Generally, N‐dopants can regulate the electronic configuration to promote the adsorption and reaction kinetics of materials [43, 44]. As graphitic N (N‐Q) and oxidized N (N‐O) can enhance the electrical conductivity and structural stability of Mo‐NHC, facilitating the transfer of Na⁺ and electrons. The proportion of edge nitrogen doping (N5 and N6) reaches as high as 60.4%, which is expected to generate abundant defects and edge active sites, thus enhancing the electronegativity and Na⁺ adsorption capability of Mo‐NHC [45]. Moreover, edge nitrogen can directly bind with single metal atoms, thereby boosting the overall electrochemical performance [46]. The formation of Mo–N bonds detected herein provides strong evidence for the existence of single Mo atom sites. The deconvoluted Mo 2d spectrum in Figure 2i reveals three distinct Mo species corresponding to Mo⁶⁺ (235.6/231.7 eV), Mo⁴⁺ (233.4/230 eV), and Mo^δ+ (232.6/228.6 eV). The coexistence of Mo⁴⁺ and Mo⁶⁺ also indicates the formation of atomic‐scale Mo sites.

The local coordination environment of Mo single atoms plays a crucial role in catalyzing redox conversion and accelerating electrochemical reaction kinetics. Therefore, synchrotron‐radiation‐based X‐ray absorption fine structure (XAFS) measurements were conducted to elucidate the coordination environment of Mo species in Mo‐NHC, where the oxidation state of Mo can be determined by the absorption edge position of the Mo K‐edge [47]. In the X‐ray absorption near‐edge structure (XANES) spectra of Mo‐NHC and the corresponding reference samples, the absorption edge energy of Mo‐NHC is higher than that of Mo foil and lies between MoO₂ and MoO₃ (Figure 2j), indicating that the oxidation state of Mo in Mo‐NHC is between these two references. This confirms that Mo exists in +4 and +6 oxidation states, consistent with Figure 2i. Moreover, the pre‐edge observed at 20006.7 eV corresponds to an electron transition from 1s to 4d, implying that Mo⁶⁺ adopts a distorted octahedral geometry. Fourier‐transformed R‐space spectra in Figures 2k and S9 further confirm the difference in local coordination geometry, where no obvious Mo–Mo scattering is detected for Mo‐NHC at ∼2.58 Å, verifying the absence of metallic Mo [47, 48]. The quantitative chemical composition of Mo in Mo‐NHC was obtained through curve fitting of Fourier Transform X‐ray Absorption Fine Structure (FT‐EXAFS) data. Compared with Mo foil, MoO₂, and MoO₃ (Figures S10–13), the FT‐EXAFS spectrum of Mo‐NHC exhibits two characteristic peaks at 1.23 Å and 2.01 Å, indicating the presence of atomically dispersed Mo species [49], corresponding to Mo–O and Mo–C coordination (Figure 2l), with average bond lengths of 1.77 Å and 2.52 Å, respectively. Wavelet‐transformed (WT‐EXAFS) spectra enable the detection of high‐resolution atomic configurations in both R‐space and k‐space. As illustrated in Figure 2m, the formation of Mo–O and Mo–C bonds is only observed near ≈6.1 Å⁻¹ (≈1.3 Å) and ≈5.4 Å⁻¹ (≈1.9 Å), with no Mo–Mo signal peak detected. This aligns with the aforementioned conclusion, indicating the presence of single Mo atom sites. Based on the above analysis, the successful formation of Mo single‐atom sites is demonstrated, which are stably anchored on the Mo‐NHC heterostructured carbon substrate via the form of Mo–O and Mo–C bonds. N₂ adsorption–desorption isotherms were employed to determine the specific surface area and pore size distribution of Mo‐NHC. The results demonstrate that Mo‐NHC exhibits a typical type I adsorption isotherm, suggesting a high proportion of micropores. Its specific surface area is as high as 1069.9 m² g⁻¹ (Figure 2n), which greatly promotes electrolyte penetration and active ion diffusion, and fully exposes active sites as well as Mo single‐atom sites favorable for electrochemical redox reactions. Furthermore, the pore size distribution reveals that Mo‐NHC possesses abundant microporous and mesoporous structures, with a corresponding pore volume ratio of up to 97.4% (Figure S14). This is expected to facilitate electron/ion transport, provide more active ion storage sites, and buffer severe volume expansion to improve structural stability. Electron paramagnetic resonance (EPR) spectroscopy was further employed to investigate the effect of N‐dopants and Mo single‐atom sites on the electronic configuration of Mo‐NHC. Mo‐NHC, Mo‐NC, and NHC all exhibit strong EPR signals near g = 2.0024. Mo‐NHC features the strongest peak intensity and the narrowest FWHM (Figure 2o), implying the highest electronegativity and more localized unpaired electrons, which is ascribed to the synergistic effect between Mo single atoms and N‐doped heterostructured carbon [49]. Compared with NHC, the higher peak intensity and narrower FWHM of Mo‐NC confirm that Mo‐SACs exert a more significant effect on the electronic configuration and electron delocalization than N‐dopants.

2.3. Investigation of Na⁺ Storage Behavior and Reaction Kinetics

An in‐depth investigation was conducted to explore the influence of Mo‐NHC's unique morphology and heterostructure, physicochemical properties, and the synergistic interaction between N‐dopants and Mo single‐atom sites on Na⁺ storage performance and electrochemical reaction kinetics. To evaluate the electrochemical reaction kinetics of the host materials and their catalytic ability to accelerate Na⁺ redox reactions, symmetric batteries based on Mo‐NHC, Mo‐NC, and NHC electrodes were assembled and cyclic voltammetry (CV) tests were conducted within the voltage range from −0.5 to 0.5 V. Figure 3a displays the typical centrosymmetric rings, reflecting highly reversible electrochemical reactions [50]. Mo‐NHC exhibits a pair of the most pronounced redox peaks at 0.2 V and −0.2 V, along with the highest response current signals. This is attributed to Mo atomic sites acting as single‐atom catalysts, which synergistically catalyze and accelerate the redox conversion of Na⁺ and rapid electron transfer with N‐dopants and heterostructured carbon. Mo‐NC shows a higher peak intensity than NHC, demonstrating that Mo single‐atom catalysts possess stronger catalytic activity than N‐dopants, which is consistent with the conclusion in Figure 2o. The derived Tafel curves reveal that Mo‐NHC not only exhibits the uppermost response current in both anodic and cathodic processes (Figure 3b), but also possesses the highest exchange current density (j, 4.3 mA cm⁻²) compared to Mo‐NC (2.1 mA cm⁻²) and NHC (1.7 mA cm⁻²), revealing its fast charge transfer and outstanding reactivity [51]. This is powerfully demonstrated by the electrochemical impedance spectroscopy (EIS) in Figure S15 and its derived Weber factor (σ), where the Mo‐NHC displays the lowest charge transfer resistance (Rct) and the highest j value derived from Rct in the high‐frequency region (inset of Figure S15a) compared to Mo‐NC and NHC, along with the smallest σ value in the low‐frequency region. According to the Arrhenius equation, Mo‐NHC exhibits the lowest activation energy required for the reaction and optimal Na⁺ and electron transport.

FIGURE 3.

Na⁺ storage performance and electrochemical kinetics of Mo‐NHC. (a) Symmetric CV curves and (b) derived Tafel curves of Mo‐NHC, Mo‐NC and NHC‐based systems. CV curves of the Mo‐NHC‐based system at (c) 0.1 mV s^{− 1} (d) under different scan rates. (e) The proportion of pseudocapacitance of the Mo‐NHC‐based system at different scan rates. (f) Rate performance of Mo‐NHC, Mo‐NC, and NHC‐based systems. (g) A comprehensive comparison of the rate performance of Mo‐NHC‐based systems with reported references. (h) GCD curves of Mo‐NHC‐based systems at different rates. (i) Cycling performance of Mo‐NHC, Mo‐NC, and NHC‐based systems at 2 C. (j) GCD curves of Mo‐NHC‐based systems under different cycles at 2 C. (k) EIS and derived exchange current density of Mo‐NHC, Mo‐NC, and NHC‐based systems after cycling. (l) GITT curves and derived Na⁺ diffusion coefficients of Mo‐NHC‐based systems. (m) Cycling performance of Mo‐NHC, Mo‐NC, and NHC‐based systems at 20 C. (n) A comprehensive comparison of the cycling performance of Mo‐NHC‐based systems with reported references.

Based on the above analysis, a half‐cell system consisting of a Mo‐NHC working electrode and a sodium foil counter electrode was assembled, and corresponding electrochemical performance measurements were conducted within the voltage range of 0.01–3.0 V to thoroughly investigate the Na⁺ storage capability of Mo‐NHC. Figure 3c presents the CV curves at 0.1 mV s⁻¹. During the first cycle, the response current gradually increases and a broad reduction peak emerges at ∼0.43 V, which is ascribed to the decomposition of the electrolyte and the formation of the solid electrolyte interphase (SEI) [52]. This peak disappears in the subsequent cycles, confirming the formation of a stable SEI. Subsequently, the response current increases sharply at ∼0.01 V and a sharp reduction peak appears, indicating that Na⁺ intercalates into the Mo‐NHC electrode and the corresponding reduction reaction of Na⁺ → Na takes place. During the following positive‐scan process, an oxidation peak emerges at ∼0.26 V, signifying the sequential occurrence of the Na → Na⁺ reaction and the deintercalation of Na⁺ from the Mo‐NHC electrode. All CV curves almost overlap except for the first cycle, demonstrating excellent stability and redox kinetics that enable rapid equilibration. The Na⁺ storage behavior and reaction kinetics of Mo‐NHC were analyzed based on the CV curves at different scan rates (0.2–1.0 mV s⁻¹). As illustrated in Figure 3d, the redox peaks gradually intensify and the polarization potential increases with increasing scan rate. Nevertheless, the CV curves retain similar profiles and well‐defined redox peaks, indicating the weak polarization effects and stable electrochemical reaction dynamics of Mo‐NHC. Specifically, the peak current and scan rate follow the relationship:

$$i=a{v}^{\mathrm{b}}$$ (1)

where i is the peak current (mA), v is the scan rate (mV s^{− 1}), and the b‐value can be obtained by fitting the plot of log(i) versus log(v). A value close to 0.5 indicates diffusion‐controlled electrochemical reactions, while a value close to 1.0 indicates pseudocapacitive behavior‐dominated electrochemical reactions [53]. The b values for Mo‐NHC at the oxidation and reduction peaks were 0.85 and 0.62, respectively, implying that the electrochemical reaction process is synergistically governed by diffusion and pseudo‐capacitance. This synergistic effect can be quantified by the following equation:

$$i=k_{1}v+k_2{v}^{0.5}$$ (2)

where k ₁ and k ₂ are constants, and k ₁ v and k ₂ v ^0.5 represent the pseudocapacitive and diffusion contributions, respectively. The calculated pseudo‐capacitive contribution at 0.2 mV s⁻¹ is 55% (Figure S17), and the pseudo‐capacitive ratio increases with increasing scan rate (Figure 3e), reaching 73% at 1 mV s⁻¹, which is expected to bring excellent rate performance. Accordingly, the diffusion coefficient of Na⁺ was calculated using the classic Randles‐Sevcik equation: [42]

$$i=269000CS{D}^{0.5}{v}^{0.5}{n}^{1.5}$$ (3)

where C is the active ion concentration, S is the active surface area of the electrode, D is the diffusion coefficient, and n is the number of transferred electrons. The peak current i is proportional to v ^0.5, and the slope k can be obtained by fitting i versus v ^0.5. Linearly fitting the peak currents to the square root of the scan rate, Figure S18 shows that Mo‐NHC exhibits a higher k‐value at the reduction peak than at the oxidation peak, with corresponding Na⁺ diffusion coefficients of 7.2 × 10⁻⁹ cm² s⁻¹ and 4.6 × 10⁻⁹ cm² s⁻¹, respectively. It indicates that the reaction kinetics during reduction (Na⁺ insertion) are favorable compared to those during oxidation (Na⁺ desertion), which is ascribed to the Na⁺ insertion process is driven by the synergistic effects of diffusion and pseudocapacitive, and the formed SEI accelerates Na⁺ transport.

Overall, the outstanding physicochemical properties and Na⁺ storage performance of Mo‐NHC are expected to deliver exceptional electrochemical performance. Figure 3f showcases the rate performance across different half‐cell systems. All three systems exhibit impressive specific discharge capacities at 0.5 C, reaching up to 477.8 mA h g⁻¹ for Mo‐NHC, 440 mA h g⁻¹ for Mo‐NHC, and 401.4 mA h g⁻¹ for Mo‐NC. Among these, the reversible capacity achieved by the Mo‐NHC‐based system represents one of the best‐reported performances, which is attributed to its unique heterogeneous structure and the synergistic effect between N‐dopants and Mo single‐atom sites. During the electrochemical reaction, this synergistic effect can anchor more Na⁺, promote its more reversible insertion/desertion, and accelerate its redox conversion, thereby ensuring superior cycle stability, discharge capacity, and rate performance. With increasing current density, the discharge capacity gradually decreases, but Mo‐NHC exhibits the smallest capacity fading. It delivers reversible capacities of 419.2, 369.6, 340.8, 313, 286.4, and 255.4 mA h g⁻¹ at 2, 4, 6, 8, 10, and 15 C, respectively. Even at a high rate of 20 C, it still maintains a high discharge capacity of 230.3 mA h g⁻¹ and an average Coulombic efficiency of 99.95%. When the current density returns to 2 C, the discharge capacity recovers and can be stably cycled, demonstrating superior reversibility. Such outstanding rate performance positions Mo‐NHC as highly competitive in both related reports and the electrochemical energy storage field, significantly outperforming most reported sodium‐ion half‐cell systems (Figure 3g). It holds substantial application potential for low‐cost and high‐performance large‐scale energy storage. Figure 3h depicts the galvanostatic charge‐discharging (GCD) curves of the Mo‐NHC‐based system at different rates. As the current density increases, the corresponding GCD curves still exhibit similar profiles and distinct charge‐discharge plateaus, demonstrating the weak polarization effects and stable rate capability of Mo‐NHC. The cycling stability at a specific current density was further investigated, and Figure 3i shows the cycling performance of the three systems at 2 C. The Mo‐NC‐based system delivers an initial specific discharge capacity of 351 mA h g⁻¹ but suffers rapid capacity decay, retaining a reversible capacity of 270.8 mA h g⁻¹ after 900 cycles. While the NHC‐based system delivers an initial specific discharge capacity of 364.2 mA h g⁻¹ with gradual capacity decay during cycling, maintaining a reversible capacity of 305.1 mA h g⁻¹ after 1200 cycles. In contrast, the Mo‐NHC‐based system exhibits a maximum initial specific discharge capacity of 416.3 mA h g⁻¹, and its capacity remains essentially stable after a brief initial decay. Even after 1400 cycles (8 months), it still delivers a high reversible capacity of 403.9 mA h g⁻¹. The corresponding GCD curves at different cycles almost overlap (Figure 3j), verifying the excellent cycling stability and structural stability of Mo‐NHC. Figure 3k displays the EIS for the three systems after cycling. Compared to Mo‐NC (87.1 Ω, 1.9 mA cm⁻²) and NHC (59.5 Ω, 2.5 mA cm⁻²), Mo‐NHC not only exhibits the lowest Rct value of 46.3 Ω and the highest exchange current density of 4.6 mA cm⁻², but also demonstrates the highest Na⁺ diffusion coefficient derived from EIS [54]. These results collectively confirm that Mo‐NHC possesses optimal reactivity and the lowest interfacial impedance, thereby ensuring fast Na⁺ transport. In addition, the diffusion kinetics of Na⁺ in Mo‐NHC, Mo‐NC, and NHC electrodes were systematically evaluated via galvanostatic intermittent titration technique (GITT). Figures 3l and S20 illustrate the corresponding sodation and desodation processes with a 10 min pulse time and a 30 min relaxation time. Analysis reveals that Mo‐NHC demonstrates the minimal IR drop and the highest diffusion coefficient, indicating the fast Na⁺ diffusion kinetics and weak polarization in Mo‐NHC. Inspiring, all three systems exhibit outstanding electrochemical performance even at a high rate of 20 C. The Mo‐NC‐based system delivers an initial capacity of 135.7 mA h g⁻¹, which decays to 83.2 mA h g⁻¹ after 7630 cycles (Figure 3m). In comparison, the NHC‐based system presents an initial capacity of 185 mA h g⁻¹ with superior long‐cycle stability over 10 000 cycles, retaining 139.2 mA h g⁻¹ after 10 730 cycles. The Mo‐NHC‐based system exhibits the most outstanding discharge capacity and ultra‐long cyclic life, achieving a high initial capacity of 241.9 mA h g⁻¹ and extremely slow capacity decay during cycling. Even after 16000 stable cycles, it still delivers a reversible capacity of 205.7 mA h g⁻¹, corresponding to an ultralow capacity fading rate of 0 000935% per cycle. The GCD curves at different cycles essentially overlap and display distinct charge‐discharge plateaus, strongly evidencing the excellent structural stability of Mo‐NHC. To the best of our knowledge, the electrochemical performance achieved by this N‐doped heterostructured carbon supported with Mo single‐atom sites represents one of the best Na⁺ storage performances reported to date, setting a new record for sodium storage in carbon‐based materials (Figure 3n).

2.4. Performance of Proof‐of‐Concept All‐Carbon Sodium Dual‐Ion Batteries

To evaluate the practical Na⁺ storage capacity and electrochemical performance of Mo‐NHC in full batteries, proof‐of‐concept all‐carbon sodium dual‐ion batteries (MoNH//G) were assembled. It consists of a Mo‐NHC anode, a self‐supporting graphite paper cathode, and an electrolyte of 4 M NaTFSI in EMC+DMC+Pyr₁₄TFSI (1:1:1, v:v:v). A series of electrochemical characterizations were then conducted. Figure 4a displays the CV curves of MoNH//G at 0.1 mV s⁻¹ that scanned from the open‐circuit voltage toward the negative potential direction. The reduction peaks at −3.17 V and −4.21 V in the first cycle are attributed to electrolyte decomposition and the formation of solid electrolyte interface, consistent with the description in Figure 3c. While the reduction peak at −4.72 V and the oxidation peak at −4.18 V are attributed to the sequential intercalation and deintercalation of active ions, respectively [55]. Subsequent CV curves essentially overlap, indicating excellent redox stability. To explore the active ion storage mechanism of the electrodes, CV measurements were conducted at various scan rates. All CV curves maintain similar profiles and well‐defined redox peaks (Figure S22), implying stable electrochemical reaction kinetics. Converting the CV curves into contour plots yields a pair of symmetric and smooth redox peaks (Figure 4b), corresponding to the highly reversible redox reactions of Na⁺ and TFSI⁻ [50]. The small potential difference between the redox peaks demonstrates that the Mo single‐atom sites possess efficient catalytic conversion capability throughout the charge/discharge process [56]. As shown in Figure S23, the peak current demonstrates a linear relationship with the square root of the scan rate, where the b‐values at the oxidation and reduction peaks are 0.52 and 0.48, respectively (Figure S24), indicating a classic diffusion‐limited process [57]. As evidenced in Figure 4c that the pseudocapacitive contribution is 31% at 0.2 mV s⁻¹ and increases to 42% at 1 mV s⁻¹ (Figure 4d). The rate performance of MoNH//G was evaluated across a voltage range of 2.5–4.8 V under different current densities. For comparative analysis, full batteries based on Mo‐NC and NHC anodes (MoN//G and NH//G) were also assembled. MoNH//G, MoN//G, and NH//G deliver high specific discharge capacities of 222.3, 208, and 197.3 mA h g⁻¹ at 0.5 C, respectively (Figure 4e). Although the discharge capacity gradually decreases with increasing rate, MoNH//G exhibits the smallest capacity decay. It delivers reversible capacities of 200.5, 182.5, 167.7, 156.7, 139, 123.7, 110.3, 92.6, and 79.3 mA h g⁻¹ at 1, 2, 4, 6, 8, 10, 15, 20, and 25 C, respectively, with a high Coulombic efficiency of 99.68% at 25 C. When the current density returns to 1 C, the discharge capacity recovers to 195.4 mA h g⁻¹, demonstrating superior rate capability and reversibility that outperforms the most reported SDIBs systems (Figure 4f). Figure 4g presents the GCD curves of MoNH//G at different rates, where the potential difference ∆V between charge and discharge plateaus increases with increasing rate. As evidenced by the differential capacitance curves in Figure S25, the oxidation and reduction peaks shift toward higher and lower potentials, respectively, accompanied by a decrease in intensity. Notably, MoNH//G maintains flat charge/discharge plateaus even at high rates, signifying suppressed polarization effects and stable reaction kinetics. The galvanostatic charge–discharge capabilities of MoNH//G, MoN//G, and NH//G were comprehensively evaluated, and Figure 4h presents the corresponding cycling performance of the three systems at 2 C. Among them, MoN//G delivers the lowest capacity of 141.7 mA h g⁻¹ and undergoes relatively severe degradation during subsequent cycles, decreasing to 91.6 mA h g⁻¹ after 700 cycles, with a capacity retention of 64.6%. NH//G delivers a capacity of 161.8 mA h g⁻¹ and decays to 121.9 mA h g⁻¹ after 700 cycles, corresponding to a capacity retention of 75.3%. Impressively, MoNH//G exhibits the highest capacity of 188.1 mA h g⁻¹ and maintains stable cycling after a slight initial decay. After 700 cycles, it still delivers a reversible capacity of 166.6 mA h g⁻¹ with a high‐capacity retention of up to 88.6%. Additionally, Figures 4i and S26 display the GCD and dQ/dV curves of MoNH//G at different cycles, where the charge/discharge plateaus and redox peaks almost overlap, strongly verifying the excellent cycling stability and low electrochemical polarization of MoNH//G. Meanwhile, the median discharge voltage of MoNH//G remains stable at 4.1 V throughout cycling, which is higher than the 3.7 V of LIBs, achieving a high energy density of 258 Wh kg⁻¹. The diffusion kinetics of active ions were further elucidated using a combination of EIS and GITT. The EIS results in Figure 4j reveal that MoNH//G shows the lowest Rs and Rct values along with the highest j value, implying optimal interfacial transport between the electrode and electrolyte, as well as within the electrode structure. Furthermore, MoNH//G also exhibits the lowest σ value, corresponding to the highest Na⁺ diffusion coefficient of 1.5 × 10⁻¹⁰ cm² s⁻¹ (Figure S27). Figure 4k shows the GITT curves of MoNH//G, which exhibit small overpotentials and voltage gaps in the charge/discharge plateau regions, indicating effectively suppressed polarization and enhanced redox reaction kinetics. The corresponding Na⁺ diffusion coefficients range from 0.4×10⁻¹⁰ cm² s⁻¹ to 4.17×10⁻⁹ cm² s⁻¹, which is consistent with the EIS results. Overall, the Na⁺ migration rate during discharging is superior to that during charging, which is attributed to the electrode activation and the formation of a stable SEI that facilitates efficient ion transfer [38]. The low electrochemical polarization, high reaction activity, and fast kinetics enable MoNH//G to exhibit excellent electrochemical performance even at a high rate of 25 C (Figure 4l). MoN//G and NH//G exhibit specific discharge capacities of 43.6 and 61.4 mA h g⁻¹, respectively, with capacities decaying to 28.9 and 46.5 mA h g⁻¹ after 5000 and 6000 cycles, respectively. Conversely, MoNH//G exhibits a high reversible capacity of 93.4 mA h g⁻¹ and shows no capacity decay after 6500 cycles. Even after 10,000 cycles, it still delivers a reversible capacity of 76.2 mA h g⁻¹ with a capacity retention of 80.4%, and the CE remains stable above 99.76%, corresponding to an extremely low‐capacity degradation of 0.00196% per cycle. Such outstanding electrochemical performance represents the best reported among all‐carbon SDIBs to date, providing an advanced material paradigm for the application of single‐atom‐supported heterostructured carbon in electrochemical energy storage. The constructed MoNH//G system demonstrates significant superiority over currently reported sodium‐based dual‐ion battery systems in key performance parameters including cyclic life, specific discharge capacity, rate capability, capacity retention, and energy density (Figure 4m), further validating the tremendous application advantages of Mo‐NHC in SDIBs.

FIGURE 4.

Electrochemical performance of the proof‐of‐concept all‐carbon SDIBs. (a) CV curves of MoNH//G at 0.1 mV s^{− 1}. (b) Contor maps of CV curves for MoNH//G at various scan rates (0.2–1.0 mV s⁻¹). (c) Pseudo‐capacitive contribution of MoNH//G at 0.1 mV s^{− 1}. (d) Pseudo‐capacitive contribution of MoNH//G at different scan rates. (e) Rate performance of MoNH//G, MoN//G, and NH//G at different current densities. (f) Comparison of rate performance between MoNH//G and previously reported SDIBs. (g) GCD curves of MoNH//G at different rates. (h) Cycling performance of MoNH//G, MoN//G, and NH//G at 2 C. (i) GCD curves of MoNH//G at different cycles. (j) EIS of MoNH//G, MoN//G, and NH//G after cycling. (k) GITT curves and corresponding Na⁺ diffusion coefficients of MoNH//G. (l) Cycling performance of MoNH//G, MoN//G, and NH//G at 25 C. (m) Comprehensive performance comparison of MoNH//G with reported SDIBs.

2.5. Research on Interface Evolution and Working Mechanisms

According to the above results and discussion, to explore the working principles and Na⁺ storage mechanisms of the constructed all‐carbon SDIBs system and electrode active materials during electrochemical reactions, the MoNH//G system was selected as the subject and systematically characterized by glow discharge optical emission spectroscopy (GD‐OES), EPR, and XPS techniques. Figure 5a shows the S‐based GD‐OES spectra of GP cathode at the initial, fully charged, and fully discharged states. No signal for sulfur is detected in the initial state, as no sulfur exists in the composition of GP cathode. Significantly, a strong S signal is observed at the fully charged state, which is ascribed to the intercalation of TFSI⁻ anions into the layered structure of GP. The S signal decreases sharply after fully discharged, indicating the reversible deintercalation of TFSI⁻ from the GP structure. The remaining S signal may originate from the residual electrolyte on the electrode surface or the decomposition of partially irreversible adsorbed/intercalated TFSI⁻ to form the cathode‐electrolyte interphase (CEI). Similarly, for the anode, an obvious Na signal is detected at the fully charged state (Figure 5b), which is attributed to the insertion of Na⁺ into the structure of Mo‐NHC. The Na signal significantly decreases after fully discharged, indicating the reversible desertion of Na⁺ from Mo‐NHC anode. The remaining signal may be attributed to the irreversible trapping or decomposition of partial Na⁺ to form the SEI [58]. XPS characterization was further employed to verify the above analysis and investigate the detailed composition of the CEI and SEI films within this system. Figure 5c presents the high‐resolution C 1s spectra of GP cathode at different charge and discharge states. In the initial state, distinct characteristic peaks assigned to C–C, C–O, and C═O are detected. After full charging, two new characteristic peaks belonging to CF₃ and ROCO₂Na emerge. The presence of CF₃ is attributed to the intercalation of TFSI⁻, while the existence of ROC₂Na is ascribed to the formation of CEI [58, 59], which aligns with the aforementioned conclusions. The CF₃ peak disappears after full discharge, confirming the reversible deintercalation of TFSI⁻. In the high‐resolution S 2p spectra, characteristic peaks assigned to TFSI⁻, N–S, and Na₂S appear at the fully charged state (Figure 5d), indicating that TFSI⁻ is not only intercalated into the GP cathode but also decomposed on its surface to form a CEI rich in inorganic components, as confirmed by the characteristic peaks assigned to S–F and Na–F detected in the high‐resolution F 1s spectra after full charging (Figure 5e). Owing to their high mechanical hardness and fast diffusion, the inorganic components containing fluorine and sulfur species serve as ideal components in the CEI layer. Surprisingly, the TFSI⁻‐related peak vanishes after full discharge, while the F‐ and S‐containing peaks constituting the CEI remain, with some peaks even exhibit increased intensity, indicating the formation of a mechanically stable and ion‐conductive CEI. For the Mo‐NHC anode, the high‐resolution C 1s spectrum exhibits distinct characteristic peaks consistent with the GP cathode in the initial state (Figure 5f). After full charging, several peaks attributable to fluorine‐ and sodium‐containing species emerge, indicating the insertion of Na⁺ and the decomposition of electrolyte, as evidenced by the distinct C–F and Na–F peaks in the high‐resolution F 1s spectra (Figure 5g) as well as the clear Na–F and Na–O peaks in the high‐resolution Na 1s spectra (Figure 5h). At the fully discharged state, these peaks still retain high intensity, implying the formation of an inorganic‐rich SEI. These fluorinated species, especially NaF, can significantly enhance the mechanical strength and form a denser interfacial layer, thereby improving the structural stability and ion transport kinetics [58]. Based on the above analysis and combined with the CV and GCD curves, the working mechanism of MoNH//G can be deduced as described in Figure 5i. During the charging process, TFSI⁻ and Na⁺ from the electrolyte migrate toward the GP cathode and Mo‐NHC anode, respectively, and participate in electrochemical reactions. Conversely, during the discharging process, TFSI⁻ and Na⁺ are reversibly desertion from cathode and anode, respectively, and return to the electrolyte. Specifically, during the sodiation process, the Mo single‐atom catalysts and N‐dopants in Mo‐NHC can effectively anchor Na⁺ and catalyze their redox conversion, while the heterojunction interface accelerates the transport and reaction kinetics of Na⁺ (Figure 5j). Furthermore, the fully charged all‐carbon SDIB can easily drive a small fan and light up an LED screen displaying the letters “SCUT” (Figure S28), demonstrating its broad practical application prospects.

FIGURE 5.

Investigation of energy storage mechanisms. (a) S element‐based GD‐OES of GP cathode. (b) Na element‐based GD‐OES of Mo‐NHC anode. The high‐resolution (c) C 1s, (d) S 2p, and (e) F1s spectra of GP cathode in the pristine, fully charged, and fully discharged states. The high‐resolution (f) C 1s, (g) Na 1s, and (h) F1s spectra of Mo‐NHC anode in the pristine, fully charged, and fully discharged states. (i) The working mechanism of the constructed all carbon SDIBs. (j) Schematic illustration of the Mo‐NHC anode during the sodiation processes.

2.6. In Situ Characterization and Structural Evolution

The outstanding electrochemical performance of MoNH//G is closely related to the excellent structural stability of the electrodes. Therefore, in situ physicochemical and electrochemical characterization was performed to monitor the evolution of electrode structure, electrode‐electrolyte interface, and interfacial impedance during charging and discharging processes. Figure 6a depicts the corresponding in situ XRD patterns of GP cathode under various charge‐discharge states. At the initial state, a distinct 002 characteristic peak is observed at 26.5°. As charging proceeds, the (002) peak splits into two individual peaks, which then shift toward lower and higher angles, respectively. After fully charged, the two split peaks shift to 22.6° and 31.1°, respectively, corresponding to the Stage I process of TFSI⁻ intercalation into the GP cathode [53]. This can be corroborated by the Raman spectra in Figure 6e, where the G‐band at 1580 cm⁻¹ gradually shifts to the right during charging accompanied by a decrease in peak intensity, confirming the insertion of TFSI⁻ and the formation of graphite intercalation compounds (GICs). Conversely, the two split peaks gradually merge and return to 26.5° during the discharge process, while the G‐band shifts back to its initial position with restored intensity, indicating the excellent reversibility of the TFSI⁻ intercalation/deintercalation process into/from the GP cathode. For the Mo‐NHC anode, the width of 002 peak gradually narrows with decreased intensity during charging (Figure 6b). Meanwhile, the intensity of the D‐band gradually weakens and the G‐band exhibits a slight leftward shift, which is ascribed to the initial adsorption of Na⁺ on the non‐graphitized carbon surface or defect active sites of Mo‐NHC, followed by intercalation into the graphitic microcrystalline structure of Mo‐NHC. This is evidenced by the EPR spectra in Figure 6c, where the Mo‐NHC electrode in its initial state exhibits a sharp Lorentzian‐shaped characteristic peak due to the abundant carbon‐centered radicals and delocalized electrons [60]. The peak gradually broadens with a significant decrease in intensity during charging, confirming the reduced defect density of Mo‐NHC. This provides strong evidence for the adsorption–intercalation of Na⁺ and the formation of quasi‐metallic sodium clusters. After fully discharged, all characteristic peaks assigned to Mo‐NHC recover to their initial intensities, and the position of 002 peak remains unchanged throughout the process, confirming the reversible desodiation of Na⁺ and the stable structure of Mo‐NHC. The evolution of morphological structure, interface layer and impedance during cycling was thoroughly investigated via SEM, TEM, and in situ EIS. Figure 6d presents the SEM images of GP cathode and Mo‐NHC anode after different cycles. The GP cathode maintains a smooth surface morphology even after 10,000 cycles, with no structural exfoliation observed. The corresponding lattice spacing remains 0.335 nm (Figure 6g), confirming the excellent structural stability of GP cathode. For the Mo‐NHC anode, it maintains an intact spherical morphology during cycling without obvious structural collapse, verifying the structural stability of Mo‐NHC. Furthermore, the surface of Mo‐NHC is relatively rough, which is attributed to the formation of the SEI layer. Consequently, the evolution of the CEI and SEI during cycling was investigated. After 5000 cycles, a thin CEI layer with a thickness of ∼2.2 nm is observed on the surface of GP cathode (Figure 6g). EDS‐mapping in Figure 6o detects the existence of N, F, and S elements, which are derived from the decomposition of TFSI⁻, indicating that the CEI is rich in inorganic components. This not only facilitates the transport of active ions, but also improves the mechanical stability. The thickness of the CEI layer increases to ∼4.1 nm after 8000 cycles (Figure 6j), which is ascribed to the continuous decomposition of electrolyte. For the Mo‐NHC anode, a distinct SEI layer with a thickness of ∼1.6 nm is observed after 5000 cycles (Figure 6k). Even after 8000 cycles, the thickness only increases to ∼1.9 nm (Figure 6l), indicating the formation of a thin and stable SEI layer, which promotes the fast diffusion of Na⁺. In situ EIS spectra also confirm this conclusion, where the charge‐transfer impedance gradually decreases during the charging process (Figure 6m). The corresponding distribution of relaxation times (DRT) reveals that the electrolyte impedance (τ₁) remains almost unchanged after a brief increase [61], the interfacial impedance (τ₂) gradually decreases, and the Rct (τ₃) exhibits a trend of increasing first and then decreasing during the charge–discharge process (Figure 6n), which is consistent with the EIS results. EDS‐mapping also detects abundant F and S elements existing in inorganic species (Figure 6p). The above conclusions and analysis clearly demonstrate the formation of a highly stable SEI layer, and the optimized electrode‐electrolyte interface effectively enhances the transport kinetics of Na⁺. In addition, Figure 6q shows the optical images of the separators for MoNH//G, MoN//G, and NH//G after 8000 cycles. In contrast to the dark yellow separators of MoN//G and NH//G, the separator of MoNH//G is light yellow, proving that the decomposition of electrolyte and corrosion of the separator are suppressed, and the overall cycling stability of the battery is effectively improved.

FIGURE 6.

Structural evolution and in situ characterization. In situ XRD characterization of (a) GP cathode and (b) Mo‐NHC anode during the charging/discharging process. (c) EPR characterization of Mo‐NHC anode during the charging/discharging process. (d) SEM images of GP cathode and Mo‐NHC anode under different cycles. In situ Raman characterization of (e) GP cathode and (f) Mo‐NHC anode during the charging/discharging process. High‐resolution TEM images of (g, i, and j) GP cathode and (h, k, and l) Mo‐NHC anode under different cycles. (m) In situ EIS and (n) corresponding DRT plots of Mo‐NHC anode during the charging/discharging process. EDS‐mapping of (o) GP cathodes and (p) Mo‐NHC anodes under different cycles. (q) Optical images of the separators after 8000 cycles of MoNH//G, MoN//G, and NH//G.

2.7. DFT Calculations and High‐Temperature/High‐Mass Loading Tests

To explain the aforementioned experimental results, DFT calculations were employed to investigate the influence of different structural models on the Na⁺ adsorption energy and the energy barriers during the Na⁺ insertion/desertion process at the microscopic level. Figure 7a illustrates the binding energies between pure graphite carbon, various nitrogen‐doped carbons, and Mo–N co‐doped carbon with Na⁺. It can be concluded that the binding energy between pristine graphite carbon and Na⁺ is 0.59 eV, indicating that Na⁺ intercalation is difficult due to the thermodynamic instability of Na‐graphite intercalation compounds [62, 63]. In contrast, the binding energy between N‐doped carbon and Na⁺ is negative, implying that Na⁺ can effectively intercalate into this structure. Specifically, compared with graphitized nitrogen (N‐Q) and oxidized nitrogen (N‐O) doping, edge nitrogen doping such as pyrrole nitrogen (N‐5) and pyridine nitrogen doping (N‐6) exhibits higher binding energies (absolute value) of −2.68 eV and −3.36 eV, respectively, which is more conducive to Na⁺ adsorption and diffusion. Notably, the existence of N‐Mo co‐doping yields the highest binding energy of −4.02 eV, indicating enhanced Na⁺ anchoring, accelerated redox conversion, and improved thermodynamic stability. Accordingly, based on the above analysis, the insertion/desertion energy barriers of Na⁺ in pure carbon (C), nitrogen‐doped carbon (NC), and Mo‐NHC structural models were further investigated. As illustrated in Figure 7b, the insertion and desertion energy barriers for Na⁺ in the pure carbon model are the highest, indicating that the electrochemical reaction is most difficult to proceed, which is consistent with the conclusion in Figure 7a. Compared with the pure carbon model, Na⁺ exhibits lower energy barriers in the nitrogen‐doped carbon model, indicating that the insertion reaction occurs more easily. This is attributed to its unique heterostructure and the synergistic interaction between the N‐dopants and Mo single‐atom sites, which significantly improves the reaction kinetics of Na⁺ and its thermodynamic stability after insertion.

FIGURE 7.

Theoretical calculation and high‐load, high‐temperature performance testing. (a) The Na⁺ binding energy and (b) Na⁺ insertion/desertion energy barriers under different carbon structural models. (c) The rate performance of MoNH//G, MoN//G, and NH//G at an active mass loading of 10.6 mg cm^{− 2}. (d) The GCD curves of MoNH//G at different rates. (e) The cycling performance of MoNH//G, MoN//G, and NH//G at 0.2 C. (f) EIS of MoNH//G, MoN//G, and NH//G after 700 cycles. (g) The GCD curves of MoNH//G under different cycles. (h) The cycling performance of MoNH//G, MoN//G, and NH//G at 5 C. (i) The comprehensive comparison of the electrochemical performance of MoNH//G under an active mass loading of 10.6 mg cm⁻² with the reported SDIBs. (j) Linear plot of the reciprocal of absolute temperature versus the logarithm of the reciprocal of Rct. The cycling performance of MoNH//G, MoN//G and NH//G under 70°C at (k) 2 C and (l) 20 C.

Stable cycling under high active mass loading and elevated temperatures represents a key performance metric and a critical challenge that must be overcome for the practical application of battery systems. Therefore, the electrochemical performances of MoNH//G, MoN//G, and NH//G were first evaluated under a high active mass loading of 10.6 mg cm⁻². As displayed in Figure 7c, all three systems exhibit satisfactory results, among which MoNH//G demonstrates the highest specific discharge capacity of 135.9 mA h g⁻¹ at 0.1 C and 66.9 mA h g⁻¹ at 5 C. When the current density returns to 0.1 C, the capacity also recovers, indicating stable rate performance. The corresponding GCD curves of MoNH//G at different rates exhibit flat charge/discharge plateaus with small voltage gaps (Figure 7d), demonstrating a weak polarization effect. Figure 7e represents the cycling performance at 0.2 C. MoN//G and NH//G deliver initial discharge capacities of 71.7 mA h g⁻¹ and 86.3 mA h g⁻¹, respectively, and retain reversible capacities of 52.1 mA h g⁻¹ and 70.3 mA h g⁻¹ after 700 cycles. Notably, MoNH//G achieves the highest initial capacity of 116.7 mA h g⁻¹ and excellent cycling stability, retaining a reversible capacity of 106.7 mA h g⁻¹ after 700 cycles with a capacity retention of up to 91.4%. Figure 7f presents the EIS after 700 cycles, where MoNH//G displays the lowest impedance and highest exchange current density, indicating the optimal electrochemical reaction kinetics. The GCD curves under different cycles almost overlap with flat charge/discharge plateaus (Figure 7g), verifying the stable cycling performance of MoNH//G. Even at a high rate of 5 C, the three systems still exhibit an ultra‐long cycling life. Compared with 32.5 mA h g⁻¹ of MoN//G and 46.5 mA h g⁻¹ of NH//G, MoNH//G displays a high initial capacity of 68.1 mA h g⁻¹ and stable cycling over 6000 cycles (Figure 7h), with a high capacity retention of 91.8% and an extremely low degradation rate of 0.00137% per cycle. The outstanding performance of MoNH//G under high active mass loading significantly outperforms most reported SDIB systems (Figure 7i), demonstrating its tremendous application potential. The electrochemical performance of these three systems at elevated temperatures was further investigated. By substituting the Rct values measured at different temperatures (30°C–70°C) into the Arrhenius equation, the activation energy (Ea) for ion migration could be calculated [64, 65].

$$lnk=lnA-\frac{Ea}{RT}$$ (4)

where A is the pre‐exponential factor, R is the gas constant, and T is the absolute temperature. All three systems exhibit a decreasing trend in Rct with increasing temperature, which indicates that the desolvation energy barrier is reduced and the ion transport kinetics are enhanced at higher temperatures. Compared with MoN//G (17.5 kJ mol⁻¹) and NH//G (12.5 kJ mol⁻¹), MoNH//G shows the lowest activation energy of 10.1 kJ mol⁻¹ (Figure 7j), implying the optimal interfacial transfer and ion migration, which is expected to bring excellent electrochemical performance at elevated temperatures. To this end, the electrochemical performance at different current densities under 70°C was investigated, and Figure 7k illustrates the cyclic performance of MoNH//G, MoN//G, and NH//G at 2 C. All three systems exhibit excellent high‐temperature performance, compared with 117.9 mA h g⁻¹ of MoN//G and 141.0 mA h g⁻¹ of NH//G, MoNH//G delivers the highest reversible capacity of 173.4 mA h g⁻¹ and maintaining stable cycling for 800 cycles with the highest capacity retention of 85.8%. Figure 7l displays the cyclic performance at a high rate of 20 C, in which MoN//G and NH//G show lower discharge capacities of 37.1 mA h g⁻¹ and 53.5 mA h g⁻¹, respectively. Impressively, MoNH//G delivers a high initial capacity of 86.0 mAh g⁻¹ and maintains stable capacity after brief degradation, retaining a reversible capacity of 70.4 mAh g⁻¹ after 4500 cycles with a low capacity degradation of 0.00403% per cycle. The discharge capacity and cycle life exhibited by MoNH//G at elevated temperatures represent the optimal performance in the field of all‐carbon‐based SDIBs. This outstanding performance demonstrates broad application prospects, proving that nitrogen‐doped heterostructured carbon supported with Mo single‐atom active sites can effectively improve the electrochemical performance of SDIBs under high active mass loading and high‐temperature conditions.

3. Conclusion

In summary, a type of supramolecular‐derived heterostructured carbon supported with Mo single atoms (Mo–NHC) is innovatively designed, which integrates the advantages of long‐range ordered graphitized carbon and short‐range disordered non‐graphitized carbon. The built‐in electric field formed at the heterogeneous interface accelerates the directional migration of electrons/ions and catalyzes the electrochemical reactions. Mo‐NHC not only achieves outstanding reversible capacity of 477.8 mA h g⁻¹, rate performance of 20 C, and ultra‐long stable cycling of 16000 cycles, but also demonstrates excellent structural and thermal stability. Moreover, the functional N‐dopants enhance the intrinsic conductivity, Na⁺ adsorption capability, and structural stability of Mo‐NHC while introducing additional active sites. The constructed all‐carbon sodium dual‐ion battery system (MoNH//G) exhibits excellent electrochemical performance: (1) High specific capacity of 222.3 mA h g⁻¹ at 0.5 C; (2) Superior rate performance and cyclic stability, achieving a reversible capacity of 93.4 mA h g⁻¹ and 10,000 stable cycles at 25 C with an extremely low degradation of 0.00196% per cycle. (3) Capable of stable cycling under high active mass loading and high temperature. Furthermore, the operating mechanism of this battery system and the structural evolution patterns of the electrodes during charging and discharging were investigated. DFT calculations demonstrated that N‐dopants and Mo single‐atom sites are conducive to anchoring Na⁺ and reducing its insertion energy barrier. The exceptional performance demonstrated by the N‐doped heterostructure carbon supported with Mo single atoms provides an advanced carbon material paradigm for electrochemical energy storage, while the proposed novel and universal synthesis strategy can be extended to other battery systems.

Author Contributions

Haisi Hua: writing – original draft, visualization, software, data curation, writing – review and editing. Zhengguo Zhang: writing – review and editing, funding acquisition, resources, supervision. Cheng Li: writing – original draft, methodology, software, visualization. Li Li: funding acquisition, resources, supervision. Hongzheng Wu: conceptualization, methodology, software, formal analysis, writing – original draft, writing – review and editing, visualization, investigation, data curation. Xuenong Gao: writing – review and editing, funding acquisition, resources, supervision, data curation. Zhenxing Liang: writing – original draft, writing – review and editing, funding acquisition, resources, supervision, methodology. Wenhui Yuan: investigation, funding acquisition, writing – original draft, writing – review and editing, visualization, data curation, resources, supervision.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supporting information

Supporting File 1: anie72810‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.