Hard carbon anodes derived from phenolic resin/sucrose cross‐linking network for high‐performance sodium‐ion batteries

Hard carbons are widely studied as anode materials for sodium‐ion batteries (SIBs) due to their high Na‐storage capacity, long cycle life, and low cost. However, the low initial coulombic efficiency (ICE) and poor cycle performance remain bottleneck concerns that necessitate a comprehensive material engineering solution. Herein, we propose a facile strategy to synthesize amorphous carbons with pseudo‐graphitic dominated crystalline, expanded interlayer spacing, and reduced surface defects via carbonization of the cross‐linking network of phenolic resin and sucrose. An elaborate structural and electrochemical characteristics analysis has been investigated against different sucrose contents and carbonization temperatures. The representative PF‐S‐55‐1200 with the optimum cross‐linking degree as well as carbonization temperature realizes a high reversible Na‐storage capacity of 323.0 mAh g−1 with an ICE as high as 86.4%, much superior to the pristine phenolic resin pyrolytic carbon with a capacity of 267.1 mAh g−1 and an ICE of 46.3%. The hybrid hard carbons also exhibit robust structural stability with a prolonged cycle lifespan evidenced by a retained capacity of 238.3 mAh g−1 at a current density of 200 mA g−1 over 1500 cycles. The proposed route promises low‐cost and high‐performance hybrid hard carbons with optimized structural configuration for advanced SIBs.

electric vehicles. 5,6 However, the deployment of LIBs for large-scale grid storage application is hampered due to the growing scarcity of lithium resources, and it has been increasingly clear that eliminating the employment of expensive metals such as Li, Co, and Ni are crucial for further reducing the cost of LIBs. Thus, sustainable but effective alternative energy storage technologies are necessary to satisfy the proportionately growing energy needs. [7][8][9] Sodium and lithium are adjoining members of the periodic table with comparative physical and chemical characteristics. As such, sodium-ion batteries (SIBs) essentially hold the promising potential to coordinate with the presentation of LIBs. More importantly, the relatively abundant Na reserves and the costeffective extraction process recognize SIBs as a suitable alternative to LIBs for large-scale energy storage applications. 10 However, the exploration of appropriate anode materials for SIBs has been an important issue considering the poor Na-storage capacity of the successful anode materials for LIBs, i.e., graphite. 11 In addition, the large ionic size of Na is also a major factor contributing to the sluggish dynamics as well as the unanticipated volume changes and structural damage during the repeating insertion/extraction process, thus deteriorating the rate and cycle performance of the electrode. 12 Therefore, the development of high-performance anode materials has been urgent for the successful application of nextgeneration SIBs. 13,14 Although numerous materials have been reported as possible anodes for SIBs in the past decade, including carbon-based materials, titanium-based compounds, metal oxides/sulfides, alloys, organic compounds, and so on, carbon materials are considered the most promising anodes due to their low cost, environmental friendliness, and synthesis ease. 15,16 The electrochemical performance of carbonaceous materials often relies on precursors and synthetic conditions that have an indefinite impact on structural characteristics such as microcrystalline, porosity, and surface chemistry. In general, hard carbons, unlike soft carbons, possess relatively more disordered structures with expanded interlayer spacing and greater defect sites, making them ideal for efficient Na ion storage.
Various precursors, including biomass, 17 carbohydrates, 18,19 resins 20,21 have been extensively explored to prepare hard carbons. Remarkable progress has been made in hard carbons in the past decade, and the reversible Na-storage capacity of advanced hard carbon anodes can reach 300 mAh g −1 . 22 However, it remains a great challenge for hard carbons to enhance the Na-storage capacity without the sacrifice of initial columbic efficiency (ICE). For example, the structure design by introducing porous or hollow structures can improve the Na-storage capacity, but the enlarged surface area and increased defect content inevitably decrease the ICE. 23,24 The large irreversible capacity resulting from the formation of solid electrolyte interphase (SEI) layers on these surface-defect sites would consume a mass of Na ions from the cathode, thus affecting the energy density and deteriorating the cycling ability of the constructed full battery.
Therefore, numerous attempts have been made to obtain hard carbons with improved comprehensive electrochemical performance, including higher Nastorage capacity, higher ICE, and excellent cycle and rate performance. Cao et al. 25 reported that heating rate played an important effect on the structural properties of hard carbons, and slow pyrolysis rate (0.5°C min −1 ) can effectively reduce the concentration of defects in sucrosederived hard carbons prepared by hydrothermal treatment and subsequent high-temperature carbonization, thus a high ICE of 86.1% was achieved. Ji et al. 26 found that the addition of a little amount of graphene oxide (GO) into sucrose (1:80) can prevent the foaming process of sucrose during the drying and dehydration process. As a result, compared with the pyrolytic carbons from pure sucrose powder, the sucrose-derived hard carbons obtained with GO as additives show a significantly decreased S BET of 5.4 from 137.2 m 2 g −1 , correspondingly, the ICE value of the hard carbon electrode increased from 74% to 83%. Employing the cross-linking reaction of two components to reduce the surface defects of hard carbons is also an effective strategy to enhance the ICE. 20,27 In our previous work, the cross-linked interaction between lignite coal and sucrose greatly reduced the S BET from 169.52 m 2 g −1 for coal-based carbon to 1.48 m 2 g −1 for the final lignite coal/ sucrose carbonized products, correspondingly, the ICE value increases from 59.9% to 82.9%. 27 Here, the ample surface functional groups are regarded as the key factors for effective cross-linking between two precursors.
Phenolic resin has achieved significant recognition as raw material to produce functional carbon materials based on its relatively high carbon content and mature manufacturing technique. However, phenolic resinderived hard carbons usually exhibit abundant surface defects due to the release of gas molecules during the pyrolysis process. In addition, the relatively ordered graphite microcrystalline in phenolic resin-derived hard carbon further limits accessible Na-storage active sites. The low ICE and unsatisfactory Na-storage capacity doubtlessly restrict the development of phenolic resinderived hard carbon in SIBs. [28][29][30] Sucrose, a type of carbohydrate with plenty of functional groups, is commonly used as a hard carbon precursor. The abundant functional groups in sucrose and phenolic resin can provide potential reactive sites for chemical crosslink, which benefit the enhancement of the structural stability of phenolic resin and restrict the release of small molecules during the high-temperature carbonization process. Herein, sucrose was introduced to modify the molecular structure of phenolic resin, and the intertwined phenolic resin/sucrose (PF/S) chemical polymerization networks, which upon carbonation, resulted in amorphous carbon material with a prominent pseudo-graphitic region and reduced surface defects. The representative PF-S-55-1200 carbon with the optimum crosslinking degree, when tested as an anode in SIBs, realized a high reversible capacity of 323.0 mAh g −1 , with a robust ICE of 86.4%. Moreover, the hard carbon heterostructure anode demonstrated improved structural and cycle stability, with a retained capacity of 238.3 mAh g −1 at a current density of 200 mA g −1 over 1500 cycles. The proposed strategy provides a comprehensive insight into the structural control and designing of new-heterostructured hard carbon anode materials, paving the pathway toward practical SIBs.

| Materials synthesis
Hard carbon heterostructures were prepared from the mixture of phenolic resin (PF) and sucrose through solvothermal treatment and a high-temperature carbonization process. In a typical procedure, a certain amount of PF was dissolved in ethanol and homogeneously mixed with sucrose aqueous solution, and then solvothermal treated at 180°C for 24 h in a Teflon autoclave to achieve molecular cross-linking. The cross-linked hybrid structures were later subjected to high-temperature carbonization in the range of 1000°C to 1400°C for 2 h under Ar flow to produce PF-S hard carbon heterostructures. The carbon materials are referred to as PF-S-X-T, where X represents the mass ratio of phenolic resin to sucrose, and T is the carbonization temperature. The pure phenolic resin pyrolytic carbon (PF-1200) and sucrose pyrolytic carbon (S-1200) were also prepared following the same solvent heat treatment and carbonization process.

| Materials characterization
The morphologies were observed with a scanning electron microscope (SEM, S-4800). High-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) patterns was collected on a transmission electron microscope (JEOL-2010). The microcrystalline structure information was obtained by using an X-ray diffractometer (XRD) with Cu radiation (XPert Pro MPD) and Raman spectra (Invia Reflex) equipped with a 514 nm argon laser. The porosity parameter was analyzed by nitrogen adsorption/desorption test (Micromeritics ASAP2460). The thermogravimetric analysis (TGA) was carried out using a TGA/DSC3 + simultaneous thermal analyzer from room temperature to 1000°C with a ramp rate of 5°C min −1 under nitrogen atmosphere. The formation mechanism of hard carbon heterostructure was analyzed by Fourier transform infrared spectrometers (FTIR, Nicolet 6700) and X-ray photoelectron spectroscopy (XPS, ESCA-LAB 250).

| Electrochemical measurements
The Electrochemical Na-storage performance of the obtained phenolic resin/sucrose hybrid hard carbons was evaluated in sodium ion half-cells. The working electrode was prepared by spreading the mixed slurry of active material and sodium carboxymethyl cellulose (CMC) binder in water solvent with a weight ratio of 95:5 onto Cu foil followed by vacuum drying at 120°C for 12 h. The C2025 type batteries were assembled in an Arfilled glove box (Mikrouna, H 2 O, O 2 < 0.1 ppm) with 1 M NaClO 4 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as electrolytes. Galvanostatic charge/discharge profiles were collected using a LAND CT-2001A battery-testing system. Cyclic voltammetry (CV) and electrochemical impedance spectrum (EIS) measurements were performed on the Bio-Logic VSP electrochemical workstation. PF-S based hard carbon heterostructures were evaluated against different experimental variables such as phenolic resin/sucrose ratio and carbonization temperature to realize a combination with an optimum electrochemical superiority.

| RESULTS AND DISCUSSION
To reveal the interaction between phenolic resin and sucrose in the solvothermal process, the functional group's types and variation in phenolic resin/sucrose cross-linked product were investigated by FTIR, as shown in Figure 1A. Compared with phenolic resin and sucrose, the disappearance of the stretching and bending vibration of -OH groups at 3562.2 cm −1 and weakened absorption bands of C-O around 1000-1460 cm −1 suggest the existence of strong hydrogen bonds 31 in the phenolic resin/sucrose cross-linked product (PF-S). In addition, the appearance of a stretching vibration peak of the C═O bond at 1704.8 cm −1 , which may be resulted from the dehydration reaction of sucrose during the solvothermal process, 32 can also facilitate the formation of hydrogen bonds and intermolecular interaction. 33,34 The variation of the surface functional groups during the solvothermal treatment was further confirmed by XPS analysis. Figure 1B and Supporting Information: Figure S1 demonstrate that the O element is present in 55.3 wt% for sucrose and 36.3 wt% for phenolic resin, respectively, which drops to 32.4 wt% for PF-S after the cross-linking process due to the dehydration reaction. The corresponding C1s spectra of sucrose and phenolic resin were further deconvoluted to C-C and C-O bands at 284.63 and 285.88 eV, respectively. However, a new C═O peak was discovered at 288.97 eV in the PF-S cross-linked matrix, correlating well with the FTIR results ( Figure 1B). 35 Similarly, in the spectrogram of O1s ( Figure 1C), there is also an obvious absorption peak of the C═O bond at 531.05 eV for PF-S in addition to the C-O bond (532.02 eV), which verifies the strong interaction between phenolic resin and sucrose.
The thermogravimetric analysis (TGA) further provides a strong basis for the cross-linking polymerization reaction between phenolic resin and sucrose in the solvothermal process. Figure 1D shows the recorded difference in the pyrolysis behavior of the materials. Here, sucrose is relatively stable below 200°C, and then loses weight rapidly around 200-400°C due to the strong dehydration, accompanied by the adhesion and foaming process. 36 The slow weight loss of sucrose above 400°C indicates its near-completed carbonization. The pyrolysis of phenolic resin contains three stages, including condensation, ether bond cleavage, and H atom detachment from the carbon skeleton. 28,37 In contrast, the PF-S (1:1) network shows a uniform and slow pyrolysis process with a significantly increased carbon yield of 36% (vs.~21% for the calculated value based on the mass ratio of phenolic resin/sucrose) at 1000°C, which can be attributed to the improved structural stability of precursor molecules due to the interaction between phenolic resin and sucrose in the solvothermal process.
The X-ray diffraction (XRD) patterns were performed to analyze the distinctive structure of the obtained F I G U R E 1 (A) FTIR spectra; (B, C) high-resolution XPS C1s and O1s profiles; and (D) TGA curves of PF, sucrose, and phenolic resin/ sucrose cross-linked product (un-carbonized). FTIR, Fourier transform infrared spectrometers; XPS, X-ray photoelectron spectroscopy.
pyrolytic carbons. Figure 2A shows the XRD pattern for the pyrolytic carbons with major peaks at~24°and~43°c orresponding to the (002) and (100) planes of carbon materials, respectively. 38 Apart from sucrose pyrolytic carbon (S-1200), which shows a relatively disordered structure with an average interlayer distance (d 002 ) of 3.89 Å, the phenolic resin pyrolytic carbon (PF-1200) and the phenolic resin/sucrose hybrid hard carbon PF-S-55-1200 display similar average interlayer distance (d 002 ) of~3.79 Å.
The microstructures of the obtained carbons were further characterized using Raman spectroscopy ( Figure 2B), where two broad bands at~1355 and 1595 cm −1 give direct information from D band (sp 3 hybridization caused by disordered and defective structure) and G band (sp 2 hybridization produced by graphite crystals). 39 The structural disorder degree of the carbons was estimated from the corresponding I D /I G ratio. Compared with PF-1200, the PF-S-55-1200 showed a relatively high I D /I G ratio of 2.10, suggesting a relatively more disordered microstructure. The D band in Raman spectra was then deconvoluted into four peaks by Gaussian-Lorentzian numerical simulation for a comprehensive analysis and interpretation of the microstructures, as shown in Figure 2C and Supporting Information: Figure S2. The D1 band at~1350 cm −1 is a prominent band with its intensity related to the disorder degree in carbonaceous materials. The D2 defect band (~1610 cm −1 ) is slightly higher in energy than the G band and originates from surface graphene layers. The D3 at~1495 cm −1 corresponds to amorphous carbon, whereas the D4 at~1215 cm −1 relates to a disordered graphitic lattice caused by sp2-sp3 bonds or vibrations of single/double carbon bonds. 40 The prominent D1 and D3 bands demonstrated the relatively disordered structure in PF-S-55-1200, recognizing its ideal structural configuration for Na ion accommodation.
The high-resolution transmission electron microscope (HRTEM) images and the corresponding selected area electron diffraction pattern (SAED) for the PF-S-55-1200 in reference to PF-1200 and S-1200 are shown in Figure 2D-F. The PF-1200 demonstrates a relatively ordered structure with a high graphitization degree, as evident by the continuous graphite crystallite stripes in HRTEM and bright diffraction rings in SAED patterns. In contrast, after hybridizing with sucrose, the microstructures of PF-S-55-1200 get amorphous with dispersing diffraction rings and no obvious long-range ordered structural features.
The structures of hard carbons are often inhomogeneous and complicated, as evidenced by the HRTEM images, which significantly adds to the difficulty of analyzing these hard carbon samples. As reported in our previous work, 41 the microcrystalline of the carbon materials for SIBs can be divided into three types according to the interlayer distance of the carbons: highly disordered phase (>0.36 nm), which enables Na ion freely access; pseudo-graphitic phase (0.36-0.40 nm), which is accessible for the insertion/extraction process of Na ion; and graphite-like phase (<0.36 nm), inaccessible for Na ion storage. Thus, a profile-fitting process was performed to simulate the characteristic of (002) peaks in the XRD pattern to differentiate the structures of PF-S derived hybrid hard carbon from the phenolic resin pyrolytic carbon (PF-1200). Each fitted peak is related to a type of carbon microcrystalline, as shown in Figure 2G-I. The PF-1200 ( Figure 2G) shows a predominant graphite-like phase (64.5%), which corroborates well with its HRTEM observation. The PF-S-55-1200, on the other hand, shows more amorphous structures with the graphite-like region decreased to 24.3%, while the dominant pseudo-graphitic region increased up to 75.7%, which guarantee the superiority of the phenolic resin/sucrose heterostructure for efficient Na ion storage. N 2 (77 K) adsorption/desorption measurements were performed to analyze the porosity structure distinction among the phenolic resin pyrolytic carbon, sucrose pyrolytic carbon, and PF-S hybrid hard carbon. The Brunauer-Emmett-Teller (BET) surface area (S BET ) and pore volume (V t ) were calculated from N 2 adsorption-desorption isotherms in Supporting Information: Figure S3. The experimental isotherms of PF-1200 show a combination of type I/II behavior with an S BET of 236.4 m 2 g −1 and V t of 0.37 cm 3 g −1 , indicating the existence of abundant micropores and large meso/macropores. In contrast, for the PF-S hybrid hard carbon, the porosity notably declines due to the enhanced structural stability of the precursor, which can effectively restrain the release of smaller gas molecules and avoid the formation of surface defects. From the very low N 2 adsorption amount in N 2 adsorption-desorption isotherms, the PF-S-55-1200 shows negligible porosity structures with a low S BET of 2.3 m 2 g −1 and a V t of 0.007 cm 3 g −1 . The decreased porosity and surface defect are favorable for reducing the side reaction with electrolytes, thus improving the ICE in the initial charge/discharge process.
The phenolic resin/sucrose ratio is found to play a vital role in the degree of cross-linking and thus affect the morphology and microstructures of the obtained hybrid hard carbons. As illustrated in Figure 3, when the phenolic resin to sucrose ratio is 7:3, the carbon shows irregular particle morphology due to insufficient sucrose concentration for effective cross-linking between the hybrid components. As the mass ratio changes to 5:5 in PF-S-55-1200, spherical structures with relatively smooth surfaces and uniform particle sizes of~2 µm were formed because of the improved cross-linking. The synthesized hard carbons could retain their spherical form when the sucrose concentration was further raised (PF-S-37-1200). However, the pyrolysis of the excessive sucrose then results in rough and fractured surfaces, which are detrimental to the intrinsic electrochemical performance of carbon materials.
The XRD and Raman spectra of the PF-S carbon heterostructures with various phenolic resin/sucrose ratios are shown in Figure 4A,B. Strangely, less or much addition of sucrose would decrease the structure disordered degree of hard carbon materials. The average d 002 values of PF-S-73-1200 and PF-S-37-1200 were calculated to be 3.73 and 3.70 Å, with an I D /I G ratio of 2.06 and 2.05 from Raman spectra, respectively. In contrast, the appropriate amount of sucrose results in a relatively disordered microcrystalline structure due to the enhanced cross-linking degree among PF-S. Thus, PF-S-64-1200, PF-S-55-1200, and PF-S-46-1200 show an increased interlayer distance of~3.8 Å with an enhanced I D /I G ratio of~2.11.
The carbonization temperature also has a significant effect on the structure of the obtained hybrid crystallites, as shown in Figure 4C,D. As the pyrolysis temperature increases from 1000°C to 1400°C, the interlayer spacing of the derived carbon gradually decreases from 4.12 to 3.72 Å, and the microcrystalline structure gradually transformed from highly disordered to relatively ordered. The corresponding Raman analysis further confirms the improved structural regularity of hard carbons as the sharpness of the typical D and G-bands increases with pyrolysis temperature. For PF-S-55-1000 and PF-S-55-1400, the I D /I G ratio decreases from 2.63 to 1.85, indicating decreased defects and improved structural order at high pyrolysis temperatures.
The electrochemical Na-storage properties of the obtained hard carbons were tested in coin cells with sodium metal foil as a counter electrode. Figure 5A-C and Supporting Information: Figure S4 show the CV curves for PF-1200, S-1200, and PF-S derived hybrid hard carbon electrodes at a scan rate of 0.1 mV s −1 . The CV curves generally show a strong redox/reduction peak in the potential range of 0.01-0.2 V versus Na/Na + and a mild hump in 0.2-1.5 V, which correspond to Na-ion insertion into carbon layers and adsorption onto surface defects/edges, respectively. 42,43 The irreversible reduction peaks at 0.25-0.75 V and~1 V that appeared in the initial cycle can be ascribed to the irreversible interfacial reactions between electrode and electrolyte as well as the formation of SEI film in the first charge/discharge process. 39 Unlike PF-1200 and S-1200, the PF-S derived hard carbon heterostructures demonstrate a smaller irreversible process, which may be ascribed to the F I G U R E 4 XRD and Raman spectra of the PF-S carbon heterostructures obtained with (A, B) different phenolic resin/sucrose ratios and (C, D) different carbonization temperatures. XRD, X-ray diffractometer. reduced surface defect in the hybrid carbon microcrystalline. Moreover, the CV curves of PF-S derived hybrid hard carbons overlap well in the subsequent cycles, indicating good reversibility for the Na ions' insertion/extraction process. Figure 5D-F shows the initial galvanostatic charge/ discharge curves of PF-1200, S-1200 and PF-S-55-1200 recorded at a constant current density of 30 mA g −1 . The voltage profiles can be divided into the plateau region below 0.1 V and the sloping region above 0.1 V, corresponding to the sharp redox/reduction peak at the low-voltage region and the weak hump at the medium range in the CV curves, respectively. PF-1200 delivered a reversible Na-storage capacity of 267.1 mAh g −1 with a low ICE of 46.3% due to the large surface area and abundant surface defects, and the sloping region accounts for 62.3% of the total capacity, while S-1200 exhibited a Na-storage capacity of 246.4 mAh g −1 with a higher ICE of 77.2%. In contrast, the PF-S derived hybrid hard carbons showed a much-improved Na-storage capacity of 323.0 mAh g −1 and ICE value of 86.4% due to the enhanced structural stability of the intertwined cross-linked PF-S macromolecules network that could restrain the release of smaller gas molecules and limit surface defect formation during the high-temperature carbonization. Moreover, the short-range ordered graphitic layers in these microcrystalline heterostructures with suitable interlayer spacing are favorable for convenient Na ion insertion/extraction, allowing for improved Na storage capacity.
Since the phenolic resin/sucrose ratio could effectively alter the cross-linking and thus the microstructures of obtained hybrid hard carbons, significant variation in the respective electrochemical Na-storage performance could be anticipated ( Figure 5G and Supporting Information: Figure S5). Unlike PF-1200, a Na-storage capacity of 264.6 mAh g −1 with an enhanced ICE of 51.6% was observed for PF-S-73-1200, with the plateau region contribution increased from 37.7% to 43.0% ( Figure 5G). This improvement is attributed to the increased pseudo-graphitic region and reduced surface defects that promote the Na ion insertion/extraction process. Supporting Information: Figure S5 further confirms this trend of improved Na-storage capacity with steadily increasing sucrose content in PF-S-73-1200, PF-S-64-1200, and PF-S-55-1200, respectively. The PF-S-55-1200 was assessed as the optimum hybrid hard carbon that could deliver the highest reversible capacity of 323.0 mAh g −1 with 64.8% plateau region capacity contribution due to its high proportion of pseudographitic region (75.7%). In addition, because of the reduced surface defects, the PF-S-55-1200 also exhibits an impressive ICE of 86.4%, which is significantly superior to most of the hard carbon anodes reported in the literature. 22 As the sucrose content rises further in the case of PF-S-46-1200 and PF-S-37-1200, the superfluous sucrose pyrolytic carbon would deteriorate the electrochemical Na-storage performance of PF-S derived hybrid hard carbons, the Na-storage capacity declines to 306.5 and 287.8 mAh g −1 , with ICE values of 85.7% and 84.7%, respectively. As seen in Figure 5G, the associated plateau capacity contribution also steadily decreases due to the reduced pseudo-graphitic regions.
The variation in the electrochemical performance of PF-S derived hybrid hard carbons was also studied against the carbonization temperature. As shown in Supporting Information: Figure S6, increasing the pyrolysis temperature from 1000°C to 1400°C, the sloping region capacity of the carbon electrode gradually decreased from 120.2 to 102.4 mAh g −1 , while the capacity of the plateau region increased, accompanied by the improved ICE. As the pyrolysis temperature reaches 1400°C, the PF-S-55-1400 electrode delivers an initial reversible sodium storage capacity of 303.6 mAh g −1 with an ICE of 86.2%. The slight decrease in reversible capacity of PF-S-55-1400 to PF-S-55-1200 can be attributed to the reduction in the carbon interlayer spacing and decrease of surface defect, which is unfavorable for efficient Na ion uptake between carbon layers and adsorption at the active sites on the surface.
As shown in Figure 5H and Supporting Information: Figure S7a, all the phenolic resin/sucrose hybrid hard carbons show satisfactory cycle stability, in particular, the PF-S-55-1200 electrode maintained a reversible capacity of 311.9 mAh g −1 after 80 cycles at a current density of 30 mA g −1 with a capacity retention of 96.6%. Moreover, it could maintain 238.3 mAh g −1 at a high current density of 200 mA g −1 with no capacity decay over 1500 cycles, justifying its superior cycling stability ( Figure 5H). The corresponding rate capability was later assessed at current densities ranging from 30 to 1000 mA g −1 with PF-S-55-1200 exhibiting a high Na-storage capacity of 311.5 and 275.7 mAh g −1 at the current density of 50 and 100 mA g −1 , respectively ( Figure 6A). However, as the current density reaches 1 A g −1 , the reversible capacity drops rapidly to 69.6 mAh g −1 . The potential curves in Figure 6B show a decline in the plateau and sloping region capacities with the current densities, and a relatively faster decay rate is observed for the plateau region, which even disappears as the current density reaches 500 and 1000 mA g −1 . This is evidence that the associated interlayer insertion-based Na storage mechanism in the plateau region has slower dynamics than the adsorption mechanism in the sloping region. 39 EIS tests were further performed to investigate the Nastorage kinetics of PF-1200, S-1200, and PF-S-55-1200 electrodes, as shown in Figure 6C. In general, the Warburg impedance (Z W ) in the low-frequency region is related to Na ion diffusion, 44,45 whereas the semicircular area in the middle and high-frequency region represents Na ion charge transfer resistance (R ct ). 46 The contact impedance R e is the electronic impedance corresponding to the spectrum intersection at the X-axis. The equivalent circuit diagram simulation estimates the charge transfer resistance R ct of the PF-1200 and S-1200 electrodes to be 94.4 Ω and 91.6 Ω, respectively, which reduces to 71.1 Ω in the case of PF-S-55-1200. Moreover, the corresponding Z W is relatively small, reflecting the loosened structure with expanded interlayer spaces of PF-S-55-1200 conducive to Na ion transportation. Although the overall performance of PF-S-55-1200 was relatively superior to other carbon counterparts, the rate performance was considered modest, which can be mainly attributed to the high sensitivity of the plateau region capacity to the polarization phenomenon at large charge/discharge rates. Thus, the next step following this research would focus on the improvement of rate performance, such as substituting the ester electrolyte with an ether-based electrolyte, which has proven the ability to solvate Na + and Na + cointercalation mechanism and realizes improved kinetics. 47 The comparison of the rate performance of PF-S derived hybrid hard carbons with distinct phenolic resin/ sucrose ratios is shown in Supporting Information: Figure S7b. Although the PF-S-73-1200 electrode has a modest sodium storage capacity at low currents, the rate performance is satisfactory with a retained capacity of 86.2 mAh g −1 at 1 A g −1 , higher than its PF-S-55-1200 counterpart (69.6 mAh g −1 ), which could be attributed to the relatively rich surface defects and dominant adsorption-driven Na storage kinetics.
CV measurements at various scan rates were performed to investigate the charge storage mechanism of phenolic resin/sucrose-based hard carbons. The relationship between scan rate and peak current could be defined by the following equations 48 : i ν a log( ) = blog( ) + log( ). (2) Here a, b are constants, while i and ν represent current and scan rate, respectively. The value of b = 1 implies the capacitive control process, while b = 0.5 relates to the diffusion control process. Figure 6D shows F I G U R E 6 (A) Rate performance of PF-1200, S-1200, and PF-S-55-1200; (B) the charging curves of PF-S-55-1200 at different current densities; (C) Nyquist plots and equivalent circuits to simulate electrochemical impedance spectrum (EIS) of PF-1200, S-1200, and PF-S-55-1200 electrodes; (D) cyclic voltammetry (CV) curves of PF-S-55-1200 electrode at different scan rates, with the relationship curves of peak current at (E) 0.1 V and (F) 1.5 V with scanning rate, CV curves at (G) 0.1 mV s −1 and (H) 2 mV s −1 with shaded region defining pseudo capacitance contribution; and (I) the corresponding histogram differentiating capacitive and diffusion controlled capacitance contribution at different scanning rates of PF-S-55-1200 electrode. the CV curve of PF-S-55-1200 at various scan rates from 0.1 to 2 mV s −1 , and the peaks at 0.1 V and the hump centered at 1.5 V were selected to study the relationship between i and ν. As shown in Figure 6E,F, the fitted b value at 0.1 V is 0.618, indicating that the plateau region corresponds to a diffusion control process, i.e., interlayer insertion process; the fitted b value at 1.5 V is 0.996, suggesting that high-voltage region belongs to the surface capacitance control process.
In addition, the distinct capacity contribution, that is, capacitive or diffusion-controlled current at different scan rates, was also estimated using Equation (3) Here, k ν 1 and k ν 2 1/2 represent the capacitive control and the diffusion control contribution to the current, respectively. At a scan rate of 0.1 mV s −1 , the capacitance contribution was calculated to be 55.0% (highlighted region), which later increased to 82.1% ( Figure 6G-I) when the scan rate enhanced to 2 mV s −1 , indicating that the capacitive-dominant process plays an important role at high rates. The co-contribution of the interlayer insertion process and surface capacitance mechanism endows the PF-S carbon heterostructures excellent Na-storage performance.

| CONCLUSION
In conclusion, hard carbon heterostructures with efficient Na-ion accommodating ability were successfully prepared via the carbonization of phenolic resin/sucrose crosslinking networks. Different from pristine phenolic resin pyrolytic carbon (PF-1200), the obtained hybrid hard carbons (PF-S) show prominent pseudo-graphitic regions with expanded interlayer space and reduced surface defects. When tested as anodes for SIBs, the representative PF-S-55-1200 carbon material delivered both a high reversible capacity of 323.0 mAh g −1 and a high ICE of 86.4%, remarkably better than the PF-1200 electrode with a capacity of 267.1 mAh g −1 and an ICE of 46.3%. In addition, the obtained hybrid hard carbon also exhibited robust structural stability and long-term cyclability with a maintained capacity of 238.3 mAh g −1 over 1500 cycles at a current density of 200 mA g −1 . The proposed strategy serves as an effective step in designing low-cost hybrid hard carbon materials with desired structural configurations and electrochemical performance for advanced SIBs.

SUPPORTING INFORMATION
Additional supporting information can be found online in the Supporting Information section at the end of this article.