MOF-Derived Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC Composite Anode Materials towards High-Performance Na-Ion Storage

: Binary transition metal selenides (BTMSs) are more promising than single transition metal selenides (TMS) as anode materials of sodium-ion batteries (SIBs). However, it is still very challenging to prepare high-performance BTMSs in the pure phase, instead of a mixture of two TMSs. In this study, a binary metal center-based MOF derived selenization strategy was developed to prepare iron–cobalt selenide (Fe 2 CoSe 4 @NC) and iron–nickel selenide (Fe 2 NiSe 4 @NC) nanocomposites in the single phase and when wrapped with carbon layers. As the anode material of SIBs, Fe 2 CoSe 4 @NC exhibits higher long-term cycling performance than Fe 2 NiSe 4 @NC, maintaining a capacity of 352 mAh g − 1 after 2100 cycles at 1.0 A g − 1 , which is ascribed to the higher percentage of the nanopores, larger lattice spacing, and faster Na+ diffusion rate in the electrode materials of the former rather than the latter.

In recent years, transition metal selenides (TMSs) have garnered extensive attention as anode materials for sodium-ion batteries (SIBs), owing to their high theoretical capacity [20,21].For example, our group has developed ZIF-67-derived CoSe 2 nanoparticles (CoSe 2 @NCF/CNTs) wrapped with N-doped and CNT-entangled carbonaceous materials [8].The overall structural morphology of CoSe 2 @N-CF/CNTs composites is well preserved, as anode materials of SIBs, even after 100 cycles at a current density of 1 A g −1 , demonstrate the effectiveness of carbonaceous material encapsulation in maintaining structural integrity.However, the electrochemical properties of TMSs anodes are constrained by challenges such as volume expansion and dissolution of polyselenide during cycling, leading to reduced conductivity and inferior electrochemical performance [17,[22][23][24].
Binary transition metal chalcogenides (BTMCs), due to their improved electrochemical performance compared to single-metal compounds, have garnered significant attention.
The improved performance of binary transition metal selenides (BTMS) as the anode materials of SIBs is ascribed to their superior conductivity, endowed by the smart choice of metal elements in combination, and coupled, with the engineering of nanostructures [8, 25,26].For instance, hierarchically porous nanospheres of binary iron-cobalt selenide (Fe 2 CoSe 4 ) were prepared using a hydrothermal method to obtain Fe-Co glycerate and further selenization using Se powder as a selenium precursor in hydrogen gas atmosphere, which exhibits an impressive rate capability and extended life cycle [8].In another study, three types of binary transition metal selenides based on nickel-cobalt, nickel-iron, and cobalt-iron combinations were prepared with nanosheet structures, and nickel-iron selenide exhibits the longest cycling performance and highest charge capacity [25].However, exploration towards the synthesis of BTMS and their application as anode materials SIBs are still very limited, since it is still very challenging to prepare a binary transition metal selenide (M1 x M2 y Se z , M1, and M2 are two different transition metal atoms) in the single phase, instead of mixed two types of metal selenides (M1 x Se y /M2 x Se z ) [27].Additionally, the mechanism behind the difference in the electrochemical performance of different combinations of these transition metal selenides remains unexplored.
Herein, we study MOF-derived bimetallic selenides, namely, iron-cobalt selenide (Fe 2 CoSe 4 @NC) and nickel-iron selenide (Fe 2 NiSe 4 @NC).Both Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC exhibited outstanding long-term cycling stability and rate capability, maintaining a capacity of 352 and 282.2 mAh g −1 after 2100 cycles at 1.0 A g −1 , respectively.Both the electrolyte's penetration of the porous electrode surface and the sodium ion conductance in the anode materials might govern the electrochemical performance of sodium-ion storage.

Results
Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC were synthesized with the selenization of a binary transition metal organic framework (MOF).Initially, a mixture of FeCo-MOF or FeNi-MOF and selenium powder was maintained at 300 • C for 4 h to ensure the complete selenization of the metal ligands.Subsequently, the sample was further annealed at 700 • C to fully carbonize the MOF structure to obtain carbon-wrapped BTMS.The crystal structure of the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC samples was initially examined using XRD, as depicted in Figure 1a,b.The diffraction pattern in Figure 1a,b exhibits several peaks that can be accurately indexed to Fe 2 CoSe 4 @NC (JCPDS 89-1967) and Fe 2 NiSe 4 @NC (JCPDS 89-1968) without any detectable impurities [8].This confirms the efficacy of our proposed strategy for the facile synthesis of BTMS in the pure phase.Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC show very similar diffraction patterns.The four diffraction peaks, with a high intensity identified at 32.9 • to 33.9 • , were ascribed to the reflection of (-2 0 2), (-1 1 2), (1 1 2), and (2 0 2), and the two prominent peaks at 44 • and 44.7 • were indexed to (-1 1 4) and (1 1 4).Another two evident peaks were identified at 51.49 • and 51.59 • , which were from the (310) and (0 2 0) planes.Although the diffraction positions of these prominent peaks are quite similar to each other, for Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC, the relative peak intensities were different from one another.For instance, the peak intensity of (-1 1 2) is slightly higher than that of (1 1 2) for Fe 2 CoSe 4 @NC, but it is the opposite case for Fe 2 NiSe 4 @NC, indicating the orientation difference of the lattice reflections of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC.Additionally, the diffraction peak position of (-1 1 4) and (1 1 4) of Fe 2 CoSe 4 @NC negatively shifted compared to the standard peak, due to the lattice extension of (-1 1 4) and (1 1 4) planes, while no apparent shift was observed for Fe 2 NiSe 4 @NC.It should also be noted that the peaks of (310) and (0 2 0) planes are well resolved for Fe 2 CoSe 4 @NC, but emerged as one peak for Fe 2 NiSe 4 @NC, indicating the higher crystallinity of Fe 2 CoSe 4 @NC compared to Fe 2 NiSe 4 @NC.
The N 2 adsorption/desorption isotherms of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC (Figure 1c) exhibit a type IV isotherm with a hysteresis loop at a relatively high pressure, indicating their mesoporous characteristics [11].Both Fe 2 CoSe 4 @NC (75.02 m 2 g −1 ) and Fe 2 NiSe 4 @NC (80.66 m 2 g −1 ) display similar specific surface areas.The pore-size dis-tribution of the Fe 2 CoSe 4 @NC sample, calculated using the Barrett−Joyner−Halenda (BJH) method, ranges from 3.8 to14.4 nm (Figure 1d), including a narrow distribution at 4.8 nm and a broad distribution at around 7.5 nm, with a much higher percentage for the smaller pores than the larger ones.On the other hand, the pore size distribution of the Fe 2 NiSe 4 @NC sample was predominantly centered around 4.3~10 nm, with the maximum peak intensity being 7.3 nm.This indicates that the pore size in Fe 2 CoSe 4 @NC is relatively smaller than that of Fe 2 NiSe 4 @NC.It has been reported that the mesoporous nature and large surface area of electrode materials can enhance the electrolyte penetration, surface contact with electrolyte, and the interaction with sodium-ions, as well as increasing electronic conductivity by reducing the ion diffusion length [8].
Inorganics 2024, 12, x FOR PEER REVIEW 3 of 13 resolved for Fe2CoSe4@NC, but emerged as one peak for Fe2NiSe4@NC, indicating the higher crystallinity of Fe2CoSe4@NC compared to Fe2NiSe4@NC.
The N2 adsorption/desorption isotherms of Fe2CoSe4@NC and Fe2NiSe4@NC (Figure 1c) exhibit a type IV isotherm with a hysteresis loop at a relatively high pressure, indicating their mesoporous characteristics [11].Both Fe2CoSe4@NC (75.02 m 2 g −1 ) and Fe2NiSe4@NC (80.66 m 2 g −1 ) display similar specific surface areas.The pore-size distribution of the Fe2CoSe4@NC sample, calculated using the Barrett−Joyner−Halenda (BJH) method, ranges from 3.8 to14.4 nm (Figure 1d), including a narrow distribution at 4.8 nm and a broad distribution at around 7.5 nm, with a much higher percentage for the smaller pores than the larger ones.On the other hand, the pore size distribution of the Fe2NiSe4@NC sample was predominantly centered around 4.3~10 nm, with the maximum peak intensity being 7.3 nm.This indicates that the pore size in Fe2CoSe4@NC is relatively smaller than that of Fe2NiSe4@NC.It has been reported that the mesoporous nature and large surface area of electrode materials can enhance the electrolyte penetration, surface contact with electrolyte, and the interaction with sodium-ions, as well as increasing electronic conductivity by reducing the ion diffusion length [8].
The morphology of the nanocomposites was examined using scanning electron microscopy (SEM), as depicted in Figure 2a,b.In Figure 2a, SEM images of Fe2CoSe4@NC reveal random nanoparticles and nano-blocks, along with small pores on the surface.On the other hand, the SEM image of Fe2NiSe4@NC displays a noticeable agglomeration of nanoparticles (Figure 2b).The high-resolution transition electron microscopy (HRTEM) image of Fe2CoSe4@NC (Figure 2c) shows the calculated lattice spacing of 2.66 Å for (202) plane, which is slightly larger than that of Fe2NiSe4@NC(2.62 Å) (Figure 2d), consistent The morphology of the nanocomposites was examined using scanning electron microscopy (SEM), as depicted in Figure 2a,b.In Figure 2a, SEM images of Fe 2 CoSe 4 @NC reveal random nanoparticles and nano-blocks, along with small pores on the surface.On the other hand, the SEM image of Fe 2 NiSe 4 @NC displays a noticeable agglomeration of nanoparticles (Figure 2b).The high-resolution transition electron microscopy (HRTEM) image of Fe 2 CoSe 4 @NC (Figure 2c) shows the calculated lattice spacing of 2.66 Å for (202) plane, which is slightly larger than that of Fe 2 NiSe 4 @NC(2.62Å) (Figure 2d), consistent with the results obtained from the XRD patterns [25].
The carbon content in the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC composites was evaluated using TGA, as shown in Figure S1.The percentages of TGA products for Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC were measured as 44.68% and 41.44%, respectively.According to the XRD (Figure S2), the TGA products of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC were identified to be (CoFe 2 )O 4 and (NiFe 2 )O 4 , respectively.Based on the stoichiometry of the chemical reaction Fe 2 CoSe 4 @NC (s) + 3 O 2 (g) = (CoFe 2 )O 4 (s) + 4 SeO 2 (g), the content of Fe 2 CoSe 4 is calculated to be approximately 92.6 wt%, suggesting a carbon content of about 7.3 wt%.Similarly, the carbon content of Fe 2 NiSe 4 @NC is calculated to be about 14.0 wt%.
The carbon content in the Fe2CoSe4@NC and Fe2NiSe4@NC composites was evaluated using TGA, as shown in Figure S1.The percentages of TGA products for Fe2CoSe4@NC and Fe2NiSe4@NC were measured as 44.68% and 41.44%, respectively.According to the XRD (Figure S2), the TGA products of Fe2CoSe4@NC and Fe2NiSe4@NC were identified to be (CoFe2)O4 and (NiFe2)O4, respectively.Based on the stoichiometry of the chemical reaction Fe2CoSe4@NC (s) + 3 O2 (g) = (CoFe2)O4 (s) + 4 SeO2 (g), the content of Fe2CoSe4 is calculated to be approximately 92.6 wt%, suggesting a carbon content of about 7.3 wt%.Similarly, the carbon content of Fe2NiSe4@NC is calculated to be about 14.0 wt%.
The storage properties of the sodium-ion of the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC anodes were assessed through cyclic voltammetry (CV) in a half-cell configuration, wherein CR2032-type cells were assembled with Na foil serving as a counter electrode.Figure 4a illustrates the first CV cycle of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC as an anode of SIBs within the voltage range of 0.5~3.0V, which was conducted at a scan rate of 0.2 mV s −1 .The significant peak near 1.10 V in the cathodic scans for the Fe 2 CoSe 4 @NC electrode, which was not observed in the subsequent scan, and can be mainly attributed to electrolyte decomposition and the formation of a solid electrolyte interphase (SEI), as well as the conversion of Fe 2 CoSe 4 @NC to Na 2 Se and Na x Fe 2 CoSe 4−y , as described in Equations ( 1)-(3) [35,44].
The Raman bands at 1350 and 1576 cm -1 are indexed to amorphous carbon (D band) and graphitic carbon (G band) [42].The ID/IG values for Fe2CoSe4@NC and Fe2NiSe4@NC were 0.97 and 0.94, respectively (Figure S4), indicating higher graphitization of Fe2CoSe4@NC than Fe2NiSe4@NC [43].The storage properties of the sodium-ion of the Fe2CoSe4@NC and Fe2NiSe4@NC anodes were assessed through cyclic voltammetry (CV) in a half-cell configuration, wherein CR2032-type cells were assembled with Na foil serving as a counter electrode.Figure 4a illustrates the first CV cycle of Fe2CoSe4@NC and Fe2NiSe4@NC as an anode of SIBs within the voltage range of 0.5~3.0V, which was conducted at a scan rate of 0.2 mV s −1 .The significant peak near 1.10 V in the cathodic scans for the Fe2CoSe4@NC electrode, which was not observed in the subsequent scan, and can be mainly attributed to electrolyte decomposition and the formation of a solid electrolyte interphase (SEI), as well as the conversion of Fe2CoSe4@NC to Na2Se and NaxFe2CoSe4−y, as described in Equations ( 1)-(3) [35,44].
Discharge procedure of Fe2CoSe4@NC: NiSe + 2Na + + 2e − → Ni + Na2Se Figure 4. Na-storage properties of the Fe2CoSe4@NC and Fe2NiSe4@NC electrodes as anodes: (a) the first and (b) second cycle of CV curves for Fe2CoSe4@NC and Fe2NiSe4@NC at 0.2 mV s −1 within 0.5~3.0V; (c) the first three cycles of the CV curves for Fe2CoSe4@NC and Fe2NiSe4@NC; and (d) the EIS spectra of Fe2CoSe4@NC and Fe2NiSe4@NC electrodes before and after cycling at 0.1 A g −1 .
In contrast to that of Fe2CoSe4@NC, Fe2NiSe4@NC exhibits two prominent cathodic peaks at 1.07 V and 0.71 V in the first cycle, respectively, which can be attributed to electrolyte decomposition, the formation of a solid electrolyte interphase (SEI), and the conversion of Fe2NiSe4@NC to NaxSe, FexSe, and NixSe, as well as the generation of Ni (NiSe + 2Na + + 2e − → Na2Se + Ni) (Equations ( 4)-( 7)) [26,45].Meanwhile, the additional cathodic peak for Fe2NiSe4@NC might indicate that the surface reaction and formation of SEI film Discharge procedure of Fe 2 CoSe 4 @NC: In contrast to that of Fe 2 CoSe 4 @NC, Fe 2 NiSe 4 @NC exhibits two prominent cathodic peaks at 1.07 V and 0.71 V in the first cycle, respectively, which can be attributed to electrolyte decomposition, the formation of a solid electrolyte interphase (SEI), and the conversion of Fe 2 NiSe 4 @NC to Na x Se, Fe x Se, and Ni x Se, as well as the generation of Ni (NiSe + 2Na + + 2e − → Na 2 Se + Ni) (Equations ( 4)-( 7)) [26,45].Meanwhile, the additional cathodic peak for Fe 2 NiSe 4 @NC might indicate that the surface reaction and formation of SEI film on Fe 2 NiSe 4 @NC is more complicated than that of the Fe 2 CoSe 4 @NC electrode.
In the anodic scan of the first cycle for the Fe 2 CoSe 4 @NC electrode, a peak and shoulder were observed at 1.350 V and 1.562 V, respectively, which are indexed to the conversion reaction of Fe 2 CoSe 4 + 8Na + + 8e − → 4Na 2 Se + 2Fe + Co.The anodic peak potential for Fe 2 CoSe 4 @NC, which emerges at 1.71 and 1.60 V, is less positive than that of Fe 2 NiSe 4 @NC, suggesting a higher oxidation potential for the reaction of Fe 2 NiSe 4 + 8Na + + 8e − → 4Na 2 Se + 2Fe + Ni.
In the second cycle of the CV scan, Fe 2 CoSe 4 @NC (Figure 4b) exhibits three cathodic peaks at 1.84, 1.24, and 0.75 eV, respectively, corresponding to sodiation reactions [8].It should be noticed that the cathodic peak intensity at 1.10 V in the first scan was much decreased in the second scan, due to the stable formation SEI film in the first scan.In the second cycle of Fe 2 NiSe 4 @NC (Figure 4b), four distinct cathodic peaks are observed at 1.70, 1.43, 1.01, and 0.77 eV, respectively, corresponding to the naturalization reactions ( 4)- (7), respectively [28,45].The cathodic peak at 0.77 eV remains very intense, indicating more complicated reactions involved in the discharging process of the Fe 2 NiSe 4 @NC electrode than the Fe 2 CoSe 4 @NC electrode.The second and third CV scans (Figure 4c) overlap very well with each other for both the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC electrodes, suggesting a reversible and stable cycling performance.
To further investigate the interfacial charge transfer kinetics of the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC electrodes, electrochemical impedance spectroscopy (EIS) was conducted within a frequency range of 0.1 Hz to 100 kHz.As depicted in Figure 4d, the Nyquist plots of both fresh electrodes exhibit a semicircle in the high-frequency region and a slanted line in the low-frequency region, corresponding to charge transfer resistance (R ct ) at the electrode-electrolyte interface and Na + diffusion process in the electrode, respectively.Fresh Fe 2 CoSe 4 @NC exhibits a significantly smaller R ct state (10.56 Ω) than that of fresh Fe 2 NiSe 4 @NC (35.25 Ω) due to the higher percentage of the smaller pore size and more complete electrolyte penetration for Fe 2 CoSe 4 @NC than the Fe 2 NiSe 4 @NC electrode, as revealed using a BET measurement (Figure 1c,d) [46].After the first cycle, the R ct values of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC decreased to 4.63 and 6.52 Ω, respectively, and a much larger variation of the R ct values for Fe 2 NiSe 4 @NC was observed, indicating a more robust structure of Fe 2 CoSe 4 @NC than Fe 2 NiSe 4 @NC.This observation is in agreement with the results obtained from CV scans.On the other hand, the slope of the slanted line observed in the low-frequency region represents the Warburg impedance (Z w ) associated with the diffusion of Na + [47,48].It is apparently observed that the slope of Fe 2 CoSe 4 @NC is much higher than that of Fe 2 NiSe 4 @NC after the first charge/discharge cycle.This indicates a faster Na + diffusion rate in the former electrode than the latter one, and also that the crystalline structure of Fe 2 CoSe 4 @NC is more conductive for Na + diffusion than Fe 2 NiSe 4 @NC, which is probably due to the larger interplanar distance of Fe 2 CoSe 4 @NC than Fe 2 NiSe 4 @NC, as observed in HRTEM.
The rate capability of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC was evaluated by varying the current densities from 0.1 to 10 A g −1 , as depicted in Figure 5a.Fe 2 CoSe 4 @NC demonstrates a reversible specific capacity of 470, 378, 362, 357, 326, and 270 mAh g −1 at a current density of 0.1, 0.5, 1, 2, 5, and 10 A g −1 , respectively.Similarly, Fe 2 NiSe 4 @NC exhibits rate capability, delivering 530, 330, 318, 315, 300, and 270 mAh g −1 -specific capacities at the current densities of 0.1, 0.5, 1.0, 2, 5, and 10, respectively.It is observed that Fe 2 CoSe 4 @NC exhibits a lower reduction in capacity than Fe 2 NiSe 4 @NC with an increase in current density, possibly due to the more robust structure of the former than the latter, which surpasses that of most single transition metal selenides reported in the literature, as illustrated in Figure 5b and Table S1.To further elucidate the exceptional rate performance of the Fe2CoSe4@NC electrode, CV scans at sweep rates from 0.2 to 1.0 mV s −1 were conducted to differentiate pseudocapacitive and diffusion-controlled contributions to energy storage capacity.As shown in Figure 6a, the CV profiles reveal two distinct pairs of cathodic and anodic peaks.The current response to the scan rate follows the relationship described below (Formulas (8) and ( 9) where a and b represent adjustable parameters.At b = 0.5, diffusion-controlled behavior prevails during the charge/discharge process, whereas when b = 1, the pseudocapacitive effect dominates.In the current study, the b values of the three reduction peaks (peaks 1, 2, and 3) and the corresponding oxidation peaks (peaks 4, 5, and 6) were determined to be As illustrated in Figure 5c, the Fe 2 CoSe 4 @NC electrode delivers a specific discharge capacity of 352 mA h g −1 after 2100 cycles at 1.0 A g −1 , much higher than that of Fe 2 NiSe 4 @NC (282.2 mA h g −1 ).Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC maintain capacities of 380 and 340 mA h g −1 (Figure S5) after 1500 cycles with a current density of 2 A g −1 (Figure S5).At a current density of 4 A g −1 , the capacities of the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC electrodes retain 370 and 310 mA h g −1 after 900 cycles, respectively (Figure S6).This indicates that the Fe 2 CoSe 4 @NC electrode possesses a higher energy density and cycle stability than Fe 2 NiSe 4 @NC.
To further elucidate the exceptional rate performance of the Fe 2 CoSe 4 @NC electrode, CV scans at sweep rates from 0.2 to 1.0 mV s −1 were conducted to differentiate pseudocapacitive and diffusion-controlled contributions to energy storage capacity.As shown in Figure 6a, the CV profiles reveal two distinct pairs of cathodic and anodic peaks.The current response to the scan rate follows the relationship described below (Formulas (8) and ( 9)): where a and b represent adjustable parameters.At b = 0.5, diffusion-controlled behavior prevails during the charge/discharge process, whereas when b = 1, the pseudocapacitive effect dominates.In the current study, the b values of the three reduction peaks (peaks 1, 2, and 3) and the corresponding oxidation peaks (peaks 4, 5, and 6) were determined to be 0.77, 0.97, 0.91, 0.85, 0.97, and 0.91, respectively, by log(i) versus log(v) plots (Figure 6b).These values are all between 0.6 to 1.0, indicating that the electrochemical reactions of the Fe 2 CoSe 4 @NC electrode are dominated by both diffusion-controlled and pseudocapacitive behaviors at a fixed voltage, which can be calculated following Equation (10): where k 1 ×v and k 2 ×v 1/2 represent the pseudocapacitive capacity and diffusion-controlled capacity, respectively.As summarized in Figure 6c, the pseudocapacitive contribution was estimated to be 90.6%,92.1%, 93.6%, 94.5%, and 95.7% at the scan rates of 0.2, 0.4, 0.6, 0.8, and 1.0, respectively, showing a continuous increase in pseudocapacitive contribution to energy storage of Fe 2 CoSe 4 @NC with increasing scan rate.As expected, the pseudocapacitive contribution dominates the charge-storage capacity at higher scan rates, which is beneficial for fast Na

Synthesis of FeCo-MOFs and FeNi-MOFs
In a typical process, Fe(NO 3 ) 3 •9H 2 O (4.5 mmol) and Co(NO 3 ) 3 •6H 2 O, (1.5 mmol) were added to a mixed solution of ethanol, DMF, and DI water (20 mL, 20 mL, and 20 mL) under vigorous stirring for about 10 min.Simultaneously, 1.5 mmol of trimesic acid and 3 g of PVP were dissolved in the same mixed solution under vigorous stirring for about 30 min.The resulting solution was then transferred into a 100 mL Teflon-lined autoclave, heated to 150 • C, and maintained for 10 h.The product was cooled to room temperature and washed three times with ethanol and DMF to obtain a metal organic framework containing iron and cobalt metal centers (denoted as FeCo-MOFs).FeNi-MOFs was also prepared using a similar protocol, with the molar ratio of iron nitrate Fe(NO 3 ) 3 •9H 2 O to Ni(NO 3 ) 3 •6H 2 O being 1 mmol:1 mmol.All other conditions remained the same.

Synthesis of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC
The FeCo-MOFs and FeNi-MOFs precursors were well ground with selenium powder (in weight ratio 1:2), placed in a silica-glazed ceramic boat, and covered with Cu foil.Selenization was carried out at 300 • C for 4 h under H 2 /Ar (10 vol% H 2 ), with a ramping rate of 2 • C min −1 .The temperature was heated to, and maintained at, 700 • C for 2 h for the carbonization of MOFs.Finally, the obtained samples are denoted as Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC.

Coin Cell Assembly
A slurry-coating procedure was adopted for the preparation of the working electrode as follows.Firstly, a homogeneous slurry was prepared by mixing Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC, carbon black (Super P, Timcal, Bironico, Switzerland), and sodium alginate binder at a mass ratio of 8:1:1, evenly pasted onto a copper foil by applying a film applicator, and was dried at 70 ºC in an electric oven overnight in order to remove the solvent.The mass loading of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC was about 1.2-1.5 mg•cm −2 on each electrode.
The coin cells of CR2032-type were built in an argon-filled glove box (Vigor-LG2400/750TS, LTD, Suzhou, China), in which the oxygen and water contents were less than 1 ppm.SIBs were assembled with a sodium tablet and glass fiber as counterpart electrode and separator, respectively.A Celgard-2400 film was used as a separator.The recipe of the commercial electrolyte is 1.0 M NaCF 3 SO 3 in diglyme.

Electrochemical Measurements
The electrochemical properties of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC as anodes of SIBs was tested using CR2032 coin cells.The galvanostatic discharge/charge (GCD) measurements were tested on a battery analysis system (CT2001A, LAND).Cyclic voltammetry (CV) tested at a scan rate of 0.2 mV s −1 , and electrochemical impedance spectra (EIS), conducted with the frequency range of 100 kHz to 0.01 Hz, were acquired with the CHI660 electrochemical workstation (Shanghai CH Instrument Co., Ltd., Shanghai, China).

Conclusions
In summary, the binary-metal selenides Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC were successfully synthesized with the preparation of a Fe/Co and Fe/Ni binary-metal organic framework using hydrothermal methods and sequential selenization binary Fe/Co and Fe/Ni MOF.The formation of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC was evidenced using XRD, XPS, and HRTEM.Fe 2 CoSe 4 @NC demonstrates higher long-term cycling stability and rate performance than Fe 2 NiSe 4 @NC, maintaining capacities of 352 and 282.2 mAh g −1 after 2100 cycles at 1.0 A g −1 , respectively.Such a higher electrochemical performance of Fe 2 CoSe 4 @NC than Fe 2 NiSe 4 @NC was ascribed to the higher portion of micropores and a higher diffusion rate of sodium-ions among the electrode composite.This study presents a novel method for synthesizing binary-metal selenides towards high-performance sodium-ion storage.

FeFigure 4 .
Figure 4. Na-storage properties of the Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC electrodes as anodes: (a) the first and (b) second cycle of CV curves for Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC at 0.2 mV s −1 within 0.5~3.0V; (c) the first three cycles of the CV curves for Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC; and (d) the EIS spectra of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC electrodes before and after cycling at 0.1 A g −1 .

14 Figure 5 .
Figure 5. Electrochemical performance of Fe2CoSe4@NC and Fe2NiSe4@NC.(a) Rate performances at current densities from 0.1 to 10 A g −1 and (c) cycling performances at a current density of 1 A g −1 of Fe2CoSe4@NC and Fe2NiSe4@NC.The Coulombic efficiency (CE, orange line) is closing to 100%, as shown in the right axis and directed by the orange arrow.(b) Rate capability of Fe2CoSe4@NC and those of reported single transition metal selenide electrodes, as cited in supporting information [29,42,49-63].

Figure 5 .
Figure 5. Electrochemical performance of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC.(a) Rate performances at current densities from 0.1 to 10 A g −1 and (c) cycling performances at a current density of 1 A g −1 of Fe 2 CoSe 4 @NC and Fe 2 NiSe 4 @NC.The Coulombic efficiency (CE, orange line) is closing to 100%, as shown in the right axis and directed by the orange arrow.(b) Rate capability of Fe 2 CoSe 4 @NC and those of reported single transition metal selenide electrodes, as cited in supporting information [29,42,49-63].
+ transfer kinetics during the intercalation/extraction process [8].Additionally, Figure 6d further illustrates the detailed pseudocapacitive portion (blue region) in comparison with the total current measured at a scan rate of 1.0 mV s −1 .Inorganics 2024, 12, x FOR PEER REVIEW 9 of 13 pseudocapacitive contribution dominates the charge-storage capacity at higher scan rates, which is beneficial for fast Na + transfer kinetics during the intercalation/extraction process [8].Additionally, Figure 6d further illustrates the detailed pseudocapacitive portion (blue region) in comparison with the total current measured at a scan rate of 1.0 mV s −1 .