Regulating Sodium Deposition Behavior by a Triple‐Gradient Framework for High‐Performance Sodium Metal Batteries

Abstract An efficient method for the synthesis of a self‐supporting carbon framework (denoted Gra‐GC‐MoSe2) is proposed with a triple‐gradient structure—in sodiophilic sites, pore volume, and electrical conductivity—which facilitates the highly efficient regulation of Na deposition. In situ and ex situ measurements, together with theoretical calculations, reveal that the gradient distribution of Se heteroatoms in MoSe2, and its derivatives tailor the sodiophilicity, while the gradient distribution of porous nanostructures homogenizes the Na+ diffusion. Therefore, Na deposition occurs from the bottom to the top of the Gra‐GC‐MoSe2 framework without dendrite formation. In addition, the gradient in electrical conductivity ensures the stripping process does not lead to dead Na. As a result, a Gra‐GC‐MoSe2 modified Na anode (Na@Gra‐GC‐MoSe2) shows impressive cycling stability with a high average Coulombic efficiency in an asymmetric cell. In symmetric cells, it also exhibits a long cycling life of 2000 h with a low polarization voltage and works stably even under a large capacity of 10 mAh cm−2. Moreover, a Na@Gra‐GC‐MoSe2|| Na3V2(PO4)3 full cell delivers a high energy density with an excellent cycling performance.


Introduction
Metallic sodium (Na) is expected to be a promising anode material for low-cost and high-energy Na metal batteries due to its highly abundant natural resources, high theoretical capacity (1166 mAh g −1 ) and low working potential (-2.71 V vs standard hydrogen electrode). [1,2]15][16][17][18][19] In particular, 3D hosts have shown promising prospects for fabricating dendrite-free NMAs.][22][23] However, Na deposition occurs preferentially on the top of the framework due to the concentrated electric field and the short Na + pathways for fast ionic diffusion result in the formation of metallic Na near the separator side, leading to the eventual formation of Na dendrites (Scheme 1a). [24][27][28][29][30] Although such sodiophilic frameworks enable more internal Na plating, the concentrated electric field and higher Na + diffusivity near the separator side still lead to Na dendrite formation on the framework surface, especially under high plating capacity.Moreover, electrons flow into the anode via the metallic case or tab, which are in direct contact with the framework bottom.The inhomogeneous electron diffusivity within the framework also affects the Na stripping process and can cause dead Na deposits. [31]Recently, gradient frameworks with a linear variation in the number of sodiophilic sites have been reported as hosts for NMAs and shown to facilitate gradient Na growth. [32,33]Nevertheless, further improvements are required since this strategy cannot tune the electric field distribution and the Na + flux. [34]Therefore, the tailored design of the framework with multiple gradients in structure and composition is necessary to further regulate Na deposition behavior by concomitantly tuning the ion/electron diffusivities as well as the number of sodiophilic sites.
In this work, we have presented an efficient method for the synthesis of a self-supporting carbon framework (denoted Gra-GC-MoSe 2 ) with a triple-gradient structure-in sodiophilic sites (Se heteroatoms, MoSe 2 and its derivatives), pore volume, and electrical conductivity.The number of sodiophilic sites and the pore volume gradually decrease from the anode to the separator, thus homogenizing Na + diffusion and inducing Na deposition from the bottom upward.In addition, the electrical conductivity of the framework gradually increases from the bottom to the top.This balances the difference in electron diffusivity and promotes the reversibility of the Na plating/stripping process (Scheme 1a, bottom).Consequently, the triple-gradient framework effectively induces Na + ions to deposit preferentially at the bottom of the host and inhibits dendrite growth, even under high plating capacity, leading to its good performance in both half-cells and full cells.

Results and Discussion
The preparation of the Gra-GC-MoSe 2 framework is illustrated in Scheme 1b (see details in the Supporting Information).Triple gradients were simultaneously realized by a simple electrospinning-pyrolysis method, which showed higher efficiency than previous strategies for the introduction of multiple-gradient structures. [24]In brief, four aqueous gelatin/(NH 4 ) 6 Mo 7 O 24 solutions with decreasing contents of Na 2 SeO 3 were used in sequence in an electrostatic spinner.
The spun product was then treated by pyrolysis and washed with water to obtain Gra-GC-MoSe 2 .This material showed superb flexibility without any damage, even after rolling and folding.The four layers of Gra-GC-MoSe 2 prepared with decreasing doses of Na 2 SeO 3 are denoted GC@MoSe 2 , GC@MoSe 2 /MoO xa, GC@MoSe 2 /MoO x -b, and GC@MoO x , respectively.Nongradient GC@MoSe 2 and GC@MoO x materials with high thickness were also prepared for comparison.
Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize the structural composition of the Gra-GC-MoSe 2 framework.As shown in Figure 1a, Gra-GC-MoSe 2 has a relatively loose porous structure with a thickness of ≈45 μm.The EDS mapping images clearly show the gradient in Se content from the bottom to the top, in contrast to the uniform dispersion of Mo and C. X-ray diffraction (XRD) and Raman spectroscopy (Figure S1, Supporting Information) not only confirm the presence of gelatin-derived carbon in all four layers but also verify the presence of the MoSe 2 gradient within the Gra-GC-MoSe 2 framework, suggesting that Na 2 SeO 3 reacted with (NH 4 ) 6 Mo 7 O 24 and gelatin-derived carbon to form MoSe 2 .X-ray photoelectron spectroscopy (XPS) confirmed that Se heteroatoms were doped in the carbon skeleton and contributed to the gradient in Se and MoSe 2 compositions (Table S2, Supporting Information), while amorphous MoO x exhibited a gradient in the opposite direction (Figure S2, Supporting Information).
In addition to the gradient in chemical composition, the Gra-GC-MoSe 2 framework also exhibits an obvious gradient in morphological structure.High-resolution SEM images of the four layers of Gra-GC-MoSe 2 (Figure 1b1-e1) show that the structures of the 1D materials change from ultrathin nanobelts (GC@MoSe 2 ) to nanofibers (GC@MoO x ).The electrical conductivity of the four   S3, Supporting Information), consistent with the XPS results.Moreover, the number of pores and nanoparticles within Gra-GC-MoSe 2 decreases from GC@MoSe 2 to GC@MoO x .N 2 absorption/desorption isotherms show that both the specific surface area and pore volume gradu-ally decrease from GC@MoSe 2 to GC@MoO x (Figure S4 and S5 and Table S1, Supporting Information).Specifically, GC@MoSe 2 has a hierarchical porous structure with a high specific surface area and pore volume of 90.0 and 0.13 cm 3 g −1 , while GC@MoO x has a mesoporous structure with a low specific surface area of 13.9 m 2 g −1 and a pore volume 0.021 cm 3 g −1 .Highresolution TEM images further indicate that the morphology of the molybdenum selenides/oxides changes from crystalline MoSe 2 nanosheets to amorphous MoO x nanoclusters, which is The activation energy of Na-ion diffusion within the Gra-GC-MoSe 2 , GC@MoSe 2 , and GC@MoO x hosts.Galvanostatic cycling performance of Na|Na symmetric cells using the Gra-GC-MoSe 2 hosts at e) different current densities, f) a current density of 0.2 mA cm −2 with a capacity of 10 mAh cm −2 , and g) a current density of 1 mA cm −2 with a capacity of 0.5 mAh cm −2 and h) Comparison with other symmetric cells reported in recent literature.i) Capacity-voltage curves of the Na@Gra-GC-MoSe 2 ||NVP and Na@Cu||NVP full cells.j) Cycling performance of the Na@Gra-GC-MoSe 2 ||NVP and Na@Cu||NVP full cell.(inset: digital photos of a Na@Gra-GC-MoSe 2 ||NVP pouch cell powering LED lights in different states. confirmed by the selected area electron diffraction (SAED) patterns (Figure 1b3-e3, inset).
The Gra-GC-MoSe 2 framework and the other two non-gradient frameworks (GC@MoSe 2 and GC@MoO x ) were then used as hosts for NMAs.Asymmetric cells were first assembled and tested to observe the initial nucleation overpotential of Na on Cu foil and different hosts (Figure 2a).The Na|Gra-GC-MoSe 2 cell exhibited a much smaller initial nucleation overpotential of 19.2 mV than the Na|Cu (278 mV), Na|GC@MoO x (30.7 mV), and Na|GC@MoSe 2 cells (30.1 mV), indicating that the Na + diffusion and electron transport within the Gra-GC-MoSe 2 framework is faster than in the other three materials. [32]Moreover, the Na|Gra-GC-MoSe 2 cell also exhibited the lowest voltage hysteresis (Figure S7, Supporting Information).In addition, the Gra-GC-MoSe 2 cell displayed an impressive cycling stability of 800 cycles with an average coulombic efficiency (CE) of 99.8% at a low current density of 0.5 mA cm −2 (Figure 2b), a superior performance to those of the other three cells.Moreover, the Gra-GC-MoSe 2 cell still works stably at higher current densities of 1 and 2 mA cm −2 (Figure S6, Supporting Information), highlighting the advantages of the triple-gradient framework.][37][38] By analyzing the temperaturedependent electrochemical impedance spectra from 253 to 333 K (Figure S9 and Table S3, Supporting Information), the kinetics during Na deposition can be quantitatively investigated based on Arrhenius plots. [39]The Na|Gra-GC-MoSe 2 cell has a much smaller activation energy of 9.3 kJ mol −1 than the Na|GC@MoSe 2 (23.5 kJ mol −1 ) and Na|GC@MoO x (24.3 kJ mol −1 ) cells, suggesting that the gradients in pore volume and electrical conductivity in the former material favor the diffusion of both Na + ions and electrons (Figure 2d).Symmetric cells were subsequently assembled in order to evaluate the galvanostatic cyclic performance of NMAs with gradient and non-gradient hosts.As shown in Figure 2e, the Gra-GC-MoSe 2 framework enabled the symmetric cell to work steadily with a low overpotential below 220 mV, even at 10 mA cm −2 .Furthermore, the Gra-GC-MoSe 2 framework afforded an ultrahigh plating/stripping capacity of 10 mAh cm −2 over 800 h (Figure 2f).In terms of cycling performance, the symmetric cell using the Gra-GC-MoSe 2 framework showed a long lifetime of over 2000 h with an ultralow voltage hysteresis (Figure 2g), which was superior to previously symmetric cells reported in recent literature (Figure 2h; Table S5, Supporting Information). [21,35,40,41]In marked contrast, symmetric cells using GC@MoSe 2, GC@MoO x , or Cu foil fluctuated obviously and failed to work after a short time (Figure 2g).
In order to evaluate the feasibility of using Gra-GC-MoSe 2modified NMAs (denoted Na@Gra-GC-MoSe 2 ) in practical applications, full cells were assembled by pairing Na@Gra-GC-MoSe 2 with Na 3 V 2 (PO 4 ) 3 (NVP).The N/P ratio of the Na@Gra-GC-MoSe 2 ||NVP was about ≈3.5.The capacity-voltage curves show that the Na@Gra-GC-MoSe 2 ||NVP full cell exhibited a large specific capacity of 117.4 mAh g −1 and a high initial CE of 98% at 0.5 C, much higher than the corresponding values for the Na||NVP full cell (38.8 mAh g −1 and 59%, Figure 2i).A discharge capacity of ≈70 mAh g −1 could still be delivered even at 5 C, implying a good rate capability (Figure S10, Supporting Information).In addition, the Na@Gra-GC-MoSe 2 ||NVP full cell also had a lower hysteresis voltage than the Na||NVP full cell, allowing higher discharge energies.A high energy density of ≈257.0Wh kg −1 can be delivered based on the total weight of Na@Gra-GC-MoSe 2 and NVP, which surpasses by far previously reported values of sodium battery systems with NVP as cathode (Table S6, Supporting Information). [42,43]Moreover, the Na@Gra-GC-MoSe 2 ||NVP full cell exhibited a remarkable cycling performance even with a low N/P ratio (Table S7, Supporting Information), with a capacity retention of ≈95% even after 400 cycles at 0.5 C (Figure 2j), showing its good prospects for practical use.The Na@Gra-GC-MoSe 2 ||NVP pouch cell could easily power LED lights due to its high energy density and showed good bending performance by virtue of the self-standing properties of the Gra-GC-MoSe 2 framework (Figure 2j, inset).
In situ XRD was employed to detect the plating position of metallic Na within the gradient and non-gradient hosts (Figure S11, Supporting Information).The XRD patterns for the bottom layer (furthest away from the separator side) of Gra-GC-MoSe 2 and Cu foam during the plating/stripping process are shown in Figure 3a.For the Gra-GC-MoSe 2 framework, the characteristic peak of Na (110) plane was detected during the deposition process but completely disappeared after the stripping process (Figure 3a, left).However, no peak characteristic of Na metal was detected for the non-gradient host (Figure 3a, right).This confirms that Na is preferentially deposited at the bottom of Gra-GC-MoSe 2 because of the homogeneous Na + flux and sodiophilicity gradient.The complete stripping of Na can be attributed to the uniform and fast electron diffusion within the Gra-GC-MoSe 2 framework resulting from the good structural reversibility of the gelatinderived carbon.The high degree of recovery of the D and G bands in the Raman spectra confirms the dynamic stability of the carbon framework during the plating/stripping process (Figure 3b; Figure S12, Supporting Information).
Ex situ Raman spectroscopy highlights the important role of MoSe 2 and Se heteroatoms in regulating Na deposition (Figure S13, Supporting Information).[46] The TEM image and corresponding SAED pattern of the Gra-GC-MoSe 2 host discharged to 0 V (versus Na + /Na) confirm the coexistence of Na 2 Se, Mo, and NaF (Figure 3d).This suggests that most of the MoSe 2 transformed into Na 2 Se and Mo during the sodiation process and could not revert to MoSe 2 .MoSe 2 and its derivatives such as Na 2 Se and Mo are all involved in the subsequent plating/stripping process.
DFT calculation results show that MoSe 2 , Mo, and Na 2 Se can spontaneously adsorb Na + ions with adsorption energies of −1.564, −2.352, and −4.463 eV, respectively (Figure 3e).In addition, Se-doped carbon has a Na + adsorption energy of −2.488 eV, demonstrating that the bottom layer of Gra-GC-MoSe 2 also has good sodiophilicity.Due to the low pore volume of the top layer (GC@MoO x ), MoO x was embedded into the nanofibers, and the surface N-doped carbon has a major effect on the Na deposition behavior.The calculated Na + adsorption energy of the N-doped carbon was only -0.943 eV, showing that the sodiophilicity of GC@MoO x is poorer than that of the underlying layers.As a result of the gradient in sodiophilicity and homogeneous Na + /electron diffusivities, Na preferentially nucleated and grew uniformly from the bottom to the top of the Gra-GC-MoSe 2 framework and exhibited high reversibility during the plating/stripping process.
COMSOL Multiphysics simulations were employed to investigate the effect of structure on the current density distributions for Na plating/stripping.Based on their morphological features, nanofibers and porous nanobelts were chosen as the models for GC@MoO x and GC@MoSe 2 , respectively.The color transition from blue to red shown in Figure 3f-i represents the change in relative current density from low to high.From the bottom view, the simulation results show that the centers of the porous nanobelts (Figure 3f) and the edges of the nanofibers (Figure 3g) exhibit the highest current densities.Similar simulation results are shown from the side view in Figure 3h,i.As a result, Na tends to plate on and strip off the surface of the nanofibers, leading to longitudinal Na growth and dendrite formation then becomes inevitable.In contrast, Na preferentially deposits in the pores of the porous nanobelts without dendrite formation.Thus, the gradients in morphology and porosity of Gra-GC-MoSe 2 can facilitate the uniform deposition of dendrite-free NMAs.
The modification of NMAs by the Gra-GC-MoSe 2 framework was verified by in situ optical microscopy.As shown in Figure 4a, Na dendrites formed quickly on the Cu surface, causing a large volume change.Although Na dendrites were not observed on the non-gradient sodiophilic framework (GC@MoSe 2 ) in the initial plating process, an obvious volumetric expansion and surface Na dendrite formation were eventually observed due to the uneven ion/electron fluxes (Figure 4b).However, a smooth surface without any dendrites was observed for the Gra-GC-MoSe 2 framework throughout the entire deposition process (Figure 4c), efficiently solving the problems of dendrite formation and infinite expansion of NMAs.In order to characterize the Na deposition behavior more clearly, cross-section and surface SEM images were obtained for the Gra-GC-MoSe 2 and GC@MoSe 2 frameworks under different Na plating capacities.During the deposition process, a metallic Na deposit continuously filled the pores from the bottom upward in Gra-GC-MoSe 2 (Figure 4d1-f1).In addition, compacted Na deposits were found at the bottom of Gra-GC-MoSe 2 (Figure S14, Supporting Information), while no deposits were found on the top of the framework (Figure 4d2-f2), proving that the triple-gradient Gra-GC-MoSe 2 can efficiently regulate Na deposition.In contrast, Na was unevenly plated in the upper layer, with obvious Na deposits on the top of the non-gradient GC@MoSe 2 framework (Figure S15, Supporting Information).The digital photos for the bottom side of the Gra-GC-MoSe 2 framework and contrast samples after Na deposition further suggested the unique sodiophilicity of Gra-GC-MoSe 2 (Figure S16, Supporting Information).After the stripping process, all of the Na deposits disappeared without any dead Na remaining and the structure of the Gra-GC-MoSe 2 framework was completely recovered, demonstrating the good reversibility of Na@Gra-GC-MoSe 2 (Figure 4g1, g2).

Conclusion
We have prepared a self-standing framework with a triplegradient structure by a facile and controllable method.The tai-lored gradients of the number of sodiophilic sites, pore volumes, and electrical conductivity homogenize Na + and electron diffusion, thus inducing Na to preferentially deposit on the side furthest away from the separator and enhancing the reversibility of the plating/stripping process.Moreover, the porous nanobelt structure provides a large space for Na growth, effectively inhibiting dendrite formation and accommodating anode expansion.Therefore, the modified NMAs exhibited excellent performance in both half-cells and full-cells.This work not only provides a new idea for the design of high-performance NMA hosts but should also promote the practical use of Na metal batteries.and c) Gra-GC-MoSe 2 at 0.5 mA cm −2 for 4 h.Ex situ cross-section and surface SEM images of the top (nearest the separator side) for the Gra-GC-MoSe 2 frameworks under Na plating capacities of (d1, d2) 1 mAh cm −2 , (e1, e2) 2 mAh cm −2 , (f1, f2) 3 mAh cm −2 at 0.5 mA cm −2 , and (g1, g2) under a Na plating/stripping capacity of 3 mAh cm −2 at 0.5 mA cm −2 .

Scheme 1 .
Scheme 1. a) Schematic illustration of Na plating on Cu foil, the electrically conducting framework, the electrically conducting sodiophilic framework, and the triple-gradient framework.b) Schematic illustration of the synthesis of the Gra-GC-MoSe 2 framework and digital photos of the products at various stages.
materials gradually increases from bottom to top (Figure 1b1-e1, inset).Transmission electron microscopy (TEM) images confirm the structural changes (Figure 1b2-e2), and the corresponding energy dispersive X-ray elemental mapping images show welldispersed C, N, Se, O, and Mo (Figure

Figure 2 .
Figure 2. a) Voltage-capacity curves in the initial cycle and b) Coulombic efficiency of Na|Cu, Na|GC@MoO x , Na|GC@MoSe 2 and Na|Gra-GC-MoSe 2 half cells.c) Comparison of the electrochemical performance of the Na|Gra-GC-MoSe 2 half-cell with other materials reported in the recent literature.d)The activation energy of Na-ion diffusion within the Gra-GC-MoSe 2 , GC@MoSe 2 , and GC@MoO x hosts.Galvanostatic cycling performance of Na|Na symmetric cells using the Gra-GC-MoSe 2 hosts at e) different current densities, f) a current density of 0.2 mA cm −2 with a capacity of 10 mAh cm −2 , and g) a current density of 1 mA cm −2 with a capacity of 0.5 mAh cm −2 and h) Comparison with other symmetric cells reported in recent literature.i) Capacity-voltage curves of the Na@Gra-GC-MoSe 2 ||NVP and Na@Cu||NVP full cells.j) Cycling performance of the Na@Gra-GC-MoSe 2 ||NVP and Na@Cu||NVP full cell.(inset: digital photos of a Na@Gra-GC-MoSe 2 ||NVP pouch cell powering LED lights in different states.

Figure 3 .
Figure 3. a) In situ XRD patterns of the Gra-GC-MoSe 2 framework (left) and non-gradient framework (right) in the Na plating/stripping process.b) In situ Raman spectra of carbon within the Gra-GC-MoSe 2 framework in the Na plating/stripping process.c) Ex situ Raman spectra of MoSe 2 within the Gra-GC-MoSe 2 framework in the Na plating/stripping process.d) High-resolution TEM image of the Gra-GC-MoSe 2 framework at 0 V (versus Na + /Na, inset: SAED pattern).e) Adsorption energies of Na atoms on MoSe 2 , Mo, Na 2 Se, Se,N-codoped carbon, and N-doped carbon.COMSOL Multiphysics simulations of the current density distributions for porous nanobelts and nanofibers from f,g) the bottom view and h,i) the side view with the schematic of Na + migration direction.