Interlayer-expanded MoSe₂/C superlattice hollow nanospheres as stable anodes for sodium/potassium ion batteries

Abstract

As a layered two-dimensional material, MoSe₂ exhibits interlayer-tunable properties and exceptional theoretical capacities, making it a promising candidate for sodium/potassium ion storage systems. Nevertheless, its inadequate conductivity and irreversible reactions during charge and discharge seriously affect its electrochemical performance. Herein, a hierarchical interlayer-expanded MoSe₂/C (IE-MoSe₂/C) hybrid architecture with interlinked hollow nanospheres is engineered via a two-stage fabrication process combining hydrothermal self-assembly and controlled pyrolysis. The interlayer spacing increases to 1.02 nm, which accelerates the transport of sodium and potassium ions. Additionally, the strong interface between carbon and MoSe₂ improves the conductivity, thereby enhancing the electrochemical kinetics of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). On the other hand, the unique hierarchical IE-MoSe₂/C structure with hollow nanospheres can effectively mitigate variations in volume during cycling. Thus, IE-MoSe₂/C exhibits outstanding electrochemical characteristics as an anode material for SIBs and PIBs. Specifically, IE-MoSe₂/C exhibits better rate capability (98 mAh g⁻¹ at 20 A g⁻¹ in SIBs) and cycling performance (269/174 mAh g⁻¹ at 2.0/5.0 A g⁻¹ over 1100 cycles in SIBs and 133/96 mAh g⁻¹ at 1.0/2.0 A g⁻¹ over 1000 cycles for PIBs). Additionally, at 0.5C, the full cell of IE-MoSe₂/C||Na₃V₂(PO₄)₃ can display a consistent capacity of 93 mAh g⁻¹, demonstrating the potential for future practical applications.

---

1. Introduction

Currently, the global transition toward renewable energy systems has propelled an exponential surge in the utilization of lithium-ion batteries (LIBs), driven by their pivotal role as high-capacity storage systems in electrified transportation and smart mobile technologies.^1–5 However, the shortage of lithium resources would hinder their further large-scale application for new energy exploration.^4–6 Sodium/potassium ion batteries (SIBs/PIBs) exhibit analogous charge-storage mechanisms to LIBs and possess Earth-abundant alkali metal resources.^7–11 Because of their elemental sustainability, these post-lithium devices are becoming attractive substitutes for grid-scale energy storage applications. Nevertheless, their greater ionic dimensions (Na⁺: 1.02 Å; K⁺: 1.38 Å; Li⁺: 0.76 Å) intrinsically create significant diffusion barriers, causing irreversible lattice strain during prolonged cycling that severely diminishes rate capabilities and cycling lifespan.^12–15 Therefore, the development of suitable Na⁺/K⁺ host materials is crucial to promote the electrochemical performance of PIBs and SIBs.

It is found that two-dimensional (2D) layered transition metal dichalcogenides (LTMDs) show promise as anode materials for SIBs and PIBs, due to their wide interlayer distances that enable efficient alkali metal ion (Na⁺/K⁺) diffusion and high theoretical capacities.^16,17 Among these, as a typical sandwich-layer structure, MoSe₂ has received extensive attention as a sodium/potassium ion host, owing to the wide interlayer spacing of 6.5 Å originating from the weak stacking of Se–Mo–Se monolayers, and good electrical conductivity based on the small band gap.^15,18 The structural merits are beneficial for the storage of larger-sized sodium/potassium ions. However, during repeated charge–discharge cycles, large sodium/potassium ions still induce interlayer expansion of MoSe₂, ultimately causing structural collapse of the 2D framework. This will induce decreased reaction kinetics and fast capacity decay. To tackle the above issues, conductive carbon compositing is considered an effective approach for enhancing the electrochemical attributes of MoSe₂/C nanocomposites in SIBs/PIBs.^19,20 Generally, the conductive carbon network in MoSe₂/C nanocomposites facilitates electron transfer, maintains structural stability and optimizes interlayer spacing, all of which can effectively boost the rate performance and cycle life. For instance, hierarchically structured MoSe₂/N–C nanorods with an expanded interlayer were synthesized via a solvothermal approach employing an ethylenediamine-modified MoO₃ precursor.²¹ Nitrogen-doped carbon (N–C) was intercalated into MoSe₂ interlayers to expand the d-spacing and served as a conductive scaffold to suppress nanosheet restacking. Thus, MoSe₂/N–C possessed superior electrochemical properties in SIBs and PIBs. On the other hand, carbon coating serves as an effective approach for modulating the crystal growth of MoSe₂. This regulation thereby enhances electron transfer kinetics and alleviates volumetric expansion during Na⁺/K⁺ insertion. For instance, ultrathin 2D MoSe₂ nanosheets featuring expanded interlayer spacing were uniformly integrated within a three-dimensional (3D) macroporous carbon framework (labeled as MoSe₂@C).²² This architecture demonstrates enhanced sodium-ion storage capacity. The MoSe₂@C nanohybrid manifests exceptional high-rate performance coupled with prolonged cyclability, attributable to multiscale electron/ion transport channels enabled by 3D-interconnected conductive networks and interfacial charge redistribution effects. This can promote efficient Na⁺ intercalation/deintercalation, accelerate electron/ion transport kinetics, and mitigate structural expansion during cycling. Consequently, carbon plays multiple roles in the construction of the MoSe₂-based hierarchical structure, thereby improving the reaction kinetics of Na⁺/K⁺ transfer and enhancing structural stability to buffer volume changes.

Herein, a hierarchical interlayer-expanded MoSe₂/C (IE-MoSe₂/C) composite with enlarged interlayer spacing and assembled hollow nanospheres was constructed using self-curled MoSe₂ layers. This hierarchical IE-MoSe₂/C structure has several advantages for boosting Na⁺/K⁺ storage. Firstly, the enlarged interlayer spacing (1.02 nm) in MoSe₂ could reduce Na⁺/K⁺ diffusion barrier energy, facilitating their insertion/extraction. Secondly, the hierarchical architecture constructed from hollow nanospheres effectively alleviates volumetric expansion, enhancing cycling stability. Thirdly, the carbon matrix can prevent the collapse of MoSe₂. Consequently, IE-MoSe₂/C displays excellent energy storage properties. In SIBs, the IE-MoSe₂/C anode demonstrates exceptional cycling stability, maintaining stable capacities of 269/174 mAh g⁻¹ at 2.0/5.0 A g⁻¹ for 1100 cycles. This robust cycling performance originates from both the expanded interlayer spacing (1.02 nm), which facilitates rapid Na⁺ diffusion, and the carbon matrix, which enhances electron transport. Remarkably, even at 20.0 A g⁻¹, a significant specific capacity of 98 mAh g⁻¹ is retained. For PIBs, the anode exhibits outstanding rate performance, delivering 133/96 mAh g⁻¹ at 1.0/2.0 A g⁻¹ over 1000 cycles, attributed to increased pseudocapacitive K⁺ storage behavior confirmed by kinetic analysis. The full cell coupled with Na₃V₂(PO₄)₃ (NVP) (IE-MoSe₂/C||NVP) further validates practical feasibility, achieving 93 mAh g⁻¹ after 300 cycles at 0.5C.

2. Results and discussion

The hierarchical IE-MoSe₂/C architecture, assembled from hollow nanospheres, was synthesized by a solvothermal and post-annealing method, as schematically shown in Fig. 1. Under hydrothermal conditions, MoSe₂ is synthesized by reacting (NH₄)₆Mo₇O₂₄·4H₂O with Se²⁻, accompanied by the insertion of ethylenediamine and D-glucose into the interlayers for subsequent carbonization. During the subsequent pyrolysis at 800 °C, the inserted organic molecules are completely converted into amorphous carbon monolayers by interlayer confinement. The sandwiched carbon monolayers can bond with the adjacent Se–Mo–Se layers and expand the interlayer spacing of MoSe₂. The carbon intercalation into the MoSe₂ interlayers produces a sandwiched MoSe₂/C superlattice structure through interfacial interaction, significantly enhancing the electronic conductivity.^24–26 The material's microstructure was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image in Fig. 2a reveals the micron-scale irregular hierarchical architecture of IE-MoSe₂/C. In Fig. 2b, it is found that the hierarchical structure is tightly assembled with numerous nanospheres, providing quite rough surfaces, which can expose larger contact areas for electrolyte penetration. The sizes of these nanospheres are determined based on a statistical method, providing an average diameter of about 35.28 nm (Fig. S1).²⁷ In the TEM image (Fig. 2c), hollow nanospheres with large cavities are observed in an irregular hierarchical structure. The hollow nanosphere characteristic effectively mitigates volume expansion during cycling, ensuring long-term structural integrity. Furthermore, high-resolution TEM (HRTEM) was used to analyze the microstructural features. As depicted in Fig. 2d, it is found that thin MoSe₂ nanolayers spontaneously assemble into onion-like structures anchored within the carbon matrix. Additionally, the (002) interlayer spacing of the IE-MoSe₂/C structure reaches 1.02 nm, significantly exceeding the standard value of 0.65 nm.^28–30 Significantly, an intercalated architecture composed of inter-overlapped carbon monolayers and MoSe₂ monolayers generates an IE-MoSe₂/C superlattice, exhibiting expanded interlayer spacings. This configuration enhances rapid kinetic processes and ensures structural integrity during sodium/potassium-ion storage. Furthermore, the absence of the long-range ordering lattice in the basal planes of IE-MoSe₂/C (Fig. 2e) suggests that the inserted carbon monolayer causes MoSe₂ to generate a large number of defects.³¹ As for pristine MoSe₂, thin nanosheets are obtained, as shown in Fig. S2. This suggests that glucose and ethylenediamine are essential for the creation of this hierarchical structure.¹⁵ The TEM image of a typical hierarchical IE-MoSe₂/C composite is displayed in Fig. 2f. Uniform dispersion of Mo, Se, C, and N elements, as evidenced by energy dispersive X-ray spectroscopy (EDX) mapping, validates the successful synthesis of the IE-MoSe₂/C composite.

Fig. 1 Schematic illustration of the formation mechanism and structure of IE-MoSe₂/C hollow nanospheres.

Fig. 2 (a and b) SEM images, (c) TEM image, (d) HRTEM image and the corresponding profile plots of lattice spacing calibration, and (e) the basal planes of IE-MoSe₂/C. (f) the TEM image and the corresponding EDX map of the IE-MoSe₂/C composite.

The X-ray diffraction (XRD) patterns (Fig. 3a) show that pristine MoSe₂ presents strong diffraction peaks at 13.6°, 31.6°, 37.8°, and 55.8°, respectively, which can be indexed to the (002), (100), (103) and (110) diffractions of 2H-MoSe₂ (PDF#29-0914).³² As for IE-MoSe₂/C, the (002) peak becomes weak and shifts to a lower angle of 8.0°.³³ The IE-MoSe₂/C interlayer spacing is determined to be 1.02 nm using the Bragg equation, which is in agreement with HRTEM observations. The component was further analyzed using Raman spectra. In Fig. 3b, the two distinct Raman peaks at 237 and 284 cm⁻¹ correspond to the out-of-plane A_1g and in-plane E_2g¹ of MoSe₂.^34,35 Notably, the A_1g peak of IE-MoSe₂/C has a slight red shift to 239 cm⁻¹, indicating that the interlayer interaction is weakened by the carbon layer insertion. Additionally, Raman analysis of the IE-MoSe₂/C composite reveals distinct vibrational signatures at 1356 and 1583 cm⁻¹, which are attributable to the disorder and graphitic lattice modes of carbonaceous materials.³⁶ This confirms the coexistence of defective carbon structures and ordered sp²-hybridized domains within the composite architecture. The intensity ratio (I_D/I_G) is about 0.98, illustrating the amorphous nature of the carbon with plenty of defects, which can supply sufficient active sites for the Na⁺/K⁺ reaction.^37–39 Thermogravimetric analysis was employed to quantify the carbon mass content of the IE-MoSe₂/C composite under an oxygen atmosphere (Fig. S3). A carbon mass ratio of 34% in the IE-MoSe₂/C composites is determined via weight changes during the thermal oxidation process converting MoSe₂ into MoO₃.⁴⁰

Fig. 3 (a) XRD patterns and (b) Raman spectra of IE-MoSe₂/C and MoSe₂; high-resolution XPS spectra of (c) Mo 3d, (d) Se 3d, (e) C 1s, and (f) N 1s for IE-MoSe₂/C.

To investigate the surface chemical states of IE-MoSe₂/C, X-ray photoelectron spectroscopy (XPS) characterization was carried out. Two distinct peaks at 229.0 eV (Mo 3d_5/2) and 232.15 eV (Mo 3d_3/2) in the deconvoluted Mo 3d spectrum (Fig. 3c) confirm the presence of Mo⁴⁺.^19,41 The broad peak at 235.2 eV is associated with the C–O–Mo bond, resulting from the dehydrogenation of the –OH group by annealing.²³ Additionally, there is a minor peak at 230.4 eV, associated with the Se 3s orbital.⁴² The Se 3d XPS spectrum (Fig. 3d) presents characteristic binding energies of 55.3 eV (Se 3d_3/2) and 54.4 eV (Se 3d_5/2), attributed to the dominant Se²⁻ oxidation state in MoSe₂.^18,43,44 Meanwhile, deconvolution of the C 1s spectrum (Fig. 3e) gives five distinct chemical states: C–Se at 290.2 eV, C=O at 288.7 eV, C–O–Mo at 287.2 eV, C–N at 285.4 eV, and C–C at 284.6 eV.²³ Notably, the C–O–Mo bond provides evidence of interfacial charge transfer between MoSe₂ and the carbon matrix.³⁹ The N 1s spectrum (Fig. 3f) has three characteristic bonding configurations: pyridinic N (398.5 eV), pyrrolic N (400.7 eV), and graphitic N (401.9 eV).¹⁵ The XPS characterization confirmed the existence of N–C bonds, which suggests that nitrogen species are successfully incorporated into the IE-MoSe₂/C composite. This N-doping can induce defective active sites and enhance charge transfer, thereby accelerating the reaction kinetics of Na⁺/K⁺ ions. Moreover, pyridinic nitrogen in the carbon matrix can produce conjugated π-electron networks, effectively boosting charge transfer kinetics.⁴⁰ Therefore, IE-MoSe₂/C could form covalent bonds with the intercalated carbon monolayers via C–Se/C–O–Mo bonds. This unique structure facilitates efficient electron/ion transport.²³

To probe the architectural merits of the IE-MoSe₂/C composite, systematic electrochemical evaluations were conducted in SIBs. Fig. 4a illustrates the first five cyclic voltammetry (CV) profiles scanned at 0.1 mV s⁻¹ from 0.01 to 3.0 V. The weak cathodic peak at 0.85 V during the first reduction scan is predominantly governed by Na⁺ insertion into MoSe₂ interlayers (forming Na_xMoSe₂), while the intense peak at 0.25 V originates from synchronous SEI film generation and subsequent phase reduction to metallic Mo/Na₂Se.^25,45 Accordingly, in the initial anodic process, the prominent oxidation peak observed at a potential of 1.67 V can be primarily attributed to the electrochemical conversion to MoSe₂.⁴⁶ The cathodic peaks shift to higher potentials and overlap well in the following four cycles. This indicates that good cycling performance can be achieved for the IE-MoSe₂/C anode in SIBs. Fig. 4b displays the galvanostatic discharge/charge (GDC) profile of IE-MoSe₂/C at 0.1 A g⁻¹, where its potential plateaus align with CV peak positions. It exhibits an exceptional initial discharge specific capacity of 520 mAh g⁻¹ and a charge specific capacity of 388 mAh g⁻¹, yielding a notably high initial coulombic efficiency (ICE) of 74.6%. The initial capacity loss is principally due to inevitable SEI film generation in the first cycle. Subsequent GDC profiles (2nd, 5th, 10th, and 50th cycles) exhibit distinctly overlapping voltage curves. This observation confirms the outstanding cycling stability for the IE-MoSe₂/C anode.

Fig. 4 Electrochemical performance of SIBs: (a) CV curves recorded at 0.1 mV s⁻¹. (b) GDC profiles obtained at 100 mA g⁻¹. (c) Rate capability assessment of the IE-MoSe₂/C and pristine MoSe₂ anodes over the range of 0.1–20.0 A g⁻¹. (d) Long-term cycling stability evaluated at 2.0 and 5.0 A g⁻¹ for the IE-MoSe₂/C anode in the half-cells. (e) GDC profiles of the NVP cathode in a half-cell and the corresponding IE-MoSe₂/C||NVP full cell at 0.2C. (f) Rate performance and (g) cycling behavior at 0.5C of the IE-MoSe₂/C||NVP full cell. Specific capacities and current densities were determined based on the mass of the NVP cathode.

Rate capability assessment of the IE-MoSe₂/C and pristine MoSe₂ anodes (Fig. 4c) reveals progressive capacity attenuation over the range of 0.1–20.0 A g⁻¹. The IE-MoSe₂/C anode delivers capacities from 368 to 116 mAh g⁻¹ upon current increase from 0.1 to 15.0 A g⁻¹. Remarkably, even under extreme 20.0 A g⁻¹ conditions, the capacity is retained at 98 mAh g⁻¹, surpassing most reported LTMD anodes.^47–50 This demonstrates exceptional high-rate tolerance. When reverting to 0.1 A g⁻¹, the IE-MoSe₂/C anode achieves full capacity recovery (383 mAh g⁻¹), which is obviously higher than that of pristine MoSe₂ (211 mAh g⁻¹). The rate capabilities of the IE-MoSe₂/C composite are superior to those of pristine MoSe₂. This suggests that IE-MoSe₂/C has superior and stable high-rate sodium storage capacity. Furthermore, Fig. S4 shows the GDC curves of IE-MoSe₂/C and pristine MoSe₂ at various current densities in SIBs. The curves retain similar shapes, except for the slope shifts, which are attributed to increased polarization at higher rates. The increased interlayer spacing and the carbon matrix of IE-MoSe₂/C may accelerate the diffusion of Na⁺ and raise the electronic conductivity. In Fig. 4d, systematic investigations were conducted on the cyclability of the IE-MoSe₂/C anode at high current densities. At 2.0 A g⁻¹, a discharge capacity of 245 mAh g⁻¹ is initially achieved. The progressive capacity attenuation over the first 40 cycles results in 7.5% capacity decay, kinetically constrained by the high electrochemical polarization. After that, the capacity increases gradually due to the activation process. Notably, the capacity is increased to 267 mAh g⁻¹ at the 800th cycle. Then, the capacity is stabilized and 269 mAh g⁻¹ is retained after 1100 cycles, demonstrating outstanding stability under high-rate cycling. It should be noted that the continuous capacity increase of the IE-MoSe₂/C electrode in the first 800 cycles can be attributed to the electrochemical activation process. Specifically, the nanospheres in the IE-MoSe₂/C hierarchical structure are tightly stacked together. During the initial cycling, the inner nanospheres are less accessible for participating in the electrochemical process. As cycling proceeds, the nanospheres in the IE-MoSe₂/C assembly become loose, as confirmed by the SEM image of the anode after 800 cycles (Fig. S5). This significantly increases the effective surface area for the electrolyte contact, which facilitates Na⁺ intercalation and deintercalation reactions, resulting in an increase in specific capacity. This capacity growth phenomenon has also been reported for MoSe₂-based anodes.^51,52 The pristine MoSe₂ anode delivers a second-cycle capacity of 284 mAh g⁻¹ (Fig. S6). However, this capacity is significantly decreased in the first 50 cycles and stabilizes at a low value of 53 mAh g⁻¹ after 200 cycles. This demonstrates that the hierarchical structure with hollow nanospheres and carbon confinement frameworks enables significant structural integrity during sodiation/desodiation processes, thus improving the cycling stability. Additionally, with a greater current of 5.0 A g⁻¹, a similar variation trend is observed. Following 1100 cycles, the IE-MoSe₂/C anode retains 90% of its original capacity, providing a stable capacity of 174 mAh g⁻¹. The remarkable capacity retention of the IE-MoSe₂/C composite demonstrates its superior high-rate sodium storage capability. Overall, the cycling stability of IE-MoSe₂/C for SIBs is superior to that of some MoSe₂-based anodes (Table S1). To validate the practical feasibility, a full cell battery was constructed utilizing commercial NVP as the cathode. Comparative GDC profiles of the NVP cathodes (half-cell) and the IE-MoSe₂/C||NVP full cells are depicted in Fig. 4e. The NVP cathode in the half-cell reveals a characteristic 3.4 V discharge plateau and delivers a capacity of 104 mAh g⁻¹ at 0.2C (where 1C = 117 mA g⁻¹), reaching its theoretical capacity.^25,53,54 In the IE-MoSe₂/C||NVP full cell with a voltage range of 1.0–3.8 V, it delivers a capacity of 99 mAh g⁻¹ at 0.2C. In Fig. 4f, for the rate performance of the IE-MoSe₂/C||NVP full cell, capacities of 99, 93, 90, 86, and 62 mAh g⁻¹ can be achieved at 0.2C, 0.5C, 1C, 2C, and 5C, respectively. Remarkably, the capacity is almost recovered (96 mAh g⁻¹) after reverting to 0.2C. For the long-term cycling at 0.5C, a capacity of 93 mAh g⁻¹ is retained over 300 cycles (Fig. 4g), indicating no capacity decay. This illustrates that exceptional reaction kinetics and cycling stability are also achieved in the full cells, demonstrating its potential practical application.

The in situ Raman test was performed with different voltage states to investigate reaction mechanisms in SIBs. As shown in Fig. 5a, the characteristic A_1g band of MoSe₂ is clearly observed for the IE-MoSe₂/C electrode before cycling. Upon discharging to 0.84 V, the A_1g peak intensity undergoes a gradual attenuation, primarily attributed to Na⁺ intercalation within MoSe₂ layers and the subsequent weakening of interlayer interaction. This demonstrates that the intercalation reaction occurs. The vanishing of the A_1g peak upon discharging to 0.44 V indicates that the conversion reaction is in progress. Throughout the charging process, the A_1g peak gradually emerges and becomes strong upon charging to 3.0 V, confirming the high reversibility.^18,55 To clearly observe the peak changes, a contour plot is derived, as shown in Fig. 5b. Consequently, an insertion reaction combined with a conversion reaction is responsible for the Na⁺ storage for the IE-MoSe₂/C anode in SIBs.

Fig. 5 (a) In situ Raman spectra and (b) the corresponding contour plots of the IE-MoSe₂/C anode with different voltages during the first discharge and charge process.

The rapid Na⁺ storage was probed via the CV method (Fig. 6a). Similar CV shapes are observed when the electrode is scanned at 0.1–4 mV s⁻¹ despite peak modulation. The i = av^b relationship quantifies storage behaviour.^56,57 Through taking the logarithm, the b value can be inferred based on the linear plots of log(i)–log(v).⁵⁸ The Na⁺ storage process can be predicted using the b value. A b-value of 1.0 indicates capacitance-dominated energy storage behaviour, whereas a diffusion-controlled process becomes predominant at b = 0.5. Anodic and cathodic peaks with b-values of 0.81 and 0.88 are shown by quantitative analysis in Fig. 6b, indicating mixed charge storage processes. This suggests that the pseudocapacitive mechanism is primarily responsible for promoting the rapid response kinetics. Additionally, the capacity contribution (k₁v) and the diffusion-controlled contribution (k₂v^1/2) ratios may be obtained using the formula i(v) = k₁v + k₂v^1/2.^59,60Fig. 6c quantifies the evolving charge storage contribution ratios in the IE-MoSe₂/C anodes. Pseudocapacitive contributions dominate 60.9% at 0.1 mV s⁻¹ and increase to 97.9% at 4 mV s⁻¹, reflecting that a capacitive process promotes the high-rate sodium storage capacity. Notably, the contribution of pseudo-capacitance for IE-MoSe₂/C is higher than for pristine MoSe₂ anodes at each scan rate, which induces a superior rate capability for the IE-MoSe₂/C anode.²⁸ Fig. S7 displays the Nyquist plots after 50 cycles in SIBs, which are composed of straight lines with low frequency and a semicircle with high frequency for the charge transfer resistance (R_ct). The reduced R_ct of the IE-MoSe₂/C anode relative to pristine MoSe₂ signifies increased conductivity. Typically, Na⁺ storage kinetics is governed by solid-state Na⁺ diffusion. The galvanostatic intermittent titration technique (GITT) is used to determine the sodium ion diffusion coefficient (D_Na). The GITT profiles, which are similar to the GDC profiles, are shown in Fig. 6d. The following formula can be used to obtain the D_Na values:

Fig. 6 Kinetics analysis in SIBs. (a) CV curves with scanning rates ranging from 0.1 to 4.0 mV s⁻¹ and (b) the linear plots of log(v)–log(i) and the inferred b values of the IE-MoSe₂/C anode. (c) The contribution of capacitance at different scan rates, (d) GITT curves and (e) D_Na values over the first whole cycling for the IE-MoSe₂/C and MoSe₂ anodes.

Fig. 6e shows the D_Na values of IE-MoSe₂/C and MoSe₂, which show similar change trends for the two electrodes. The reduction in D_Na values observed alongside the phase transformation demonstrates a strong correlation with the characteristic oxidation/reduction features in the CV profiles.^61,62 The D_Na values for the IE-MoSe₂/C electrode are from 1.2 × 10⁻¹¹ to 2.1 × 10⁻¹¹ cm² s⁻¹, which are larger than those for pristine MoSe₂ (0.7 × 10⁻¹¹–1.4 × 10⁻¹¹ cm² s⁻¹). As a result, enhanced reaction kinetics can be achieved for IE-MoSe₂/C owing to its expanded interlayer spacing and carbon modification, which can provide superior rate and cycling performance.

Given the superior Na⁺ storage capabilities, the IE-MoSe₂/C architecture was strategically adapted for K⁺ storage. Fig. 7 comprehensively characterizes the performance of PIBs. Fig. 7a describes the first five CV profiles recorded at 0.1 mV s⁻¹. During the initial reduction phase, K⁺ intercalation into the MoSe₂ interlamellar structure occurs at 1.0 V, forming K_xMoSe₂. The pronounced reduction signal observed at 0.07 V is linked to the phase transformation of K_xMoSe₂ into metallic molybdenum and K₂Se compounds, accompanied by SEI layer generation. Anodic scanning shows a peak at 1.79 V (Mo → MoSe₂ conversion).⁶³ In the following cycles, the reduction peaks shift to higher voltages, which is similar to that for the SIBs. The high overlapping of the subsequent CV curves reveals the good reversibility and cycling stability of the IE-MoSe₂/C anode. At 100 mA g⁻¹ (Fig. 7b), the initial GDC cycles show 377/247 mAh g⁻¹ discharge/charge capacities (66% ICE), attributed to SEI formation losses. Beyond the first cycle, the charge–discharge profiles exhibit minimal variation. Furthermore, the GDC curves with different currents are shown in Fig. 7c. The voltage–capacity curves show similar features, accompanied by gradual slope evolution as current densities escalate, originating from polarization effects. Rate capability assessment of IE-MoSe₂/C (Fig. 7d) reveals progressive capacity attenuation upon current increase (0.1 → 5.0 A g⁻¹), decreasing from 248 to 88 mAh g⁻¹. Notably, upon reverting to the baseline 0.1 A g⁻¹, the IE-MoSe₂/C anode demonstrates complete capacity recovery (246 mAh g⁻¹). In contrast to pristine MoSe₂, only less than 70% of the initial capacity can be recovered (Fig. 7d). For the cycling performance at 1.0 and 2.0 A g⁻¹ (Fig. 7e), it is found that the cycling performance is similar to that in SIBs. At 1.0 A g⁻¹, the capacity reached 172 mAh g⁻¹ during the 5th cycle. By the 1000th cycle, the capacity remained stable at approximately 133 mAh g⁻¹, with a capacity decay rate of only 0.023% per cycle. Even under high-current cycling (2.0 A g⁻¹), the capacity stabilizes at 96 mAh g⁻¹ after 1000 cycles, retaining 72% of its initial value. Conversely, the MoSe₂ electrode exhibits limited cycling stability under continuous charge–discharge cycling (Fig. S8), maintaining only a low capacity of 21 mAh g⁻¹ after 400 cycles. Notably, the cycling stability of the IE-MoSe₂/C anode is excellent, compared with the previously reported MoSe₂-based anodes (Table S1).

Fig. 7 Electrochemical performance of the PIBs. (a) CV curves at 0.1 mV s⁻¹, (b) the selected GDC profiles at 100 mA g⁻¹, and (c) GDC profiles with different current densities of the IE-MoSe₂/C anode. (d) Rate performance of the IE-MoSe₂/C and MoSe₂ anodes. (e) Cycling performance at 1.0 A g⁻¹ and 2.0 A g⁻¹, (f) CV curves with scanning rates ranging from 0.1 to 1.0 mV s⁻¹, and (g) the linear plots of log(v)–log(i) for the IE-MoSe₂/C anode. (h) The contribution of capacitance for the IE-MoSe₂/C and MoSe₂ anodes at various scan rates.

Kinetic analysis via CV could infer high-rate K⁺ storage promotion for IE-MoSe₂/C. In Fig. 7f, CV profiles acquired at 0.1–1 mV s⁻¹ reveal b-values of 0.80 (anodic) and 0.81 (cathodic) (Fig. 7g). Thus, the fast reaction kinetics of the IE-MoSe₂/C anode in PIBs is also dominated by the pseudo-capacitance behaviour.⁶³ Meanwhile, the pseudo-capacitance contribution for the IE-MoSe₂/C anode increases from 51.6% to 76.6% at 0.1 to 1.0 mV s⁻¹, as shown in Fig. 7h, which is higher than that of the MoSe₂ anode at each scan rate. The enhanced reaction kinetics observed in PIBs are attributable to the specific 3D hierarchical structure of the IE-MoSe₂/C composite anode. This can induce the outstanding rate performance and prolonged cycling stability in PIBs for the IE-MoSe₂/C anode. Moreover, Nyquist plots after 50 cycles in PIBs are displayed in Fig. S9. The reduced R_ct of the IE-MoSe₂/C anode indicates the increased conductivity, which can induce the enhanced reaction kinetics for the IE-MoSe₂/C anode in PIBs.

3. Conclusion

In summary, an IE-MoSe₂/C hierarchical structure with expanded interlayer spacing and assembled from hollow nanospheres is constructed. Due to its unique structure with robust interfacial coupling between MoSe₂ and carbon monolayers, the composite significantly boosts electronic conductivity and accelerates Na⁺/K⁺ insertion/extraction kinetics. Furthermore, the hierarchical structure with hollow nanospheres can effectively mitigate the volume expansion, resulting in enhanced structural stability during cycling in SIBs and PIBs. Owing to such structural merits, the IE-MoSe₂/C composite manifests superior high-rate capability and ultralong cyclability. Stable capacities of 269/174 mAh g⁻¹ can be attained at 2.0/5.0 A g⁻¹ in SIBs, while a capacity fading rate of 0.023% per cycle is achieved in PIBs at high current densities (1.0 A g⁻¹). This demonstrates the exceptional high-rate Na⁺/K⁺ storage capability owing to the increased D_Na, enhanced pseudocapacitive contribution and higher conductivity. Moreover, in the IE-MoSe₂/C||NVP full cell, it also delivers outstanding electrochemical performance. This study presents a viable method for preparing high-performance anode materials for SIBs and PIBs.

Author contributions

Qiannian Li: methodology, investigation, formal analysis, visualization, data curation, and writing – original draft. Yanting Xia: conceptualization, investigation, visualization, data curation, and writing – original draft. Jinmao Fang: supervision, investigation, conceptualization, visualization, and data curation. Junwei Chen: resources and writing – review & editing. Yan Zhang: formal analysis and writing – review & editing. Wenpei Kang: data curation and writing – review & editing. Jun Xu: formal analysis, funding acquisition, resources, project administration, supervision, and writing – review & editing.

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.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: Experimental section, Fig. S1–S9, and Table S1. See DOI: https://doi.org/10.1039/d5qi01642h.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 52271208 and 51972092) and the Fundamental Research Funds for the Central Universities (Grant No. JZ2024HGTG0305).

References

1. Y. Wan, Y. Liu, D. Chao, W. Li and D. Zhao, Recent advances in hard carbon anodes with high initial Coulombic efficiency for sodium-ion batteries, Nano Mater. Sci., 2023, 5, 189–201.
2. F. Song, J. Hu, G. Li, J. Wang, S. Chen, X. Xie, Z. Wu and N. Zhang, Room-temperature assembled MXene-based aerogels for high mass-loading sodium-ion storage, Nano–Micro Lett., 2021, 14, 37.
3. Y. Tian, Y. Xu, S. Guo, B. Xu, Z. Zhao, X. Yuan, Y. Wang, J. Li, X. Wang, P. Wang and Z. Liu, Upcycling spent lithium-ion batteries: constructing bifunctional catalysts featuring long-range order and short-range disorder for lithium-oxygen batteries, Adv. Mater., 2025, 37, 2418963.
4. D. Li, H. Liu, J. Li, R. Hou, Y. Li, Z. Sun, L. Wang and W. Han, MXene-modified Sb₂Se₃@NCR anode for high-performance potassium-ion batteries with enhanced stability and low-temperature tolerance, Energy Storage Mater., 2025, 80, 104355.
5. Z. Tian, W. Sun, J. Yu, J. Yuan, J. Chen, Y. Liu, Y. Ding, X. Hu and Z. Wen, Vacancy-rich ternary iron phosphoselenide multicavity nanorods: a highly reversible and fast anode for sodium-ion batteries, Adv. Funct. Mater., 2024, 34, 2404320.
6. J. Xu, X. Cai, S. Cai, Y. Shao, C. Hu, S. Lu and S. Ding, High-energy lithium-ion batteries: recent progress and a promising future in applications, Energy Environ. Mater., 2023, 6, e12450.
7. F. Li and Z. Zhou, Micro/nanostructured materials for sodium ion batteries and capacitors, Small, 2018, 14, 1702961.
8. C. Wang, L. Liu, S. Zhao, Y. Liu, Y. Yang, H. Yu, S. Lee, G.-H. Lee, Y.-M. Kang, R. Liu, F. Li and J. Chen, Tuning local chemistry of P2 layered-oxide cathode for high energy and long cycles of sodium-ion battery, Nat. Commun., 2021, 12, 2256.
9. G. Zhu, X. Tian, H.-C. Tai, Y.-Y. Li, J. Li, H. Sun, P. Liang, M. Angell, C.-L. Huang, C.-S. Ku, W.-H. Hung, S.-K. Jiang, Y. Meng, H. Chen, M.-C. Lin, B.-J. Hwang and H. Dai, Rechargeable Na/Cl₂ and Li/Cl₂ batteries, Nature, 2021, 596, 525–530.
10. F. Wang, T. Zhang, T. Zhang, T. He and F. Ran, Recent progress in improving rate performance of cellulose-derived carbon materials for sodium-ion batteries, Nano–Micro Lett., 2024, 16, 148.
11. C. Ma, X. Tang, H. Ben, W. Jiang, X. Shao, G. Wang and B. Sun, Promoting reaction kinetics and boosting sodium storage capability via constructing stable heterostructures for sodium-ion batteries, Adv. Funct. Mater., 2025, 35, 2412879.
12. R. Shao, Z. Sun, L. Wang, J. Pan, L. Yi, Y. Zhang, J. Han, Z. Yao, J. Li, Z. Wen, S. Chen, S.-L. Chou, D.-L. Peng and Q. Zhang, Resolving the origins of superior cycling performance of antimony anode in sodium-ion batteries: a comparison with lithium-ion batteries, Angew. Chem., Int. Ed., 2024, 63, e202320183.
13. F. Wang, Z. Jiang, Y. Zhang, Y. Zhang, J. Li, H. Wang, Y. Jiang, G. Xing, H. Liu and Y. Tang, Revitalizing sodium-ion batteries via controllable microstructures and advanced electrolytes for hard carbon, eScience, 2024, 4, 100181.
14. P. Liang, D. Pan, X. Hu, K. R. Yang, Y. Liu, Z. Huo, Z. Bo, L. Xu, J. Xu and Z. Wen, Se-regulated MnS porous nanocubes encapsulated in carbon nanofibers as high-performance anode for sodium-ion batteries, Nano–Micro Lett., 2025, 17, 237.
15. J. Ge, L. Fan, J. Wang, Q. Zhang, Z. Liu, E. Zhang, Q. Liu, X. Yu and B. Lu, MoSe₂/N-doped carbon as anodes for potassium-ion batteries, Adv. Funct. Mater., 2018, 8, 1801477.
16. D. T. Pham, T. T. Vu, S. Kim, B. Sambandam, V. Mathew, J. Lim and J. Kim, A versatile pyramidal hauerite anode in congeniality diglyme-based electrolytes for boosting performance of Li- and Na-ion batteries, Adv. Energy Mater., 2019, 9, 1900710.
17. Y. Su, X.-Y. Liu, R. Zhang, S. Zhang, J. Wang, Y.-D. Qian, Z.-C. Jian, Y.-F. Zhu, J.-F. Mao, S. Xu, S. Dou and Y. Xiao, A dual-confinement strategy based on encapsulated Ni-CoS₂ in CNTs with few-layer MoS₂ scaffolded in rGO for boosting sodium storage via rapid electron/ion transports, Energy Storage Mater., 2024, 71, 103638.
18. J. Ge, M. Huang, C. Li, X. Ji, X. Meng, H. Tan, H. Liu and W. Zhou, The charge self-regulation effect induced by microcrystalline-amorphous heterointerface network toward fast charging sodium ion batteries, Adv. Energy Mater., 2025, 15, 2405288.
19. W. Wang, B. Jiang, C. Qian, F. Lv, J. Feng, J. Zhou, K. Wang, C. Yang, Y. Yang and S. Guo, Pistachio-shuck-like MoSe₂/C core/shell nanostructures for high-performance potassium-ion storage, Adv. Mater., 2018, 30, 1801812.
20. M. Yousaf, Y. Wang, Y. Chen, Z. Wang, A. Firdous, Z. Ali, N. Mahmood, R. Zou, S. Guo and R. P. S. Han, A 3D trilayered CNT/MoSe₂/C heterostructure with an expanded MoSe₂ interlayer spacing for an efficient sodium storage, Adv. Energy Mater., 2019, 9, 1900567.
21. X. Zhang, Y. Xiong, L. Zhang, Z. Hou and Y. Qian, Hierarchical interlayer-expanded MoSe₂/N–C nanorods for high-rate and long-life sodium and potassium-ion batteries, Inorg. Chem. Front., 2021, 8, 1271–1278.
22. Y. Liu, N. Wang, X. Zhao, Z. Fang, X. Zhang, Y. Liu, Z. Bai, S. Dou and G. Yu, Hierarchical nanoarchitectured hybrid electrodes based on ultrathin MoSe₂ nanosheets on 3D ordered macroporous carbon frameworks for high-performance sodium-ion batteries, J. Mater. Chem. A, 2020, 8, 2843–2850.
23. L. Liu, B. Li, J. Wang, H. Du, Z. Du and W. Ai, Molecular intercalation enables phase transition of MoSe₂ for durable Na-ion storage, Small, 2024, 20, 2309647.
24. C. Zhang, J. Shang, H. Dong, E. H. Ang, L. Tai, M. Aizudin, X. Wang, H. Geng and H. Gu, Modulation of MoS₂ interlayer dynamics by in situ N-doped carbon intercalation for high-rate sodium-ion half/full batteries, Nanoscale, 2021, 13, 18322–18331.
25. J. Xu, J. Jiang, H. Tang, Z. Chen, J. Chen, Y. Zhang and C.-S. Lee, Interlayer engineering and electronic regulation of MoSe₂ nanosheets rolled hollow nanospheres for high-performance sodium-ion half/full batteries, Adv. Powder Mater., 2024, 3, 100169.
26. J. Xu, J. Zhang, W. Zhang and C.-S. Lee, Interlayer nanoarchitectonics of two-dimensional transition-metal dichalcogenides nanosheets for energy storage and conversion applications, Adv. Energy Mater., 2017, 7, 1700571.
27. K. Wang, H. Du, S. He, L. Liu, K. Yang, J. Sun, Y. Liu, Z. Du, L. Xie, W. Ai and W. Huang, Kinetically controlled, scalable synthesis of γ-FeOOH nanosheet arrays on nickel foam toward efficient oxygen evolution: the key role of in situ-generated γ-NiOOH, Adv. Mater., 2021, 33, 2005587.
28. J. Tao, Z. Yan, D. Wang, W. Zhong, Y. Yang, J. Li, Y. Lin and Z. Huang, Rational designing of MoSe₂ nanosheets in carbon framework for high-performance potassium-ion batteries, Chem. Eng. J., 2022, 448, 137658.
29. J. S. Lee, J.-S. Park, K. W. Baek, R. Saroha, S. H. Yang, Y. C. Kang and J. S. Cho, Coral-like porous microspheres comprising polydopamine-derived N-doped C-coated MoSe₂ nanosheets composited with graphitic carbon as anodes for high-rate sodium- and potassium-ion batteries, Chem. Eng. J., 2023, 456, 141118.
30. X. Ni, Z. Cui, H. Luo, H. Chen, C. Liu, Q. Wu and A. Ju, Hollow multi-nanochannel carbon nanofibers@MoSe₂ nanosheets composite as flexible anodes for high performance lithium-ion batteries, Chem. Eng. J., 2021, 404, 126249.
31. W. Feng, X. Wei, F. Cao, Y. Li, X. Zhang, Y. Li, W. Liu, J. Han, D. Kong and L. Zhi, Defective MoSSe with local-expanded structure for high-rate potassium ion battery, Energy Storage Mater., 2024, 65, 103186.
32. Y. Shi, C. Hua, B. Li, X. Fang, C. Yao, Y. Zhang, Y.-S. Hu, Z. Wang, L. Chen, D. Zhao and G. D. Stucky, Highly ordered mesoporous crystalline MoSe₂ material with efficient visible-light-driven photocatalytic activity and enhanced lithium storage performance, Adv. Funct. Mater., 2013, 23, 1832–1838.
33. J.-N. Yang, H. Tian, S.-Q. Li, K.-X. Wang and J.-S. Chen, MoSe₂ hybrid superlattice with expanded interlayer spacing and enriched 1T phase for aqueous zinc ion batteries, Chem. Eng. J., 2025, 516, 164105.
34. X. Zhao, Q. Liu, C. Zhong, Y. Li, Q. Chen, D. Chao and M. Chen, Activated proton storage in molybdenum selenide through electronegativity regulation, Adv. Funct. Mater., 2022, 32, 2205874.
35. R. Kang, D. Zhang, H. Wang, B. Zhang, X. Zhang, G. Chen, Y. Du and J. Zhang, Synergistic optimization of electronic and lattice structures through Ti-intercalation and Se-vacancy engineering for high-performance aluminum storage, Energy Environ. Sci., 2024, 17, 7135–7146.
36. K. Xu, Y.-H. Li, X. Wang, Y.-P. Cao, S.-T. Wang, L. Cao, Q.-T. Zhang, Z.-F. Wang and J. Yang, Unlocking the structure and anion synergistic modulation of MoSe₂ anode for ultra-stable and high-rate sodium-ion storage, Rare Met., 2025, 44, 1661–1673.
37. T. Tian, L.-L. Lu, Y.-C. Yin, F. Li, T.-W. Zhang, Y.-H. Song, Y.-H. Tan and H.-B. Yao, Multiscale designed niobium titanium oxide anode for fast charging lithium ion batteries, Adv. Funct. Mater., 2021, 31, 2007419.
38. G. Xu, L. Yang, Z. Yan, Z. Huang, X. Li, G. Guo, Y. Tian, L. Yang, J. Huang, Y. Liang and S. Chou, Multiscale structural NaTi₂(PO₄)₃ anode for sodium-ion batteries with long cycle, high areal capacity, and wide operation temperature, Carbon Energy, 2024, 6, e552.
39. Z. Li, L. Yu, X. Tao, Y. Li, L. Zhang, X. He, Y. Chen, S. Xiong, W. Hu, J. Li, J. Wang, H. Jin and S. Wang, Honeycomb-structured MoSe₂/rGO composites as high-performance anode materials for sodium-ion batteries, Small, 2024, 20, 2304124.
40. J. Xu, J. Jiang, S. Cao, S. Li, Y. Ma, J. Chen, Y. Zhang and X. Lu, Amorphous carbon intercalated MoS₂ nanosheets embedded on reduced graphene oxide for excellent high-rate and ultralong cycling sodium storage, EcoMat, 2024, 6, e12479.
41. J. Li, Y. He, Y. Dai, H. Zhang, Y. Zhang, S. Gu, X. Wang, T. Gao, G. Zhou and L. Xu, Heterostructure interface construction of cobalt/molybdenum selenides toward ultra-stable sodium-ion half/full batteries, Adv. Funct. Mater., 2024, 34, 2406915.
42. D. Damien, A. Anil, D. Chatterjee and M. M. Shaijumon, Direct deposition of MoSe₂ nanocrystals onto conducting substrates: towards ultra-efficient electrocatalysts for hydrogen evolution, J. Mater. Chem. A, 2017, 5, 13364–13372.
43. F. Niu, J. Yang, N. Wang, D. Zhang, W. Fan, J. Yang and Y. Qian, MoSe₂-covered N,P-doped carbon nanosheets as a long-life and high-rate anode material for sodium-ion batteries, Adv. Funct. Mater., 2017, 27, 1700522.
44. Y. Wang, W. Kang, X. Pu, Y. Liang, B. Xu, X. Lu, D. Sun and Y. Cao, Template-directed synthesis of Co₂P/MoSe₂ in a N-doped carbon hollow structure for efficient and stable sodium/potassium ion storage, Nano Energy, 2022, 93, 106897.
45. S. Tao, X. Zhang, Z. Gao, T.-Y. Chen, H. Min, H. Yang, H.-Y. Chen, X. Shen, J. Wang and H. Yang, Dynamic electronic and ionic transport actuated by cobalt-doped MoSe₂/rGO for superior potassium-ion batteries, Small, 2023, 19, 2304200.
46. J. Guo, J. Yang, J. Guan, X. Chen, Y. Zhu, H. Fu, Q. Liu, B. Wei and H. Geng, Interface and electronic structure dual-engineering on MoSe₂ with multi-ion/electron transportation channels for boosted sodium-ion half/full batteries, Chem. Eng. J., 2022, 450, 138007.
47. X. Liu, M. Wang, B. Qin, Y. Zhang, Z. Liu and H. Fan, 2D–2D MXene/ReS₂ hybrid from Ti₃C₂T_x MXene conductive layers supporting ultrathin ReS2 nanosheets for superior sodium storage, Chem. Eng. J., 2022, 431, 133796.
48. X. Zhang, C. Shen, H. Wu, Y. Han, X. Wu, W. Ding, L. Ni, G. Diao and M. Chen, Filling few-layer ReS₂ in hollow mesoporous carbon spheres for boosted lithium/sodium storage properties, Energy Storage Mater., 2020, 26, 457–464.
49. G. Du, M. Tao, W. Gao, Y. Zhang, R. Zhan, S. Bao and M. Xu, Preparation of MoS₂/Ti₃C₂T_x composite as anode material with enhanced sodium/lithium storage performance, Inorg. Chem. Front., 2019, 6, 117–125.
50. L. Liu, J. Xu, J. Sun, S. He, K. Wang, Y. Chen, S. Dou, Z. Du, H. Du, W. Ai and W. Huang, A stable and ultrafast K ion storage anode based on phase-engineered MoSe₂, Chem. Commun., 2021, 57, 3885–3888.
51. X. Wang, J. Bai, D. Wang, X. Shen, X. Zhang, Z. Xia, Q. Zheng and H. Yu, Construction of an MoSe₂/MoS₂ nanosheet heterostructure for enhanced sodium storage performance, J. Mater. Chem. C, 2025, 13, 12554–12563.
52. Z. Li, J. Yan, Q. Li, A. Xu, J. Sun, Y. Wang, X. Zhang, X. Sun, F. Jiang and Y. Zhou, Te doped 1T/2H-MoSe₂ nanosheets with rich defects as advanced anode materials for high-rate sodium ion half/full batteries, Inorg. Chem. Front., 2024, 11, 2017–2028.
53. J. Xu, F. Cao, X. Yang, X. Chen, Y. Zhang, J. Chen, L. He and W. Kang, Sandwiched ReS₂ nanocables with dual carbon coating for efficient K⁺/Na⁺ storage performance, J. Colloid Interface Sci., 2024, 669, 825–834.
54. Y. Li, Q. Guan, J. Cheng and B. Wang, Flexible high energy density sodium dual-ion battery with long cycle life, Energy Environ. Mater., 2022, 5, 1285–1293.
55. Z. Huang, G. Wang, S. Xie, W. Zhang, J. Wang, Z. Lin, G. Wang, H. Chu, Y. Zhong, Y. Huang, J. Xu, S. Xiong and S. Huang, Regulating ion transfer dynamics and potassium polyselenide dissolution in dual-defect MoSe_2(x@NC for ultrafast and stable potassium-ion storage, Adv. Funct. Mater., 2025, 35, 2424278.
56. X. Hu, Y. Liu, J. Li, G. Wang, J. Chen, G. Zhong, H. Zhan and Z. Wen, Self-assembling of conductive interlayer-expanded WS₂ nanosheets into 3D hollow hierarchical microflower bud hybrids for fast and stable sodium storage, Adv. Funct. Mater., 2020, 30, 1907677.
57. X. Xu, R. Zhao, B. Chen, L. Wu, C. Zou, W. Ai, H. Zhang, W. Huang and T. Yu, Progressively exposing active facets of 2D nanosheets toward enhanced pseudocapacitive response and high-rate sodium storage, Adv. Mater., 2019, 31, 1900526.
58. K. Ma, H. Jiang, Y. Hu and C. Li, 2D nanospace confined synthesis of pseudocapacitance-dominated MoS₂-in-Ti₃C₂ superstructure for ultrafast and stable Li/Na-ion batteries, Adv. Funct. Mater., 2018, 28, 1804306.
59. K. Yao, Z. Xu, J. Huang, M. Ma, L. Fu, X. Shen, J. Li and M. Fu, Bundled defect-rich MoS₂ for a high-rate and long-life sodium-ion battery: achieving 3D diffusion of sodium ion by vacancies to improve kinetics, Small, 2019, 15, 1805405.
60. T. Yao, H. Wang, Y. Qin, J.-W. Shi and Y. Cheng, Enhancing pseudocapacitive behavior of MOF-derived TiO_2(x@Carbon nanocubes via Mo-doping for high-performance sodium-ion capacitors, Composites, Part B, 2023, 253, 110557.
61. M. Jia, W. Zhang, X. Cai, X. Zhan, L. Hou, C. Yuan and Z. Guo, Re-understanding the galvanostatic intermittent titration technique: pitfalls in evaluation of diffusion coefficients and rational suggestions, J. Power Sources, 2022, 543, 231843.
62. W. Huang, D. T. Boyle, Y. Li, Y. Li, A. Pei, H. Chen and Y. Cui, Nanostructural and electrochemical evolution of the solid-electrolyte interphase on CuO nanowires revealed by cryogenic-electron microscopy and impedance spectroscopy, ACS Nano, 2019, 13, 737–744.
63. J. Xu, Y. Xia, J. Fang, Y. Zhang, W. Kang and Z. Chen, MoSe₂/C superlattice hollow nanospheres embedded in rGO nanosheets for efficient potassium-ion storage, Chem. Eng. J., 2025, 522, 167432.

---